diff --git a/.github/scripts/check_plugin_api.py b/.github/scripts/check_plugin_api.py
index b0ee5462..b19726fc 100644
--- a/.github/scripts/check_plugin_api.py
+++ b/.github/scripts/check_plugin_api.py
@@ -32,6 +32,7 @@
     "agent_env.providers.env_state.env_state_provider.EnvStateProvider",
     "agent_env.providers.env_providers.env_provider.EnvironmentProvider",
     "agent_env.explorer.plugin.ExplorerPlugin",
+    "agent_env.runner.runner.Runner",
 )
 USED = (
     "agent_env.task_step.context.TaskStepContext", "agent_env.plugins",
@@ -39,6 +40,11 @@
     # What a plugin reads of recorded runs.
     "agent_env.task.store.task_instances", "agent_env.task.store.count_task_instances",
     "agent_env.task.store.find_task_instance", "agent_env.task.store.TaskInstance",
+    # What a runner returns and the run index it records each run in.
+    "agent_env.runner.runner.RunStatus", "agent_env.runner.runner.RunHandle", "agent_env.runner.runner.RunRecord",
+    "agent_env.runner.store.ensure_indexes", "agent_env.runner.store.insert_run", "agent_env.runner.store.get_run",
+    "agent_env.runner.store.list_runs", "agent_env.runner.store.count_runs", "agent_env.runner.store.active_runs",
+    "agent_env.runner.store.set_running", "agent_env.runner.store.mark_terminal",
     # What an environment provider reads to deploy a built-in env.
     "agent_env.env.envs.mcp_server.MCPServerEnv.docker_image_artifact", "agent_env.env.envs.mcp_server.MCPServerEnv.environment_name",
     "agent_env.env.envs.website.WebsiteEnv.backend_docker_image_artifact",
diff --git a/AGENTS.md b/AGENTS.md
index 57f012f5..e62bdcf6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -104,7 +104,7 @@ and deserialized through a registry.
 | `config/` | The `Config` singleton (`get_config`, `configure`, `reset_config`) in `config/runtime.py`, file discovery in `config/loader.py`, and `load_impl`, which resolves `module:Class` pointers. `agent_env.store` re-exports the config names for compatibility. |
 | `providers/` | `providers/sandbox_providers/` holds the sandbox providers `local`, `modal`, `modal_vm`, `e2b`, `sail_vm` (the `sail` extra), `smol_vm` (the `smol` extra), `tensorlake` (the `tensorlake` extra), `tenki` (the `tenki` extra), `vercel` (the `vercel` extra); `[sandbox] default` and `agent_default` accept a comma-separated fallback chain, whose deploys skip a provider that can't run them. A sandbox provider declares what it runs (`ON_THIS_MACHINE`, and for VMs on this machine the address their requests to it arrive at, `REACHES_HOST_AT`; `CONTAINER_ACCEPTS`, `PRIVATE_NETWORK`; `SUPPORTED_SANDBOX_MODES`, derived from whether it implements `create_sandbox` and `create_vm`; and `SANDBOX_ACCEPTS`, the images an agent's sandbox runs, which defaults to those a VM loads when it implements `create_vm` and to `CONTAINER_ACCEPTS` otherwise, so only a provider that implements `create_vm` yet gives an agent a container, like the local one, declares it), each sandbox carries the `platform` of the images it runs (linux/amd64 unless its provider sets another when it creates it; None, any, unchecked, for a local sandbox), and code outside `providers/sandbox_providers/` reads those, never which provider it has. `providers/env_providers/` holds the environment providers: `EnvironmentProvider` (an env's containers and state store), `EnvironmentGatewayProvider` (`gateway`, an env's default), which renders a docker-compose for the gateway and its MCP servers inside the sandbox, and `EnvironmentServerProvider` (`server`), one MCP server in its own container with no gateway in front and no state store; `providers/env_state/` holds env-state providers (`local_postgres` built in). |
 | `a2a_agent/` | The `A2AAgent` entity (`a2a_agent`), its stores and the validator steps. The protocol package provides the agent-side framework. |
-| `runner/` | The `[runner]` seam: `Runner.submit()` returns a `RunHandle` (`run_id`, `instance_id`) before the run starts; `LocalRunner` is built in. |
+| `runner/` | The `[runner]` seam: `Runner.submit()` returns a `RunHandle` (`run_id`, `instance_id`) before the run starts; `LocalRunner` is built in. `runner/store.py` is the run index every runner writes and the explorer reads. |
 | `bundle/` | A bundle is a folder of entities (`envs/`, `agents/`, `artifacts/`, `skills/`, `tasks/<name>.json`, `evals/<name>.toml`) that runs without a registry: `agent-env run` plans it, writes it into the `@local` namespace (reusing each version whose inputs haven't changed) and runs its evals or tasks; `agent-env bundle init` writes a stored task or eval as one, with its references pinned. |
 | `eval/` | `Eval` (`eval`), a versioned list of task references, and its store. |
 | `plugins/` | Entry-point plugins (see Extension points): the group names, `inventory()`, which `agent-env plugin list/show/check` report, `load_failures` and `settings`. The top level is public; the underscored modules are internal. |
@@ -164,6 +164,10 @@ orchestrators call it the same way. What callers rely on:
   through `record_task_failure()`, retries through `undo_steps()`.
 - The runner writes its run identifier to `context.metadata["workflow_id"]` before calling `run()`;
   core stores it opaquely with the instance.
+- A runner records every run it accepts in the run index (`agent_env.runner.store`): `insert_run` at
+  submit, `set_running` when it starts, `mark_terminal` when it ends. The explorer lists, counts and
+  groups runs from that collection and never calls the runner to ask, so a run missing there is
+  invisible and one never marked terminal shows as in flight.
 - Per-run overrides travel in `context.metadata["user_overrides"]` (`litellm_api_key`,
   `a2a_agent_id`, `agent_sandbox`, `env_sandbox`, `env_state_type`, and more), never in `os.environ`;
   steps fall back to the configured `[model]` key. `step_params` maps a step id to overrides of that
diff --git a/src/agent_env/explorer/routers/runs.py b/src/agent_env/explorer/routers/runs.py
index 3241e18e..f8da1d0f 100644
--- a/src/agent_env/explorer/routers/runs.py
+++ b/src/agent_env/explorer/routers/runs.py
@@ -235,7 +235,7 @@ def list_runs(
     from agent_env.runner import store as run_store
 
