diff --git a/README.md b/README.md
index e6b62cd..bab614b 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,53 @@
-# crucible-agent-maintain-only-latest
+# FastAPI current-status step
 
-> Maintain only the latest check result for each monitored target in concurrency-safe memory and expose it through the specified FastAPI current-status endpoint.
+Python/FastAPI implementation for step 3. It keeps one in-memory completed result per target behind a single `asyncio.Lock` and emits `current_status_implementation` while preserving all prior contract outputs.
 
-A tool, not a standing agent service — see `input.schema.json`/`output.schema.json` for its contract. Generated by crucible-agent-decomposer for a plan gap step; language and structure are whatever the capability actually needs, not a fixed layout.
+## Integrate with the prior service/checker
+
+Create one coordinator for the FastAPI process and include its router:
+
+```python
+coordinator = CurrentStatusCoordinator()
+app.include_router(build_current_status_router(coordinator))
+```
+
+The prior target-create path must call `await coordinator.register_target(target_id)`. The prior target-delete path must call `await coordinator.delete_target(target_id)`. The scheduler/check path calls:
+
+```python
+published = await coordinator.run_check(target_id, target, prior_checker)
+```
+
+`prior_checker` is an async callable returning a JSON-compatible mapping. `published=False` means the result became stale. These lifecycle hooks are deliberately explicit so target deletion and status cleanup share the coordinator's critical section.
+
+## REST contract
+
+`GET /targets/{target_id}/status` returns:
+
+* `404` with `{"detail":{"code":"target_not_found","target_id":"..."}}` when the target does not exist.
+* `200` with `state="not_checked"` and `result=null` when it exists but has no completed check.
+* `200` with `state="checked"` and the latest result otherwise.
+
+Only one `_statuses[target_id]` value can exist. A UUID incarnation prevents an in-flight check from publishing after delete/recreate. A per-incarnation sequence prevents an older overlapping check from replacing a newer-started check. Deletion removes status while holding the same lock.
+
+## Contract transformer
+
+```bash
+python -m current_status < input.json > output.json
+```
+
+The three input objects are deep-copied unchanged and `current_status_implementation` is appended.
+
+## Tests and exact verification command
+
+Files: `tests/test_api.py`, `tests/test_concurrency.py`, and `tests/test_transform.py`.
+
+```bash
+python -m pip install -e '.[test]'
+python -m pytest -q
+```
+
+The tests audit latest-only retention, REST not-found/no-check/latest responses, deletion cleanup, overlapping-check ordering, delete/recreate resurrection prevention, and output preservation/emission.
+
+## Execution evidence
+
+No shell/test runner is available to the generation call, so this commit does **not** claim that pytest was executed there. The exact reproducible command is above; committed tests are the auditable evidence supplied by this generation step.