feat(agent): implement tool-controlled display protocol (Steps 2-3)

[mbleigh]

Summary

Implement tool-controlled display protocol (Steps 2-3) for AgentProtocol / useAgentSession. This allows tools to provide structured visual metadata (name, description, result summary, and rich results like diffs) directly to the UI, replacing ad-hoc logic in the frontend.

Details

• Protocol Changes: Added ToolDisplay interface and related types (DisplayText, DisplayDiff, DisplayContent) to packages/core/src/agent/types.ts.
• Cleanup: Removed legacy displayContent property from ToolUpdate and ToolResponse in favor of the new display: ToolDisplay.
• Centralized Logic: Added populateToolDisplay utility in packages/core/src/agent/tool-display-utils.ts to handle the transition and provide a consistent way for translators to build display metadata.
• Translators: Updated event-translator.ts and legacy-agent-session.ts to use the new display property.
• CLI Support: Updated IndividualToolCallDisplay and useAgentStream to consume and store the new display property.
• Type Safety: Renamed Message to AgentMessage in the protocol types to avoid conflicts with other Message definitions in the codebase.

Related Issues

Related to #25046 (Steps 2 & 3)

How to Validate

1. Run core tests: npm run test -w @google/gemini-cli-core
2. Run CLI tests: npm run test -w @google/gemini-cli
3. Verify type check: npm run typecheck

Pre-Merge Checklist

• Added/updated tests
• Noted breaking changes (Internal protocol change, removed displayContent)
• Validated on MacOS

[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 implements a new tool-controlled display protocol for the AgentProtocol and useAgentSession. By centralizing display logic and introducing structured types for visual metadata, this change enables tools to communicate rich results—such as diffs—directly to the frontend, replacing previous ad-hoc implementations and improving overall maintainability.

Highlights

• Protocol Enhancement: Introduced the ToolDisplay interface and related types to standardize how tools provide visual metadata and rich results to the UI.
• API Cleanup: Replaced the legacy displayContent property in ToolUpdate and ToolResponse with the new display field for better consistency.
• Centralized Utilities: Added populateToolDisplay to handle the transition logic and ensure consistent metadata construction across translators.
• Type Safety: Renamed Message to AgentMessage within the protocol types to prevent naming conflicts.

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[github-actions]

Size Change: +1.9 kB (+0.01%)

Total Size: 34.1 MB

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[gemini-code-assist]

Code Review

This pull request refactors the tool display logic by replacing the generic displayContent with a structured ToolDisplay object, facilitating better handling of diverse output types like text and diffs. The changes include new utility functions in core and updates to the CLI and UI layers to adopt the new schema. Feedback identifies potential data loss in the non-interactive CLI and stale UI states in the React hooks when handling non-text results, as well as the need to merge rather than overwrite display metadata to prevent accidental data loss.

[adamfweidman]

LGTM!