diff --git a/pyproject.toml b/pyproject.toml
index 3e5dc2c..0a512c5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.21.2"
+version = "0.21.3"
 description = "Worldtree Conversation API debug console (web + headless CLI) — multi-pane observability"
 readme = "README.md"
 requires-python = ">=3.12"
diff --git a/src/ratatoskr/wt.py b/src/ratatoskr/wt.py
new file mode 100644
index 0000000..674da14
--- /dev/null
+++ b/src/ratatoskr/wt.py
@@ -0,0 +1,92 @@
+"""worldtree-sdk adapter — ratatoskr's single chokepoint onto the Conversation-API SDK.
+
+Slice-1 foundation of the SDK cutover (issue #20;
+`docs/contracts/worldtree_sdk_cutover.contract.md`). A thin adapter that owns ONE
+`WorldtreeClient`, built over ratatoskr's own injected `httpx.AsyncClient`
+transport, and translates the SDK's error floor into ratatoskr's caller-semantic
+exceptions. Two hard invariants anchor it:
+
+  * **INV-CUT-1** — ratatoskr owns the injected transport's lifecycle; the SDK is
+    given it as `transport=` (so `_owns_client=False`) and MUST NOT close it.
+  * **INV-CUT-2** — the adapter raises ratatoskr's caller-semantic exceptions; the
+    ROUTE is the discriminator (never the error body).
+
+This slice ships only the construction chokepoint (`build_client`) and the error
+adapter's DEFAULT rule (`translate_error`: `ApiError` → `SessionApiFailed`, every
+discriminated `WorldtreeError` subclass passing through unchanged). Route-specific
+error rows (`AgentNotFound`, `InvalidCursor`, `AuthoredHistoryUnavailable`, ...)
+and the CLI / web / TUI surface wiring land in later slices.
+
+Transient-migration note (DEC-4, incremental cutover): a same-named
+`SessionApiFailed` still lives in the retiring `ratatoskr.sessions` wrapper and
+serves the old hand-rolled path until slice-2 deletes it. No surface wires the
+adapter in this slice, so the two never meet at runtime; slice-2 reconciles them
+(deletes the `sessions.py` copy, points presenters at this one, which additionally
+carries the SDK's parsed `error_code`).
+"""
+
+from __future__ import annotations
+
+import httpx
+from worldtree_sdk import ApiError, AuthProvider, WorldtreeClient
+
+
+class SessionApiFailed(Exception):
+    """The adapter's DEFAULT caller-semantic error (INV-CUT-2 default row): any SDK
+    `ApiError` not mapped to a more specific ratatoskr exception surfaces here,
+    carrying the SDK's cleanly-parsed `status` / `error_code` / `body` verbatim.
+    The `body` is already UTF-8-byte-bounded and bearer-scrubbed by the SDK."""
+
+    def __init__(self, *, status: int, error_code: str, body: str | None = None) -> None:
+        super().__init__(
+            f"worldtree API failed: status={status}, error_code={error_code!r}"
+        )
+        self.status = status
+        self.error_code = error_code
+        self.body = body
+
+
+def build_client(
+    base_url: str,
+    *,
+    api_key: AuthProvider,
+    admin_key: AuthProvider | None = None,
+    transport: httpx.AsyncClient,
+) -> WorldtreeClient:
+    """Construct the adapter's `WorldtreeClient` over a ratatoskr-owned transport.
+
+    `transport` is REQUIRED and ratatoskr-owned: injecting it sets the SDK's
+    `_owns_client=False`, so `WorldtreeClient.aclose()` never closes it — ratatoskr
+    owns the lifecycle exactly as today (INV-CUT-1). ratatoskr's `api_key` /
+    `admin_key` map to the SDK's per-request `auth` / `admin_auth` providers; the
+    injected transport carries ratatoskr's User-Agent / timeout (wired by the
+    caller in slice-2), NOT the Authorization header — the SDK adds auth per
+    request.
+    """
+    return WorldtreeClient(
+        base_url,
+        auth=api_key,
+        admin_auth=admin_key,
+        transport=transport,
+    )
+
+
+def translate_error(exc: BaseException) -> BaseException:
+    """Map an SDK exception to ratatoskr's caller-semantic exception (INV-CUT-2).
+
+    Foundation scope — the § Error map DEFAULT plus discriminated passthrough:
+      * SDK `ApiError` (the undiscriminated REST floor) → `SessionApiFailed`
+        carrying `status` / `error_code` / `body`.
+      * Every other exception — the SDK's discriminated `WorldtreeError` subclasses
+        (`AgentNotAvailable`, `SessionRetired`, `ResumeError`, `Cancel*`, ...) and
+        any non-SDK error — passes through by IDENTITY, unchanged.
