fix(agent): strip x-stainless headers from OAuth Anthropic requests to prevent 429

agent/anthropic_adapter.py

@@ -724,6 +724,38 @@ def _build_anthropic_client_with_bearer_hook(
     return _anthropic_sdk.Anthropic(**kwargs)
 
 
+# ── HTTP client helper for OAuth requests ──────────────────────────────
+
+def _build_oauth_http_client(timeout):
+    """Create an httpx.Client that strips ``x-stainless-*`` headers per-request.
+
+    Anthropic's infrastructure uses the ``x-stainless-*`` headers injected by
+    the SDK (language, runtime, OS, package version) as a fingerprint to
+    distinguish "Claude Code CLI" requests from "third-party SDK" requests.
+    Subscription OAuth tokens are rate-limited (HTTP 429) when these SDK
+    headers are present, even when the UA and other identity headers match
+    Claude Code's fingerprint. Stripping them lets the request flow through
+    the same pipeline as the genuine CLI (which uses axios, not the Anthropic
+    Python SDK).
+
+    The hook is applied at the httpx level so the SDK is unaware of the
+    removal — exactly the pattern used for the Azure Entra ID bearer hook
+    in ``_build_anthropic_client_with_bearer_hook``.
+    """
+    import httpx
+
+    client = httpx.Client(timeout=timeout)
+
+    def _strip_stainless(request: httpx.Request) -> None:
+        """Strip all x-stainless-* headers before the request goes on the wire."""
+        to_remove = [k for k in request.headers if k.lower().startswith("x-stainless-")]
+        for k in to_remove:
+            del request.headers[k]
+
+    client.event_hooks["request"].append(_strip_stainless)
+    return client
+
+
 def build_anthropic_client(
     api_key,
     base_url: str = None,
@@ -837,8 +869,21 @@ def build_anthropic_client(
         # OAuth access token / setup-token → Bearer auth + Claude Code identity.
         # Anthropic routes OAuth requests based on user-agent and headers;
         # without Claude Code's fingerprint, requests get intermittent 500s.
+        #
+        # Additionally, the Anthropic Python SDK injects ``x-stainless-*``
+        # headers (SDK fingerprint: language, runtime, OS, package version)
+        # into every request. Anthropic's infrastructure uses these headers
+        # to differentiate "Claude Code CLI" (which uses axios) from
+        # "third-party SDK" requests. Subscription OAuth tokens are then
+        # rate-limited (HTTP 429) when the SDK headers are present, even when
+        # the UA and other identity headers match Claude Code's fingerprint.
+        # A custom httpx.Client with a request hook strips these headers so
+        # the request indistinguishable from the genuine CLI's traffic.
         all_betas = common_betas + _OAUTH_ONLY_BETAS
         kwargs["auth_token"] = api_key
+        kwargs["http_client"] = _build_oauth_http_client(
+            Timeout(timeout=float(_read_timeout), connect=10.0),
+        )
         kwargs["default_headers"] = {
             "anthropic-beta": ",".join(all_betas),
             "user-agent": f"claude-code/{_get_claude_code_version()} (external, cli)",

tests/agent/test_anthropic_oauth_ua_prefix.py

@@ -110,3 +110,78 @@ def test_token_refresh_ua_not_throttled(self):
             "refresh_anthropic_oauth_pure should send the shared "
             "_OAUTH_TOKEN_USER_AGENT (non-claude-code) on the token endpoint"
         )
+
+
+class TestOAuthStainlessHeaders:
+    """Regression tests for GH-70039: OAuth subscription tokens get 429 when
+    Anthropic SDK's x-stainless-* headers leak the client fingerprint.
+    """
+
+    def test_oauth_client_strips_stainless_headers(self):
+        """build_anthropic_client with an OAuth token must provide a custom
+        http_client that strips x-stainless-* headers from outgoing requests.
+
+        Anthropic uses these SDK headers to differentiate \"Claude Code CLI\"
+        (axios) from \"third-party SDK\" (Python) requests. Subscription OAuth
+        tokens are rate-limited (429) when the headers are present.
+        """
+        from agent.anthropic_adapter import _build_oauth_http_client, _is_oauth_token
+        import httpx
+
+        assert _is_oauth_token("sk-ant-oat-foobar"), "sk-ant-oat* should be OAuth"
+        assert _is_oauth_token("eyJhbGciOi"), "eyJ* JWT should be OAuth"
+        assert _is_oauth_token("cc-abc123"), "cc-* should be OAuth"
+        assert not _is_oauth_token("sk-ant-api03-..."), "sk-ant-api* should NOT be OAuth"
+
+        timeout = httpx.Timeout(timeout=900.0, connect=10.0)
+        client = _build_oauth_http_client(timeout)
+        assert client is not None
+
+        # Verify the hook is registered
+        assert len(client.event_hooks.get("request", [])) >= 1
+        hook = client.event_hooks["request"][0]
+        assert callable(hook)
+
+        # Test the hook directly — simulate what the SDK would attach
+        mock_request = httpx.Request("POST", "https://api.anthropic.com/v1/messages")
+        mock_request.headers["x-stainless-lang"] = "python"
+        mock_request.headers["x-stainless-runtime"] = "CPython"
+        mock_request.headers["x-stainless-runtime-version"] = "3.11.15"
+        mock_request.headers["x-stainless-package-version"] = "0.50.0"
+        mock_request.headers["x-stainless-arch"] = "arm64"
+        mock_request.headers["x-stainless-os"] = "macOS"
+        # Keep these — should NOT be stripped
+        mock_request.headers["user-agent"] = "claude-code/2.1.217 (external, cli)"
+        mock_request.headers["x-app"] = "cli"
+        mock_request.headers["anthropic-beta"] = "claude-code-20250219"
+
+        hook(mock_request)
+
+        # Verify all x-stainless-* headers are gone
+        for key in mock_request.headers:
+            assert not key.lower().startswith("x-stainless-"), (
+                f"x-stainless header should be stripped: {key}"
+            )
+
+        # Verify our identity headers are untouched
+        assert mock_request.headers["user-agent"] == "claude-code/2.1.217 (external, cli)"
+        assert mock_request.headers["x-app"] == "cli"
+        assert mock_request.headers["anthropic-beta"] == "claude-code-20250219"
+
+        client.close()
+
+    def test_build_anthropic_client_uses_custom_http_client_for_oauth(self):
+        """OAuth branch of build_anthropic_client must set http_client kwarg."""
+        from agent.anthropic_adapter import build_anthropic_client
+
+        mock_sdk = MagicMock()
+        with patch("agent.anthropic_adapter._get_anthropic_sdk", return_value=mock_sdk):
+            build_anthropic_client(
+                "sk-ant-oat-some-token",
+                "https://api.anthropic.com",
+            )
+
+        call_kwargs = mock_sdk.Anthropic.call_args[1]
+        assert "http_client" in call_kwargs, (
+            "OAuth client must provide a custom http_client to strip x-stainless-* headers"
+        )