diff --git a/libs/hackbot-client/hackbot_client/__init__.py b/libs/hackbot-client/hackbot_client/__init__.py
index 1f3eecd1e9..12fa3438cd 100644
--- a/libs/hackbot-client/hackbot_client/__init__.py
+++ b/libs/hackbot-client/hackbot_client/__init__.py
@@ -1,8 +1,9 @@
 from hackbot_client.client import HackbotClient
-from hackbot_client.models import RunRef, RunStatus
+from hackbot_client.models import RunRef, RunStatus, TriggeredRun
 
 __all__ = [
     "HackbotClient",
     "RunRef",
     "RunStatus",
+    "TriggeredRun",
 ]
diff --git a/libs/hackbot-client/hackbot_client/client.py b/libs/hackbot-client/hackbot_client/client.py
index 4668b8ec6b..aa4306f402 100644
--- a/libs/hackbot-client/hackbot_client/client.py
+++ b/libs/hackbot-client/hackbot_client/client.py
@@ -7,7 +7,7 @@
 
 import httpx
 
-from hackbot_client.models import RunRef
+from hackbot_client.models import TriggeredRun
 
 
 class HackbotClient:
@@ -27,12 +27,12 @@ async def trigger_run(
         *,
         on_behalf_of: str | None = None,
         dedupe_key: str | None = None,
-    ) -> RunRef:
+    ) -> TriggeredRun:
         """Create an agent run and return the API's typed run reference.
 
         `dedupe_key` keys the work the run does, and a key belongs to one run
         for good: repeated triggers carrying it are no-ops, answered with the
-        same run reference.
+        same run reference and `is_new=False`.
         """
         headers = {"X-API-Key": self._api_key}
         if on_behalf_of is not None:
@@ -49,4 +49,8 @@ async def trigger_run(
             )
 
         response.raise_for_status()
-        return RunRef.model_validate(response.json())
+        # The API distinguishes the two outcomes only by status code: `201` for the
+        # run this request started, `200` for one a `dedupe_key` collapsed onto.
+        return TriggeredRun.model_validate(
+            {**response.json(), "is_new": response.status_code == 201}
+        )
diff --git a/libs/hackbot-client/hackbot_client/models.py b/libs/hackbot-client/hackbot_client/models.py
index 8100013a59..afc4c89d87 100644
--- a/libs/hackbot-client/hackbot_client/models.py
+++ b/libs/hackbot-client/hackbot_client/models.py
@@ -20,3 +20,16 @@ class RunRef(BaseModel):
     run_id: UUID
     agent: str
     status: RunStatus
+
+
+class TriggeredRun(RunRef):
+    """The run a trigger resolved to, and whether that request is what started it.
+
+    `is_new` is a property of the request rather than of the run, and says
+    nothing about the run's `status`: the same run is `is_new` to the request
+    that created it and not to every later request carrying its `dedupe_key`.
+    That is why it is not on the API's own `RunRef`. Subclasses `RunRef` so a
+    caller that only wants the run keeps reading `run_id` off it unchanged.
+    """
+
+    is_new: bool
diff --git a/libs/hackbot-client/tests/test_client.py b/libs/hackbot-client/tests/test_client.py
index 58d1e132a2..5a3bba61b5 100644
--- a/libs/hackbot-client/tests/test_client.py
+++ b/libs/hackbot-client/tests/test_client.py
@@ -58,6 +58,8 @@ async def test_trigger_run_posts_inputs_and_returns_typed_reference(monkeypatch)
     assert run.run_id == UUID(RUN_ID)
     assert run.agent == "bug-fix"
     assert run.status is RunStatus.pending
+    # `201`: this request is what started it.
+    assert run.is_new is True
     assert captured == {
         "timeout": 30.0,
         "url": "https://hackbot.example/agents/bug-fix/runs",
@@ -97,3 +99,38 @@ async def test_trigger_run_rejects_an_invalid_success_response(monkeypatch):
 
     with pytest.raises(ValidationError):
         await _client().trigger_run("bug-fix", {"bug_id": 1234})
+
+
+async def test_trigger_run_reports_a_deduplicated_run_as_not_new(monkeypatch):
+    # `200` rather than `201`: the key already belonged to this run, so the
+    # request that got this answer started nothing.
+    _capture_post(
+        monkeypatch,
+        httpx.Response(
+            200,
+            json={"run_id": RUN_ID, "agent": "bug-fix", "status": "running"},
+        ),
+    )
+
+    run = await _client().trigger_run(
+        "bug-fix", {"bug_id": 1234}, dedupe_key="push:autoland:abc123"
+    )
+
+    assert run.run_id == UUID(RUN_ID)
+    assert run.is_new is False
+
+
+async def test_trigger_run_sends_the_dedupe_key_as_a_query_parameter(monkeypatch):
+    captured = _capture_post(
+        monkeypatch,
+        httpx.Response(
+            201,
+            json={"run_id": RUN_ID, "agent": "bug-fix", "status": "pending"},
+        ),
+    )
+
+    await _client().trigger_run(
+        "bug-fix", {"bug_id": 1234}, dedupe_key="push:autoland:abc123"
+    )
+
+    assert captured["params"] == {"dedupe_key": "push:autoland:abc123"}