feat(acp): add session/list and session/fork support

[remorses]

Closes #7978

Summary

• Add session/list (unstable) with cursor-based pagination
• Add session/fork (unstable) to fork existing sessions
• Update @agentclientprotocol/sdk to 0.12.0

Smoke test script: https://gist.github.com/remorses/ddec691b72cef0ae128dc440505596f6

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[Copilot]

Pull request overview

This PR adds support for two ACP draft/unstable methods: session/list for listing existing sessions with cursor-based pagination, and session/fork for creating new sessions branched from existing ones. The implementation includes updating the SDK from version 0.5.1 to 0.12.0 to support these new capabilities.

Changes:

• Added session/list (unstable) with cursor-based pagination sorted by most recently updated
• Added session/fork (unstable) to create new sessions from existing ones
• Updated @agentclientprotocol/sdk dependency from 0.5.1 to 0.12.0
• Refactored session replay logic into a reusable private method
• Improved defensive programming in the providers call with optional chaining

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 5 comments.

File	Description
packages/opencode/src/acp/agent.ts	Implements unstable_listSessions and unstable_forkSession methods, adds sessionCapabilities to initialization, refactors replay logic, and improves error handling
packages/opencode/package.json	Updates @agentclientprotocol/sdk from 0.5.1 to 0.12.0
bun.lock	Updates lock file with new SDK version and peer dependency changes

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[nonsleepr]

Hey @remorses! 👋

I've been working on the same session/list feature and noticed your PR. Rather than duplicate efforts, I'd love to collaborate.

What I have:

• 113 tests covering ACP functionality including session/list pagination edge cases
• Upgraded to SDK 0.13.0 (vs 0.12.0 in this PR)

One design difference to discuss:

Your implementation loads all sessions then slices in memory:

const all = await Session.list({ cwd: ... })
return all.slice(start, start + limit)

My implementation uses backend pagination via the SDK's start/limit params:

const result = await Session.list({ cwd, start, limit })

The backend approach scales better for users with many sessions (doesn't load everything into memory).

My branch: https://github.com/nonsleepr/opencode/tree/feat/acp-session-list

Happy to:

1. Contribute my test suite to this PR
2. Help update to SDK 0.13.0
3. Discuss the pagination approach with you/maintainers

Let me know if you'd like to collaborate! If you're busy, I can also open a separate PR and let maintainers decide which approach to merge.

[remorses]

Hey @nonsleepr, thanks for the feedback! We updated the implementation to use cursor-based pagination following the ACP spec. Also added session/resume support.

Regarding your test suite - we took a look but decided not to include them since they rely heavily on mocks, which doesn't align with our testing approach.

We'll keep this PR moving forward. Feel free to review the latest changes!