dotnet/runtime · error · Error

Unknown object id ${objId}

Error message

Unknown object id ${objId}

What it means

Thrown by mono_wasm_call_function_on() in debug.ts when request.objectId starts with the prefix 'dotnet:cfo_res:' (meaning it references a cached call-function-on result) but is not present in the in-memory _call_function_res_cache. Results from previous Runtime.callFunctionOn calls are cached by an incrementing id (dotnet:cfo_res:N); referencing one that was never created or was released via mono_wasm_release_object fails here.

Source

Thrown at src/mono/browser/runtime/debug.ts:238

    });
    return proxy;
}

export function mono_wasm_call_function_on (request: CallRequest): CFOResponse {
    forceThreadMemoryViewRefresh();

    if (request.arguments != undefined && !Array.isArray(request.arguments))
        throw new Error(`"arguments" should be an array, but was ${request.arguments}`);

    const objId = request.objectId;
    const details = request.details;
    let proxy: any = {};

    if (objId.startsWith("dotnet:cfo_res:")) {
        if (objId in _call_function_res_cache)
            proxy = _call_function_res_cache[objId];
        else
            throw new Error(`Unknown object id ${objId}`);
    } else {
        proxy = _create_proxy_from_object_id(objId, details);
    }

    const fn_args = request.arguments != undefined ? request.arguments.map(a => JSON.stringify(a.value)) : [];

    const fn_body_template = `const fn = ${request.functionDeclaration}; return fn.apply(proxy, [${fn_args}]);`;
    const fn_defn = new Function("proxy", fn_body_template);
    const fn_res = fn_defn(proxy);

    if (fn_res === undefined)
        return { type: "undefined" };

    if (Object(fn_res) !== fn_res) {
        if (typeof (fn_res) == "object" && fn_res == null)
            return { type: typeof (fn_res), subtype: `${fn_res}`, value: null };
        return { type: typeof (fn_res), description: `${fn_res}`, value: `${fn_res}` };
    }

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Re-acquire the object id from a fresh Runtime.callFunctionOn / evaluation before operating on it.
2. Avoid calling mono_wasm_release_object on ids you still intend to use.
3. Reload the page if ids are stale after a navigation/restart.

Example fix

null

null

null

try {
  return mono_wasm_call_function_on(request);
} catch (e) {
  if (String(e).includes('Unknown object id')) {
    // stale cfo_res handle; re-acquire via a fresh callFunctionOn/evaluate
    return { type: 'undefined' };
  }
  throw e;
}

When it happens

Trigger: A CDP client sending Runtime.callFunctionOn with an objectId from a previous session/page load (the cache is in-memory and resets per page). Calling against an id after mono_wasm_release_object freed it. A client fabricating an objectId string.

Common situations: DevTools reusing object handles across a page reload or after the object group was released. Long-lived inspector sessions in a test harness that assume stable object ids.

Related errors

• Could not find any object with id ${objectId}
• Failed on mono_wasm_send_dbg_command_with_parms
• Failed on mono_wasm_send_dbg_command
• "arguments" should be an array, but was ${request.arguments}
• Failed on mono_wasm_get_dbg_command_info

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/ed120a5a13588f4b. Report an issue: GitHub.