dotnet/runtime · error · Error

Failed on mono_wasm_send_dbg_command

Error message

Failed on mono_wasm_send_dbg_command

What it means

Thrown by mono_wasm_send_dbg_command() in debug.ts when the native Mono runtime marks a debugger command response as failed (res_ok=false). This is the simpler variant of the proxy command (no value/valtype parameters); it marshals a base64 command buffer to the heap and reads the reply that native pushed through mono_wasm_add_dbg_command_received. It is invoked by the proxy getters generated in _create_proxy_from_object_id (debug.ts:198) and other debugger read paths.

Source

Thrown at src/mono/browser/runtime/debug.ts:100

    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 {
    const { res_ok, res } = commands_received.remove(0);

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

export function mono_wasm_debugger_resume (): void {
    forceThreadMemoryViewRefresh();
}

export function mono_wasm_detach_debugger (): void {
    forceThreadMemoryViewRefresh();

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Retry the inspection/evaluation once the runtime is no longer loading or paused.
2. Reload the page to reset the native command id space and the proxy cache.
3. Verify the served runtime .wasm matches the diagnostics/debug JS modules (rebuild and hard-refresh).
4. If it persists for a specific object, that object reference is stale; release it and re-acquire from the live runtime.

Example fix

null

null

null

try {
  return mono_wasm_send_dbg_command(id, command_set, command, command_parameters);
} catch (e) {
  console.warn('debugger read failed', { id, command_set, command }, e);
  return undefined;
}

When it happens

Trigger: A proxy property getter firing against a dotnet: object id whose native target is invalid, collected, or belongs to a paused/transitioning runtime. Issuing a debugger read command during module initialization, GC, or thread memory relocation (forceThreadMemoryViewRefresh is called but native can still fail). A stale command id reused after the command buffer was freed.

Common situations: Expanding a .NET object in the DevTools Watch/Scope pane while the runtime is loading or paused at a breakpoint. Evaluating expressions referencing objects from a previous debug session after hot reload. Serving a mismatched runtime wasm + debug JS bundle.

Related errors

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

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/bfd9d5494d942b26. Report an issue: GitHub.