dotnet/runtime · error · Error

Failed on mono_wasm_send_dbg_command_with_parms

Error message

Failed on mono_wasm_send_dbg_command_with_parms

What it means

Thrown by `mono_wasm_send_dbg_command_with_parms` when the recorded debug-command response has `res_ok === false`. The runtime queued the response via `mono_wasm_add_dbg_command_received`; a false ok flag means the native debugger side rejected the command (bad parameters, unknown command set, or a debugger protocol error). The wrapper turns that into a thrown Error so the debugger proxy surfaces it.

Solutions

1. Verify the `command_set` and `command` numbers match the runtime's supported debugger protocol version.
2. Check that `command_parameters` is a valid base64 string of the expected length and that `length` matches it.
3. Re-issue the command after re-attaching the debugger; an expired session id is the most common cause.
4. Match the dotnetDebugger/BrowserDebugProxy build to the runtime build.

Example fix

// before: stale id after the runtime continued
mono_wasm_send_dbg_command_with_parms(oldId, set, cmd, b64, len, vt, val); // throws

// after: refresh the object id within the same paused state
const fresh = mono_wasm_send_dbg_command(curId, set, cmd, b64);
mono_wasm_send_dbg_command_with_parms(fresh.id, set, cmd, b64, len, vt, val);

// Validate parameter shape before sending
function isValidParams (command_parameters, length, valtype, newvalue) {
  return typeof command_parameters === 'string' &&
    command_parameters.length === length &&
    Number.isInteger(valtype) &&
    newvalue !== undefined;
}

function isLiveDebuggerObjectId (objId: unknown): boolean {
  return typeof objId === 'number' && Number.isFinite(objId) && objId >= 0;
}

try {
  return mono_wasm_send_dbg_command_with_parms(id, set, cmd, params, len, vt, val);
} catch (e) {
  if (e instanceof Error && /Failed on mono_wasm_send_dbg_command_with_parms/.test(e.message)) {
    // refresh ids and retry once within the same paused state
    return mono_wasm_send_dbg_command_with_parms(freshId, set, cmd, params, len, vt, val);
  }
  throw e;
}

When it happens

Trigger: Calling `mono_wasm_send_dbg_command_with_parms(id, command_set, command, command_parameters, length, valtype, newvalue)` (debug.ts:79) where the native runtime records a failing response for `id`. This occurs when the proxy issues a set-value command with an invalid id, wrong value type, malformed base64 `command_parameters`, or an unsupported `command_set`/`command` pair.

Common situations: Stale debugger proxy talking to a runtime with a different debugger protocol version; trying to set a property/getter on an object id whose session has expired; sending parameters with the wrong `valtype` enum value; heap corruption resetting the debug buffer.

Related errors

• Failed on mono_wasm_send_dbg_command
• Failed on mono_wasm_get_dbg_command_info
• args must be an object, but got
• event.eventName is a required parameter, in event
• event must be an object, but got

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/02b7fdafc6d45d79. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/debug.ts:87

        _debugger_buffer_len = Math.max(command_parameters.length, _debugger_buffer_len, 256);
        _debugger_buffer = malloc(_debugger_buffer_len);
    }
    const byteCharacters = atob(command_parameters);
    const heapU8 = localHeapViewU8();
    for (let i = 0; i < byteCharacters.length; i++) {
        heapU8[<any>_debugger_buffer + i] = byteCharacters.charCodeAt(i);
    }
}

export function mono_wasm_send_dbg_command_with_parms (id: number, command_set: number, command: number, command_parameters: string, length: number, valtype: number, newvalue: number): CommandResponseResult {
    forceThreadMemoryViewRefresh();

    mono_wasm_malloc_and_set_debug_buffer(command_parameters);
    cwraps.mono_wasm_send_dbg_command_with_parms(id, command_set, command, _debugger_buffer, length, valtype, newvalue.toString());

    const { res_ok, res } = commands_received.remove(id);
    if (!res_ok)
        throw new Error("Failed on mono_wasm_send_dbg_command_with_parms");
    return res;
}

export function mono_wasm_send_dbg_command (id: number, command_set: number, command: number, command_parameters: string): CommandResponseResult {
    forceThreadMemoryViewRefresh();

    mono_wasm_malloc_and_set_debug_buffer(command_parameters);
    cwraps.mono_wasm_send_dbg_command(id, command_set, command, _debugger_buffer, command_parameters.length);

    const { res_ok, res } = commands_received.remove(id);

    if (!res_ok)
        throw new Error("Failed on mono_wasm_send_dbg_command");
    return res;

}

export function mono_wasm_get_dbg_command_info (): CommandResponseResult {

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