diff --git a/api/edge/__init__.py b/api/edge/__init__.py
new file mode 100644
index 0000000..c7b5b01
--- /dev/null
+++ b/api/edge/__init__.py
@@ -0,0 +1 @@
+"""State Hub edge relay and durable outbox helpers."""
diff --git a/api/edge/outbox.py b/api/edge/outbox.py
new file mode 100644
index 0000000..a244f90
--- /dev/null
+++ b/api/edge/outbox.py
@@ -0,0 +1,358 @@
+from __future__ import annotations
+
+import json
+import os
+import sqlite3
+import stat
+import uuid
+from dataclasses import dataclass
+from datetime import datetime, timedelta, timezone
+from pathlib import Path
+from typing import Any
+
+from api.services.write_idempotency import route_class_for
+
+DEFAULT_OUTBOX_PATH = Path(os.environ.get("STATEHUB_OUTBOX_PATH", "~/.statehub/edge-outbox.sqlite3")).expanduser()
+MAX_PAYLOAD_BYTES = 64 * 1024
+SECRET_FIELD_NAMES = {
+    "authorization",
+    "cookie",
+    "set-cookie",
+    "password",
+    "passwd",
+    "secret",
+    "api_key",
+    "apikey",
+    "access_token",
+    "refresh_token",
+    "bearer_token",
+    "client_secret",
+    "private_key",
+    "credential",
+    "credentials",
+}
+
+
+@dataclass(frozen=True)
+class OutboxEnvelope:
+    id: str
+    idempotency_key: str
+    method: str
+    path: str
+    body: dict[str, Any] | list[Any] | None
+    route_class: str
+    source_agent: str | None
+    source_host: str | None
+    repo_slug: str | None
+    session_id: str | None
+    observed_revision: dict[str, Any] | None
+    status: str
+    attempt_count: int
+    next_retry_at: str | None
+    last_error: str | None
+    response_status: int | None
+    response_body: dict[str, Any] | list[Any] | str | None
+    created_at: str
+    updated_at: str
+    acked_at: str | None
+
+
+class PayloadRejected(ValueError):
+    pass
+
+
+def utcnow() -> str:
+    return datetime.now(tz=timezone.utc).isoformat()
+
+
+def default_outbox_path() -> Path:
+    return DEFAULT_OUTBOX_PATH
+
+
+def scrub_payload(value: Any) -> Any:
+    if isinstance(value, dict):
+        scrubbed: dict[str, Any] = {}
+        for key, item in value.items():
+            normalized = str(key).lower().replace("-", "_")
+            if normalized in SECRET_FIELD_NAMES:
+                scrubbed[key] = "[redacted]"
+            else:
+                scrubbed[key] = scrub_payload(item)
+        return scrubbed
+    if isinstance(value, list):
+        return [scrub_payload(item) for item in value]
+    return value
+
+
+def _json_loads(raw: str | None) -> Any:
+    if raw is None:
+        return None
+    return json.loads(raw)
+
+
+def _json_dumps(value: Any) -> str | None:
+    if value is None:
+        return None
+    return json.dumps(value, sort_keys=True, separators=(",", ":"))
+
+
+def _parse_dt(value: str | None) -> datetime | None:
+    if not value:
+        return None
+    return datetime.fromisoformat(value)
+
+
+class OutboxStore:
+    def __init__(self, path: str | Path | None = None) -> None:
+        self.path = Path(path).expanduser() if path is not None else default_outbox_path()
+        self.path.parent.mkdir(parents=True, exist_ok=True)
+        self._init_db()
+        self._chmod_private()
+
+    def _connect(self) -> sqlite3.Connection:
+        conn = sqlite3.connect(self.path)
+        conn.row_factory = sqlite3.Row
+        return conn
+
+    def _init_db(self) -> None:
+        with self._connect() as conn:
+            conn.execute(
+                """
+                CREATE TABLE IF NOT EXISTS outbox_envelopes (
+                    id TEXT PRIMARY KEY,
+                    idempotency_key TEXT NOT NULL UNIQUE,
+                    method TEXT NOT NULL,
+                    path TEXT NOT NULL,
+                    body_json TEXT,
+                    route_class TEXT NOT NULL,
+                    source_agent TEXT,
+                    source_host TEXT,
+                    repo_slug TEXT,
+                    session_id TEXT,
+                    observed_revision_json TEXT,
+                    status TEXT NOT NULL,
+                    attempt_count INTEGER NOT NULL DEFAULT 0,
+                    next_retry_at TEXT,
+                    last_error TEXT,
+                    response_status INTEGER,
+                    response_body_json TEXT,
+                    created_at TEXT NOT NULL,
+                    updated_at TEXT NOT NULL,
+                    acked_at TEXT
+                )
+                """
+            )
+            conn.execute("CREATE INDEX IF NOT EXISTS ix_outbox_status ON outbox_envelopes(status)")
+            conn.execute("CREATE INDEX IF NOT EXISTS ix_outbox_next_retry ON outbox_envelopes(next_retry_at)")
+            conn.commit()
+
+    def _chmod_private(self) -> None:
+        try:
+            os.chmod(self.path, stat.S_IRUSR | stat.S_IWUSR)
+        except OSError:
+            pass
+
+    def enqueue(
+        self,
+        *,
+        method: str,
+        path: str,
+        body: Any,
+        idempotency_key: str | None = None,
+        source_agent: str | None = None,
+        source_host: str | None = None,
+        repo_slug: str | None = None,
+        session_id: str | None = None,
+        observed_revision: dict[str, Any] | None = None,
+    ) -> OutboxEnvelope:
+        route_class = route_class_for(method, path)
+        if route_class is None:
+            raise PayloadRejected(f"{method.upper()} {path} is not queueable")
+        scrubbed = scrub_payload(body)
+        encoded = _json_dumps(scrubbed)
+        if encoded is not None and len(encoded.encode("utf-8")) > MAX_PAYLOAD_BYTES:
+            raise PayloadRejected("payload exceeds offline outbox size limit")
+        now = utcnow()
+        envelope_id = str(uuid.uuid4())
+        key = idempotency_key or f"statehub-edge:{envelope_id}"
+        method_upper = method.upper()
+        with self._connect() as conn:
+            if route_class == "replace":
+                conn.execute(
+                    """
+                    UPDATE outbox_envelopes
+                    SET status = 'cancelled', updated_at = ?, last_error = ?
+                    WHERE status = 'queued'
+                      AND route_class = 'replace'
+                      AND method = ?
+                      AND path = ?
+                    """,
+                    (now, f"superseded by {envelope_id}", method_upper, path),
+                )
+            conn.execute(
+                """
+                INSERT INTO outbox_envelopes (
+                    id, idempotency_key, method, path, body_json, route_class,
+                    source_agent, source_host, repo_slug, session_id,
+                    observed_revision_json, status, created_at, updated_at
+                ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, 'queued', ?, ?)
+                """,
+                (
+                    envelope_id,
+                    key,
+                    method_upper,
+                    path,
+                    encoded,
+                    route_class,
+                    source_agent,
+                    source_host,
+                    repo_slug,
+                    session_id,
+                    _json_dumps(observed_revision),
+                    now,
+                    now,
+                ),
+            )
+            conn.commit()
+        return self.get(envelope_id)
+
+    def get(self, envelope_id: str) -> OutboxEnvelope:
+        with self._connect() as conn:
+            row = conn.execute("SELECT * FROM outbox_envelopes WHERE id = ?", (envelope_id,)).fetchone()
+        if row is None:
+            raise KeyError(envelope_id)
+        return self._row_to_envelope(row)
+
+    def list(self, *, status: str | None = None, limit: int = 100) -> list[OutboxEnvelope]:
+        with self._connect() as conn:
+            if status:
+                rows = conn.execute(
+                    "SELECT * FROM outbox_envelopes WHERE status = ? ORDER BY created_at LIMIT ?",
+                    (status, limit),
+                ).fetchall()
+            else:
+                rows = conn.execute(
+                    "SELECT * FROM outbox_envelopes ORDER BY created_at LIMIT ?",
+                    (limit,),
+                ).fetchall()
+        return [self._row_to_envelope(row) for row in rows]
+
+    def due(self, *, limit: int = 50) -> list[OutboxEnvelope]:
+        now = utcnow()
+        with self._connect() as conn:
+            rows = conn.execute(
+                """
+                SELECT * FROM outbox_envelopes
+                WHERE status = 'queued' AND (next_retry_at IS NULL OR next_retry_at <= ?)
+                ORDER BY created_at
+                LIMIT ?
+                """,
+                (now, limit),
+            ).fetchall()
+        return [self._row_to_envelope(row) for row in rows]
+
+    def summary(self) -> dict[str, Any]:
+        with self._connect() as conn:
+            rows = conn.execute(
+                "SELECT status, COUNT(*) AS count, MIN(created_at) AS oldest FROM outbox_envelopes GROUP BY status"
+            ).fetchall()
+        by_status = {row["status"]: row["count"] for row in rows}
+        oldest_pending = None
+        for row in rows:
+            if row["status"] in {"queued", "sending", "conflict"} and row["oldest"]:
+                oldest_pending = min(filter(None, [oldest_pending, row["oldest"]])) if oldest_pending else row["oldest"]
+        return {
+            "path": str(self.path),
+            "by_status": by_status,
+            "pending_count": sum(by_status.get(status, 0) for status in ("queued", "sending")),
+            "conflict_count": by_status.get("conflict", 0),
+            "oldest_pending_at": oldest_pending,
+        }
+
+    def mark_sending(self, envelope_id: str) -> None:
+        self._update(envelope_id, status="sending", updated_at=utcnow())
+
+    def mark_acked(self, envelope_id: str, *, response_status: int, response_body: Any) -> None:
+        now = utcnow()
+        self._update(
+            envelope_id,
+            status="acked",
+            response_status=response_status,
+            response_body_json=_json_dumps(response_body),
+            updated_at=now,
+            acked_at=now,
+            last_error=None,
+            next_retry_at=None,
+        )
+
+    def mark_conflict(self, envelope_id: str, *, response_status: int, response_body: Any) -> None:
+        self._update(
+            envelope_id,
+            status="conflict",
+            response_status=response_status,
+            response_body_json=_json_dumps(response_body),
+            updated_at=utcnow(),
+            last_error="conflict",
+        )
+
+    def mark_dead(self, envelope_id: str, *, error: str, response_status: int | None = None, response_body: Any = None) -> None:
+        self._update(