+
+    Route-specific rows (a 404 on `sessions.create` → `AgentNotFound`, a 404 on
+    `sessions.write_history` → `AuthoredHistoryUnavailable`, ...) are the ROUTE's
+    to add at its call-site in later slices, never inferred from the body here.
+    """
+    if isinstance(exc, ApiError):
+        return SessionApiFailed(
+            status=exc.status, error_code=exc.error_code, body=exc.body
+        )
+    return exc
diff --git a/tests/test_wt.py b/tests/test_wt.py
new file mode 100644
index 0000000..8878c64
--- /dev/null
+++ b/tests/test_wt.py
@@ -0,0 +1,85 @@
+"""Unit tests for the worldtree-sdk adapter (`ratatoskr.wt`) — slice-1 foundation.
+
+Covers the two foundation surfaces (issue #20 cutover contract, slice 1):
+  * `build_client` — construction wiring + injected-transport ownership (INV-CUT-1:
+    the SDK must never close ratatoskr's transport).
+  * `translate_error` — the § Error map DEFAULT (`ApiError` → `SessionApiFailed`)
+    plus discriminated-`WorldtreeError` passthrough (INV-CUT-2).
+
+No ratatoskr surface (CLI / web / TUI) is exercised here — that wiring lands in
+slice 2. These tests hit no network (WorldtreeClient does no I/O at construction).
+"""
+
+from __future__ import annotations
+
+import httpx
+from worldtree_sdk import AgentNotAvailable, ApiError, WorldtreeClient
+
+from ratatoskr.wt import SessionApiFailed, build_client, translate_error
+
+
+class TestBuildClient:
+    async def test_constructs_worldtree_client(self) -> None:
+        transport = httpx.AsyncClient()
+        try:
+            client = build_client(
+                "https://wt.example:8081", api_key="ck-test", transport=transport
+            )
+            assert isinstance(client, WorldtreeClient)
+            assert client.base_url == "https://wt.example:8081"
+        finally:
+            await transport.aclose()
+
+    async def test_injected_transport_is_ratatoskr_owned(self) -> None:
+        # INV-CUT-1 [hard]: aclose() on the SDK client must NOT close ratatoskr's
+        # transport — ratatoskr owns the lifecycle exactly as it does today.
+        transport = httpx.AsyncClient()
+        client = build_client("https://wt.example", api_key="ck", transport=transport)
+        await client.aclose()
+        assert client.closed is True
+        assert transport.is_closed is False
+        await transport.aclose()
+
+    async def test_admin_key_optional(self) -> None:
+        transport = httpx.AsyncClient()
+        try:
+            # Absent admin_key → admin_auth=None; still constructs.
+            without_admin = build_client(
+                "https://wt.example", api_key="ck", transport=transport
+            )
+            assert isinstance(without_admin, WorldtreeClient)
+            # Present admin_key → constructs (admin surface available in later slices).
+            with_admin = build_client(
+                "https://wt.example", api_key="ck", admin_key="ak", transport=transport
+            )
+            assert isinstance(with_admin, WorldtreeClient)
+        finally:
+            await transport.aclose()
+
+
+class TestTranslateError:
+    def test_apierror_maps_to_session_api_failed_default(self) -> None:
+        exc = ApiError("some_code", "boom", status=500, body="raw-body")
+        mapped = translate_error(exc)
+        assert isinstance(mapped, SessionApiFailed)
+        assert mapped.status == 500
+        assert mapped.error_code == "some_code"
+        assert mapped.body == "raw-body"
+
+    def test_apierror_with_no_body_maps_cleanly(self) -> None:
+        exc = ApiError("nope", "no body", status=404)
+        mapped = translate_error(exc)
+        assert isinstance(mapped, SessionApiFailed)
+        assert mapped.status == 404
+        assert mapped.error_code == "nope"
+        assert mapped.body is None
+
+    def test_discriminated_subclass_passes_through_unchanged(self) -> None:
+        # Discriminated WorldtreeError subclasses are already the right semantic
+        # type — the adapter passes them through by identity (no re-wrap).
+        exc = AgentNotAvailable("agent_not_available", "gone", status=409)
+        assert translate_error(exc) is exc
+
+    def test_non_worldtree_error_passes_through_unchanged(self) -> None:
+        exc = ValueError("unrelated")
+        assert translate_error(exc) is exc
diff --git a/uv.lock b/uv.lock
index f09c39f..602cf8d 100644
--- a/uv.lock
+++ b/uv.lock
@@ -472,7 +472,7 @@ wheels = [
 
 [[package]]
 name = "ratatoskr"
-version = "0.21.2"
+version = "0.21.3"
 source = { editable = "." }
 dependencies = [
     { name = "httpx" },