diff --git a/AGENTS.md b/AGENTS.md
index 3a406d577..6e0170258 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -368,7 +368,8 @@ orchestrators call it the same way. What callers rely on:
   in the proxy, which records the turn and checks it against the model calls the caller records. Its
   record is tagged `context.metadata["episode"]` and names the original task, so it isn't resumable
   with `start_step`. The episode tears down what its run deployed, and an abandoned one is recorded
-  `cancelled`.
+  `cancelled`. It records itself in the run index as a runner would (runner `episode`, in the run group
+  `start(run_group_id=...)` names), so the explorer lists it.
 
 ## Tests
 
diff --git a/src/agent_env/explorer/routers/runs.py b/src/agent_env/explorer/routers/runs.py
index f8da1d0f0..1113dd74d 100644
--- a/src/agent_env/explorer/routers/runs.py
+++ b/src/agent_env/explorer/routers/runs.py
@@ -222,7 +222,17 @@ async def start_runs(task_id: EntityId, body: StartRunsRequest | None = None) ->
 
 @router.post("/{task_id}/cancel-run")
 async def cancel_run(task_id: EntityId, workflow_id: str = Query(...)) -> dict:
-    canceled = await get_runner().cancel(workflow_id)
+    """Cancel a run of the configured runner. A run another process records, such as an episode a trainer plays, is
+    refused with 409: only the process running it can stop it, so asking this runner would silently do nothing."""
+    from agent_env.runner import store as run_store
+
+    runner = get_runner()
+    record = run_store.get_run(workflow_id)
+    if record is not None and record.runner != runner.type:
+        raise HTTPException(status_code=409, detail=(
+            f"run {workflow_id} is run by {record.runner!r}, not by this explorer's {runner.type!r} runner: stop it "
+            "where it runs (an episode, from the trainer that plays it)"))
+    canceled = await runner.cancel(workflow_id)
     return {"workflow_id": workflow_id, "canceled": canceled}
 
 
diff --git a/src/agent_env/explorer/ui/src/components/task-detail-page.tsx b/src/agent_env/explorer/ui/src/components/task-detail-page.tsx
index ea7bfa9be..064884c43 100644
--- a/src/agent_env/explorer/ui/src/components/task-detail-page.tsx
+++ b/src/agent_env/explorer/ui/src/components/task-detail-page.tsx
@@ -1087,7 +1087,15 @@ export function TaskDetailPage({
           )}/cancel-run?workflow_id=${encodeURIComponent(workflowId)}`,
           { method: 'POST' },
         );
-        if (!res.ok) throw new Error(`Cancel failed (${res.status})`);
+        if (!res.ok) {
+          // A refusal says why (an episode is stopped by the trainer that plays it); show that, not just the status.
+          const body = (await res.json().catch(() => null)) as { detail?: unknown } | null;
+          throw new Error(
+            typeof body?.detail === 'string'
+              ? body.detail
+              : `Cancel failed (${res.status})`,
+          );
+        }
         // Cancel endpoint flips Mongo to `cancelled` synchronously; refresh
         // so the row transitions without waiting for the next background poll.
         fetchRunGroups();
diff --git a/src/agent_env/task/episode/run.py b/src/agent_env/task/episode/run.py
index 2b082cd69..4f9e594ba 100644
--- a/src/agent_env/task/episode/run.py
+++ b/src/agent_env/task/episode/run.py
@@ -2,7 +2,10 @@
 
 import asyncio
 import logging
+import os
+import socket
 from collections import deque
+from datetime import datetime, timezone
 from typing import Any
 
 import httpx
@@ -18,6 +21,8 @@
 from agent_env.a2a_agent import protocol
 from agent_env.a2a_agent.a2a_agent import A2AAgent
 from agent_env.providers.sandbox_providers.sandbox_provider import sandbox_request_headers_for_url
+from agent_env.runner import store as run_store
+from agent_env.runner.runner import RunRecord, RunStatus, run_outcome
 from agent_env.store.routing import namespace_routing, run_scope
 from agent_env.task.episode.proxy import InProcessProxy, ProxyClient, ProxyRefused
 from agent_env.task.store import record_task_cancelled
@@ -249,10 +254,17 @@ async def _drive(self, running: asyncio.Event) -> None:
         resolves."""
         running.set()
         with namespace_routing():
+            failure: BaseException | None = None
             try:
+                # Before any await, so start() returns with the run listed; a write that fails is the run's error, and
+                # the finally below still takes the episode out of _live.
+                _record_run(self._task, self.id, self._context.metadata)
                 await self._task.run(context=self._context, instance_id=self.id)
                 self._ran = True
                 self._notify()
+            except BaseException as e:
+                failure = e
+                raise
             finally:
                 self._tearing_down = True
                 try:
@@ -264,7 +276,19 @@ async def _drive(self, running: asyncio.Event) -> None:
                     if self._abandoned is not None:
                         await asyncio.to_thread(_record_cancelled, self._task.id, self.id, self._abandoned)
                 finally:
-                    del _live[self.id]
+                    try:
+                        await asyncio.to_thread(_record_ended, self._task.id, self.id, *self._outcome(failure))
+                    finally:
+                        del _live[self.id]
+
+    def _outcome(self, failure: BaseException | None) -> tuple[RunStatus, str | None]:
+        """How the run ended, by the rule every runner records a run's end by (``run_outcome``: a step that failed
+        without failing the task fails the run too), but cancelled when it was abandoned or cancelled."""
+        if self._abandoned is not None:
+            return RunStatus.CANCELED, self._abandoned
+        if isinstance(failure, asyncio.CancelledError):
+            return RunStatus.CANCELED, "the episode was cancelled"
+        return run_outcome(self._context, failure)
 