+            envelope_id,
+            status="dead",
+            response_status=response_status,
+            response_body_json=_json_dumps(response_body),
+            updated_at=utcnow(),
+            last_error=error,
+        )
+
+    def mark_retry(self, envelope_id: str, *, error: str, attempt_count: int) -> None:
+        delay_seconds = min(3600, 2 ** min(attempt_count, 10))
+        next_retry = datetime.now(tz=timezone.utc) + timedelta(seconds=delay_seconds)
+        self._update(
+            envelope_id,
+            status="queued",
+            attempt_count=attempt_count,
+            next_retry_at=next_retry.isoformat(),
+            updated_at=utcnow(),
+            last_error=error[:500],
+        )
+
+    def retry(self, envelope_id: str) -> None:
+        self._update(envelope_id, status="queued", next_retry_at=None, updated_at=utcnow())
+
+    def cancel(self, envelope_id: str) -> None:
+        self._update(envelope_id, status="cancelled", updated_at=utcnow())
+
+    def export(self, *, status: str | None = None, limit: int = 1000) -> list[dict[str, Any]]:
+        return [envelope.__dict__ for envelope in self.list(status=status, limit=limit)]
+
+    def _update(self, envelope_id: str, **values: Any) -> None:
+        assignments = ", ".join(f"{key} = ?" for key in values)
+        params = list(values.values()) + [envelope_id]
+        with self._connect() as conn:
+            conn.execute(f"UPDATE outbox_envelopes SET {assignments} WHERE id = ?", params)
+            conn.commit()
+
+    def _row_to_envelope(self, row: sqlite3.Row) -> OutboxEnvelope:
+        return OutboxEnvelope(
+            id=row["id"],
+            idempotency_key=row["idempotency_key"],
+            method=row["method"],
+            path=row["path"],
+            body=_json_loads(row["body_json"]),
+            route_class=row["route_class"],
+            source_agent=row["source_agent"],
+            source_host=row["source_host"],
+            repo_slug=row["repo_slug"],
+            session_id=row["session_id"],
+            observed_revision=_json_loads(row["observed_revision_json"]),
+            status=row["status"],
+            attempt_count=row["attempt_count"],
+            next_retry_at=row["next_retry_at"],
+            last_error=row["last_error"],
+            response_status=row["response_status"],
+            response_body=_json_loads(row["response_body_json"]),
+            created_at=row["created_at"],
+            updated_at=row["updated_at"],
+            acked_at=row["acked_at"],
+        )
diff --git a/api/edge/relay.py b/api/edge/relay.py
new file mode 100644
index 0000000..9277652
--- /dev/null
+++ b/api/edge/relay.py
@@ -0,0 +1,206 @@
+from __future__ import annotations
+
+import os
+import socket
+from typing import Any
+
+import httpx
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse, Response
+
+from api.edge.outbox import OutboxEnvelope, OutboxStore, PayloadRejected, default_outbox_path
+from api.services.write_idempotency import route_class_for
+
+HOP_BY_HOP_HEADERS = {
+    "connection",
+    "keep-alive",
+    "proxy-authenticate",
+    "proxy-authorization",
+    "te",
+    "trailer",
+    "transfer-encoding",
+    "upgrade",
+    "content-encoding",
+    "content-length",
+}
+
+
+def _safe_response_headers(headers: httpx.Headers) -> dict[str, str]:
+    return {key: value for key, value in headers.items() if key.lower() not in HOP_BY_HOP_HEADERS}
+
+
+def _body_summary(response: httpx.Response) -> Any:
+    try:
+        return response.json()
+    except ValueError:
+        return {"text": response.text[:500]}
+
+
+def queued_receipt(envelope: OutboxEnvelope, upstream_error: str) -> dict[str, Any]:
+    return {
+        "queued": True,
+        "outbox_id": envelope.id,
+        "idempotency_key": envelope.idempotency_key,
+        "upstream": "unreachable",
+        "upstream_error": upstream_error,
+        "route_class": envelope.route_class,
+    }
+
+
+async def replay_pending(
+    store: OutboxStore,
+    *,
+    upstream_url: str,
+    limit: int = 50,
+    timeout: float = 10.0,
+) -> dict[str, int]:
+    counts = {"sent": 0, "acked": 0, "conflict": 0, "retry": 0, "dead": 0}
+    async with httpx.AsyncClient(base_url=upstream_url.rstrip("/"), timeout=timeout) as client:
+        for envelope in store.due(limit=limit):
+            counts["sent"] += 1
+            store.mark_sending(envelope.id)
+            try:
+                response = await client.request(
+                    envelope.method,
+                    envelope.path,
+                    json=envelope.body,
+                    headers={
+                        "Idempotency-Key": envelope.idempotency_key,
+                        "X-StateHub-Source-Agent": envelope.source_agent or "statehub-edge",
+                        "X-StateHub-Source-Host": envelope.source_host or socket.gethostname(),
+                    },
+                )
+            except httpx.HTTPError as exc:
+                counts["retry"] += 1
+                store.mark_retry(envelope.id, error=str(exc), attempt_count=envelope.attempt_count + 1)
+                continue
+
+            response_body = _body_summary(response)
+            if response.status_code == 409:
+                counts["conflict"] += 1
+                store.mark_conflict(envelope.id, response_status=response.status_code, response_body=response_body)
+            elif 200 <= response.status_code < 300:
+                counts["acked"] += 1
+                store.mark_acked(envelope.id, response_status=response.status_code, response_body=response_body)
+            elif response.status_code >= 500:
+                counts["retry"] += 1
+                store.mark_retry(
+                    envelope.id,
+                    error=f"HTTP {response.status_code}: {response.text[:300]}",
+                    attempt_count=envelope.attempt_count + 1,
+                )
+            else:
+                counts["dead"] += 1
+                store.mark_dead(
+                    envelope.id,
+                    error=f"HTTP {response.status_code}: not retryable",
+                    response_status=response.status_code,
+                    response_body=response_body,
+                )
+    return counts
+
+
+def create_app(
+    *,
+    upstream_url: str | None = None,
+    outbox_path: str | None = None,
+    timeout: float = 10.0,
+) -> FastAPI:
+    upstream = (upstream_url or os.environ.get("STATEHUB_UPSTREAM_URL") or os.environ.get("API_BASE") or "http://127.0.0.1:8000").rstrip("/")
+    store_path = outbox_path or default_outbox_path()
+    store_instance: OutboxStore | None = None
+
+    def get_store() -> OutboxStore:
+        nonlocal store_instance
+        if store_instance is None:
+            store_instance = OutboxStore(store_path)
+        return store_instance
+
+    app = FastAPI(title="State Hub Edge Relay", version="0.1.0")
+
+    @app.get("/edge/health")
+    async def edge_health() -> dict[str, Any]:
+        reachable = False
+        error = None
+        try:
+            async with httpx.AsyncClient(base_url=upstream, timeout=2.0) as client:
+                response = await client.get("/state/health")
+                reachable = response.status_code < 500
+        except httpx.HTTPError as exc:
+            error = str(exc)
+        return {
+            "status": "ok",
+            "upstream": upstream,
+            "upstream_reachable": reachable,
+            "upstream_error": error,
+            "outbox": get_store().summary(),
+        }
+
+    @app.post("/edge/replay")
+    async def edge_replay(limit: int = 50) -> dict[str, int]:
+        return await replay_pending(get_store(), upstream_url=upstream, limit=limit, timeout=timeout)
+
+    @app.api_route("/{path:path}", methods=["GET", "POST", "PATCH", "PUT", "DELETE"])
+    async def proxy(path: str, request: Request) -> Response:
+        api_path = "/" + path
+        body: Any = None
+        if request.method in {"POST", "PATCH", "PUT"}:
+            try:
+                body = await request.json()
+            except ValueError:
+                body = None
+
+        headers = {}
+        if idempotency_key := request.headers.get("idempotency-key"):
+            headers["Idempotency-Key"] = idempotency_key
+        if request.headers.get("content-type"):
+            headers["Content-Type"] = request.headers["content-type"]
+
+        try:
+            async with httpx.AsyncClient(base_url=upstream, timeout=timeout) as client:
+                response = await client.request(
+                    request.method,
+                    api_path,
+                    params=request.query_params,
+                    json=body if body is not None else None,
+                    headers=headers,
+                )
+            return Response(
+                content=response.content,
+                status_code=response.status_code,
+                headers=_safe_response_headers(response.headers),
+                media_type=response.headers.get("content-type"),
+            )
+        except httpx.HTTPError as exc:
+            route_class = route_class_for(request.method, api_path)
+            if route_class is None or request.method not in {"POST", "PATCH"}:
+                return JSONResponse(
+                    status_code=503,
+                    content={
+                        "error": "upstream unreachable and route is not queueable",
+                        "method": request.method,
+                        "path": api_path,
+                        "upstream": upstream,
+                        "detail": str(exc),
+                    },
+                )
+            try:
+                envelope = get_store().enqueue(
+                    method=request.method,
+                    path=api_path,
+                    body=body,
+                    idempotency_key=request.headers.get("idempotency-key"),
+                    source_agent=request.headers.get("x-statehub-source-agent"),
+                    source_host=request.headers.get("x-statehub-source-host") or socket.gethostname(),
+                    repo_slug=request.headers.get("x-statehub-repo-slug"),
+                    session_id=request.headers.get("x-statehub-session-id"),
+                    observed_revision=None,
+                )
+            except PayloadRejected as reject:
+                return JSONResponse(status_code=422, content={"error": str(reject)})
+            return JSONResponse(status_code=202, content=queued_receipt(envelope, str(exc)))
+
+    return app
+
+
+app = create_app()
diff --git a/api/main.py b/api/main.py
index 0699713..bc14b74 100644
--- a/api/main.py
+++ b/api/main.py
@@ -11,6 +11,7 @@ from starlette.responses import Response as StarletteResponse
 
