Implement session forking functionality

[shuv1337]

Problem/Context

The OpenCode TUI and Desktop/Web UI support session forking - the ability to create a new session that branches off from an existing session at a specific message. This feature is currently missing from the OpenCode Web application, limiting users' ability to explore alternative conversation paths or restart from specific points in their sessions.

Acceptance Criteria

• Add forkSession API endpoint to opencode-http-api.ts
• Add forkSession server function to opencode-server-fns.ts
• Add forkSession method to openCodeService in opencode-client.ts
• Add UI component/trigger for session forking (e.g., button on messages)
• Handle navigation to newly forked session
• Test forking at different message points in a session
• Error handling for invalid session IDs or message IDs

Implementation Details

Backend API Integration

Based on the sst/opencode repository, the fork API is available at:

• Endpoint: POST /session/:id/fork
• Parameters:
  • sessionID (path param): The source session to fork
  • messageID (optional, body): The message to fork from (if not provided, forks from latest)

Reference Implementation (sst/opencode):

// Server endpoint: /session/:id/fork
export const fork = fn(
  z.object({
    sessionID: Identifier.schema("session"),
    messageID: Identifier.schema("message").optional(),
  }),
  async (input) => {
    const session = await createNext({
      directory: Instance.directory,
    })
    const msgs = await messages(input.sessionID)
    for (const msg of msgs) {
      if (input.messageID && msg.info.id >= input.messageID) break
      // Copy messages to new session
    }
    return session
  }
)

Files to Modify

1. src/lib/opencode-http-api.ts - Add forkSession function
2. src/lib/opencode-server-fns.ts - Add server function wrapper
3. src/lib/opencode-client.ts - Add service method
4. UI Components - Add fork button/trigger to message components

Integration Points

• Fork button should be available on each message in the conversation
• When forking from a specific message, only messages up to that point are copied
• User should be automatically navigated to the new forked session
• Consider adding a visual indicator that a session was forked from another

Reference Implementations

• sst/opencode fork implementation: https://github.com/sst/opencode/blob/dev/packages/opencode/src/session/index.ts#L123
• Session fork API endpoint: https://github.com/sst/opencode/blob/dev/packages/opencode/src/server/server.ts#L538
• SDK types for fork: https://github.com/sst/opencode/blob/dev/packages/sdk/js/src/gen/types.gen.ts#L1756

Tasks

• Research existing message UI components to determine optimal fork trigger placement
• Implement backend API integration for session forking
• Add fork functionality to the OpenCode service layer
• Design and implement UI components for session forking
• Add proper error handling and user feedback
• Test forking functionality across different scenarios
• Update TypeScript types as needed
• Consider adding session history/parentage tracking in UI