google-gemini/gemini-cli · error

LegacyAgentSession.send() only supports message sends for th

Error message

LegacyAgentSession.send() only supports message sends for the moment.

What it means

Thrown by the legacy session adapter's `send` when the incoming `AgentSend` payload has no `message` field. The adapter only knows how to translate full message sends into the legacy streaming protocol; other payload kinds (in-stream updates, elicitation responses, cancellations) are explicitly unsupported and rejected at the boundary.

Source

Thrown at packages/core/src/agent/legacy-agent-session.ts:111

      this._scheduler = scheduler;
    }
  }

  get events(): readonly AgentEvent[] {
    return this._events;
  }

  subscribe(callback: (event: AgentEvent) => void): Unsubscribe {
    this._subscribers.add(callback);
    return () => {
      this._subscribers.delete(callback);
    };
  }

  async send(payload: AgentSend): Promise<{ streamId: string }> {
    const message = 'message' in payload ? payload.message : undefined;
    if (!message) {
      throw new Error(
        'LegacyAgentSession.send() only supports message sends for the moment.',
      );
    }

    if (this._activeStreamId) {
      // TODO: Interactive may eventually allow selected in-stream sends such as
      // updates or elicitation responses. Keep rejecting all concurrent sends
      // here until we define those correlation semantics.
      throw new Error(
        'LegacyAgentSession.send() cannot be called while a stream is active.',
      );
    }

    this._beginNewStream();
    const streamId = this._translationState.streamId;
    const parts = contentPartsToGeminiParts(message.content);
    const userMessage = this._makeUserMessageEvent(
      message.content,

View on GitHub (pinned to 5024443c72)

Solutions

1. Only forward `{ message: ... }` payloads to `LegacyAgentSession.send`; route other payload kinds to a session implementation that supports them.
2. Add a type guard that narrows the payload to the message variant before calling send.
3. If you must use the legacy session, translate the unsupported payload into a new message send instead.
4. Upgrade the session to a non-legacy implementation if you need full `AgentSend` coverage.

Example fix

// before
await legacySession.send({ update: { value: 1 } }); // throws

// after
function isMessageSend(p: AgentSend): p is { message: unknown } {
  return 'message' in p;
}
if (isMessageSend(payload)) {
  await legacySession.send(payload);
} else {
  // route to a session that supports the payload kind
}

function isMessageSend(p: AgentSend): p is { message: unknown } {
  return 'message' in p && p.message != null;
}

if (!isMessageSend(payload)) {
  throw new Error('LegacyAgentSession only accepts message sends; got: ' + Object.keys(payload).join(','));
}
await legacySession.send(payload);

function isMessageSend(p: AgentSend): p is { message: unknown } {
  return typeof p === 'object' && p !== null && 'message' in p && (p as { message: unknown }).message != null;
}

try {
  await legacySession.send(payload);
} catch (e) {
  if (e instanceof Error && e.message.includes('only supports message sends')) {
    // route the payload to a session that handles its kind
  }
  throw e;
}

When it happens

Trigger: Calling `legacySession.send(payload)` with a payload that is not a `{ message: ... }` shape — e.g. an `{ update: ... }`, `{ response: ... }`, or an empty object. The check is `'message' in payload ? payload.message : undefined`, then `if (!message) throw`.

Common situations: New code written against the newer `AgentSend` union being routed into a `LegacyAgentSession` (e.g. an adapter layer or a config that still selects the legacy implementation); a generic dispatcher that forwards any payload type without filtering; a payload that was built with optional chaining and silently dropped the `message` field.

Related errors

• Unknown eventId: ${options.eventId}
• Cannot resume from eventId ${options.eventId} before agent_s
• LegacyAgentSession.send() cannot be called while a stream is
• Invalid taskId: ${taskId}
• Security violation: Null byte detected in path.

AI-assisted analysis of google-gemini/gemini-cli@5024443c72 (2026-08-12). Data as JSON: /api/errors/b745a8fbb0293e24. Report an issue: GitHub.