fix: acp/zed race condition between MCP initialisation and prompt

[kartikangiras]

Summary

Fix a race condition in ACP/Zed mode where GeminiAgent.prompt could be handled before MCP server connections had finished initialising, causing the model to produce inconsistent responses.

Details

Config.initialize() intentionally starts MCP server connections in the background so the interactive CLI prompt appears quickly.

Two code paths violated this contract:

config.ts set interactive: true without experimentalZedIntegration: true, so MCP always ran in the background and was never awaited — even though the a2a-server is not a terminal and has no reason to skip waiting.

zedIntegration.ts had no guard to wait for MCP to be ready before invoking the model.

Related Issues

fixes #18893

How to Validate

• Configure one or more MCP servers in ~/.gemini/settings.json.
• Start the CLI in ACP mode: [gemini --experimental-acp].
• Send a prompt immediately after session initialisation (before MCP would have had time to connect in the old code).
• Verify the model consistently has access to the configured MCP tools on the very first response.
• Run the unit tests: npx vitest run [zedIntegration.test.ts] packages/a2a-server/src/config/config.test.ts

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello @kartikangiras, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical race condition in the ACP/Zed mode that caused inconsistent model responses due to premature handling of prompts before MCP server connections were fully initialized. The changes ensure that the system reliably waits for MCP initialization, providing a more stable and consistent user experience. Additionally, unit tests were added to validate the fix.

Highlights

• Race Condition Fix: Addressed a race condition in ACP/Zed mode where GeminiAgent.prompt could be handled before MCP server connections were fully initialized, leading to inconsistent model responses.
• MCP Initialization Handling: Ensured that the system waits for MCP to be ready before invoking the model, preventing issues related to uninitialized MCP server connections.
• Configuration Updates: Modified config.ts to properly await MCP initialization, especially in non-interactive or Zed integration scenarios.
• Testing Improvements: Added a unit test to verify that MCP initialization is awaited before processing a prompt in zedIntegration.test.ts.

Changelog

• packages/a2a-server/src/config/config.ts
  • Ensured MCP initialization is awaited before proceeding with configuration loading.
  • Added waitForMcpInit to Config class.
• packages/cli/src/zed-integration/zedIntegration.ts
  • Modified Session class to await MCP initialization before processing prompts.
• packages/core/src/config/config.ts
  • Added a promise to track MCP initialization status.
  • Added waitForMcpInit method to await MCP initialization.

Activity

• The pull request addresses issue Acp/ZedIntegration Race between mcp initialization and first prompt #18893.
• Testing includes unit tests in zedIntegration.test.ts and packages/a2a-server/src/config/config.test.ts.
• Validation steps include configuring MCP servers, starting the CLI in ACP mode, and verifying consistent access to MCP tools.

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[gemini-code-assist]

Code Review

This pull request addresses a race condition where prompts could be handled before MCP server initialization was complete. This is fixed by introducing a mechanism to await MCP initialization within the Config class. This mechanism is then used in the a2a-server and Zed integration code paths to ensure MCP servers are ready before processing prompts. The changes include corresponding updates to tests to validate the new behavior.

[scidomino]

Remove the mcpInitialization variable and just immediately assign it to this.mcpInitializationPromise

[kartikangiras]

i have assigned it to the this.mcpInitializationPromise

[scidomino]

please revert the changes to package-lock.json.

[kartikangiras]

@scidomino i have reverted the changes to package-lock.json.