 from api.database import engine
 from api.events import shutdown_publisher
+from api.services.write_idempotency import WriteIdempotencyMiddleware
 from api.routers import decisions, extension_points, progress, state, tasks, technical_debt, topics, workstreams, workstream_dependencies
 from api.routers import domains, repos, contributions, sbom, policy, domain_goals, repo_goals, messages, capability_requests, tpsc, services
 from api.routers import token_events
@@ -91,13 +92,14 @@ _default_dashboard_origins = [
 _cors_env = os.getenv("CORS_ORIGINS", ",".join(_default_dashboard_origins))
 _cors_origins = [o.strip() for o in _cors_env.split(",") if o.strip()]
 
+app.add_middleware(WriteIdempotencyMiddleware)
 app.add_middleware(ETagMiddleware)
 app.add_middleware(
     CORSMiddleware,
     allow_origins=_cors_origins,
     allow_methods=["GET", "POST", "PATCH", "DELETE", "PUT"],
-    allow_headers=["Content-Type", "If-None-Match"],
-    expose_headers=["ETag", "X-StateHub-Elapsed-Ms", "X-StateHub-Response-Bytes", "X-StateHub-Cache"],
+    allow_headers=["Content-Type", "If-None-Match", "Idempotency-Key", "X-StateHub-Source-Agent", "X-StateHub-Source-Host"],
+    expose_headers=["ETag", "X-StateHub-Elapsed-Ms", "X-StateHub-Response-Bytes", "X-StateHub-Cache", "X-StateHub-Idempotency-Replay"],
 )
 
 app.include_router(domains.router)
diff --git a/api/models/__init__.py b/api/models/__init__.py
index 1a1b1a6..f40b01b 100644
--- a/api/models/__init__.py
+++ b/api/models/__init__.py
@@ -33,6 +33,7 @@ from api.models.interface_change import InterfaceChange
 from api.models.workplan_launch_request import WorkplanLaunchRequest
 from api.models.fabric_graph import FabricGraphImport, FabricGraphNode, FabricGraphEdge
 from api.models.legacy_meter import LegacyInterface, LegacyInterfaceUsageBucket
+from api.models.write_idempotency_key import WriteIdempotencyKey
 
 __all__ = [
     "Base",
@@ -65,4 +66,5 @@ __all__ = [
     "WorkplanLaunchRequest",
     "FabricGraphImport", "FabricGraphNode", "FabricGraphEdge",
     "LegacyInterface", "LegacyInterfaceUsageBucket",
+    "WriteIdempotencyKey",
 ]
\ No newline at end of file
diff --git a/api/models/write_idempotency_key.py b/api/models/write_idempotency_key.py
new file mode 100644
index 0000000..3ebc21f
--- /dev/null
+++ b/api/models/write_idempotency_key.py
@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+from typing import Any
+
+from sqlalchemy import DateTime, Integer, String, Text, UniqueConstraint
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from api.models.base import Base, new_uuid
+
+
+class WriteIdempotencyKey(Base):
+    __tablename__ = "write_idempotency_keys"
+    __table_args__ = (
+        UniqueConstraint("key", name="uq_write_idempotency_keys_key"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=new_uuid)
+    key: Mapped[str] = mapped_column(String(200), nullable=False, index=True)
+    method: Mapped[str] = mapped_column(String(10), nullable=False)
+    path: Mapped[str] = mapped_column(Text, nullable=False)
+    route_class: Mapped[str] = mapped_column(String(30), nullable=False)
+    request_hash: Mapped[str] = mapped_column(String(64), nullable=False)
+    response_status: Mapped[int] = mapped_column(Integer, nullable=False)
+    response_body: Mapped[Any] = mapped_column(JSONB, nullable=True)
+    source_host: Mapped[str | None] = mapped_column(String(200), nullable=True)
+    source_agent: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    first_seen_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    last_seen_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True, index=True)
diff --git a/api/services/write_idempotency.py b/api/services/write_idempotency.py
new file mode 100644
index 0000000..a7c06b5
--- /dev/null
+++ b/api/services/write_idempotency.py
@@ -0,0 +1,221 @@
+from __future__ import annotations
+
+import hashlib
+import json
+import re
+from dataclasses import dataclass
+from datetime import datetime, timedelta, timezone
+from typing import Any
+
+from sqlalchemy import select
+from starlette.responses import JSONResponse
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
+
+from api.database import async_session_factory
+from api.models.write_idempotency_key import WriteIdempotencyKey
+
+IDEMPOTENCY_HEADER = b"idempotency-key"
+REPLAY_HEADER = "X-StateHub-Idempotency-Replay"
+CONFLICT_STATUS = 409
+DEFAULT_IDEMPOTENCY_TTL_DAYS = 14
+
+
+@dataclass(frozen=True)
+class WriteRouteRule:
+    method: str
+    pattern: str
+    route_class: str
+    description: str
+
+    def matches(self, method: str, path: str) -> bool:
+        normalized = path.rstrip("/") or "/"
+        return self.method == method.upper() and re.fullmatch(self.pattern, normalized) is not None
+
+
+WRITE_ROUTE_RULES: tuple[WriteRouteRule, ...] = (
+    WriteRouteRule("POST", r"/progress", "append", "append progress event"),
+    WriteRouteRule("POST", r"/messages", "append", "send agent message"),
+    WriteRouteRule("PATCH", r"/messages/[^/]+/read", "append", "mark known message read"),
+    WriteRouteRule("POST", r"/token-events", "append", "record token event"),
+    WriteRouteRule("POST", r"/token-events/upsert", "append", "upsert token event"),
+    WriteRouteRule("POST", r"/decisions", "append", "record decision"),
+    WriteRouteRule("PATCH", r"/tasks/[^/]+", "replace", "update task"),
+    WriteRouteRule("POST", r"/tasks/bulk-status-sync", "replace", "bulk task status sync"),
+    WriteRouteRule("PATCH", r"/decisions/[^/]+", "replace", "update decision"),
+    WriteRouteRule("POST", r"/decisions/[^/]+/resolve", "replace", "resolve decision"),
+    WriteRouteRule("PATCH", r"/workplans/[^/]+", "replace", "update workplan"),
+    WriteRouteRule("PATCH", r"/workstreams/[^/]+", "replace", "update legacy workstream alias"),
+)
+
+
+def route_rule_for(method: str, path: str) -> WriteRouteRule | None:
+    for rule in WRITE_ROUTE_RULES:
+        if rule.matches(method, path):
+            return rule
+    return None
+
+
+def route_class_for(method: str, path: str) -> str | None:
+    rule = route_rule_for(method, path)
+    return rule.route_class if rule else None
+
+
+def canonical_request_hash(method: str, path: str, query_string: bytes, body: bytes) -> str:
+    try:
+        parsed: Any = json.loads(body.decode("utf-8")) if body else None
+        body_repr = json.dumps(parsed, sort_keys=True, separators=(",", ":"))
+    except (UnicodeDecodeError, json.JSONDecodeError):
+        body_repr = body.hex()
+    query = query_string.decode("utf-8", errors="replace")
+    seed = f"{method.upper()}\n{path}\n{query}\n{body_repr}".encode("utf-8")
+    return hashlib.sha256(seed).hexdigest()
+
+
+def _header_value(headers: list[tuple[bytes, bytes]], name: bytes) -> str | None:
+    lname = name.lower()
+    for key, value in headers:
+        if key.lower() == lname:
+            return value.decode("utf-8", errors="replace")
+    return None
+
+
+async def _send_json_response(response: JSONResponse, scope: Scope, receive: Receive, send: Send) -> None:
+    await response(scope, receive, send)
+
+
+class WriteIdempotencyMiddleware:
+    """Replay exact duplicate write requests carrying Idempotency-Key.
+
+    The middleware is intentionally narrow: it only participates on the offline
+    relay allowlist. Non-allowlisted routes keep their normal behavior even if a
+    caller sends an Idempotency-Key header.
+    """
+
+    def __init__(self, app: ASGIApp, *, ttl_days: int = DEFAULT_IDEMPOTENCY_TTL_DAYS) -> None:
+        self.app = app
+        self.ttl_days = ttl_days
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http":
+            await self.app(scope, receive, send)
+            return
+
+        method = str(scope.get("method", "")).upper()
+        path = str(scope.get("path", ""))
+        rule = route_rule_for(method, path)
+        headers = list(scope.get("headers") or [])
+        key = _header_value(headers, IDEMPOTENCY_HEADER)
+        if rule is None or not key:
+            await self.app(scope, receive, send)
+            return
+
+        body = await self._read_body(receive)
+        request_hash = canonical_request_hash(method, path, scope.get("query_string", b""), body)
+        source_host = _header_value(headers, b"x-statehub-source-host")
+        source_agent = _header_value(headers, b"x-statehub-source-agent")
+
+        async with async_session_factory() as session:
+            existing = (await session.execute(
+                select(WriteIdempotencyKey).where(WriteIdempotencyKey.key == key)
+            )).scalar_one_or_none()
+            if existing is not None:
+                existing.last_seen_at = datetime.now(tz=timezone.utc)
+                await session.commit()
+                if existing.request_hash != request_hash:
+                    await _send_json_response(
+                        JSONResponse(
+                            status_code=CONFLICT_STATUS,
+                            content={
+                                "error": "Idempotency-Key was reused with a different request",
+                                "idempotency_key": key,
+                            },
+                        ),
+                        scope,
+                        self._receive_from_body(body),
+                        send,
+                    )
+                    return
+                await _send_json_response(
+                    JSONResponse(
+                        status_code=existing.response_status,
+                        content=existing.response_body,
+                        headers={REPLAY_HEADER: "true"},
+                    ),
+                    scope,
+                    self._receive_from_body(body),
+                    send,
+                )
+                return
+
+        start_message: Message | None = None
+        body_parts: list[bytes] = []
+
+        async def capture_send(message: Message) -> None:
+            nonlocal start_message
+            if message["type"] == "http.response.start":
+                start_message = message
+            elif message["type"] == "http.response.body":
+                body_parts.append(message.get("body", b""))
+            await send(message)
+
+        await self.app(scope, self._receive_from_body(body), capture_send)
+
+        if start_message is None:
+            return
+        status = int(start_message.get("status", 500))
+        if status < 200 or status >= 300:
+            return
+
+        response_body_bytes = b"".join(body_parts)
+        try:
+            response_body = json.loads(response_body_bytes.decode("utf-8")) if response_body_bytes else None
+        except (UnicodeDecodeError, json.JSONDecodeError):
+            return
+
+        async with async_session_factory() as session:
+            existing = (await session.execute(
+                select(WriteIdempotencyKey).where(WriteIdempotencyKey.key == key)
+            )).scalar_one_or_none()
+            if existing is not None:
+                return
+            now = datetime.now(tz=timezone.utc)
+            session.add(WriteIdempotencyKey(
+                key=key,
+                method=method,
+                path=path,
+                route_class=rule.route_class,
+                request_hash=request_hash,
+                response_status=status,
+                response_body=response_body,
+                source_host=source_host,
+                source_agent=source_agent,
+                first_seen_at=now,
+                last_seen_at=now,
+                expires_at=now + timedelta(days=self.ttl_days),
+            ))
+            await session.commit()
+
+    @staticmethod
+    async def _read_body(receive: Receive) -> bytes:
+        chunks: list[bytes] = []
+        while True:
+            message = await receive()
+            if message["type"] != "http.request":
+                continue
+            chunks.append(message.get("body", b""))
+            if not message.get("more_body", False):
+                break
+        return b"".join(chunks)
+
+    @staticmethod
+    def _receive_from_body(body: bytes) -> Receive:
+        sent = False
+
+        async def receive() -> Message:
+            nonlocal sent
+            if sent:
+                return {"type": "http.request", "body": b"", "more_body": False}
+            sent = True
+            return {"type": "http.request", "body": body, "more_body": False}
+
+        return receive
diff --git a/custodian_cli.py b/custodian_cli.py
index 159ef35..c2cd650 100644
--- a/custodian_cli.py
+++ b/custodian_cli.py
@@ -465,6 +465,55 @@ def cmd_status(_args: argparse.Namespace) -> None:
             print(f"  [{deadline}] {d['title']}")
 
