diff --git a/docsgpt/deploy/commands.py b/docsgpt/deploy/commands.py
index b8f063aa..94a144f6 100644
--- a/docsgpt/deploy/commands.py
+++ b/docsgpt/deploy/commands.py
@@ -739,6 +739,12 @@ def dev(args, context: Optional[Context] = None) -> int:
             f"port {args.port} is already in use, so the API cannot bind it. Stop what is on it "
             f"(a previous `docsgpt dev`, or `docsgpt down` for an install), or pass --port."
         )
+    if getattr(args, "mock_llm", False) and args.port == dev_module.MOCK_LLM_PORT:
+        # Both checks below would pass: the port really is free, and then two children want it.
+        raise DeployError(
+            f"port {args.port} is where the mock LLM listens, so the API cannot have it too. "
+            "Give the API another port with --port."
+        )
     if getattr(args, "mock_llm", False) and not _port_is_free(dev_module.MOCK_LLM_PORT):
         # It starts first and the others are pointed at it, so a busy port here would surface as the
         # API talking to someone else's server, or as a child exiting once everything else is up.
diff --git a/tests/deploy/test_doctor.py b/tests/deploy/test_doctor.py
index c0a52b28..4acca1d5 100644
--- a/tests/deploy/test_doctor.py
+++ b/tests/deploy/test_doctor.py
@@ -66,6 +66,20 @@ class TestDevCommand:
                 _run(["dev", "--mock-llm", "--port", str(self._free_port())], _context())
         assert started == [], "nothing is spawned when the mock LLM has nowhere to listen"
 
+    def test_the_api_cannot_be_given_the_mock_llm_port(self, monkeypatch, tmp_path):
+        """Both free-port checks pass here: the port is free, and then two children want it."""
+        from docsgpt.deploy import dev as dev_module
+
+        self._checkout(monkeypatch, tmp_path)
+        (tmp_path / "scripts").mkdir()
+        (tmp_path / "scripts" / "mock_llm.py").write_text("", encoding="utf-8")
+        started = []
+        monkeypatch.setattr(dev_module, "run", lambda children, **kwargs: started.append(children) or 0)
+        monkeypatch.setattr(dev_module, "MOCK_LLM_PORT", self._free_port())
+        with pytest.raises(DeployError, match="mock LLM listens"):
+            _run(["dev", "--mock-llm", "--port", str(dev_module.MOCK_LLM_PORT)], _context())
+        assert started == [], "nothing is spawned when the two would collide"
+
     def test_it_runs_the_children_it_planned_and_says_where(self, monkeypatch, tmp_path, capsys):
         from docsgpt.deploy import dev as dev_module