diff --git a/docs/runbook.md b/docs/runbook.md
index fc81eb1..0136854 100644
--- a/docs/runbook.md
+++ b/docs/runbook.md
@@ -358,6 +358,27 @@ Legacy values are still accepted: `catchup` → `catchup_all`,
 > brief outage at trigger time silently dropped the fire with no recovery and no
 > log line. The `daily-statehub-wsjf-triage` definition now uses `catchup_latest`.
 
+## State Hub write idempotency (ACTIVITY-WP-0014 T05)
+
+Every State Hub write from activity-core (report-sink progress, ops-evidence
+progress, schedule-miss alerts) carries a stable **`Idempotency-Key`** header
+derived deterministically from the write's identity
+(`run_id:instruction_id:event_type`, or `schedule_miss:activity_id:last_fired`
+for miss alerts). This makes writes safe to **buffer and replay** under the
+planned State Hub *beachhead* (per-machine read cache + write outbox): a flush —
+possibly retried after an outage — cannot create duplicate progress/triage
+events once State Hub / the beachhead honours the header.
+
+The guarantee lives on the write, not on a live dedup read. The read-based
+`_progress_exists` check is now best-effort only: if State Hub is unreachable it
+returns `False` (proceed to the keyed write) rather than hard-failing. The header
+passes untouched through the `actcore-state-hub-bridge` proxy and is ignored by
+State Hub versions that do not yet honour it.
+
+> The queue/cache itself is **not** built in activity-core — it belongs to the
+> state-hub beachhead. activity-core only emits the key. See the proposal sent to
+> the `state-hub` agent.
+
 ## Troubleshooting
 
 ### Worker fails to start: "ACTCORE_DB_URL is required"
diff --git a/src/activity_core/ops_evidence_sinks.py b/src/activity_core/ops_evidence_sinks.py
index 9299280..803479f 100644
--- a/src/activity_core/ops_evidence_sinks.py
+++ b/src/activity_core/ops_evidence_sinks.py
@@ -8,6 +8,7 @@ from typing import Any
 import httpx
 
 from activity_core.context_resolvers.ops_inventory import _sanitize_url
+from activity_core.state_hub_write import idempotency_headers
 
 _DEFAULT_STATE_HUB_URL = "http://127.0.0.1:8000"
 _INTER_HUB_SINK_TYPES = {
@@ -121,6 +122,7 @@ def _post_state_hub_progress(
     resp = httpx.post(
         f"{base_url}/progress/",
         json=body,
+        headers=idempotency_headers(run_id, context_key, event_type),
         timeout=float(sink.get("timeout_seconds", 10.0)),
     )
     resp.raise_for_status()
@@ -136,12 +138,17 @@ def _post_state_hub_progress(
 
 
 def _progress_exists(base_url: str, event_type: str, idempotency_key: str) -> bool:
-    resp = httpx.get(
-        f"{base_url}/progress/",
-        params={"limit": 100},
-        timeout=10.0,
-    )
-    resp.raise_for_status()
+    # Best-effort optimisation only; the Idempotency-Key header on the write is the
+    # real dedup guarantee. Do not hard-fail if State Hub is unreachable here.
+    try:
+        resp = httpx.get(
+            f"{base_url}/progress/",
+            params={"limit": 100},
+            timeout=10.0,
+        )
+        resp.raise_for_status()
+    except httpx.HTTPError:
+        return False
     for item in resp.json():
         detail = item.get("detail") or {}
         if (
diff --git a/src/activity_core/report_sinks.py b/src/activity_core/report_sinks.py
index ad6dd1b..66fa5a7 100644
--- a/src/activity_core/report_sinks.py
+++ b/src/activity_core/report_sinks.py
@@ -11,6 +11,8 @@ from zoneinfo import ZoneInfo
 