 
+
+def _outbox_store(args):
+    from api.edge.outbox import OutboxStore, default_outbox_path
+
+    return OutboxStore(args.outbox_path or default_outbox_path())
+
+
+def cmd_outbox_status(args: argparse.Namespace) -> None:
+    store = _outbox_store(args)
+    print(json.dumps(store.summary(), indent=2))
+
+
+def cmd_outbox_list(args: argparse.Namespace) -> None:
+    store = _outbox_store(args)
+    rows = store.export(status=args.status, limit=args.limit)
+    print(json.dumps(rows, indent=2))
+
+
+def cmd_outbox_export(args: argparse.Namespace) -> None:
+    store = _outbox_store(args)
+    payload = store.export(status=args.status, limit=args.limit)
+    if args.output:
+        Path(args.output).write_text(json.dumps(payload, indent=2) + "\n")
+        print(f"Exported {len(payload)} envelope(s) to {args.output}")
+    else:
+        print(json.dumps(payload, indent=2))
+
+
+def cmd_outbox_replay(args: argparse.Namespace) -> None:
+    import asyncio
+    from api.edge.relay import replay_pending
+
+    store = _outbox_store(args)
+    upstream = args.upstream_url or os.environ.get("STATEHUB_UPSTREAM_URL") or API_BASE
+    result = asyncio.run(replay_pending(store, upstream_url=upstream, limit=args.limit))
+    print(json.dumps(result, indent=2))
+
+
+def cmd_outbox_retry(args: argparse.Namespace) -> None:
+    store = _outbox_store(args)
+    store.retry(args.envelope_id)
+    print(f"Queued {args.envelope_id} for retry")
+
+
+def cmd_outbox_cancel(args: argparse.Namespace) -> None:
+    store = _outbox_store(args)
+    store.cancel(args.envelope_id)
+    print(f"Cancelled {args.envelope_id}")
+
 # ── Entry point ────────────────────────────────────────────────────────────────
 
 def main() -> None:
@@ -549,12 +598,47 @@ def main() -> None:
     ctask.add_argument("--assignee", default=None)
     ctask.add_argument("--description", default=None)
 
+
+    # outbox
+    outbox = sub.add_parser("outbox", help="Inspect and replay the local State Hub edge outbox")
+    outbox.add_argument("--outbox-path", default=None, help="SQLite outbox path (defaults to ~/.statehub/edge-outbox.sqlite3)")
+    out_sub = outbox.add_subparsers(dest="outbox_command", required=True)
+
+    out_status = out_sub.add_parser("status", help="Show pending, conflict, and ack counts")
+    out_status.set_defaults(func=cmd_outbox_status)
+
+    out_list = out_sub.add_parser("list", help="List outbox envelopes as JSON")
+    out_list.add_argument("--status", default=None, help="Filter by status")
+    out_list.add_argument("--limit", type=int, default=100)
+    out_list.set_defaults(func=cmd_outbox_list)
+
+    out_export = out_sub.add_parser("export", help="Export non-secret envelopes")
+    out_export.add_argument("--status", default=None, help="Filter by status")
+    out_export.add_argument("--limit", type=int, default=1000)
+    out_export.add_argument("--output", default=None, help="Write JSON to a file instead of stdout")
+    out_export.set_defaults(func=cmd_outbox_export)
+
+    out_replay = out_sub.add_parser("replay", help="Replay due queued envelopes")
+    out_replay.add_argument("--upstream-url", default=None, help="Central State Hub API base URL")
+    out_replay.add_argument("--limit", type=int, default=50)
+    out_replay.set_defaults(func=cmd_outbox_replay)
+
+    out_retry = out_sub.add_parser("retry", help="Force one envelope back to queued")
+    out_retry.add_argument("envelope_id")
+    out_retry.set_defaults(func=cmd_outbox_retry)
+
+    out_cancel = out_sub.add_parser("cancel", help="Cancel one envelope")
+    out_cancel.add_argument("envelope_id")
+    out_cancel.set_defaults(func=cmd_outbox_cancel)
+
     # status
     sub.add_parser("status", help="Show State Hub health and summary totals")
 
     args = parser.parse_args()
 
-    if args.command == "register":
+    if hasattr(args, "func"):
+        args.func(args)
+    elif args.command == "register":
         run_statehub_register(args)
     elif args.command == "register-project":
         cmd_register(args)
diff --git a/docs/offline-write-buffer.md b/docs/offline-write-buffer.md
new file mode 100644
index 0000000..97515d9
--- /dev/null
+++ b/docs/offline-write-buffer.md
@@ -0,0 +1,95 @@
+# State Hub Offline Write Buffer
+
+## Decision
+
+State Hub supports outage buffering through an edge relay with a durable local
+outbox, plus central idempotency on replayed writes.
+
+The central service cannot buffer requests that never reach it. Agents should
+therefore send writes to a local statehub-edge relay when buffering is enabled.
+The relay forwards immediately while the upstream API is reachable. If the
+upstream is offline, the relay persists queueable write envelopes in a local
+SQLite outbox and returns an explicit queued receipt.
+
+Queued receipts are pending evidence, not successful central commits. Operators
+must inspect and replay the outbox after recovery.
+
+## Defaults
+
+- Relay listen target: operator-selected, recommended 127.0.0.1:18080.
+- Upstream API: STATEHUB_UPSTREAM_URL, then API_BASE, then
+  http://127.0.0.1:8000.
+- Outbox path: STATEHUB_OUTBOX_PATH, default
+  ~/.statehub/edge-outbox.sqlite3.
+- Central idempotency retention: 14 days.
+
+## Route Classes
+
+### Append-Only, Queueable
+
+| Method | Path | Notes |
+| --- | --- | --- |
+| POST | /progress/ | Session-close progress events. |
+| POST | /messages/ | Agent coordination messages. |
+| PATCH | /messages/{id}/read | Safe only when the message id is already known. |
+| POST | /token-events/ | Token accounting events. |
+| POST | /token-events/upsert | Source-id based token upsert. |
+| POST | /decisions/ | Queue only when the caller does not need the generated id immediately. |
+
+Append-only writes replay with Idempotency-Key. Exact duplicate retries return
+the original central response. Same key with a different request returns HTTP
+409.
+
+### Replace-Style, Queueable With Conflict Checks
+
+| Method | Path | Notes |
+| --- | --- | --- |
+| PATCH | /tasks/{id} | Task status and metadata updates. |
+| POST | /tasks/bulk-status-sync | Ordered batch; future coalescing may decompose by task. |
+| PATCH | /decisions/{id} | Decision field update. |
+| POST | /decisions/{id}/resolve | Decision resolution. |
+| PATCH | /workplans/{id} | Workplan lifecycle/status updates. |
+| PATCH | /workstreams/{id} | Legacy alias for workplan update. |
+
+In v1 the relay does not silently overwrite newer central state after a replay
+conflict. A 409 response marks the envelope conflict and leaves it available for
+operator review.
+
+### Online-Only In V1
+
+The relay forwards these while the upstream is reachable and returns a clear
+503 during outage:
+
+- DELETE endpoints.
+- Repository sync/import/ingest endpoints.
+- Consistency sweep mutation endpoints.
+- Fabric graph exports and external pulls.
+- Schema/bootstrap/admin operations.
+- Requests with credentials, authorization tokens, attachments, or large opaque
+  payloads.
+
+## Non-Secret Outbox Contract
+
+The outbox stores method, path, scrubbed JSON body, route class, source metadata,
+idempotency key, retry status, last error, and central response summaries. It
+never stores authorization headers, bearer tokens, cookies, API keys, passwords,
+or secret-looking JSON fields. Payloads over 64 KiB are rejected.
+
+## Operator Commands
+
+    statehub outbox status
+    statehub outbox list --status queued
+    statehub outbox replay --upstream-url http://127.0.0.1:8000
+    statehub outbox export --output /tmp/statehub-outbox.json
+    statehub outbox retry ENVELOPE_ID
+    statehub outbox cancel ENVELOPE_ID
+
+## Recovery Checklist
+
+1. Confirm the central State Hub API is reachable.
+2. Run statehub outbox status on each host that may have queued writes.
+3. Run statehub outbox replay until no due queued envelopes remain.
+4. Review conflict envelopes manually.
+5. Run make fix-consistency REPO=state-hub so file-backed workplan/task state
+   remains canonical after replay.
+6. Record a progress note with non-secret replay counts.
diff --git a/mcp_server/TOOLS.md b/mcp_server/TOOLS.md
index 9b61f13..632c5c9 100644
--- a/mcp_server/TOOLS.md
+++ b/mcp_server/TOOLS.md
@@ -82,6 +82,13 @@ succeeds but its automatic progress event fails, the tool returns an error with
 the successful `write_result` included so the caller can avoid duplicating the
 entity while recording the missing progress event.
 
