diff --git a/pyproject.toml b/pyproject.toml
index 79a9ae9..94f3bb8 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -29,6 +29,7 @@ dependencies = [
   "alembic >= 1.13",
   "blake3 >= 0.4",
   "cbor2 >= 5.6",
+  "jcs >= 0.2",
   "typer >= 0.12",
   "structlog >= 24.1",
   "pydantic >= 2.7",
@@ -89,6 +90,10 @@ mypy_path = "src"
 explicit_package_bases = true
 namespace_packages = true
 
+[[tool.mypy.overrides]]
+module = ["jcs"]
+ignore_missing_imports = true
+
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
diff --git a/src/artifactstore/manifest/__init__.py b/src/artifactstore/manifest/__init__.py
index 9b87e45..36ca0fa 100644
--- a/src/artifactstore/manifest/__init__.py
+++ b/src/artifactstore/manifest/__init__.py
@@ -1,5 +1,37 @@
-"""Package manifest model and canonical-CBOR codec.
+"""Package manifest model, codec, and projections (ADR-0003).
 
-Real implementation lands in ARTIFACT-STORE-WP-0001-T010. See ADR-0003 for
-the canonicalisation pin (RFC 8949 §4.2.2).
+The canonical wire format is CBOR with deterministic encoding (RFC 8949).
+JCS (RFC 8785) is the JSON projection used for display and signing-tool
+interop. A package's external identifier is the BLAKE3 digest over its
+canonical CBOR bytes (ADR-0001).
 """
+
+from artifactstore.manifest.codec import decode, encode
+from artifactstore.manifest.digest import manifest_digest
+from artifactstore.manifest.model import (
+    MANIFEST_VERSION,
+    FileEntry,
+    Manifest,
+    Package,
+    Provenance,
+    RetentionHold,
+    RetentionSummary,
+    StorageReceipt,
+)
+from artifactstore.manifest.projection import cbor_from_jcs, jcs_projection
+
+__all__ = [
+    "MANIFEST_VERSION",
+    "FileEntry",
+    "Manifest",
+    "Package",
+    "Provenance",
+    "RetentionHold",
+    "RetentionSummary",
+    "StorageReceipt",
+    "cbor_from_jcs",
+    "decode",
+    "encode",
+    "jcs_projection",
+    "manifest_digest",
+]
diff --git a/src/artifactstore/manifest/codec.py b/src/artifactstore/manifest/codec.py
new file mode 100644
index 0000000..b16bc6a
--- /dev/null
+++ b/src/artifactstore/manifest/codec.py
@@ -0,0 +1,63 @@
+"""Canonical CBOR codec for the manifest (ADR-0003).
+
+Canonicalisation uses cbor2's deterministic encoding (RFC 8949): definite-
+length encoding, shortest-form integers, sorted map keys. The same input
+manifest always produces the same byte sequence, which is the property the
+manifest digest (ADR-0001) and signature flows rely on.
+"""
+
+from __future__ import annotations
+
+from dataclasses import asdict
+from typing import Any
+
+import cbor2
+
+from artifactstore.manifest.model import (
+    FileEntry,
+    Manifest,
+    Package,
+    Provenance,
+    RetentionHold,
+    RetentionSummary,
+    StorageReceipt,
+)
+
+__all__ = ["decode", "encode"]
+
+
+def encode(manifest: Manifest) -> bytes:
+    """Serialise a :class:`Manifest` to canonical CBOR bytes."""
+    payload = asdict(manifest)
+    return cbor2.dumps(payload, canonical=True)
+
+
+def decode(data: bytes) -> Manifest:
+    """Parse canonical CBOR bytes back into a :class:`Manifest`."""
+    payload = cbor2.loads(data)
+    if not isinstance(payload, dict):
+        raise ValueError("manifest must decode to a CBOR map")
+    return _from_dict(payload)
+
+
+def _from_dict(payload: dict[str, Any]) -> Manifest:
+    package = Package(**payload["package"])
+    files = [FileEntry(**f) for f in payload["files"]]
+    receipts = [StorageReceipt(**r) for r in payload["storage_receipts"]]
+    rs_raw = payload["retention_summary"]
+    holds = [RetentionHold(**h) for h in rs_raw["active_holds"]]
+    retention_summary = RetentionSummary(
+        retention_class=rs_raw["retention_class"],
+        expires_at=rs_raw["expires_at"],
+        active_holds=holds,
+        last_retention_event_sequence=rs_raw["last_retention_event_sequence"],
+    )
+    provenance = Provenance(**payload["provenance"])
+    return Manifest(
+        manifest_version=payload["manifest_version"],
+        package=package,
+        files=files,
+        storage_receipts=receipts,
+        retention_summary=retention_summary,
+        provenance=provenance,
+    )
diff --git a/src/artifactstore/manifest/digest.py b/src/artifactstore/manifest/digest.py
new file mode 100644
index 0000000..c9484c3
--- /dev/null
+++ b/src/artifactstore/manifest/digest.py
@@ -0,0 +1,19 @@
+"""Manifest digest helper (ADR-0001 + ADR-0003)."""
