diff --git a/components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py b/components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py
index c8ee67534..2b420ec3a 100644
--- a/components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py
+++ b/components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py
@@ -152,6 +152,7 @@ async def query(
             stderr=asyncio.subprocess.PIPE,
             cwd=self._cwd,
             env=env,
+            limit=10 * 1024 * 1024,  # 10 MB — default 64 KB is too small for large MCP tool responses
         )
 
         # Start concurrent stderr streaming
diff --git a/components/runners/ambient-runner/tests/test_gemini_session.py b/components/runners/ambient-runner/tests/test_gemini_session.py
index 94fbaf260..5ac60edb5 100644
--- a/components/runners/ambient-runner/tests/test_gemini_session.py
+++ b/components/runners/ambient-runner/tests/test_gemini_session.py
@@ -78,6 +78,44 @@ async def _wait():
 # ------------------------------------------------------------------
 
 
+class TestWorkerSubprocessConfig:
+    """Verify subprocess configuration passed to create_subprocess_exec."""
+
+    @pytest.mark.asyncio
+    async def test_stream_buffer_limit_raised(self):
+        """The subprocess must use a 10 MB buffer limit to handle large MCP tool responses."""
+        worker = GeminiSessionWorker(model="gemini-2.5-flash", api_key="key1")
+        proc = _make_mock_process(
+            stdout_lines=[b'{"type":"result"}\n'],
+            stderr_lines=[],
+        )
+
+        with patch("asyncio.create_subprocess_exec", return_value=proc) as mock_exec:
+            async for _ in worker.query("test"):
+                pass
+
+            call_kwargs = mock_exec.call_args[1]
+            assert call_kwargs["limit"] == 10 * 1024 * 1024
+
+    @pytest.mark.asyncio
+    async def test_large_stdout_line_does_not_crash(self):
+        """A real subprocess outputting >64 KB on one line must not raise ValueError.
+
+        Without the 10 MB limit the default 64 KB StreamReader would blow up with:
+            ValueError: Separator is found, but chunk is longer than limit
+        """
+        payload = "x" * 100_000  # 100 KB — well above the old 64 KB default
+        proc = await asyncio.create_subprocess_exec(
+            "python3", "-c", f'print("{payload}")',
+            stdout=asyncio.subprocess.PIPE,
+            limit=10 * 1024 * 1024,
+        )
+        line = await proc.stdout.readline()
+        await proc.wait()
+        assert len(line) > 64 * 1024
+        assert proc.returncode == 0
+
+
 class TestWorkerCommandConstruction:
     """Verify the CLI command built by query()."""