diff --git a/docs/harness-contract.md b/docs/harness-contract.md
new file mode 100644
index 0000000..68bf621
--- /dev/null
+++ b/docs/harness-contract.md
@@ -0,0 +1,86 @@
+# Harness contract
+
+The interface a concrete harness backend ("rein") implements so glas-harness
+can route to it, per `docs/adr/ADR-001-rein-harness-family.md`. Mirrors
+sand-boxer's `SandboxExtension` ABC (`provision`/`wait_ready`/`teardown`) at
+the harness level, one layer up: sand-boxer establishes *where* dangerous
+work runs, this contract governs *how an agent session runs* on top of it.
+
+## Interface
+
+```python
+class Rein(ABC):
+    """Base class for concrete harness backends (reins)."""
+
+    @abstractmethod
+    def start_session(
+        self, profile: HarnessProfile, inputs: dict[str, str], sandbox: SandboxHandle
+    ) -> dict[str, str]:
+        """Begin an agent session bound to a sandbox. Returns a session handle."""
+
+    @abstractmethod
+    def dispatch_tool(
+        self, session: dict[str, str], tool_call: ToolCall
+    ) -> ToolResult:
+        """Run one tool call under the session's policy. Returns result + audit envelope."""
+
+    @abstractmethod
+    def end_session(self, session: dict[str, str]) -> dict[str, str]:
+        """Close the session. Returns a summary (commit sha, tokens, duration, outcome)."""
+```
+
+- `start_session` resolves the rein's own setup (persona/blueprint loading,
+  credential acquisition) against the sandbox handle glas-harness already
+  obtained from sand-boxer — the rein never provisions or tears down a
+  sandbox itself.
+- `dispatch_tool` enforces the tool-profile allow-list glas-harness resolved
+  (the same allow-list concept rein-aharness's `profiles.py` already
+  implements today, just invoked through the contract instead of only the
+  rein's own CLI).
+- `end_session` is where the existing "commit exists = success" signal
+  (rein-aharness) or an equivalent per-rein success criterion is reported
+  back to glas-harness for State Hub posting. glas-harness posts the audit
+  event; the rein returns the facts.
+
+## Session lifecycle (who calls what)
+
+```
+glas-harness                              rein                    sand-boxer
+────────────                              ────                    ──────────
+resolve harness.* profile
+request sandbox                    ──────────────────────────────▶ create()
+receive sandbox handle             ◀──────────────────────────────
+start_session(profile, in, sbx)  ─▶ acquire creds, load persona
+                                  ◀─ session handle
+dispatch_tool(session, call)     ─▶ run under allow-list
+                                  ◀─ result + audit envelope
+  (repeat dispatch_tool ...)
+end_session(session)             ─▶ verify success, summarize
+                                  ◀─ summary
+post State Hub event
+request sandbox teardown           ──────────────────────────────▶ destroy()
+```
+
+glas-harness owns the *outer* loop (profile resolution, sandbox
+request/teardown, State Hub reporting, actor attribution). The rein owns
+the *inner* loop (its own agentic reasoning, whatever that looks like —
+Claude Code CLI subprocess for `rein-aharness`, a direct OpenRouter
+tool-use loop for `rein-openweights`).
+
+## Actor attribution
+
+Every `dispatch_tool` call and lifecycle transition carries an actor
+(`adm`/`agt`/`atm`) per glas-harness INTENT.md's audit requirement.
+glas-harness stamps this at dispatch time; reins do not need their own
+actor model.
+
+## Open questions this contract does not resolve yet
+
+- Whether `start_session` also carries scheduling/blueprint-sourcing
+  parameters (kaizen-agentic blueprint id, activity-core task id) as part
+  of `inputs`, or whether that stays entirely rein-side — tracked in
+  `rein-aharness/workplans/HARNESS-WP-0002-T04`.
+- The exact `SandboxHandle`/`ToolCall`/`ToolResult` field shapes — sketched
+  here at the level needed to unblock `GLAS-WP-0001-T04` (the
+  rein-aharness parity proof); refine once that task is underway rather
+  than speculatively now.
diff --git a/workplans/GLAS-WP-0001-harness-router-foundation.md b/workplans/GLAS-WP-0001-harness-router-foundation.md
index f904079..5a27f65 100644
--- a/workplans/GLAS-WP-0001-harness-router-foundation.md
+++ b/workplans/GLAS-WP-0001-harness-router-foundation.md
@@ -25,7 +25,7 @@ harness level: something like `start_session`/`dispatch_tool`/
 
 ```task
 id: GLAS-WP-0001-T01
-status: todo
+status: done
 priority: high
 state_hub_task_id: "e4e6e7e4-ad30-42b7-a278-39d24ec5cecc"
 ```