🤖 feat: copy plan file when forking workspace

[ThomasK33]

Copy plan file when forking workspace so the forked workspace can continue working with the same plan context.

Changes:

• Add copyPlanFile helper in src/node/utils/runtime/helpers.ts (mirrors existing movePlanFile pattern)
• Call copyPlanFile in workspaceService.fork() after session files are copied

---

📋 Implementation Plan

Plan: Copy Plan File on Workspace Fork

Summary

Add functionality to copy the source workspace's plan file when forking, allowing the forked workspace to continue working with the same plan context.

Problem

When a workspace is forked via /fork, the following files are copied:

• chat.jsonl (chat history)
• partial.json (partial streaming state)
• session-usage.json (usage tracking)

However, the plan file (~/.mux/plans/{project}/{workspace}.md) is not copied. This means the forked workspace starts without the plan context, even though it inherits the chat history where the plan was discussed.

Solution

Add a copyPlanFile helper function and invoke it during the fork operation in workspaceService.fork().

Implementation

1. Add copyPlanFile helper in src/node/utils/runtime/helpers.ts

Create a new function following the same pattern as movePlanFile:

/**
 * Copy a plan file from one workspace to another (e.g., during fork).
 * Silently succeeds if source file doesn't exist.
 */
export async function copyPlanFile(
  runtime: Runtime,
  sourceWorkspaceName: string,
  targetWorkspaceName: string,
  projectName: string
): Promise<void> {
  const sourcePath = getPlanFilePath(sourceWorkspaceName, projectName);
  const targetPath = getPlanFilePath(targetWorkspaceName, projectName);

  try {
    await runtime.stat(sourcePath);
    await execBuffered(runtime, `cp "${sourcePath}" "${targetPath}"`, { cwd: "/tmp", timeout: 5 });
  } catch {
    // No plan file to copy, that's fine
  }
}

2. Update src/node/services/workspaceService.ts

1. Add import for copyPlanFile:

   import { movePlanFile, copyPlanFile } from "@/node/utils/runtime/helpers";

2. Call copyPlanFile in the fork() method after successfully copying session files (~line 924):

   // Copy plan file if it exists (uses workspace name, not ID)
   await copyPlanFile(runtime, sourceMetadata.name, newName, projectName);

File Changes

File	Change
src/node/utils/runtime/helpers.ts	Add copyPlanFile function (~15 LoC)
src/node/services/workspaceService.ts	Import + call copyPlanFile (~3 LoC)

Net LoC estimate: ~+18 lines

Testing

The existing test patterns for movePlanFile in rename operations provide a template. The change is straightforward:

• copyPlanFile follows the same error-handling pattern as movePlanFile
• Silently succeeds if no plan file exists (idempotent)
• Uses runtime abstraction, so works for both local and SSH runtimes

Manual verification:

1. Create a workspace with a plan
2. Fork it with /fork new-name
3. Verify the new workspace has a copy of the plan file at ~/.mux/plans/{project}/new-name.md

Edge Cases

• No plan file exists: Silent success (matches movePlanFile behavior)
• Target file already exists: cp will overwrite, which is correct for forks
• SSH runtime: Works via runtime abstraction (same as existing movePlanFile)

---

Generated with mux • Model: anthropic:claude-opus-4-5 • Thinking: high

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