 import httpx
 
+from activity_core.state_hub_write import idempotency_headers
+
 _DEFAULT_STATE_HUB_URL = "http://127.0.0.1:8000"
 _THE_CUSTODIAN_ROOT = Path("/home/worsch/the-custodian")
 _FORBIDDEN_CUSTODIAN_ROOTS = (
@@ -149,6 +151,7 @@ def _post_state_hub_progress(
     resp = httpx.post(
         f"{base_url}/progress/",
         json=body,
+        headers=idempotency_headers(run_id, instruction_id, event_type),
         timeout=float(sink.get("timeout_seconds", 10.0)),
     )
     resp.raise_for_status()
@@ -167,12 +170,18 @@ def _progress_exists(
     instruction_id: str,
     event_type: str,
 ) -> bool:
-    resp = httpx.get(
-        f"{base_url}/progress/",
-        params={"limit": 100},
-        timeout=10.0,
-    )
-    resp.raise_for_status()
+    # Best-effort read-dedup optimisation only. The Idempotency-Key header on the
+    # write is the real guarantee; if State Hub is unreachable here we must not
+    # hard-fail — proceed to the (keyed) write rather than raising.
+    try:
+        resp = httpx.get(
+            f"{base_url}/progress/",
+            params={"limit": 100},
+            timeout=10.0,
+        )
+        resp.raise_for_status()
+    except httpx.HTTPError:
+        return False
     for item in resp.json():
         detail = item.get("detail") or {}
         if (
diff --git a/src/activity_core/schedule_health.py b/src/activity_core/schedule_health.py
index 2aa4813..d363812 100644
--- a/src/activity_core/schedule_health.py
+++ b/src/activity_core/schedule_health.py
@@ -24,6 +24,7 @@ from uuid import UUID
 import httpx
 
 from activity_core.schedule_manager import schedule_id
+from activity_core.state_hub_write import idempotency_headers
 
 _DEFAULT_STATE_HUB_URL = "http://127.0.0.1:8000"
 
@@ -176,7 +177,14 @@ def post_missed_fire_alert(
     if workstream_id:
         body["workstream_id"] = workstream_id
 
