Setting ToolChoice to 'none' in lifecycle hook has no effect

[desmondpp]

Describe the bug

As far as I can tell there's no way to give the agent the flexibility to call a tool as often as necessary to answer a user prompt, but cap it to prevent runaway calls. While this can be achieved through prompt engineering, I had mixed results so hoped to have a mechanism in the agent SDK. I attempted to do so by adding a lifecycle hook and tracking the tool calls. When the calls are > N, I set toolChoice to none. However despite this the tool continued to be called.

Debug information

• Agents SDK version: (e.g. v0.0.7)
• Runtime environment (e.g. Node.js 22.16.0)

Repro steps

Include a system prompt telling the agent to call the tool at least 3 times (to trigger the code below).

export function createKnowledgeCenterAgent(model: string) {
  const searchTool = createKnowledgeCenterSearchTool();

  const agent = new Agent<KnowledgeCenterAgentContext, typeof KnowledgeCenterAgentOutputSchema>({
    name: 'Knowledge Center Agent',
    instructions: KNOWLEDGE_CENTER_PROMPT,
    model,
    tools: [searchTool],
    outputType: KnowledgeCenterAgentOutputSchema,
  });
  let toolCallCount = 0;
  agent.on('agent_tool_end', () => {
    if (++toolCallCount >= 2) {
      agent.modelSettings.toolChoice = 'none';
    }
  });

  return agent;
}

Expected behavior

I expected the tool calls to discontinue once toolChoice was set to none.