     records = run_store.list_runs(task_id=task_id, limit=limit, offset=offset)
-    total = docs().count("runs", Filter.of(task_id=task_id))
+    total = run_store.count_runs(task_id)
     return PaginatedResponse(
         items=[r.to_dict() for r in records], total=total, limit=limit, offset=offset,
         has_more=offset + len(records) < total,
diff --git a/src/agent_env/runner/runner.py b/src/agent_env/runner/runner.py
index 321df72e..1771e68f 100644
--- a/src/agent_env/runner/runner.py
+++ b/src/agent_env/runner/runner.py
@@ -87,7 +87,7 @@ def from_dict(cls, data: dict) -> "RunRecord":
 
 
 class Runner(ABC):
-    """Dispatches task runs and reports on them.
+    """Dispatches task runs and reports on them, recording each run in ``agent_env.runner.store``.
 
     ``type`` is persisted on every run record: a hub restarted under a different
     ``[runner]`` reports on a run it does not own rather than misreporting it.
diff --git a/src/agent_env/runner/store.py b/src/agent_env/runner/store.py
index 884bfb28..d30b0b6c 100644
--- a/src/agent_env/runner/store.py
+++ b/src/agent_env/runner/store.py
@@ -1,9 +1,8 @@
-"""Run-record persistence for the in-process runner.
+"""The run index every ``[runner]`` writes: ``insert_run`` at submit, ``set_running`` when the run
+starts, ``mark_terminal`` when it ends. The explorer lists, counts and groups runs from here and never
+asks the runner, so a run missing here is invisible and one never marked terminal stays in flight.
 
-Stored as a collection so the hub can list runs and report their status — not a durable
-work queue: the runner executes ``Task.run()`` in-process and holds the run's asyncio task
-in memory, so there is no lease or claim here. A run left non-terminal by a dead process
-is failed at ``start()``, not resumed. A durable, lease-based runner is a follow-up.
+It is an index, not a work queue: there is no lease or claim here.
 """
 
 from __future__ import annotations
@@ -59,6 +58,10 @@ def list_runs(
     return [RunRecord.from_dict(d) for d in docs]
 
 
+def count_runs(task_id: Optional[str] = None) -> int:
+    return _docs().count(RUNS_COLLECTION, Filter.of(task_id=task_id) if task_id else Filter())
+
+
 def active_runs(runner: str, limit: int = 100_000) -> list[RunRecord]:
     """Every non-terminal (QUEUED/RUNNING) run for ``runner`` — used at startup to reconcile
     orphans a dead process left behind, no matter how many terminal runs precede them."""
diff --git a/tst/unit/runner/run_store_test.py b/tst/unit/runner/run_store_test.py
index f167fe6d..5c1eff2a 100644
--- a/tst/unit/runner/run_store_test.py
+++ b/tst/unit/runner/run_store_test.py
@@ -1,4 +1,4 @@
-"""Run-store invariants for the in-process runner: a terminal run is never overwritten,
+"""Run-index invariants: a terminal run is never overwritten,
 and set_running only advances a still-queued one."""
 
 import pytest
@@ -48,3 +48,11 @@ def test_set_running_advances_only_a_queued_run(docs):
     run_store.insert_run(_record("c", RunStatus.CANCELED))
     run_store.set_running("c")
     assert run_store.get_run("c").status == RunStatus.CANCELED
+
+
+def test_count_runs_counts_one_task_or_every_task(docs):
+    run_store.insert_run(_record("a", RunStatus.QUEUED))
+    run_store.insert_run(RunRecord(run_id="b", runner="other", task_id="u", task_version=1, instance_id="j"))
+    assert run_store.count_runs("t") == 1
+    assert run_store.count_runs("missing") == 0
+    assert run_store.count_runs() == 2