fix: use max_completion_tokens for GitHub Copilot

tests/agent/test_auxiliary_client.py

@@ -57,6 +57,18 @@ def codex_auth_dir(tmp_path, monkeypatch):
     return codex_dir
 
 
+class TestAuxiliaryMaxTokensParam:
+    def test_uses_max_completion_tokens_for_github_copilot_custom_base(self):
+        with patch("agent.auxiliary_client._resolve_custom_runtime", return_value=("https://api.githubcopilot.com", "key", None)), \
+             patch("agent.auxiliary_client._read_nous_auth", return_value=None):
+            assert auxiliary_max_tokens_param(2048) == {"max_completion_tokens": 2048}
+
+    def test_uses_max_completion_tokens_for_github_copilot_custom_base_path(self):
+        with patch("agent.auxiliary_client._resolve_custom_runtime", return_value=("https://api.githubcopilot.com/chat/completions", "key", None)), \
+             patch("agent.auxiliary_client._read_nous_auth", return_value=None):
+            assert auxiliary_max_tokens_param(2048) == {"max_completion_tokens": 2048}
+
+
 class TestNormalizeAuxProvider:
     def test_maps_github_copilot_aliases(self):
         assert _normalize_aux_provider("github") == "copilot"

tests/run_agent/test_run_agent.py

@@ -3666,6 +3666,18 @@ class TestMaxTokensParam:
         result = agent._max_tokens_param(4096)
         assert result == {"max_completion_tokens": 4096}
 
+    def test_returns_max_completion_tokens_for_github_copilot(self, agent):
+        """GitHub Copilot's OpenAI-compatible API rejects max_tokens for newer models."""
+        agent.base_url = "https://api.githubcopilot.com"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_completion_tokens": 4096}
+
+    def test_returns_max_completion_tokens_for_github_copilot_path(self, agent):
+        """Detect Copilot by hostname even when the configured URL includes a path."""
+        agent.base_url = "https://api.githubcopilot.com/chat/completions"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_completion_tokens": 4096}
+
 
 class TestAzureOpenAIRouting:
     """Verify Azure OpenAI endpoints stay on chat_completions for gpt-5.x."""