     async def _wait(self, changed: asyncio.Future[None]) -> None:
         """Until the run ends or what was ``changed`` when the caller last looked changes again."""
@@ -368,6 +392,28 @@ def _episode_of(context: TaskStepContext) -> Episode:
     return _live[context.metadata["episode"]["id"]]
 
 
+def _record_run(task: Task, instance_id: str, metadata: dict[str, Any]) -> None:
+    """The episode in the run index as a runner records the runs it accepts, so the explorer lists it, in its run group
+    when ``start`` was given one; in the local stores for an ``@local`` task, as ``Task.run`` scopes its records."""
+    now = datetime.now(timezone.utc).isoformat()
+    with run_scope(task.id):
+        run_store.insert_run(RunRecord(
+            run_id=_run_id(instance_id), runner="episode", task_id=task.id, task_version=task.version,
+            instance_id=instance_id, status=RunStatus.RUNNING, started_at_utc=now,
+            owner={"host": socket.gethostname(), "pid": os.getpid(), "seen": now},
+            overrides={"metadata": {key: metadata[key] for key in ("episode", "run_group_id", "seed")
+                                    if key in metadata}}))
+
+
+def _record_ended(task_id: str, instance_id: str, status: RunStatus, error: str | None) -> None:
+    with run_scope(task_id):
+        run_store.mark_terminal(_run_id(instance_id), status, error=error)
+
+
+def _run_id(instance_id: str) -> str:
+    return f"episode-{instance_id}"
+
+
 def _record_cancelled(task_id: str, instance_id: str, reason: str) -> None:
     """Marked where the run recorded its instance, as ``Task.run`` scopes it: an ``@local`` task's in the local
     stores."""
diff --git a/src/agent_env/task/episode/spec.py b/src/agent_env/task/episode/spec.py
index 4f795b62f..e45ff2e3b 100644
--- a/src/agent_env/task/episode/spec.py
+++ b/src/agent_env/task/episode/spec.py
@@ -56,12 +56,15 @@ def prompts(self) -> tuple[tuple[str, str | None], ...]:
                 prompts.append((step.prompt, system_prompts[step.agent_name]))
         return tuple(prompts)
 
-    async def start(self, *, sandbox: str | None = None) -> Episode:
+    async def start(self, *, sandbox: str | None = None, run_group_id: str | None = None) -> Episode:
         """Start one run of the task, a new episode each call. It returns once the run's record exists, before
         anything is deployed; ``next_turn()`` is where the deploys and setup are waited for. ``sandbox`` is the
-        provider its envs, agents, sandboxes and judge deploy on, as ``agent-env run --sandbox`` sets it."""
+        provider its envs, agents, sandboxes and judge deploy on, as ``agent-env run --sandbox`` sets it. The run is in
+        the run index, which the explorer lists; ``run_group_id`` groups it with the other runs given the same id."""
         episode_id = uuid.uuid4().hex
         metadata: dict[str, Any] = {"episode": {"id": episode_id, "trainer_agents": list(self.trainer_agents)}}
+        if run_group_id:
+            metadata["run_group_id"] = run_group_id
         if self._seed:
             metadata["seed"] = dict(self._seed)
         if sandbox:
diff --git a/tst/unit/explorer/explorer_app_test.py b/tst/unit/explorer/explorer_app_test.py
index 797010a09..521ff8365 100644
--- a/tst/unit/explorer/explorer_app_test.py
+++ b/tst/unit/explorer/explorer_app_test.py
@@ -328,6 +328,18 @@ async def run(self, **kw):
     assert client.post(f"{base}/cancel-run", params={"workflow_id": single["workflow_id"]}).status_code == 200
 