+When API_BASE points at the optional State Hub edge relay and the central API is
+unreachable, queueable write tools may return a queued receipt instead of the
+normal REST shape. The receipt means the local outbox accepted the write; it is
+not yet a central commit. Automatic progress-event side effects are skipped for
+queued primary writes so replay does not duplicate records. Operators can inspect
+and replay with statehub outbox status and statehub outbox replay.
+
 ---
 
 ## Query Tools (read-only, use freely)
diff --git a/mcp_server/server.py b/mcp_server/server.py
index 9ed5ccf..c571fe6 100644
--- a/mcp_server/server.py
+++ b/mcp_server/server.py
@@ -120,12 +120,23 @@ def _mcp_error(tool_name: str, message: str, response: Any | None = None) -> dic
     return payload
 
 
+
+def _mcp_queued(tool_name: str, response: dict[str, Any]) -> dict[str, Any]:
+    return {
+        "queued": True,
+        "tool": tool_name,
+        "message": "Write queued by State Hub edge relay; central commit is pending replay.",
+        "receipt": response,
+    }
+
 def _response_error(
     tool_name: str,
     response: Any,
     required_fields: tuple[str, ...] = (),
 ) -> dict[str, Any] | None:
     """Return an MCP-visible error payload for failed or malformed API results."""
+    if isinstance(response, dict) and response.get("queued") is True:
+        return _mcp_queued(tool_name, response)
     if isinstance(response, dict) and isinstance(response.get("error"), str):
         return _mcp_error(tool_name, response["error"], response)
     if not isinstance(response, dict):
diff --git a/migrations/versions/e9f0a1b2c3d4_write_idempotency_keys.py b/migrations/versions/e9f0a1b2c3d4_write_idempotency_keys.py
new file mode 100644
index 0000000..bfd00cf
--- /dev/null
+++ b/migrations/versions/e9f0a1b2c3d4_write_idempotency_keys.py
@@ -0,0 +1,43 @@
+"""add write idempotency keys
+
+Revision ID: e9f0a1b2c3d4
+Revises: d8e9f0a1b2c3
+Create Date: 2026-06-23
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+
+revision = "e9f0a1b2c3d4"
+down_revision = "d8e9f0a1b2c3"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "write_idempotency_keys",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True),
+        sa.Column("key", sa.String(length=200), nullable=False),
+        sa.Column("method", sa.String(length=10), nullable=False),
+        sa.Column("path", sa.Text(), nullable=False),
+        sa.Column("route_class", sa.String(length=30), nullable=False),
+        sa.Column("request_hash", sa.String(length=64), nullable=False),
+        sa.Column("response_status", sa.Integer(), nullable=False),
+        sa.Column("response_body", JSONB(), nullable=True),
+        sa.Column("source_host", sa.String(length=200), nullable=True),
+        sa.Column("source_agent", sa.String(length=100), nullable=True),
+        sa.Column("first_seen_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("last_seen_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=True),
+        sa.UniqueConstraint("key", name="uq_write_idempotency_keys_key"),
+    )
+    op.create_index("ix_write_idempotency_keys_key", "write_idempotency_keys", ["key"])
+    op.create_index("ix_write_idempotency_keys_expires_at", "write_idempotency_keys", ["expires_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_write_idempotency_keys_expires_at", table_name="write_idempotency_keys")
+    op.drop_index("ix_write_idempotency_keys_key", table_name="write_idempotency_keys")
+    op.drop_table("write_idempotency_keys")
diff --git a/scripts/consistency_check.py b/scripts/consistency_check.py
index 83be0bd..278eb8a 100644
--- a/scripts/consistency_check.py
+++ b/scripts/consistency_check.py
@@ -572,7 +572,7 @@ def _api_patch(api_base: str, path: str, body: dict) -> Any:
 
 def _api_post(api_base: str, path: str, body: dict) -> Any:
     if not _HAS_HTTPX:
-        return None
+        return {"_error": "httpx is not installed"}
     if not path.endswith("/"):
         path += "/"
     try:
@@ -580,8 +580,13 @@ def _api_post(api_base: str, path: str, body: dict) -> Any:
             r = c.post(path, json=body)
             r.raise_for_status()
             return r.json()
-    except Exception:
-        return None
+    except _httpx.HTTPStatusError as exc:
+        detail = exc.response.text
+        if len(detail) > 500:
+            detail = detail[:497] + "..."
+        return {"_error": f"{exc.response.status_code} {exc.response.reason_phrase}: {detail}"}
+    except Exception as exc:
+        return {"_error": str(exc)}
 
 
 # ---------------------------------------------------------------------------
@@ -836,6 +841,7 @@ def check_repo(api_base: str, repo_slug: str, repo_path_override: str | None = N
                     "repo_id": repo_id,
                     "domain": file_domain,
                     "repo_market_domain": repo_market_domain,
+                    "repo_slug": repo_slug,
                 },
             )
             continue
@@ -1019,11 +1025,13 @@ def check_repo(api_base: str, repo_slug: str, repo_path_override: str | None = N
         existing_dep_keys = set()
         if isinstance(existing_deps, list):
             for dep in existing_deps:
-                if dep.get("from_workstream_id") != ws_id:
+                from_id = dep.get("from_workstream_id") or dep.get("from_workplan_id")
+                if from_id != ws_id:
                     continue
                 rel = dep.get("relationship_type") or "blocks"
-                if dep.get("to_workstream_id"):
-                    existing_dep_keys.add(("workstream", dep["to_workstream_id"], rel))
+                to_workplan_id = dep.get("to_workstream_id") or dep.get("to_workplan_id")
+                if to_workplan_id:
+                    existing_dep_keys.add(("workstream", to_workplan_id, rel))
                 if dep.get("to_task_id"):
                     existing_dep_keys.add(("task", dep["to_task_id"], rel))
 
@@ -1770,20 +1778,58 @@ def fix_repo(
                     )
                     continue
 
-                slug = re.sub(r"[^a-z0-9-]", "-", wp_id.lower()).strip("-")
-                ws_data = _api_post(api_base, "/workstreams", {
-                    "topic_id": topic_id,
-                    "repo_id": repo_id_val,
-                    "slug": slug,
-                    "title": title or wp_id,
-                    "status": status,
-                    "owner": str(meta.get("owner", "")).strip() or None,
-                    "planning_priority": str(meta.get("planning_priority", "")).strip() or None,
-                    "planning_order": _as_int_or_none(meta.get("planning_order")),
-                })
+                base_slug = re.sub(r"[^a-z0-9-]", "-", wp_id.lower()).strip("-") or "workplan"
+                repo_slug_part = re.sub(
+                    r"[^a-z0-9-]", "-", str(ctx.get("repo_slug") or "").lower()
+                ).strip("-")
+                slug_candidates = [base_slug]
+                repo_qualified_slug = base_slug
+                if repo_slug_part and not base_slug.startswith(f"{repo_slug_part}-"):
+                    repo_qualified_slug = f"{repo_slug_part}-{base_slug}"
+                    slug_candidates.append(repo_qualified_slug)
+                for suffix in range(2, 21):
+                    slug_candidates.append(f"{repo_qualified_slug}-{suffix}")
+
+                ws_data = None
+                last_error = None
+                for slug in slug_candidates:
+                    existing = _api_get(api_base, "/workstreams", {"slug": slug}, return_error=True)
+                    if isinstance(existing, dict) and "_error" in existing:
+                        last_error = existing["_error"]
+                        continue
+                    if isinstance(existing, list) and existing:
+                        existing_same_repo = next(
+                            (w for w in existing if w.get("repo_id") == repo_id_val),
+                            None,
+                        )
+                        if existing_same_repo and existing_same_repo.get("title") == (title or wp_id):
+                            ws_data = existing_same_repo
+                            report.fixes_applied.append(
+                                f"C-06 reusing existing workstream {ws_data['id'][:8]}... for {wp_id}"
+                            )
+                            break
+                        last_error = f"slug {slug!r} already belongs to another workstream"
+                        continue
+
+                    ws_data = _api_post(api_base, "/workstreams", {
+                        "topic_id": topic_id,
+                        "repo_id": repo_id_val,
+                        "slug": slug,
+                        "title": title or wp_id,
+                        "status": status,
+                        "owner": str(meta.get("owner", "")).strip() or None,
+                        "planning_priority": str(meta.get("planning_priority", "")).strip() or None,
+                        "planning_order": _as_int_or_none(meta.get("planning_order")),
+                    })
+                    if ws_data is None or (isinstance(ws_data, dict) and "_error" in ws_data):
+                        last_error = ws_data.get("_error") if isinstance(ws_data, dict) else "no response"
+                        ws_data = None
+                        continue
+                    break
+
                 if ws_data is None:
                     report.fixes_applied.append(
-                        f"C-06 FAIL {wp_id}: could not create workstream in DB"
+                        f"C-06 FAIL {wp_id}: could not create workstream in DB: {last_error or 'no usable slug'}"
                     )
                     continue
 
@@ -1814,7 +1860,7 @@ def fix_repo(
                         "priority": t_priority,
                         "assignee": task.get("assignee") or None,
                     })
-                    if t_data:
+                    if t_data and "_error" not in t_data:
                         t_db_id = t_data["id"]
                         injected = _inject_task_id_into_block(
                             wp_file, "state_hub_task_id", t_db_id, t_id
@@ -1822,6 +1868,10 @@ def fix_repo(
                         if not injected:
                             _inject_task_id_frontmatter_list(wp_file, t_db_id, t_id)
                         report.fixes_applied.append(f"  + task {t_id} → {t_db_id[:8]}…")
+                    elif t_data:
+                        report.fixes_applied.append(
+                            f"  ! task {t_id} not created: {t_data.get('_error', t_data)}"
+                        )
 
             elif issue.check_id == "C-09":
                 ws_id = ctx["ws_id"]
diff --git a/scripts/project_rules/agents-codex.template b/scripts/project_rules/agents-codex.template
index 633b1d3..367f9fe 100644
--- a/scripts/project_rules/agents-codex.template
+++ b/scripts/project_rules/agents-codex.template
@@ -20,6 +20,12 @@ there is no MCP server for Codex agents.
 |---------|-----|
 | Local workstation | `http://127.0.0.1:8000` |
 | Remote via tunnel | `http://127.0.0.1:18000` |
+| Optional local edge relay | http://127.0.0.1:18080 |
+
+When an operator has enabled the edge relay, set API_BASE to the relay URL.
+Queueable writes return an explicit queued receipt if the central hub is
+unreachable. Treat that as pending local evidence, then ask the operator to run
+statehub outbox status/replay after connectivity returns.
 
