Clients take a flow's id or key: one resolver in the SDK, id first, explicit id:/key:, a warning for reads and a refusal for changes when a name means two flows, deleted flows found by their last key; sync creates by POST, retries a create race through the drift checks and reports the id; MCP gets create_flow and resolves every flow argument with the caller's token; displays show flow_name; an engine from before ids is named as such

backend/tests/api/routes/test_sync.py

@@ -213,6 +213,70 @@ def test_sync_refuses_a_flow_it_did_not_create(api, flows):
         theirs.name = "train"
 
 
+def test_two_first_syncs_of_one_key_end_with_one_flow(api, flows):
+    """The second looked before the first had created the flow.
+
+    Its create is refused for the key, so it looks again and updates the
+    first one's flow, checked first as any stored flow is: a canvas edit made
+    in between is refused rather than overwritten.
+    """
+
+    class Late(Client):
+        looked = False
+
+        def find_flow(self, key):
+            if self.looked:
+                return super().find_flow(key)
+            # What it saw before the other sync created the flow.
+            self.looked = True
+            return None
+
+    target = flows["train"]
+    target.name = "raced"
+    try:
+        first = sync([target], api, origin=ORIGIN)[0]
+        second = sync([target], Late(http=api.http, token=api.token), origin=ORIGIN)[0]
+
+        assert first.created and not second.created
+        assert second.id == first.id
+        assert [f["id"] for f in api.flows() if f["name"] == "raced"] == [first.id]
+
+        api.put_source(first.id, "fit", "def process(dataset, lr):\n    return {}\n")
+        with pytest.raises(SyncError, match="edited on the canvas"):
+            sync([target], Late(http=api.http, token=api.token), origin=ORIGIN)
+    finally:
+        target.name = "train"
+        found = api.find_flow("raced")
+        if found:
+            api.http.delete(f"{PREFIX}/flows/{found['id']}")
+
+
+def test_a_synced_flow_is_addressed_by_its_key_under_an_issued_id(api, flows):
+    """The id is the engine's own, opaque; the client takes the key and
+    sends the id, and the history answers with both."""
+    report = sync([flows["train"]], api, origin=ORIGIN)[0]
+    train = flow_id(api, "train")
+    assert report.id == train
+    assert train != "train" and train.startswith("f_")
+
+    [made] = api.import_runs(
+        [
+            {
+                "flow": "train",
+                "external_id": "elsewhere:1",
+                "params": {"lr": 0.05},
+                "created_at": "2026-03-01T10:00:00Z",
+                "result": {"score": 0.9},
+            }
+        ]
+    )
+    rows = api.runs(flow="train")
+    assert made["id"] in {row["id"] for row in rows}
+    assert {(row["flow"], row["flow_name"]) for row in rows} == {(train, "train")}
+    exported = api.export_runs(flow="train")
+    assert {(row["flow"], row["flow_name"]) for row in exported} == {(train, "train")}
+
+
 def test_use_stores_the_rewiring_it_was_given(api, flows):
     sync([flows["finetune"]], api, origin=ORIGIN)