feat(acp): add /compress slash command

[dorukozgen]

Summary

Adds /compress as a first-class ACP slash command so longer-running ACP sessions can compact their history before hitting context-window limits. Today this works in the TUI but not over ACP, so the agent silently processes /compress as plain user text, burning tokens and producing no real compaction. This PR ports the TUI's existing command behaviour to the ACP command registry, giving ACP clients parity with the TUI in a self-contained ~90 lines of new code plus tests.

Details

The new CompressCommand mirrors packages/cli/src/ui/commands/compressCommand.ts:

• Same name and aliases: /compress, /summarize, /compact.
• Same force-compress semantics, calling geminiClient.tryCompressChat(promptId, /*force*/ true).
• Same telemetry shape coming back: originalTokenCount, newTokenCount, compressionStatus.

ACP returns the result as text inside CommandExecutionResponse.data since the protocol doesn't yet carry typed compression metadata. Each CompressionStatus enum value maps to a distinct, user-readable message so clients can render structured feedback:

• COMPRESSED → token delta line
• NOOP → "No compression needed"
• CONTENT_TRUNCATED → fall-back path with token delta
• COMPRESSION_FAILED_* → specific failure reason for each enum value
• Unknown or future status → generic failure

This is intentionally the smallest possible step toward #23945. It does not refactor acpClient.ts to use GeminiClient (the longer-term fix flagged effort/large on the issue), it does not add auto-compression checks before prompts, it does not add a token-count overflow guard, and it does not wire tryMaskToolOutputs() into the ACP path. Those are separate, independently reviewable changes.

Related Issues

Related to #23945

How to Validate

# From repo root, after `npm ci` and a one-time core build:
npm run build --workspace=@google/gemini-cli-core

# Lint the new + modified files
cd packages/cli
npx eslint src/acp/commands/compress.ts \
           src/acp/commands/compress.test.ts \
           src/acp/acpCommandHandler.ts \
           src/acp/acpCommandHandler.test.ts
# Expected: no errors.

# Run the new + extended test files
npx vitest run src/acp/commands/compress.test.ts \
              src/acp/acpCommandHandler.test.ts
# Expected: Test Files 2 passed, Tests 11 passed.

Manual smoke test against a live Gemini session over ACP:

1. Start the CLI in ACP mode: gemini --experimental-acp.
2. Run a handful of prompts to grow the conversation a bit.
3. From the connected ACP client, send /compress (or /summarize, or /compact).
4. Expected: the registry intercepts the slash command, tryCompressChat runs, and the response text reads Compressed conversation: <pre> → <post> tokens.

Edge cases the test file exercises: NOOP returns the "no compression needed" message, CONTENT_TRUNCATED surfaces the truncation fall-back, each COMPRESSION_FAILED_* enum value emits its specific reason, and a thrown error from the client is caught and reported via getErrorMessage.

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

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[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 adds the /compress slash command to the Agent Communication Protocol (ACP), allowing users to compact their conversation history and manage context-window limits effectively. By porting existing TUI compression logic to the ACP command registry, this change provides parity between the CLI's interactive mode and ACP-connected clients, while also introducing robust error handling and status reporting for the compression process.

Highlights

• New ACP Command: Introduced the /compress slash command to the ACP registry, enabling conversation history compaction over the ACP protocol.
• Command Parity: Implemented the new command with the same aliases (/summarize, /compact) and logic as the existing TUI implementation, ensuring consistent behavior across interfaces.
• Structured Feedback: Added comprehensive handling for compression statuses, providing user-readable text responses for success, no-op, truncation, and various failure scenarios.
• Testing: Added a new test suite for the CompressCommand and updated the command handler tests to verify command registration and alias resolution.

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[gemini-code-assist]

Code Review

This pull request introduces the /compress command (with /summarize and /compact aliases) to the ACP (Agent Context Protocol) implementation, allowing users to replace conversation history with a model-generated summary to manage context window limits. The changes include the new command implementation, registration in the CommandHandler, and comprehensive unit tests. The review feedback correctly identifies a potential runtime error due to missing null checks on the geminiClient and the result of tryCompressChat, recommending defensive programming practices consistent with the project's interface contracts.

[dorukozgen]

Code Review

This pull request introduces the /compress command (with /summarize and /compact aliases) to the ACP (Agent Context Protocol) implementation, allowing users to replace conversation history with a model-generated summary to manage context window limits. The changes include the new command implementation, registration in the CommandHandler, and comprehensive unit tests. The review feedback correctly identifies a potential runtime error due to missing null checks on the geminiClient and the result of tryCompressChat, recommending defensive programming practices consistent with the project's interface contracts.

Thanks for the catch. Pushed e45c8df with the optional chain on geminiClient and an explicit not-available branch, plus a test for that path. Worth flagging that the interface still declares geminiClient as non-optional in AgentLoopContext today, so this is more of a forward-looking safeguard than a current bug fix. Matching the TUI's compress pattern feels like the right call here either way.

[dorukozgen]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements the /compress command and its aliases, /summarize and /compact, within the ACP command handler. This feature enables conversation history compression via model-generated summaries to optimize context window usage. The changes include the new CompressCommand implementation, its registration in the command handler, and extensive unit tests for various success and failure states. I have no feedback to provide.

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This PR will be closed in 7 days if it remains without that designation. We encourage you to find and contribute to existing 'help wanted' issues in our backlog! Thank you for your understanding.

[gemini-cli]

This pull request is being closed as it has been open for 14 days without a 'help wanted' designation. We encourage you to find and contribute to existing 'help wanted' issues in our backlog! Thank you for your understanding.