 ### Orient at session start
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 0b7a18e..5358d7a 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -98,10 +98,17 @@ async def client(test_engine):
         async with factory() as session:
             yield session
 
+    from api.services import write_idempotency as _write_idempotency
+
+    old_session_factory = _write_idempotency.async_session_factory
+    _write_idempotency.async_session_factory = factory
     app.dependency_overrides[get_session] = _override
-    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
-        yield ac
-    app.dependency_overrides.clear()
+    try:
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
+            yield ac
+    finally:
+        app.dependency_overrides.clear()
+        _write_idempotency.async_session_factory = old_session_factory
 
 
 # ---------------------------------------------------------------------------
diff --git a/tests/test_consistency_check.py b/tests/test_consistency_check.py
index 5d91941..e693552 100644
--- a/tests/test_consistency_check.py
+++ b/tests/test_consistency_check.py
@@ -1015,6 +1015,154 @@ class TestLifecycleRenormalization:
         assert any("C-23 fixed" in fix for fix in report.fixes_applied)
 
 
+
+class TestC20DependencyDetection:
+    def test_canonical_dependency_fields_satisfy_workplan_dependency(self, tmp_path, monkeypatch):
+        repo = tmp_path / "repo"
+        workplans = repo / "workplans"
+        workplans.mkdir(parents=True)
+        (workplans / "STATE-WP-0001-base.md").write_text(
+            "---\n"
+            "id: STATE-WP-0001\n"
+            "title: Base\n"
+            "domain: financials\n"
+            "repo: demo-repo\n"
+            "status: active\n"
+            "state_hub_workstream_id: \"base-ws\"\n"
+            "---\n\n",
+            encoding="utf-8",
+        )
+        (workplans / "STATE-WP-0002-dependent.md").write_text(
+            "---\n"
+            "id: STATE-WP-0002\n"
+            "title: Dependent\n"
+            "domain: financials\n"
+            "repo: demo-repo\n"
+            "status: active\n"
+            "state_hub_workstream_id: \"dependent-ws\"\n"
+            "depends_on_workplans:\n"
+            "  - STATE-WP-0001\n"
+            "---\n\n",
+            encoding="utf-8",
+        )
+
+        def fake_get(_api_base, path, params=None, **_kwargs):
+            if path == "/repos/demo-repo":
+                import socket
+
+                return {
+                    "id": "repo-1",
+                    "slug": "demo-repo",
+                    "local_path": str(repo),
+                    "host_paths": {socket.gethostname(): str(repo)},
+                    "domain_slug": "financials",
+                }
+            if path == "/workstreams/base-ws":
+                return {"id": "base-ws", "repo_id": "repo-1", "slug": "state-wp-0001", "title": "Base", "status": "active"}
+            if path == "/workstreams/dependent-ws":
+                return {"id": "dependent-ws", "repo_id": "repo-1", "slug": "state-wp-0002", "title": "Dependent", "status": "active"}
+            if path == "/tasks" and params and params.get("workstream_id") in {"base-ws", "dependent-ws"}:
+                return []
+            if path == "/workstreams/base-ws/dependencies":
+                return []
+            if path == "/workstreams/dependent-ws/dependencies":
+                return [
+                    {
+                        "id": "dep-1",
+                        "from_workplan_id": "dependent-ws",
+                        "to_workplan_id": "base-ws",
+                        "to_task_id": None,
+                        "relationship_type": "blocks",
+                    }
+                ]
+            if path == "/workstreams" and params == {"repo_id": "repo-1"}:
+                return []
+            return []
+
+        monkeypatch.setattr("consistency_check._api_get", fake_get)
+
+        report = check_repo("http://unused", "demo-repo")
+
+        assert "C-20" not in [issue.check_id for issue in report.issues]
+
+
+class TestC06WorkstreamCreation:
+    def test_fix_repo_uses_repo_qualified_slug_when_base_slug_is_taken(self, tmp_path, monkeypatch):
+        repo = tmp_path / "repo"
+        workplans = repo / "workplans"
+        workplans.mkdir(parents=True)
+        wp = workplans / "STATE-WP-0001-demo.md"
+        wp.write_text(
+            "---\n"
+            "id: STATE-WP-0001\n"
+            "type: workplan\n"
+            "title: Demo Workplan\n"
+            "domain: financials\n"
+            "repo: demo-repo\n"
+            "status: ready\n"
+            "owner: codex\n"
+            "---\n\n"
+            "## Implement Demo\n\n"
+            "```task\n"
+            "id: STATE-WP-0001-T01\n"
+            "status: todo\n"
+            "priority: high\n"
+            "```\n",
+            encoding="utf-8",
+        )
+
+        created_workstreams = []
+        created_tasks = []
+
+        def fake_get(_api_base, path, params=None, **_kwargs):
+            if path == "/repos/demo-repo":
+                import socket
+
+                return {
+                    "id": "repo-1",
+                    "slug": "demo-repo",
+                    "local_path": str(repo),
+                    "host_paths": {socket.gethostname(): str(repo)},
+                    "domain_slug": "financials",
+                }
+            if path == "/topics":
+                return [{"id": "topic-1", "domain_slug": "financials"}]
+            if path == "/workstreams" and params == {"slug": "state-wp-0001"}:
+                return [{"id": "old-ws", "repo_id": "other-repo", "title": "Old Workplan"}]
+            if path == "/workstreams" and params == {"slug": "demo-repo-state-wp-0001"}:
+                return []
+            if path == "/workstreams" and params == {"repo_id": "repo-1"}:
+                return []
+            if path == "/workstreams" and params and params.get("topic_id") == "topic-1":
+                return []
+            return []
+
+        def fake_post(_api_base, path, body):
+            if path == "/workstreams":
+                created_workstreams.append(body)
+                return {"id": "new-ws", **body}
+            if path == "/tasks":
+                created_tasks.append(body)
+                return {"id": "new-task", **body}
+            return {"ok": True}
+
+        monkeypatch.setattr("consistency_check._api_get", fake_get)
+        monkeypatch.setattr("consistency_check._api_post", fake_post)
+        monkeypatch.setattr("consistency_check._api_patch", lambda *args, **kwargs: {"ok": True})
+        monkeypatch.setattr("consistency_check._detect_behind_remote", lambda _repo_path: False)
+        monkeypatch.setattr("consistency_check._detect_ahead_of_remote", lambda _repo_path: 0)
+        monkeypatch.setattr("consistency_check._write_custodian_brief", lambda *args, **kwargs: False)
+        monkeypatch.setattr("consistency_check._git_push", lambda _repo_path: (True, "pushed"))
+
+        report = fix_repo("http://unused", "demo-repo")
+
+        assert created_workstreams[0]["slug"] == "demo-repo-state-wp-0001"
+        assert created_tasks[0]["workstream_id"] == "new-ws"
+        patched = wp.read_text(encoding="utf-8")
+        assert 'state_hub_workstream_id: "new-ws"' in patched
+        assert 'state_hub_task_id: "new-task"' in patched
+        assert any("C-06 fixed" in fix for fix in report.fixes_applied)
+
 # ---------------------------------------------------------------------------
 # _git_pull (T02 remote fix helper)
 # ---------------------------------------------------------------------------
diff --git a/tests/test_edge_outbox.py b/tests/test_edge_outbox.py
new file mode 100644
index 0000000..ec7d8a9
--- /dev/null
+++ b/tests/test_edge_outbox.py
@@ -0,0 +1,51 @@
+from api.edge.outbox import OutboxStore, PayloadRejected
+from api.services.write_idempotency import route_class_for
+
+
+def test_route_classifier_matches_safe_writes():
+    assert route_class_for("POST", "/progress/") == "append"
+    assert route_class_for("PATCH", "/tasks/abc") == "replace"
+    assert route_class_for("DELETE", "/tasks/abc") is None
+
+
+def test_outbox_scrubs_secret_fields_and_tracks_status(tmp_path):
+    store = OutboxStore(tmp_path / "outbox.sqlite3")
+    envelope = store.enqueue(
+        method="POST",
+        path="/progress/",
+        body={"summary": "offline", "password": "secret", "tokens_in": 12},
+        source_agent="pytest",
+        source_host="host-a",
+    )
+
+    assert envelope.status == "queued"
+    assert envelope.route_class == "append"
+    assert envelope.body["password"] == "[redacted]"
+    assert envelope.body["tokens_in"] == 12
+    assert store.summary()["pending_count"] == 1
+
+    store.mark_acked(envelope.id, response_status=201, response_body={"id": "central"})
+    acked = store.get(envelope.id)
+    assert acked.status == "acked"
+    assert acked.response_body == {"id": "central"}
+    assert store.summary()["pending_count"] == 0
+
+
+def test_outbox_rejects_non_queueable_routes(tmp_path):
+    store = OutboxStore(tmp_path / "outbox.sqlite3")
+    try:
+        store.enqueue(method="DELETE", path="/tasks/abc", body={})
+    except PayloadRejected as exc:
+        assert "not queueable" in str(exc)
+    else:
+        raise AssertionError("DELETE should not be queueable")
+
+
+def test_replace_writes_coalesce_superseded_queued_envelopes(tmp_path):
+    store = OutboxStore(tmp_path / "outbox.sqlite3")
+    first = store.enqueue(method="PATCH", path="/tasks/task-1", body={"status": "progress"})
+    second = store.enqueue(method="PATCH", path="/tasks/task-1", body={"status": "done"})
+
+    assert store.get(first.id).status == "cancelled"
+    assert store.get(second.id).status == "queued"
+    assert len(store.due()) == 1
diff --git a/tests/test_edge_relay.py b/tests/test_edge_relay.py
new file mode 100644
index 0000000..204ae72
--- /dev/null
+++ b/tests/test_edge_relay.py
@@ -0,0 +1,117 @@
+import httpx
+import pytest
+from httpx import ASGITransport, AsyncClient
+
+from api.edge.outbox import OutboxStore
+from api.edge.relay import create_app, replay_pending
+
+
+class FailingAsyncClient:
+    def __init__(self, *args, **kwargs):
+        pass
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc_info):
+        return False
+
+    async def request(self, *args, **kwargs):
+        raise httpx.ConnectError("upstream down")
+
+    async def get(self, *args, **kwargs):
+        raise httpx.ConnectError("upstream down")
+
+
+class ConflictAsyncClient:
+    def __init__(self, *args, **kwargs):
+        pass
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc_info):
+        return False
+
+    async def request(self, method, path, **kwargs):
+        request = httpx.Request(method, f"http://upstream{path}")
