fix: stop leaking upstream auth-error bodies into user comments

get_github_token_for_user folded the raw upstream response text into the
error string that becomes a Slack/Linear comment (AUTH-RESP-LEAK-01). Log the
full body server-side only and return a generic "GitHub auth failed (status
<code>)". Also document the accepted shared-installation-token blast radius on
the bot-token-only path (AUTHZ-003).

agent/utils/auth.py

@@ -195,8 +195,10 @@ async def get_github_token_for_user(ls_user_id: str, tenant_id: str) -> dict[str
             return {"error": f"Unexpected auth result: {response_data}"}
 
     except httpx.HTTPStatusError as e:
+        # Log the full upstream body server-side only; the returned error becomes a
+        # user-facing Slack/Linear comment, so never echo the raw response text.
         logger.error("GitHub auth API HTTP error: %s - %s", e.response.status_code, e.response.text)
-        return {"error": f"HTTP error: {e.response.status_code} - {e.response.text}"}
+        return {"error": f"GitHub auth failed (status {e.response.status_code})"}
     except Exception as e:  # noqa: BLE001
         logger.error("GitHub auth API call failed: %s: %s", type(e).__name__, str(e))
         return {"error": str(e)}
@@ -383,7 +385,13 @@ async def _resolve_dashboard_user_token(
 
 
 async def _resolve_bot_installation_token(thread_id: str) -> tuple[str, str | None]:
-    """Get a GitHub App installation token and cache it for the thread."""
+    """Get a GitHub App installation token and cache it for the thread.
+
+    AUTHZ-003 (accepted): in bot-token-only mode every run shares one GitHub App
+    installation token, so its blast radius is the whole installation rather than
+    a single user. This is a documented, accepted prod posture for this
+    single-tenant deployment, not a defect.
+    """
     bot_token, expires_at = await get_github_app_installation_token_with_expiry()
     if not bot_token:
         raise RuntimeError(

tests/test_auth_error_leak.py

@@ -0,0 +1,55 @@
+"""AUTH-RESP-LEAK-01: upstream auth-error bodies must not reach user-facing text."""
+
+from __future__ import annotations
+
+import asyncio
+from typing import Any
+
+import httpx
+import pytest
+
+from agent.utils import auth
+
+_SECRET_BODY = "SENSITIVE-UPSTREAM-BODY-9999"
+
+
+class _Resp:
+    def __init__(self, status_code: int, text: str) -> None:
+        self.status_code = status_code
+        self.text = text
+
+    def raise_for_status(self) -> None:
+        raise httpx.HTTPStatusError(
+            "boom",
+            request=httpx.Request("POST", "http://example.test"),
+            response=self,  # type: ignore[arg-type]
+        )
+
+    def json(self) -> Any:
+        return {}
+
+
+class _Client:
+    def __init__(self, resp: _Resp) -> None:
+        self._resp = resp
+
+    async def __aenter__(self) -> _Client:
+        return self
+
+    async def __aexit__(self, *_a: Any) -> None:
+        return None
+
+    async def post(self, *_a: Any, **_k: Any) -> _Resp:
+        return self._resp
+
+
+def test_http_error_returns_generic_message(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(auth, "GITHUB_OAUTH_PROVIDER_ID", "provider-id")
+    monkeypatch.setattr(auth, "X_SERVICE_AUTH_JWT_SECRET", "jwt-secret")
+    monkeypatch.setattr(httpx, "AsyncClient", lambda *a, **k: _Client(_Resp(500, _SECRET_BODY)))
+
+    result = asyncio.run(auth.get_github_token_for_user("ls-user", "tenant"))
+
+    assert "error" in result
+    assert _SECRET_BODY not in result["error"]
+    assert "500" in result["error"]