fix(providers): make GitHub Copilot backend work with GPT-5/o-series models

Calling GitHub Copilot with `gpt-5.*` / `o*` models (e.g.
`github_copilot/gpt-5.4`, `github_copilot/gpt-5.4-mini`) failed with a
chain of misleading errors:

  1. `Unsupported parameter: 'max_tokens' is not supported with this
     model. Use 'max_completion_tokens' instead.`
  2. `model "gpt-5.4-mini" is not accessible via the /chat/completions
     endpoint` (`unsupported_api_for_model`).
  3. `The requested model is not supported.` (`model_not_supported`)
     even after routing to /responses.

Root causes (each one masked the next):

  * The `github_copilot` ProviderSpec did not opt into
    `supports_max_completion_tokens`, so `_build_kwargs` always sent the
    legacy `max_tokens` parameter that GPT-5/o-series reject.
  * `_should_use_responses_api` was hard-gated to
    `spec.name == "openai"` plus a direct-OpenAI base URL, so the
    GitHub Copilot backend always went through /chat/completions even
    for models the Copilot gateway exposes only via /responses
    (e.g. `gpt-5.4-mini`).
  * When /responses did fail on github_copilot, the existing
    "compatibility marker" heuristic silently fell back to
    /chat/completions — which can never succeed for these models — so
    the real upstream error was hidden.
  * `_build_responses_body` did not honour `spec.strip_model_prefix`,
    so the request body sent `model="github_copilot/gpt-5.4-mini"`
    (with the routing prefix), which the Copilot gateway rejects with
    `model_not_supported`. (`_build_kwargs` already stripped it; this
    branch was missed.)

Fix:

  * registry.py: set `supports_max_completion_tokens=True` on the
    `github_copilot` spec so requests use `max_completion_tokens`.
  * openai_compat_provider.py:
      - `_should_use_responses_api` now also allows the
        `github_copilot` spec, and skips the direct-OpenAI base check
        for it (the Copilot gateway is its own base URL).
      - `_build_responses_body` now strips the model routing prefix
        when `spec.strip_model_prefix` is set, matching `_build_kwargs`.
      - `chat` / `chat_stream` no longer fall back from /responses to
        /chat/completions on the `github_copilot` spec: the fallback
        cannot succeed for GPT-5/o-series and would mask the real
        gateway error.

Tests:

  * tests/cli/test_commands.py: switched the
    `test_github_copilot_provider_refreshes_client_api_key_before_chat`
    fixture model from `gpt-5.1` to `gpt-4` so it continues to exercise
    the /chat/completions code path it was designed for (gpt-5.1 now
    correctly routes to /responses on github_copilot).
  * `pytest tests/providers/ tests/cli/test_commands.py` — 314 passed.
  * Verified end-to-end against the live Copilot gateway with both
    `github_copilot/gpt-5.4` and `github_copilot/gpt-5.4-mini`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

nanobot/providers/openai_compat_provider.py

@@ -446,10 +446,11 @@ class OpenAICompatProvider(LLMProvider):
         reasoning_effort: str | None,
     ) -> bool:
         """Use Responses API only for direct OpenAI requests that benefit from it."""
-        if self._spec and self._spec.name != "openai":
-            return False
-        if not _is_direct_openai_base(self._effective_base):
+        if self._spec and self._spec.name not in ("openai", "github_copilot"):
             return False
+        if self._spec is None or self._spec.name != "github_copilot":
+            if not _is_direct_openai_base(self._effective_base):
+                return False
 
         model_name = (model or self.default_model).lower()
         wants = False
@@ -527,6 +528,8 @@ class OpenAICompatProvider(LLMProvider):
     ) -> dict[str, Any]:
         """Build a Responses API body for direct OpenAI requests."""
         model_name = model or self.default_model
+        if self._spec and self._spec.strip_model_prefix:
+            model_name = model_name.split("/")[-1]
         sanitized_messages = self._sanitize_messages(self._sanitize_empty_content(messages))
         instructions, input_items = convert_messages(sanitized_messages)
 
@@ -987,6 +990,11 @@ class OpenAICompatProvider(LLMProvider):
                     self._record_responses_success(model, reasoning_effort)
                     return result
                 except Exception as responses_error:
+                    if self._spec and self._spec.name == "github_copilot":
+                        # Copilot gateway exposes GPT-5/o-series only via /responses;
+                        # falling back to /chat/completions cannot succeed and would
+                        # hide the real error.
+                        raise
                     if not self._should_fallback_from_responses_error(responses_error):
                         raise
                     self._record_responses_failure(model, reasoning_effort)
@@ -1045,6 +1053,11 @@ class OpenAICompatProvider(LLMProvider):
                         reasoning_content=reasoning_content,
                     )
                 except Exception as responses_error:
+                    if self._spec and self._spec.name == "github_copilot":
+                        # Copilot gateway exposes GPT-5/o-series only via /responses;
+                        # falling back to /chat/completions cannot succeed and would
+                        # hide the real error.
+                        raise
                     if not self._should_fallback_from_responses_error(responses_error):
                         raise
                     self._record_responses_failure(model, reasoning_effort)

nanobot/providers/registry.py

@@ -223,6 +223,7 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
         default_api_base="https://api.githubcopilot.com",
         strip_model_prefix=True,
         is_oauth=True,
+        supports_max_completion_tokens=True,
     ),
     # DeepSeek: OpenAI-compatible at api.deepseek.com
     ProviderSpec(

tests/cli/test_commands.py

@@ -421,13 +421,13 @@ async def test_github_copilot_provider_refreshes_client_api_key_before_chat():
     })
 
     with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI", return_value=mock_client):
-        provider = GitHubCopilotProvider(default_model="github-copilot/gpt-5.1")
+        provider = GitHubCopilotProvider(default_model="github-copilot/gpt-4")
 
     provider._get_copilot_access_token = AsyncMock(return_value="copilot-access-token")
 
     response = await provider.chat(
         messages=[{"role": "user", "content": "hi"}],
-        model="github-copilot/gpt-5.1",
+        model="github-copilot/gpt-4",
         max_tokens=16,
         temperature=0.1,
     )