feat(core): introduce AgentSession and rename stream events to agent events

[mbleigh]

Summary

This PR updates the AgentProtocol interface to use a subscription-based model and introduces AgentSession, a wrapper that provides an AsyncIterable API (sendStream) for consuming agent events. It also renames stream_start and stream_end events to agent_start and agent_end to better reflect their role in indicating agent activity.

Details

• Refactored AgentProtocol to replace the stream() method with a subscribe() method.
• Added AgentSession in packages/core/src/agent/agent-session.ts to provide a higher-level AsyncIterable interface.
• Updated MockAgentProtocol to support the new interface and ensure correct event ordering (e.g., streamId is returned before agent_start is emitted).
• Renamed events: StreamStart -> AgentStart, StreamEnd -> AgentEnd.
• Updated all related tests and added new tests for AgentSession.

Related Issues

Related to #20995

How to Validate

• Run npm test -w @google/gemini-cli-core to verify agent and session tests.
• Run npm run preflight to ensure no regressions across the monorepo.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on MacOS:
  • npm run

[gemini-cli]

Hi @mbleigh, thank you so much for your contribution to Gemini CLI! We really appreciate the time and effort you've put into this.

We're making some updates to our contribution process to improve how we track and review changes. Please take a moment to review our recent discussion post: Improving Our Contribution Process & Introducing New Guidelines.

Key Update: Starting January 26, 2026, the Gemini CLI project will require all pull requests to be associated with an existing issue. Any pull requests not linked to an issue by that date will be automatically closed.

Thank you for your understanding and for being a part of our community!

[gemini-code-assist]

Summary of Changes

Hello, 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 significantly enhances the agent event handling mechanism by introducing a new AgentSession class, which provides an AsyncIterable interface for event consumption. It also refactors the underlying AgentProtocol to a more modern subscription-based model and renames key event types for clarity. These changes aim to improve the developer experience when interacting with agent events, making asynchronous event processing more intuitive and robust.

Highlights

• New AgentSession Class: Introduced a new AgentSession class that wraps the AgentProtocol to provide a more convenient AsyncIterable API for consuming agent events via its sendStream method.
• AgentProtocol Refactoring: The core AgentProtocol interface was refactored from a stream() method to a subscription-based subscribe() method, allowing for a more flexible event consumption model.
• Event Renaming: The stream_start and stream_end events have been renamed to agent_start and agent_end respectively, to better reflect their role in indicating agent activity.
• Mock Agent Protocol Updates: The MockAgentProtocol (formerly MockAgentSession) was updated to support the new subscription interface and ensure correct event ordering, particularly that streamId is returned before agent_start is emitted.

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[gemini-code-assist]

Code Review

This pull request is a significant and well-executed refactoring of the agent communication protocol. The transition from a stateful stream() method to a subscription-based AgentProtocol is a solid architectural improvement, and the new AgentSession provides a convenient AsyncIterable wrapper for consumers. The code is well-structured, and the tests have been thoroughly updated to reflect the changes. I have one suggestion regarding a potential performance issue in the event queue processing logic, which could be important for streams with a high volume of events.

[github-actions]

Size Change: -4 B (0%)

Total Size: 26.3 MB

Filename	Size	Change
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./bundle/chunk-EBXZJDKB.js	13.5 MB	+13.5 MB (new file)	🆕
./bundle/chunk-V37XEM2C.js	3.64 MB	+3.64 MB (new file)	🆕
./bundle/core-KMD3PLV2.js	41.8 kB	+41.8 kB (new file)	🆕
./bundle/devtoolsService-BZEBKFTT.js	27.7 kB	+27.7 kB (new file)	🆕
./bundle/interactiveCli-RZABCUSG.js	1.61 MB	+1.61 MB (new file)	🆕
./bundle/oauth2-provider-AEJVJZHT.js	9.19 kB	+9.19 kB (new file)	🆕

ℹ️ View Unchanged

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