-    resp = httpx.post(f"{base_url}/progress/", json=body, timeout=timeout_seconds)
+    # Dedup repeated alerts for the same missed window (same schedule + last fire).
+    last_fired = health.last_fired_at.isoformat() if health.last_fired_at else "none"
+    resp = httpx.post(
+        f"{base_url}/progress/",
+        json=body,
+        headers=idempotency_headers("schedule_miss", health.activity_id, last_fired),
+        timeout=timeout_seconds,
+    )
     resp.raise_for_status()
     data = resp.json()
     return {
diff --git a/src/activity_core/state_hub_write.py b/src/activity_core/state_hub_write.py
new file mode 100644
index 0000000..2b45138
--- /dev/null
+++ b/src/activity_core/state_hub_write.py
@@ -0,0 +1,34 @@
+"""Idempotency-keyed State Hub writes (ACTIVITY-WP-0014 T05).
+
+Under the State Hub *beachhead* model, a write may be buffered locally while
+central State Hub is unreachable and **flushed later, possibly with retries**.
+To keep that flush safe — no duplicate progress / triage events — every write
+carries a stable ``Idempotency-Key`` header derived deterministically from the
+write's identity. The guarantee lives on the write itself and does **not** depend
+on a live dedup read, so it holds even when the beachhead is serving offline.
+
+activity-core does not implement the queue/cache (that is state-hub's beachhead);
+it only emits the key so the beachhead / State Hub can dedup on flush. The header
+passes untouched through the existing ``actcore-state-hub-bridge`` proxy and is
+ignored by State Hub versions that do not yet honour it.
+"""
+
+from __future__ import annotations
+
+IDEMPOTENCY_HEADER = "Idempotency-Key"
+
+
+def idempotency_key(*parts: str | None) -> str:
+    """Build a stable, header-safe idempotency key from identity parts.
+
+    Empty/None parts are kept as empty segments so the key shape is stable across
+    calls. Whitespace and control characters are collapsed to keep the value a
+    valid single-line HTTP header.
+    """
+    raw = ":".join((p or "") for p in parts)
+    return "".join(ch if 0x20 < ord(ch) < 0x7F else "_" for ch in raw) or "_"
+
+
+def idempotency_headers(*parts: str | None) -> dict[str, str]:
+    """Return the header dict to attach to a State Hub write."""
+    return {IDEMPOTENCY_HEADER: idempotency_key(*parts)}
diff --git a/tests/test_state_hub_write.py b/tests/test_state_hub_write.py
new file mode 100644
index 0000000..207eaf9
--- /dev/null
+++ b/tests/test_state_hub_write.py
@@ -0,0 +1,81 @@
+"""ACTIVITY-WP-0014 T05: idempotency-keyed State Hub writes."""
+
+from __future__ import annotations
+
+import httpx
+import pytest
+
+from activity_core import report_sinks
+from activity_core.state_hub_write import (
+    IDEMPOTENCY_HEADER,
+    idempotency_headers,
+    idempotency_key,
+)
+
+
+def test_key_is_stable_and_deterministic() -> None:
+    a = idempotency_key("run1", "daily-triage-report", "daily_triage")
+    b = idempotency_key("run1", "daily-triage-report", "daily_triage")
+    assert a == b == "run1:daily-triage-report:daily_triage"
+
+
+def test_key_shape_stable_with_missing_parts() -> None:
+    assert idempotency_key("run1", None, "daily_triage") == "run1::daily_triage"
+
+
+def test_key_sanitizes_control_and_whitespace() -> None:
+    key = idempotency_key("run 1", "a\tb", "x\n")
+    assert "\t" not in key and "\n" not in key and " " not in key
+
+
+def test_headers_carry_the_key() -> None:
+    headers = idempotency_headers("run1", "i", "e")
+    assert headers == {IDEMPOTENCY_HEADER: "run1:i:e"}
+
+
+def test_distinct_identities_get_distinct_keys() -> None:
+    assert idempotency_key("r", "i", "daily_triage") != idempotency_key(
+        "r", "i", "schedule_miss"
+    )
+
+
+def test_progress_exists_is_best_effort_on_connection_error(monkeypatch) -> None:
+    """A down State Hub must not hard-fail the dedup read; it returns False so the
+    keyed write can still proceed."""
+
+    def _boom(*args, **kwargs):
+        raise httpx.ConnectError("Connection refused")
+
+    monkeypatch.setattr(report_sinks.httpx, "get", _boom)
+    assert (
+        report_sinks._progress_exists(
+            "http://127.0.0.1:8000", "run1", "daily-triage-report", "daily_triage"
+        )
+        is False
+    )
+
+
+def test_report_sink_post_sends_idempotency_header(monkeypatch) -> None:
+    """The state-hub-progress write carries a stable Idempotency-Key header."""
+    captured: dict[str, object] = {}
+
+    monkeypatch.setattr(report_sinks, "_progress_exists", lambda *a, **k: False)
+
+    class _Resp:
+        def raise_for_status(self) -> None: ...
+        def json(self) -> dict[str, str]:
+            return {"id": "pid-1"}
+
+    def _capture_post(url, json, headers, timeout):  # noqa: A002
+        captured["headers"] = headers
+        return _Resp()
+
+    monkeypatch.setattr(report_sinks.httpx, "post", _capture_post)
+
+    payload = {"run_id": "run1", "activity_id": "act1", "scheduled_for": None}
+    report_entry = {"instruction_id": "daily-triage-report", "report": {"summary": "s"}}
+    sink = {"event_type": "daily_triage"}
+
+    result = report_sinks._post_state_hub_progress(payload, report_entry, sink)
+    assert result["status"] == "posted"
+    assert captured["headers"][IDEMPOTENCY_HEADER] == "run1:daily-triage-report:daily_triage"
diff --git a/workplans/ACTIVITY-WP-0014-schedule-misfire-robustness.md b/workplans/ACTIVITY-WP-0014-schedule-misfire-robustness.md
index 9ae43da..28cc264 100644
--- a/workplans/ACTIVITY-WP-0014-schedule-misfire-robustness.md
+++ b/workplans/ACTIVITY-WP-0014-schedule-misfire-robustness.md
@@ -171,12 +171,14 @@ down. This is handed off to state-hub (see the coordination message / proposal);
 
 activity-core's only responsibilities under this model are thin:
 
-- **Idempotent writes (do now, in-repo):** attach a stable idempotency key
-  (e.g. `run_id` + `instruction_id` + `event_type`) to every State Hub write so a
-  beachhead flush — possibly replayed after an outage — cannot create duplicate
-  `daily_triage`/progress events. The report sink already does a read-based dedup
-  check (`_progress_exists`); make the guarantee explicit and not dependent on a
-  live read.
+- **Idempotent writes — DONE (2026-06-23, in-repo):** added
+  `activity_core/state_hub_write` (`idempotency_headers`); every State Hub write
+  (report-sink, ops-evidence, schedule-miss) now sends a stable `Idempotency-Key`
+  header derived from `run_id:instruction_id:event_type`. The read-based
+  `_progress_exists` dedup is now best-effort (returns `False` on connection
+  error instead of hard-failing), so the guarantee lives on the keyed write, not
+  a live read. Tests in `tests/test_state_hub_write.py`; documented in
+  `docs/runbook.md`.
 - **Adopt the beachhead endpoint (blocked on state-hub):** keep `STATE_HUB_URL`
   pointed at the local beachhead, and **retire the bespoke
   `actcore-state-hub-bridge` proxy** (the inline `hostNetwork` proxy in