`conversation_initiation_client_data` with `agent.prompt` causes immediate disconnect, while `sendContextualUpdate` works

[ibrahimkholil6210]

Hi team,

We are seeing a disconnect issue when setting agent.prompt during startSession overrides.

What works:

• Set firstMessage in overrides.
• Send prompt later via sendContextualUpdate(...).

What fails:

• Set prompt in overrides (ConversationOverrides.agent.prompt) during startSession(...).
• Session reaches connected and returns conversationId, then disconnects immediately.

Observed behavior:

• status=connecting -> status=connected -> immediate disconnect.
• Disconnect reasons seen: agent, then error during cleanup.
• From ElevenLabs console: Invalid message received.

Repro (simplified):

await client.startSession(
  agentId: '...',
  overrides: ConversationOverrides(
    agent: AgentOverrides(
      firstMessage: 'Good evening...',
      prompt: 'Random string', // adding this triggers the issue
      language: 'en',
    ),
  ),
);

If we remove prompt from overrides and instead do:

client.sendContextualUpdate('Random string');

the session stays stable.

Environment:

• Flutter app on Android (Pixel 6 Pro)
• elevenlabs_agents package
• Bluetooth speaker may be connected (but issue also appears without BT in our tests)

Question:

• Is agent.prompt in conversation_initiation_client_data currently restricted/validated differently from contextual updates?
• Is there a required payload shape/size limit for prompt at init time?

[kraenhansen]

Just to be absolutely sure: You've configured the agent to allow overriding the prompt?

[p-karcz]

I encountered the same error and I confirm my agent is configured to allow overriding the prompt

[kraenhansen]

@ibrahimkholil6210 & @p-karcz are either of you using our SDKs for other platforms? If no, that's perfectly fine, if yes - do they exhibit the same behaviour?

[p-karcz]

@kraenhansen yes, before integrating the flutter sdk I used both iOS and Android SDKs and the issue was not present there - I was able to pass prompt to AgentOverrides with no issues

[harunr-plateron]

I'm also facing this same error and also this is the error that I see in the eleven labs SDK

[harunr-plateron]

There's a workaround for this @ibrahimkholil6210. You can send it as a part of dynamic variables and use it in the eleven labs dashboard.