 
+def test_a_run_another_runner_records_is_refused_cancelling_with_where_to_stop_it(client):
+    task_id = "@local/~/dock/crate"
+    run_store.insert_run(RunRecord(run_id="episode-abc", runner="episode", task_id=task_id, task_version=1,
+                                   instance_id="abc", status=RunStatus.RUNNING))
+
+    refused = client.post(f"/api/v1/tasks/{quote(task_id, safe='')}/cancel-run", params={"workflow_id": "episode-abc"})
+
+    assert refused.status_code == 409
+    assert "run by 'episode'" in refused.json()["detail"] and "trainer that plays it" in refused.json()["detail"]
+    assert run_store.get_run("episode-abc").status == "RUNNING"
+
+
 def test_a_bundle_runs_namespaced_instance_id_is_one_encoded_segment(client):
     task_id, instance_id = "@local/~/triage/runs", "@local/~/triage/runs-7f3a9c2e"
     store = get_config().get_document_store()
diff --git a/tst/unit/task/episode/run_test.py b/tst/unit/task/episode/run_test.py
index a51290470..8e18231d9 100644
--- a/tst/unit/task/episode/run_test.py
+++ b/tst/unit/task/episode/run_test.py
@@ -25,6 +25,7 @@
 from agentenv_protocol.a2a_agent import proxy as proxy_module
 
 import agent_env.task.episode.run as episode_run
+from agent_env.runner import store as run_store
 from agent_env.config import get_config
 from agent_env.config.runtime import Config
 from agent_env.env.env import DeployedEnv
@@ -446,6 +447,55 @@ async def test_an_episode_abandoned_as_it_starts_is_recorded_cancelled_with_the_
             await ask()
 
 
+@pytest.mark.asyncio
+async def test_an_episode_is_in_the_run_index_running_then_ended_in_the_group_it_was_started_in(teardowns):
+    played = await _spec(ITEMS, ALICE, TURN, REPLIES, seed={"week": "3"}).start(run_group_id="rg-train-0")
+    run = run_store.get_run(f"episode-{played.id}")
+    assert (run.runner, run.task_id, run.task_version, run.instance_id, run.status) == (
+        "episode", "dock", 4, played.id, "RUNNING")
+    assert run.overrides["metadata"] == {"episode": {"id": played.id, "trainer_agents": ["alice"]},
+                                         "run_group_id": "rg-train-0", "seed": {"week": "3"}}
+    await (await played.next_turn()).end("Added a crate.")
+    await played.result()
+    abandoned = await _spec(ITEMS, ALICE, TURN, REPLIES).start()
+    await abandoned.abandon("the trainer moved on")
+    failed = await _spec({**ITEMS, "fail": True}, ALICE, TURN, REPLIES).start()
+    with pytest.raises(RuntimeError):
+        await failed.result()
+
+    ended = {episode: run_store.get_run(f"episode-{episode.id}") for episode in (played, abandoned, failed)}
+    assert [(run.status, run.error) for run in ended.values()] == [
+        ("COMPLETED", None), ("CANCELED", "the trainer moved on"),
+        ("FAILED", "RuntimeError: the env didn't come up")]
+    assert all(run.finished_at_utc for run in ended.values())
+    assert ended[abandoned].overrides["metadata"].keys() == {"episode"}
+
+
+@pytest.mark.asyncio
+async def test_an_episode_whose_step_fails_without_failing_the_task_is_recorded_failed_as_a_runner_would(teardowns):
+    spare = {"id": "spare", "type": "episode_test_env", "env_id": "spare", "fail": True, "fail_task_on_error": False}
+    episode = await _spec(ITEMS, ALICE, TURN, REPLIES, spare).start()
+    await (await episode.next_turn()).end("Added a crate.")
+
+    assert await episode.result() == {"replies": 1.0}
+    run = run_store.get_run(f"episode-{episode.id}")
+    assert run.status == "FAILED" and "the env didn't come up" in run.error
+
+
+@pytest.mark.asyncio
+async def test_a_run_record_that_cant_be_written_is_the_runs_error_and_leaves_no_live_episode(teardowns, monkeypatch):
+    def refused(record):
+        raise RuntimeError("the run index is down")
+
+    monkeypatch.setattr(episode_run.run_store, "insert_run", refused)
+    episode = await _spec(ITEMS, ALICE, TURN, REPLIES).start()
+
+    with pytest.raises(RuntimeError, match="the run index is down"):
+        await episode.next_turn()
+    assert episode.id not in episode_run._live
+    assert len(teardowns.finished) == 1
+
+
 @pytest.mark.asyncio
 async def test_an_episode_abandoned_mid_turn_drops_the_turn(teardowns):
     episode = await _spec(ITEMS, ALICE, TURN, REPLIES).start()