diff --git a/src/shard_wiki/adapters/__init__.py b/src/shard_wiki/adapters/__init__.py
new file mode 100644
index 0000000..9635b25
--- /dev/null
+++ b/src/shard_wiki/adapters/__init__.py
@@ -0,0 +1,6 @@
+"""adapters/ — the shard adapter contract (bottom waist) and concrete adapters."""
+
+from shard_wiki.adapters.contract import CONTRACT_VERSION, ShardAdapter
+from shard_wiki.adapters.folder import FolderAdapter
+
+__all__ = ["ShardAdapter", "FolderAdapter", "CONTRACT_VERSION"]
diff --git a/src/shard_wiki/adapters/contract.py b/src/shard_wiki/adapters/contract.py
new file mode 100644
index 0000000..ac4da20
--- /dev/null
+++ b/src/shard_wiki/adapters/contract.py
@@ -0,0 +1,52 @@
+"""The shard adapter contract — the bottom narrow waist (CoreArchitectureBlueprint §6, TSD §A).
+
+A backend participates by implementing :class:`ShardAdapter`. ``shard_id``, ``profile`` and
+``read`` are mandatory; everything else is an optional capability that defaults to raising
+:class:`~shard_wiki.model.NotSupported` — so a limited backend is honest about what it can't do
+(graceful degradation, I-8) and core never assumes a capability it wasn't given (capability-as-
+data, I-3). Declared profiles are verified by the conformance suite (T4), never taken on trust.
+"""
+
+from __future__ import annotations
+
+import abc
+from collections.abc import Iterable
+
+from shard_wiki.model import CapabilityProfile, NotSupported, Page
+
+__all__ = ["ShardAdapter", "CONTRACT_VERSION"]
+
+CONTRACT_VERSION = "0.1"
+
+
+class ShardAdapter(abc.ABC):
+    """Versioned interface a backend implements to attach as a shard."""
+
+    contract_version: str = CONTRACT_VERSION
+
+    @property
+    @abc.abstractmethod
+    def shard_id(self) -> str:
+        """Stable id scoping every Identity this shard mints."""
+
+    @abc.abstractmethod
+    def profile(self) -> CapabilityProfile:
+        """The (to-be-verified) capability profile of this binding."""
+
+    @abc.abstractmethod
+    def keys(self) -> Iterable[str]:
+        """The stable page keys this shard offers (the handle half of Identity)."""
+
+    @abc.abstractmethod
+    def read(self, key: str) -> Page:
+        """Read one page by its stable key. Raises ``KeyError`` if absent."""
+
+    # --- optional capability verbs: honest NotSupported by default ---
+    def write(self, key: str, body: str) -> Page:  # noqa: ARG002
+        raise NotSupported(f"{type(self).__name__} does not support write")
+
+    def diff(self, key: str, other: str) -> str:  # noqa: ARG002
+        raise NotSupported(f"{type(self).__name__} does not support diff")
+
+    def notify(self):
+        raise NotSupported(f"{type(self).__name__} does not support notify")
diff --git a/src/shard_wiki/adapters/folder.py b/src/shard_wiki/adapters/folder.py
new file mode 100644
index 0000000..3a30c2b
--- /dev/null
+++ b/src/shard_wiki/adapters/folder.py
@@ -0,0 +1,91 @@
+"""FolderAdapter — a read-only file-store shard over a directory of Markdown.
+
+The home-case substrate: a plain folder of ``.md`` files. The relative path (sans extension,
+``/``-separated) is the stable page **key**; the file is the page **body**; mtime gives a
+freshness stamp. Read-only in this slice (overlay/write-through come later); declared profile
+reflects exactly that (read-only, file-store, path addressing, no native history/query).
