feat(mcp): forward session ID and plugin _meta to MCP tool calls

[Sathvik-1007]

Issue for this PR

Closes #6279
Closes #17084

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

MCP servers currently have no way to know which opencode session called them. This adds two things:

1. Built-in session context: opencode now always includes _meta: { sessionID } in every callTool() request. MCP servers receive the caller's session ID automatically via the standard MCP _meta field.

2. Plugin-extensible _meta: the tool.execute.before hook output now includes an optional _meta field. Plugins can add custom metadata (correlation IDs, trace context, etc.) that gets forwarded to downstream MCP servers alongside the built-in sessionID.

When no plugin sets _meta, MCP servers still receive { sessionID }. When a plugin adds fields, they merge with the built-in ones.

Files changed (3):

• packages/opencode/src/mcp/index.ts — read _meta from execute opts, forward to client.callTool()
• packages/opencode/src/session/prompt.ts — seed _meta with { sessionID }, pass through hook and into execute
• packages/plugin/src/index.ts — add _meta to tool.execute.before hook output type

How did you verify your code works?

• All 236 session tests pass (0 fail, 624 assertions)
• Typecheck: zero new errors (existing llm.ts errors are from upstream feat(llm): integrate GitLab DWS tool approval with permission system #19955, present on bare dev)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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

I found a related PR that may be a duplicate or predecessor:

Related PR Found:

• PR feat(mcp): forward plugin-set _meta to downstream MCP tool calls #21539: "feat(mcp): forward plugin-set _meta to downstream MCP tool calls" (feat(mcp): forward plugin-set _meta to downstream MCP tool calls #21539)

This PR appears to be addressing the same core feature of forwarding metadata (_meta) to MCP tool calls. PR #21624 (the current PR) seems to be an enhancement that expands on this by also including the session ID as built-in context alongside plugin-extensible metadata.

Why they're related:

• Both PRs focus on forwarding _meta to MCP tool calls
• Both involve the tool.execute.before hook modifications
• PR feat(mcp): forward session ID and plugin _meta to MCP tool calls #21624 adds automatic session ID inclusion (sessionID in _meta), while feat(mcp): forward plugin-set _meta to downstream MCP tool calls #21539 appears to focus on plugin-set metadata
• The current PR (feat(mcp): forward session ID and plugin _meta to MCP tool calls #21624) references issues Feature: Pass session identifier to MCP servers #6279 and [FEATURE]: Support _meta when calling downstream MCP tools #17084, which are likely the base requirements

You may want to check if PR #21539 is still open and whether #21624 supersedes it or if they should be combined.

[Sathvik-1007]

Relationship to #21539

This PR supersedes #21539. Same 3 files, same approach. Diff comparison:

• mcp/index.ts — identical changes (read _meta from opts, forward to callTool())
• plugin/src/index.ts — identical (_meta added to hook output type)
• prompt.ts — only difference: feat(mcp): forward plugin-set _meta to downstream MCP tool calls #21539 leaves _meta empty (plugin must set it). This PR seeds _meta: { sessionID: ctx.sessionID } so every MCP server gets session context built-in, no plugin needed. Plugins can still extend it.

Net: this is #21539 + built-in sessionID. If this merges, #21539 can close.

cc @moghtader — credit to you for the plugin _meta hook approach, adopted here.