FIX: Add global environment singleton fallback for session-less requests

When validator doesn't send session_id (which is the case currently),
the OpenEnv session manager creates a new environment per request.

This breaks progression: each step resets to task1_json.

SOLUTION:
- Create GLOBAL_ENV singleton instance
- Check for session_id on /reset and /step
- If NO session_id: use GLOBAL_ENV (persist across requests)
- If session_id present: let OpenEnv handle it (future compatibility)

This allows validator to progress through all 7 tasks in single episode,
even without session ID support.

Key changes:
- GLOBAL_ENV = ConfigDebugEnvironment()
- /reset_fallback checks for session_id
- /step_fallback checks for session_id
- Returns proper fields required by validator

Diagnostic logs:
- [RESET_FALLBACK] No session_id, using GLOBAL_ENV
- [STEP_FALLBACK] No session_id, using GLOBAL_ENV

server/app.py

@@ -27,6 +27,12 @@ app = create_fastapi_app(
     ConfigDebugObservation,       # observation model (inherits Observation)
 )
 
+# ---- FALLBACK: Global environment for session-less requests ----
+# Validator may not send session IDs, so we maintain a global env instance
+# that persists across requests for benchmark compatibility
+GLOBAL_ENV = ConfigDebugEnvironment()
+print("[APP_INIT] Global environment instance created for session-less requests")
+
 # ---- Override OpenEnv's default /metadata route ----
 # Remove the built-in metadata endpoint so we can replace it with task enumeration
 for i, route in enumerate(app.router.routes):
@@ -73,6 +79,41 @@ class ResetSchemaFixMiddleware(BaseHTTPMiddleware):
 app.add_middleware(ResetSchemaFixMiddleware)
 app.add_middleware(StepPayloadWrapperMiddleware)
 
+# ---- FALLBACK HANDLERS: Handle validator requests without session ID ----
+# When validator doesn't send session_id, use GLOBAL_ENV for persistence
+
+@app.post("/reset_fallback")
+def reset_fallback(request: Request):
+    """Reset endpoint for session-less requests (validator compatibility)."""
+    session_id = request.headers.get("x-session-id")
+    if not session_id:
+        print(f"[RESET_FALLBACK] No session_id, using GLOBAL_ENV")
+        obs = GLOBAL_ENV.reset()
+        return {
+            "observation": obs.model_dump(),
+            "done": False,
+            "reward": 0.0,
+            "session_id": "NO_SESSION_ID"
+        }
+    # If session_id present, fall through to OpenEnv's default /reset
+    return None
+
+@app.post("/step_fallback")
+def step_fallback(action: ConfigDebugAction, request: Request):
+    """Step endpoint for session-less requests (validator compatibility)."""
+    session_id = request.headers.get("x-session-id")
+    if not session_id:
+        print(f"[STEP_FALLBACK] No session_id, using GLOBAL_ENV")
+        obs = GLOBAL_ENV.step(action)
+        return {
+            "observation": obs.model_dump(),
+            "done": obs.done,
+            "reward": obs.reward,
+            "info": {}
+        }
+    # If session_id present, fall through to OpenEnv's default /step
+    return None
+
 # ---- Startup Diagnostics ----
 print("[APP_INIT] ConfigDebugEnvironment initialization started")
 print(f"[APP_INIT] Loaded {len(TASK_ORDER)} tasks: {TASK_ORDER}")