---

name: preview description: "See what breaks BEFORE you edit. Type errors, affected callers, required tests." allowed-tools: mcp__preview__preview_edit

preview

Know the consequences of your changes before you make them. The ultimate safety net.

The Problem

You're about to change a function. Will it break types? Who calls it? What tests need to run?

Without preview:

1. Make the change
2. Run type checker - find errors
3. Fix errors
4. Run tests - more failures
5. Fix those too
6. Repeat...

With preview - know all this BEFORE you edit.

First: preview_edit

Copypreview_edit({
  file: 'src/api.ts',
  edit_type: 'symbol',
  symbol: 'fetchUser',
  new_content: 'async function fetchUser(id: string, options?: RequestOptions): Promise<User> { ... }'
})

Returns:

• Type errors - Will this break the build?
• Affected callers - Who calls this function?
• Related tests - What tests should you run?
• Impact summary - How risky is this change?

Edit Types

Why This Wins

When to Use

Before Risky Changes

Copy// Changing a widely-used function? Preview first.
preview_edit({
  file: 'src/core/utils.ts',
  edit_type: 'symbol',
  symbol: 'formatDate',
  new_content: '...',
  check_types: true,
  analyze_callers: true
})

Before Signature Changes

Copy// Adding a required parameter? See who breaks.
preview_edit({
  file: 'src/api.ts',
  edit_type: 'symbol',
  symbol: 'createUser',
  new_content: 'async function createUser(data: UserData, options: CreateOptions) { ... }'
})

Before Refactoring

Copy// Want to simplify a function? Know the impact.
preview_edit({
  file: 'src/handlers.ts',
  edit_type: 'symbol',
  symbol: 'processRequest',
  new_content: '...',
  find_tests: true
})

What You Get

Type Errors (Predicted)

Copy- src/routes.ts:45 - Argument of type '{}' is not assignable to parameter 'options'
- src/handlers.ts:23 - Property 'timeout' is missing in type 'RequestOptions'

Affected Callers

Copy- handleLogin (src/auth.ts:34) - uses fetchUser
- getUserProfile (src/profile.ts:12) - uses fetchUser
- testUserFetch (tests/api.test.ts:45) - uses fetchUser

Related Tests

Copy- tests/api.test.ts (high confidence - naming convention)
- tests/integration/user.test.ts (high confidence - imports source)

Options

The Workflow

1. Think - "I want to change this function"
2. Preview - preview_edit({ ... }) - see consequences
3. Decide - Is this change safe? What needs updating?
4. Edit - Make the change with confidence
5. Verify - Run the tests you now know to run

Integration

Use with other tools:

• After preview → syntax.edit_symbol to make the change
• After preview → test-runner.run_related_tests to verify
• After preview → types.check_file to confirm no errors

Not a Replacement

Preview shows you consequences. You still need to:

• syntax.edit_symbol - actually make the change
• types.check_file - verify after editing
• test-runner.run_tests - run the tests

Preview is your pre-flight check. The other tools are the flight.