+
+from __future__ import annotations
+
+from artifactstore.identity import PRIMARY_ALGORITHM, ContentAddress, digest_bytes
+from artifactstore.manifest.codec import encode
+from artifactstore.manifest.model import Manifest
+
+__all__ = ["manifest_digest"]
+
+
+def manifest_digest(manifest: Manifest) -> ContentAddress:
+    """Return the canonical content address of a manifest.
+
+    Defined as the primary digest (default BLAKE3) over the manifest's
+    canonical CBOR bytes. This is the package's external identifier.
+    """
+    cbor_bytes = encode(manifest)
+    return digest_bytes(cbor_bytes, primary=PRIMARY_ALGORITHM).primary.content_address
diff --git a/src/artifactstore/manifest/model.py b/src/artifactstore/manifest/model.py
new file mode 100644
index 0000000..0e30af2
--- /dev/null
+++ b/src/artifactstore/manifest/model.py
@@ -0,0 +1,97 @@
+"""Manifest data model (ADR-0003).
+
+Field types are restricted to CBOR-/JSON-compatible primitives (``str``,
+``int``, ``bool``, ``None``, ``list``, ``dict``) so the canonical CBOR
+encoding and the JCS JSON projection round-trip losslessly.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+MANIFEST_VERSION = 1
+
+
+@dataclass(frozen=True, slots=True)
+class FileEntry:
+    """One stored file in a package."""
+
+    id: str
+    relative_path: str
+    media_type: str
+    size_bytes: int
+    digest_algorithm: str
+    digest_primary_hex: str
+    digest_sha256_hex: str
+
+
+@dataclass(frozen=True, slots=True)
+class StorageReceipt:
+    """A record of where a file's bytes are stored."""
+
+    file_id: str
+    backend_id: str
+    content_address: str
+    retrieval_tier: str
+    status: str
+
+
+@dataclass(frozen=True, slots=True)
+class RetentionHold:
+    """An active hold preventing deletion eligibility."""
+
+    hold_id: str
+    reason: str
+    actor: str
+    applied_at: str
+
+
+@dataclass(frozen=True, slots=True)
+class RetentionSummary:
+    """Retention state summary as of manifest write time."""
+
+    retention_class: str
+    expires_at: str | None
+    active_holds: list[RetentionHold]
+    last_retention_event_sequence: int | None
+
+
+@dataclass(frozen=True, slots=True)
+class Package:
+    """Package-level metadata."""
+
+    id: str
+    name: str
+    producer: str
+    subject: str
+    retention_class: str
+    status: str
+    created_at: str
+    finalized_at: str | None
+    expires_at: str | None
+    metadata: dict[str, Any]
+    metadata_schema_id: str | None
+
+
+@dataclass(frozen=True, slots=True)
+class Provenance:
+    """Provenance fields recorded at ingest time."""
+
+    source_commits: dict[str, str]
+    tool_versions: dict[str, str]
+    environment: dict[str, str]
+    ingest_actor: str
+    ingest_timestamps: dict[str, str]
+
+
+@dataclass(frozen=True, slots=True)
+class Manifest:
+    """The complete v1 manifest payload."""
+
+    manifest_version: int
+    package: Package
+    files: list[FileEntry]
+    storage_receipts: list[StorageReceipt]
+    retention_summary: RetentionSummary
+    provenance: Provenance
diff --git a/src/artifactstore/manifest/projection.py b/src/artifactstore/manifest/projection.py
new file mode 100644
index 0000000..dd28186
--- /dev/null
+++ b/src/artifactstore/manifest/projection.py
@@ -0,0 +1,35 @@
+"""JCS (RFC 8785) projection of the manifest.
+
+Used for human display, signing-tool interop, and as the JSON form of the
+manifest. Round-trips losslessly with the canonical CBOR form as long as
+manifest values use only JSON-compatible primitive types.
+"""
+
+from __future__ import annotations
+
+import json
+from dataclasses import asdict
+
+import cbor2
+import jcs
+
+from artifactstore.manifest.model import Manifest
+
+__all__ = ["cbor_from_jcs", "jcs_projection"]
+
+
+def jcs_projection(manifest: Manifest) -> bytes:
+    """Serialise a :class:`Manifest` to canonical JSON (RFC 8785)."""