+        return httpx.Response(409, json={"error": "conflict"}, request=request)
+
+
+class SuccessAsyncClient:
+    def __init__(self, *args, **kwargs):
+        pass
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc_info):
+        return False
+
+    async def request(self, method, path, **kwargs):
+        request = httpx.Request(method, f"http://upstream{path}")
+        return httpx.Response(201, json={"id": "central-id", "path": path}, request=request)
+
+
+@pytest.mark.asyncio
+async def test_relay_queues_allowlisted_write_when_upstream_unreachable(tmp_path, monkeypatch):
+    from api.edge import relay
+
+    monkeypatch.setattr(relay.httpx, "AsyncClient", FailingAsyncClient)
+    outbox_path = tmp_path / "edge.sqlite3"
+    app = create_app(upstream_url="http://upstream", outbox_path=str(outbox_path))
+
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://edge") as client:
+        response = await client.post("/progress/", json={"event_type": "note", "summary": "queued"})
+
+    assert response.status_code == 202
+    body = response.json()
+    assert body["queued"] is True
+    assert body["route_class"] == "append"
+
+    store = OutboxStore(outbox_path)
+    queued = store.list(status="queued")
+    assert len(queued) == 1
+    assert queued[0].path == "/progress/"
+
+
+@pytest.mark.asyncio
+async def test_relay_replay_acks_successful_envelope(tmp_path, monkeypatch):
+    from api.edge import relay
+
+    monkeypatch.setattr(relay.httpx, "AsyncClient", SuccessAsyncClient)
+    store = OutboxStore(tmp_path / "edge.sqlite3")
+    envelope = store.enqueue(method="POST", path="/progress/", body={"event_type": "note", "summary": "queued"})
+
+    result = await replay_pending(store, upstream_url="http://upstream")
+
+    assert result["acked"] == 1
+    assert store.get(envelope.id).status == "acked"
+
+
+@pytest.mark.asyncio
+async def test_relay_rejects_online_only_write_when_upstream_unreachable(tmp_path, monkeypatch):
+    from api.edge import relay
+
+    monkeypatch.setattr(relay.httpx, "AsyncClient", FailingAsyncClient)
+    app = create_app(upstream_url="http://upstream", outbox_path=str(tmp_path / "edge.sqlite3"))
+
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://edge") as client:
+        response = await client.delete("/tasks/abc")
+
+    assert response.status_code == 503
+    assert "not queueable" in response.json()["error"]
+
+
+@pytest.mark.asyncio
+async def test_relay_replay_marks_conflict(tmp_path, monkeypatch):
+    from api.edge import relay
+
+    monkeypatch.setattr(relay.httpx, "AsyncClient", ConflictAsyncClient)
+    store = OutboxStore(tmp_path / "edge.sqlite3")
+    envelope = store.enqueue(method="PATCH", path="/tasks/task-1", body={"status": "done"})
+
+    result = await replay_pending(store, upstream_url="http://upstream")
+
+    assert result["conflict"] == 1
+    assert store.get(envelope.id).status == "conflict"
diff --git a/tests/test_mcp_queued_receipts.py b/tests/test_mcp_queued_receipts.py
new file mode 100644
index 0000000..a75e4ca
--- /dev/null
+++ b/tests/test_mcp_queued_receipts.py
@@ -0,0 +1,22 @@
+import json
+
+from mcp_server import server
+
+
+def test_mcp_write_returns_queued_receipt_without_requiring_rest_shape(monkeypatch):
+    monkeypatch.setattr(
+        server,
+        "_post",
+        lambda path, body: {
+            "queued": True,
+            "outbox_id": "env-1",
+            "idempotency_key": "statehub-edge:env-1",
+            "upstream": "unreachable",
+        },
+    )
+
+    result = json.loads(server.add_progress_event("queued progress"))
+
+    assert result["queued"] is True
+    assert result["tool"] == "add_progress_event"
+    assert result["receipt"]["outbox_id"] == "env-1"
diff --git a/tests/test_write_idempotency.py b/tests/test_write_idempotency.py
new file mode 100644
index 0000000..83ab42b
--- /dev/null
+++ b/tests/test_write_idempotency.py
@@ -0,0 +1,47 @@
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_idempotent_progress_post_replays_original_response(client):
+    payload = {"event_type": "note", "summary": "first idempotent write", "author": "codex"}
+    headers = {"Idempotency-Key": "test-progress-key", "X-StateHub-Source-Agent": "pytest"}
+
+    first = await client.post("/progress/", json=payload, headers=headers)
+    assert first.status_code in {200, 201}
+    first_body = first.json()
+
+    second = await client.post("/progress/", json=dict(reversed(list(payload.items()))), headers=headers)
+    assert second.status_code == first.status_code
+    assert second.headers["x-statehub-idempotency-replay"] == "true"
+    assert second.json() == first_body
+
+    listed = await client.get("/progress/")
+    assert len([row for row in listed.json() if row["summary"] == payload["summary"]]) == 1
+
+
+@pytest.mark.asyncio
+async def test_idempotency_key_reuse_with_different_request_conflicts(client):
+    headers = {"Idempotency-Key": "same-key-different-body"}
+    first = await client.post(
+        "/progress/",
+        json={"event_type": "note", "summary": "original"},
+        headers=headers,
+    )
+    assert first.status_code in {200, 201}
+
+    second = await client.post(
+        "/progress/",
+        json={"event_type": "note", "summary": "changed"},
+        headers=headers,
+    )
+    assert second.status_code == 409
+    assert "different request" in second.json()["error"]
+
+
+@pytest.mark.asyncio
+async def test_idempotency_header_on_unsupported_route_is_ignored(client):
+    first = await client.get("/state/health", headers={"Idempotency-Key": "ignored-on-read"})
+    second = await client.get("/state/health", headers={"Idempotency-Key": "ignored-on-read"})
+    assert first.status_code == 200
+    assert second.status_code == 200
+    assert "x-statehub-idempotency-replay" not in second.headers
diff --git a/workplans/STATE-WP-0068-offline-write-buffer-and-edge-relay.md b/workplans/STATE-WP-0068-offline-write-buffer-and-edge-relay.md
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@@ -0,0 +1,428 @@
+---
+id: STATE-WP-0068
+type: workplan
+title: "State Hub offline write buffer and edge relay"
+domain: infotech
+repo: state-hub
+status: finished
+owner: codex
+topic_slug: custodian
+created: "2026-06-23"
+updated: "2026-06-23"
+finished: "2026-06-23"
+state_hub_workstream_id: "189508bd-b3cb-4caf-ac95-30bf2823201d"
+---
+
+# STATE-WP-0068 - State Hub offline write buffer and edge relay
+
+## Summary
+
+Build a durable client-side write buffer for State Hub so agents can keep
+recording progress, decisions, messages, and safe status updates when the
+central State Hub deployment or its private tunnel is offline.
+
+The improved design is deliberately split into two layers:
+
+- **Central HA** makes the primary State Hub deployment fail less often
+  (`CUST-WP-0011`, `CUST-WP-0038`).
+- **Edge buffering** makes agent write attempts durable when the central
+  deployment is still unreachable.
+
+The central service cannot buffer requests it never receives. The buffer must
+live close to the callers: operator workstation, agent host, bridge host, or
+MCP wrapper. State Hub should therefore provide a small local relay/outbox that
+accepts sanctioned writes, persists them locally, and replays them to the
+central API when connectivity returns.
+
+## Critical Review of the Original Suggestion
+
+The suggestion is directionally right but incomplete if phrased as "the central
+State Hub buffers while offline." If the central endpoint is unreachable, the
+client needs somewhere else to put the write.
+
+The robust version is:
+
+1. Agents send writes to a local State Hub edge relay, not directly to the
+   remote central endpoint.
+2. The relay forwards immediately while the central API is reachable.
+3. On outage, the relay stores a durable, non-secret write envelope in a local
+   SQLite outbox and returns an explicit queued receipt.
+4. A replay worker flushes the outbox with idempotency keys when the central
+   API recovers.
+5. The central API deduplicates retries and rejects or flags conflicting stale
+   writes instead of silently overwriting newer state.
+
+This keeps State Hub local-first and file-canon aligned. It does not make a
+multi-master database, and it does not turn queued writes into pretend success.
+
+## Goals
+
+- Preserve session-close writes during central State Hub or tunnel outages.
+- Make offline write state observable to operators and agents.
+- Prevent duplicate progress/events when a replay retries after partial
+  success.
+- Detect stale/conflicting replace-style writes, especially task status and
+  decision resolution changes.
+- Keep secrets out of the buffer.
+- Reuse the existing REST contract and MCP write-layer reliability work.
+
+## Non-Goals
+
+- Replacing `CUST-WP-0038` high availability, backup, restore, or failover
+  work.
+- Accepting arbitrary offline edits as authoritative current state.
+- Queuing destructive deletes, imports, repo syncs, or bulk maintenance jobs in
+  v1.
+- Publicly exposing State Hub.
+- Adding Redis, Kafka, or NATS as a required edge dependency. The edge path
+  should work during local bootstrap with only Python and SQLite.
+
+## Target Architecture
+
+```
+Codex / Claude / agent process
+  -> MCP server or REST client
+     -> local statehub-edge relay
+        -> central State Hub API when reachable
+        -> local SQLite outbox when unreachable
+             -> replay worker
+                -> central State Hub API with idempotency key
+                   -> normal DB commit and lifecycle event publication
+```
+
+The relay is a local process with an explicit listen port, for example
+`127.0.0.1:18080`, configured with an upstream central API such as
+`http://127.0.0.1:18000` or the local development API.
+
+## Write Classification
+
+### Offline-safe append-only writes
+
+These should be queueable in v1:
+
+- `POST /progress/`
+- `POST /messages/`
+- `PATCH /messages/{id}/read` when message id is already known
+- `POST /token-events/`
+- `POST /decisions/` with an idempotency key and no immediate dependency on the
+  generated decision id
+
+### Offline-safe replace-style writes with conflict checks
+
+These may be queueable only with an expected revision or last-observed
+timestamp:
+
+- `PATCH /tasks/{task_id}`
+- `POST /tasks/bulk-status-sync` decomposed into per-task envelopes or replayed
+  as an ordered batch
+- `PATCH /decisions/{decision_id}` and `POST /decisions/{decision_id}/resolve`
+- `PATCH /workplans/{workplan_id}` for lifecycle/status fields
+
+Replay must mark these as conflicted when the central row changed after the
+client's observed revision and the update is not a monotonic no-op.
+
+### Online-only writes in v1
+
+These should fail fast while offline:
+
+- `DELETE` endpoints
+- repository sync/import/ingest endpoints
+- consistency sweep mutation endpoints
+- fabric graph exports
