Claude Code: Add robust error handling with stderr logging

- Wrap main execution loop in try-except with full traceback
- Log all startup/debug output to stderr (not buffered stdout)
- Add KeyboardInterrupt handling for graceful shutdown
- Update entrypoint.sh to explicitly flush stderr

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -24,11 +24,11 @@ def get_app():
 
     print(">>> CAIN: Creating FastAPI app...", flush=True)
 
-    # Configure stdout logging
+    # Configure stderr logging for real-time debugging (stdout can be buffered)
     logging.basicConfig(
         level=logging.INFO,
         format='%(asctime)s - %(levelname)s - %(message)s',
-        handlers=[logging.StreamHandler(sys.stdout)]
+        handlers=[logging.StreamHandler(sys.stderr)]
     )
     logger = logging.getLogger(__name__)
     logger.info(">>> CAIN: FastAPI initialization starting")
@@ -83,6 +83,19 @@ except Exception as e:
 
 if __name__ == "__main__":
     import uvicorn
-    port = int(os.environ.get('PORT', 7860))
-    print(f">>> CAIN: Starting uvicorn on port {port}...", flush=True)
-    uvicorn.run(app, host="0.0.0.0", port=port)
+
+    # Wrap main execution in try-except for graceful error handling
+    try:
+        port = int(os.environ.get('PORT', 7860))
+        print(f">>> CAIN: Starting uvicorn on port {port}...", flush=True, file=sys.stderr)
+        uvicorn.run(app, host="0.0.0.0", port=port)
+    except Exception as e:
+        # Log full stack trace to stderr before exiting
+        print(f">>> CAIN: FATAL - Main process crashed!", flush=True, file=sys.stderr)
+        print(f">>> CAIN: Error: {type(e).__name__}: {e}", flush=True, file=sys.stderr)
+        print(f">>> CAIN: Full traceback:", flush=True, file=sys.stderr)
+        traceback.print_exc(file=sys.stderr)
+        sys.exit(1)
+    except KeyboardInterrupt:
+        print("\n>>> CAIN: Interrupted by user", flush=True, file=sys.stderr)
+        sys.exit(0)

entrypoint.sh

@@ -41,9 +41,9 @@ echo "=========================================="
 
 # Auto-restart loop: if uvicorn crashes, wait 2s and restart
 while true; do
-    echo "[$(date -u +"%Y-%m-%dT%H:%M:%SZ")] Starting uvicorn on 0.0.0.0:${PORT:-7860}..."
-    uvicorn app:app --host 0.0.0.0 --port "${PORT:-7860}" 2>&1
-    EXIT_CODE=$?
+    echo "[$(date -u +"%Y-%m-%dT%H:%M:%SZ")] Starting uvicorn on 0.0.0.0:${PORT:-7860}..." >&2
+    uvicorn app:app --host 0.0.0.0 --port "${PORT:-7860}" 2>&1 | tee >(cat >&2)
+    EXIT_CODE=${PIPESTATUS[0]}
 
     echo "[$(date -u +"%Y-%m-%dT%H:%M:%SZ")] uvicorn exited with code: $EXIT_CODE"