+    payload = asdict(manifest)
+    return jcs.canonicalize(payload)  # type: ignore[no-any-return]
+
+
+def cbor_from_jcs(jcs_bytes: bytes) -> bytes:
+    """Decode JCS JSON bytes and re-encode as canonical CBOR.
+
+    Used to validate the JCS↔CBOR round-trip property: ``cbor_from_jcs`` of
+    a JCS projection must equal the canonical CBOR encoding of the same
+    manifest (after re-decoding).
+    """
+    payload = json.loads(jcs_bytes)
+    return cbor2.dumps(payload, canonical=True)
diff --git a/tests/unit/test_manifest.py b/tests/unit/test_manifest.py
new file mode 100644
index 0000000..23efaf8
--- /dev/null
+++ b/tests/unit/test_manifest.py
@@ -0,0 +1,167 @@
+"""Tests for :mod:`artifactstore.manifest` (ARTIFACT-STORE-WP-0001-T010)."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import cbor2
+import pytest
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from artifactstore.identity import PRIMARY_ALGORITHM, digest_bytes
+from artifactstore.manifest import (
+    MANIFEST_VERSION,
+    FileEntry,
+    Manifest,
+    Package,
+    Provenance,
+    RetentionHold,
+    RetentionSummary,
+    StorageReceipt,
+    cbor_from_jcs,
+    decode,
+    encode,
+    jcs_projection,
+    manifest_digest,
+)
+
+
+def _sample_manifest(
+    *,
+    file_count: int = 1,
+    holds: int = 0,
+    metadata: dict[str, Any] | None = None,
+) -> Manifest:
+    files = [
+        FileEntry(
+            id=f"file-{i:04d}",
+            relative_path=f"reports/r-{i}.json",
+            media_type="application/json",
+            size_bytes=42 + i,
+            digest_algorithm=PRIMARY_ALGORITHM,
+            digest_primary_hex="ab" * 32,
+            digest_sha256_hex="cd" * 32,
+        )
+        for i in range(file_count)
+    ]
+    receipts = [
+        StorageReceipt(
+            file_id=f.id,
+            backend_id="local",
+            content_address=f"{f.digest_algorithm}:{f.digest_primary_hex}",
+            retrieval_tier="hot",
+            status="recorded",
+        )
+        for f in files
+    ]
+    holds_list = [
+        RetentionHold(
+            hold_id=f"hold-{i:04d}",
+            reason="audit-prep",
+            actor="ops@example.org",
+            applied_at="2026-05-15T12:00:00Z",
+        )
+        for i in range(holds)
+    ]
+    return Manifest(
+        manifest_version=MANIFEST_VERSION,
+        package=Package(
+            id="pkg-0001",
+            name="guide-board cmis run",
+            producer="guide-board",
+            subject="kontextual-engine",
+            retention_class="raw-evidence",
+            status="finalized",
+            created_at="2026-05-15T10:00:00Z",
+            finalized_at="2026-05-15T11:00:00Z",
+            expires_at="2027-05-15T11:00:00Z",
+            metadata=metadata if metadata is not None else {},
+            metadata_schema_id=None,
+        ),
+        files=files,
+        storage_receipts=receipts,
+        retention_summary=RetentionSummary(
+            retention_class="raw-evidence",
+            expires_at="2027-05-15T11:00:00Z",
+            active_holds=holds_list,
+            last_retention_event_sequence=None,
+        ),
+        provenance=Provenance(
+            source_commits={"guide-board": "abc1234"},
+            tool_versions={"guide-board": "0.1.0"},
+            environment={"runner": "ci"},
+            ingest_actor="codex",
+            ingest_timestamps={"received_at": "2026-05-15T10:00:00Z"},
+        ),
+    )
+
+
+def test_cbor_round_trip_simple() -> None:
+    m = _sample_manifest(file_count=3, holds=1)
+    assert decode(encode(m)) == m
+
+
+def test_canonical_cbor_is_byte_stable_across_calls() -> None:
+    m = _sample_manifest(
+        file_count=5,
+        holds=0,
+        metadata={"zz": "z", "aa": "a", "mm": "m"},
+    )
+    assert encode(m) == encode(m)
+
+
+def test_jcs_round_trip_via_cbor() -> None:
+    m = _sample_manifest(file_count=2, holds=2, metadata={"key": "value", "n": 7})
+    cbor_a = encode(m)
+    jcs_bytes = jcs_projection(m)
+    cbor_b = cbor_from_jcs(jcs_bytes)
+    m_decoded = decode(cbor_b)
+    cbor_c = encode(m_decoded)
+    assert cbor_a == cbor_c
+
+
+def test_manifest_digest_is_blake3_of_canonical_cbor() -> None:
+    m = _sample_manifest()
+    ca = manifest_digest(m)
+    expected = digest_bytes(encode(m)).primary.content_address
+    assert ca == expected
+    assert str(ca).startswith("blake3:")
+
+
+def test_decode_rejects_non_map_cbor() -> None:
+    bad = cbor2.dumps([1, 2, 3], canonical=True)
+    with pytest.raises(ValueError):
+        decode(bad)
+
+
+def test_jcs_projection_is_canonical_json() -> None:
+    m = _sample_manifest(file_count=1, holds=0, metadata={"b": 2, "a": 1})
+    jcs_bytes = jcs_projection(m)
+    # JCS sorts object keys lexicographically; verify metadata keys appear
+    # in alphabetical order in the serialised output.
