diff --git a/docsgpt/deploy/commands.py b/docsgpt/deploy/commands.py
index 758fc715..b8f063aa 100644
--- a/docsgpt/deploy/commands.py
+++ b/docsgpt/deploy/commands.py
@@ -739,6 +739,13 @@ 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 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.
+        raise DeployError(
+            f"port {dev_module.MOCK_LLM_PORT} is already in use, so the mock LLM cannot bind it. "
+            "Stop what is on it, or leave --mock-llm off and point DocsGPT at a real provider."
+        )
     children = dev_module.plan(args, checkout)
     print(f"DocsGPT from {checkout}")
     for child in children:
diff --git a/tests/deploy/test_doctor.py b/tests/deploy/test_doctor.py
index ac046a85..c0a52b28 100644
--- a/tests/deploy/test_doctor.py
+++ b/tests/deploy/test_doctor.py
@@ -49,6 +49,23 @@ class TestDevCommand:
                 _run(["dev", "--port", str(port)], _context())
         assert started == [], "nothing is spawned when the port is taken"
 
+    def test_a_busy_mock_llm_port_is_refused_before_anything_starts(self, monkeypatch, tmp_path):
+        """It starts first and the rest are pointed at it, so a clash cannot wait until spawn time."""
+        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)
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as held:
+            held.bind(("127.0.0.1", 0))
+            held.listen(1)
+            monkeypatch.setattr(dev_module, "MOCK_LLM_PORT", held.getsockname()[1])
+            with pytest.raises(DeployError, match="mock LLM cannot bind"):
+                _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_it_runs_the_children_it_planned_and_says_where(self, monkeypatch, tmp_path, capsys):
         from docsgpt.deploy import dev as dev_module