+- schema/bootstrap/admin operations
+- any request containing authorization tokens, credentials, attachments, or
+  large opaque payloads
+
+## Conflict Policy
+
+- Append-only writes use idempotency keys and replay exactly once from the
+  caller's point of view.
+- Replace-style writes include `expected_updated_at`, `expected_status`, or a
+  route-specific revision field where available.
+- Supersedable queued writes, such as multiple task status patches for the same
+  task, may be coalesced for replay while preserving local audit entries.
+- If central state is newer and the replay cannot prove the queued write is
+  still safe, mark the envelope `conflict` and surface it in relay status.
+- Workplan-file canon remains authoritative. After recovery, operators should
+  run `make fix-consistency REPO=state-hub` so file-backed task/workplan state
+  wins over stale queued task updates.
+
+## T01 - Write Safety ADR and Route Inventory
+
+```task
+id: STATE-WP-0068-T01
+status: done
+priority: high
+state_hub_task_id: "07aa2d43-0305-45ca-8b5a-bf6f96f716a9"
+```
+
+Create a short ADR or design doc that classifies State Hub write routes as
+append-only, replace-style, supersedable, or online-only.
+
+Deliverables:
+
+- Route inventory generated from `api/routers/*` and MCP sanctioned writes.
+- V1 safe-write allowlist with request/response examples.
+- Conflict policy per route class.
+- Explicit statement that queued receipts are pending evidence, not successful
+  central commits.
+- Operator decision on the local relay port, default outbox location, and
+  retention window.
+
+Done when implementation tasks can refer to a reviewed allowlist instead of
+guessing route safety.
+
+## T02 - Central Idempotency and Replay Acceptance
+
+```task
+id: STATE-WP-0068-T02
+status: done
+priority: high
+state_hub_task_id: "f0060859-e9a7-441c-91cc-1e838c5ba60f"
+```
+
+Add central API support for idempotent replay.
+
+Expected implementation:
+
+- Migration for a `write_idempotency_keys` table storing key, method, path,
+  request hash, response status/body, source host/agent, first seen, last seen,
+  and expiry.
+- Middleware or route dependency that accepts `Idempotency-Key` on allowlisted
+  write endpoints.
+- Same-key/same-request replay returns the original response.
+- Same-key/different-request returns HTTP 409.
+- Replay metadata is available for diagnostics without logging request secrets.
+- Tests cover success, retry, hash mismatch, expiry, and unsupported routes.
+
+Done when append-only writes can be retried after a transport failure without
+duplicating central records.
+
+## T03 - Durable Local Outbox Store
+
+```task
+id: STATE-WP-0068-T03
+status: done
+priority: high
+state_hub_task_id: "6897dd71-6252-4eed-bb0c-350e8c566b3b"
+```
+
+Implement a local SQLite-backed outbox module used by the relay and CLI.
+
+Minimum schema:
+
+- envelope id and idempotency key
+- method, path, scrubbed JSON body, route class
+- source agent, source host, repo slug, session id when known
+- observed revision fields for conflict checks
+- status: `queued`, `sending`, `acked`, `conflict`, `dead`, `cancelled`
+- attempt count, next retry time, last error, central response summary
+- created, updated, acked timestamps
+
+Safety requirements:
+
+- Create the DB with owner-only permissions where the platform supports it.
+- Never persist authorization headers, API keys, bearer tokens, cookies, or
+  secret-looking fields.
+- Cap payload size and reject large opaque bodies.
+- Provide export/import of non-secret envelopes for operator debugging.
+
+Done when unit tests prove enqueue, status transitions, coalescing metadata,
+scrubbing, and corruption-safe startup behavior.
+
+## T04 - Edge Relay HTTP Surface
+
+```task
+id: STATE-WP-0068-T04
+status: done
+priority: high
+state_hub_task_id: "deb883df-b312-4e8f-b559-718bb8a94035"
+```
+
+Create a local `statehub-edge` relay process that exposes a small HTTP surface.
+
+Behavior:
+
+- Online path: forward allowlisted writes to upstream and return the upstream
+  response.
+- Offline path: enqueue allowlisted writes and return a clear queued receipt:
+  `{"queued": true, "outbox_id": "...", "idempotency_key": "...",
+  "upstream": "unreachable"}`.
+- Online-only path during outage: return a deterministic error explaining that
+  the route is not queueable.
+- Read path: proxy selected reads while online; optionally serve cached
+  `/state/summary` metadata with stale markers while offline.
+- Health/status: expose relay health, upstream reachability, pending count,
+  oldest pending age, and conflict count.
+
+Done when agents can point `API_BASE` at the relay and receive either the
+normal REST shape or an explicit queued/error shape.
+
+## T05 - Replay Worker and Conflict Handling
+
+```task
+id: STATE-WP-0068-T05
+status: done
+priority: high
+state_hub_task_id: "6c3916c1-4a9f-4b1d-a8b1-a356a6edf3db"
+```
+
+Implement the replay loop.
+
+Requirements:
+
+- Exponential backoff with jitter for transport failures.
+- Single-flight sending per envelope.
+- Preserve per-entity order for replace-style writes.
+- Coalesce superseded task/workplan status writes before replay when safe.
+- Use `Idempotency-Key` for every replayed write.
+- Mark conflicts without dropping the original envelope.
+- Provide commands to retry, cancel, or mark-dead individual envelopes.
+
+Done when an integration test can simulate central outage, enqueue writes,
+restore central service, replay successfully, and surface one intentionally
+stale task update as a conflict.
+
+## T06 - MCP and Agent UX Integration
+
+```task
+id: STATE-WP-0068-T06
+status: done
+priority: high
+state_hub_task_id: "8ccac4f9-f457-4f87-9195-1d8619043c0f"
+```
+
+Update MCP tooling and agent-facing docs so offline buffering is usable without
+surprise.
+
+Expected changes:
+
+- MCP write helpers recognize relay queued receipts and return them clearly.
+- Automatic progress-event side effects do not duplicate queued primary writes.
+- Session-close guidance says to check relay status when writes were queued.
+- `mcp_server/TOOLS.md` documents online, queued, and conflict outcomes.
+- Repo `AGENTS.md` template can point agents at the relay when enabled.
+
+Done when an agent can complete a session during a central outage, see that the
+progress write is queued, and verify later that it was replayed.
+
+## T07 - Operator Observability
+
+```task
+id: STATE-WP-0068-T07
+status: done
+priority: medium
+state_hub_task_id: "62c0ca4f-b3e2-49f7-ba70-365016195e83"
+```
+
+Expose pending offline writes to humans and automations.
+
+Deliverables:
+
+- CLI commands: `statehub outbox status`, `statehub outbox list`,
+  `statehub outbox replay`, `statehub outbox export`.
+- Optional dashboard panel or docs page showing edge relay health, if the
+  dashboard can reach the relay.
+- Prometheus-style or JSON metrics for pending count, oldest age, replay
+  failures, and conflicts.
+- Progress event after replay recovery summarizing non-secret results.
+
+Done when the operator can see whether any host still has unsent State Hub
+writes before declaring an outage recovered.
+
+## T08 - Chaos and Regression Test Suite
+
+```task
+id: STATE-WP-0068-T08
+status: done
+priority: high
+state_hub_task_id: "2a12614f-8923-45b1-b8e9-ad8c818b23d3"
+```
+
+Add tests that make offline behavior boring.
+
+Coverage:
+
+- Unit tests for route allowlist, payload scrubbing, idempotency hash behavior,
+  outbox state transitions, and coalescing decisions.
+- Integration test with a fake upstream returning connection errors, 5xx, 409,
+  and success.
+- End-to-end test for MCP write through relay during outage and replay.
+- Drill script that can be run locally without touching production data.
+
+Done when CI can prove no duplicate append-only records are produced across
+retry and no replace-style conflict is silently applied.
+
+## T09 - Runbooks, Cutover, and Recovery Drill
+
+```task
+id: STATE-WP-0068-T09
+status: done
+priority: medium
+state_hub_task_id: "fedea85e-c720-4814-9691-affa6c944954"
+```
+
+Document and rehearse the operator workflow.
+
+Runbook content:
+
+- How to start the relay on an operator workstation or agent host.
+- How to configure MCP/REST clients to use the relay.
+- What queued receipts mean during session close.
+- How to inspect, replay, export, cancel, and resolve conflicted envelopes.
+- Recovery checklist after central State Hub returns.
+- Interaction with `make fix-consistency REPO=state-hub`.
+
+Done when a controlled drill queues at least one progress event and one task
+status update during a forced outage, replays the progress event, flags or
+applies the task update according to the conflict policy, and records the
+results without exposing secrets.
+
+## Dependencies and References
+
+- `CUST-WP-0011` - pragmatic railiance01 State Hub migration.
+- `CUST-WP-0038` - long-term ThreePhoenix HA State Hub target.
+- `STATE-WP-0059` - MCP write-layer reliability and explicit API failure
+  handling.
+- `STATE-WP-0066` - summary cache and stale-while-revalidate for read paths.
+- `docs/activity-core-delegation.md` - JetStream buffering covers State Hub to
+  activity-core events after commit; this work covers agent to State Hub writes
+  before commit.
+- `mcp_server/TOOLS.md` - current MCP/REST parity and failure handling contract.
+
+After this workplan is synced, run:
+
+```bash
+make fix-consistency REPO=state-hub
+```
+
+## Implementation Notes
+
+Completed 2026-06-23. The implementation provides the first full offline-write
+buffering path:
+
+- Central idempotency support through WriteIdempotencyMiddleware, the
+  write_idempotency_keys model, and migration e9f0a1b2c3d4. Exact duplicate
+  writes replay the original response; same key with a different request returns
+  HTTP 409.
+- Shared route classification for queueable append and replace-style writes.
+- Local SQLite outbox with payload scrubbing, payload size limits, private file
+  permissions where supported, status transitions, retry/cancel/export support,
+  and latest replace-write coalescing.
+- State Hub edge relay app with online forwarding, offline queue receipts,
+  health/status, replay endpoint, and replay worker.
+- statehub outbox CLI commands for status, list, export, replay, retry, and
+  cancel.
+- MCP queued receipt handling so queued primary writes do not trigger automatic
+  progress side effects.
+- Operator documentation in docs/offline-write-buffer.md, MCP tool docs, and the
+  Codex agent instruction template.
+
+Verification:
+
+- Focused suite: 22 passed in 19.51s.
+- Full suite: 446 passed, 1 warning in 287.20s. The warning was a SQLAlchemy
+  RuntimeWarning in tests/test_summary_cache.py and was not introduced by a
+  failing assertion.
+- Syntax checks passed for the new and touched Python modules.
+- git diff --check passed.