+    text = jcs_bytes.decode("utf-8")
+    assert text.find('"a":1') < text.find('"b":2')
+
+
+@settings(suppress_health_check=[HealthCheck.too_slow], max_examples=25)
+@given(
+    file_count=st.integers(min_value=0, max_value=5),
+    hold_count=st.integers(min_value=0, max_value=3),
+)
+def test_property_cbor_round_trip(file_count: int, hold_count: int) -> None:
+    m = _sample_manifest(file_count=file_count, holds=hold_count)
+    assert decode(encode(m)) == m
+
+
+@settings(suppress_health_check=[HealthCheck.too_slow], max_examples=25)
+@given(
+    file_count=st.integers(min_value=0, max_value=3),
+    hold_count=st.integers(min_value=0, max_value=2),
+)
+def test_property_jcs_to_cbor_round_trip(file_count: int, hold_count: int) -> None:
+    m = _sample_manifest(file_count=file_count, holds=hold_count)
+    cbor_a = encode(m)
+    cbor_b = cbor_from_jcs(jcs_projection(m))
+    cbor_c = encode(decode(cbor_b))
+    assert cbor_a == cbor_c
diff --git a/uv.lock b/uv.lock
index 19615df..8d5348f 100644
--- a/uv.lock
+++ b/uv.lock
@@ -70,6 +70,7 @@ dependencies = [
     { name = "blake3" },
     { name = "cbor2" },
     { name = "fastapi" },
+    { name = "jcs" },
     { name = "pydantic" },
     { name = "pydantic-settings" },
     { name = "sqlalchemy" },
@@ -108,6 +109,7 @@ requires-dist = [
     { name = "fastapi", specifier = ">=0.115" },
     { name = "httpx", marker = "extra == 'dev'", specifier = ">=0.27" },
     { name = "hypothesis", marker = "extra == 'dev'", specifier = ">=6.100" },
+    { name = "jcs", specifier = ">=0.2" },
     { name = "mypy", marker = "extra == 'dev'", specifier = ">=1.10" },
     { name = "pydantic", specifier = ">=2.7" },
     { name = "pydantic-settings", specifier = ">=2.4" },
@@ -505,6 +507,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/cb/b1/3846dd7f199d53cb17f49cba7e651e9ce294d8497c8c150530ed11865bb8/iniconfig-2.3.0-py3-none-any.whl", hash = "sha256:f631c04d2c48c52b84d0d0549c99ff3859c98df65b3101406327ecc7d53fbf12", size = 7484 },
 ]
 
+[[package]]
+name = "jcs"
+version = "0.2.1"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/47/e5/9d547f0d42ba00f68eec773aa7b2145e0c0335eb632cbaf519f480a429af/jcs-0.2.1.tar.gz", hash = "sha256:9f20360b2f3b0a410d65493b448f96306d80e37fb46283f3f4aa5db2c7c1472b", size = 6886 }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/c0/d4/9a99bc15266a842bd14a1913afdb05182888ebab035666c1ce8a64537ca2/jcs-0.2.1-py3-none-any.whl", hash = "sha256:e23a3e1de60f832d33cd811bb9c3b3be79219cdf95f63b88f0972732c3fa8476", size = 7603 },
+]
+
 [[package]]
 name = "librt"
 version = "0.11.0"
diff --git a/workplans/ARTIFACT-STORE-WP-0001-service-baseline.md b/workplans/ARTIFACT-STORE-WP-0001-service-baseline.md
index 64affab..3eca839 100644
--- a/workplans/ARTIFACT-STORE-WP-0001-service-baseline.md
+++ b/workplans/ARTIFACT-STORE-WP-0001-service-baseline.md
@@ -118,7 +118,7 @@ Acceptance:
 
 ```task
 id: ARTIFACT-STORE-WP-0001-T010
-status: todo
+status: done
 priority: high
 state_hub_task_id: "8b45a3d9-aa19-4ae8-afe0-687417bf12d0"
 ```