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Plan mode lets you review and refine the agent’s approach before any code changes happen. Instead of diving straight into implementation, the agent writes a plan to a file, proposes it for your review, and waits for approval.

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How It Works

  1. Toggle to Plan Mode: Press Cmd+Shift+M (Mac) or Ctrl+Shift+M (Windows/Linux), or use the mode switcher in the UI.
  2. Agent Writes Plan: In plan mode, all file edit tools (file_edit_*) are restricted to only modify the plan file. The agent can still read any file in the workspace to gather context.
  3. Propose for Review: When ready, the agent calls propose_plan to present the plan in the chat UI with rendered markdown.
  4. Edit Externally: Click the Edit button on the latest plan to open it in your preferred editor (nvim, VS Code, etc.). Your changes are automatically detected.
  5. Iterate or Execute: Provide feedback in chat, or switch to Exec mode (Cmd+Shift+M) to implement the plan.

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External Edit Detection

When you edit the plan file externally and send a message, mux automatically detects the changes and informs the agent with a diff. This uses a timestamp-based polling approach:
  1. State Tracking: When propose_plan runs, it records the plan file’s content and modification time.
  2. Change Detection: Before each LLM query, mux checks if the file’s mtime has changed.
  3. Diff Injection: If modified, mux computes a diff and injects it into the context so the agent sees exactly what changed.
This means you can make edits in your preferred editor, return to mux, send a message, and the agent will incorporate your changes.

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Plan File Location

Plans are stored in a dedicated directory: 
~/.mux/plans/<workspace-id>.md
The file is created when the agent first writes a plan and persists across sessions.

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UI Features

The propose_plan tool call in chat includes:
  • Rendered Markdown: View the plan with proper formatting.
  • Edit Button: Opens the plan file in your external editor (latest plan only).
  • Copy Button: Copy plan content to clipboard.
  • Show Text/Markdown Toggle: Switch between rendered and raw views.
  • Start Here: Replace chat history with this plan as context (useful for long sessions).

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Workflow Example

User: "Add user authentication to the app"
         │
         ▼
┌─────────────────────────────────────┐
│  Agent reads codebase, writes plan  │
│  to ~/.mux/plans/<id>.md            │
└─────────────────────────────────────┘
         │
         ▼
┌─────────────────────────────────────┐
│  Agent calls propose_plan           │
│  (plan displayed in chat)           │
└─────────────────────────────────────┘
         │
         ├─────────────────────────────────┐
         │                                 │
         ▼                                 ▼
┌─────────────────────┐       ┌─────────────────────────┐
│  User provides      │       │  User clicks "Edit"     │
│  feedback in chat   │       │  → edits in nvim/vscode │
└─────────────────────┘       └─────────────────────────┘
         │                                 │
         │                                 │ (external edits)
         │                                 ▼
         │                    ┌─────────────────────────┐
         │                    │  User sends message     │
         │                    │  → mux detects changes  │
         │                    │  → diff injected        │
         │                    └─────────────────────────┘
         │                                 │
         ▼                                 ▼
┌─────────────────────────────────────────────────────────┐
│  Agent revises plan based on feedback                   │
│  (cycles back to propose_plan)                          │
└─────────────────────────────────────────────────────────┘
         │
         │  (when satisfied)
         ▼
┌─────────────────────────────────────┐
│  User switches to Exec mode         │
│  (Cmd+Shift+M)                      │
└─────────────────────────────────────┘
         │
         ▼
┌─────────────────────────────────────┐
│  Agent implements the plan          │
└─────────────────────────────────────┘

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Customizing Plan Mode Behavior

Use scoped instructions to customize how the agent behaves in plan mode: 
## Mode: Plan

When planning:

- Focus on goals and trade-offs
- Propose alternatives with pros/cons
- Attach LoC estimates to each approach

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CLI Usage

Plan mode is also available via the CLI: 
mux run --mode plan "Design a caching strategy for the API"
See CLI documentation for more options.