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterable
+from datetime import datetime, timezone
+from pathlib import Path
+
+from shard_wiki.adapters.contract import ShardAdapter
+from shard_wiki.model import (
+    AccessGrant,
+    Addressing,
+    AttachmentMode,
+    CapabilityProfile,
+    ContentOpacity,
+    History,
+    Identity,
+    MergeModel,
+    NativeQuery,
+    OperationalEnvelope,
+    Page,
+    Placement,
+    Substrate,
+    Translation,
+    Verb,
+    WriteGranularity,
+)
+from shard_wiki.provenance import Liveness, ProvenanceEnvelope, Staleness
+
+__all__ = ["FolderAdapter"]
+
+
+class FolderAdapter(ShardAdapter):
+    def __init__(self, shard_id: str, root: str | Path) -> None:
+        self._shard_id = shard_id
+        self._root = Path(root)
+
+    @property
+    def shard_id(self) -> str:
+        return self._shard_id
+
+    def profile(self) -> CapabilityProfile:
+        return CapabilityProfile(
+            substrate=Substrate.FILES,
+            attachment_mode=AttachmentMode.FILE_STORE,
+            write_granularity=WriteGranularity.NONE,
+            content_opacity=ContentOpacity.TRANSPARENT,
+            operational_envelope=OperationalEnvelope.LOCAL_UNBOUNDED,
+            access_grant=AccessGrant.OPEN,
+            liveness=Liveness.STATIC,
+            history=History.NONE,
+            merge_model=MergeModel.NONE,
+            addressing=Addressing.PATH,
+            native_query=NativeQuery.NONE,
+            translation=Translation.NATIVE,
+            supported_verbs=frozenset({Verb.READ}),
+        ).validate()
+
+    def _path_for(self, key: str) -> Path:
+        return self._root / f"{key}.md"
+
+    def keys(self) -> Iterable[str]:
+        for p in sorted(self._root.rglob("*.md")):
+            yield p.relative_to(self._root).with_suffix("").as_posix()
+
+    def read(self, key: str) -> Page:
+        path = self._path_for(key)
+        if not path.is_file():
+            raise KeyError(key)
+        body = path.read_text(encoding="utf-8")
+        mtime = datetime.fromtimestamp(path.stat().st_mtime, tz=timezone.utc)
+        envelope = ProvenanceEnvelope(
+            source_shard=self._shard_id,
+            liveness=Liveness.STATIC,
+            staleness=Staleness.FRESH,
+            source_rev=mtime.isoformat(),
+            observed_at=datetime.now(tz=timezone.utc),
+        )
+        rel = path.relative_to(self._root).as_posix()
+        return Page(
+            identity=Identity(self._shard_id, key),
+            body=body,
+            envelope=envelope,
+            placements=(Placement(self._shard_id, rel),),
+        )
diff --git a/tests/test_folder_adapter.py b/tests/test_folder_adapter.py
new file mode 100644
index 0000000..c2ab0e5
--- /dev/null
+++ b/tests/test_folder_adapter.py
@@ -0,0 +1,44 @@
+"""Tests for the FolderAdapter (SHARD-WP-0007 T3)."""
+
+import pytest
+
+from shard_wiki.adapters import FolderAdapter
+from shard_wiki.model import Identity, NotSupported, Verb
+
+
+def _make_shard(tmp_path, files: dict[str, str]) -> FolderAdapter:
+    for rel, text in files.items():
+        p = tmp_path / rel
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_text(text, encoding="utf-8")
+    return FolderAdapter("shardA", tmp_path)
+
+
+def test_keys_and_read(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "# Home", "sub/Topic.md": "topic body"})
+    assert set(shard.keys()) == {"Home", "sub/Topic"}
+    page = shard.read("sub/Topic")
+    assert page.identity == Identity("shardA", "sub/Topic")
+    assert page.body == "topic body"
+    assert page.envelope.source_shard == "shardA"
+    assert page.envelope.source_rev is not None  # mtime stamp
+    assert page.placements[0].path == "sub/Topic.md"
+
+
+def test_read_missing_raises_keyerror(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    with pytest.raises(KeyError):
+        shard.read("Nope")
+
+
+def test_profile_is_valid_and_read_only(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    prof = shard.profile()  # .validate() already called inside
+    assert prof.supports(Verb.READ)
+    assert not prof.supports(Verb.WRITE)
+
+
+def test_unsupported_write_is_honest(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    with pytest.raises(NotSupported):
+        shard.write("Home", "new")
diff --git a/workplans/SHARD-WP-0007-foundation-implementation.md b/workplans/SHARD-WP-0007-foundation-implementation.md
index 129ebbb..db882f6 100644
--- a/workplans/SHARD-WP-0007-foundation-implementation.md
+++ b/workplans/SHARD-WP-0007-foundation-implementation.md
@@ -71,7 +71,7 @@ identity stability vs content change; profile validation accepts/rejects.
 
 ```task
 id: SHARD-WP-0007-T3
-status: todo
+status: done
 priority: high
 state_hub_task_id: "f6e35ddb-ab1e-406a-82f8-563244455f6b"
 ```