---

name: Unison Development description: Write, test, and update Unison code using MCP tools. Use when working with Unison language files (.u extension), UCM operations, or Unison projects. Enforces TDD with strict typechecking via development skill. Used as a part of the XP skill.

Unison Development

• Uses development skill for TDD workflow, enhanced with Unison-specific tooling.
• Use the unison MCP server commands for all operations.

Core Principles

1. NEVER run UCM commands on the command line — use MCP tools only
2. Code is stored by the Unison Code Manager, not Git
3. TDD is mandatory — be aware UCM may enter "Handling typecheck errors after update"
4. Always use fully qualified names in scratch.u
5. Never create multiple scratch files
6. ALWAYS wait for user confirmation after update before continuing
7. ALWAYS Typecheck code with unison MCP server before adding to scratch file
8. Write code to the scratch.u file in the current directory after it typechecks
9. After successful update, you may delete the scratch file

Workflow

1. Research & Understanding

Use MCP tools to explore before writing:

• view-definitions for existing implementations
• search-definitions-by-name for related functions
• docs for library functions

2. Branch

Ask the user to create feature branch before beginning.

3. Clear scratch files

Ask the user if they want scratch.u deleted.

4. Write Tests First

Use test.verify, labeled, and ensureEqual:

CopyprojectName.module.tests.featureTest : '{IO, Exception} [Result]
projectName.module.tests.featureTest = do
  test.verify do
    labeled "Description" do
      use test ensureEqual
      result = performOperation testData
      ensureEqual result expectedValue

5. Typecheck Incrementally

Copymcp__unison__typecheck-code with {"text": "code here"}

Iterate until clean — fix type errors, add imports, verify effects.

6. Add to scratch.u with Fully Qualified Names

• WRONG: foo : Text -> ...
• CORRECT: projectName.module.foo : Text -> ...

Why: Without Fully Qualified Names, Unison creates new function instead of modifying existing functions.

Typecheck output indicators:

• + (added) — new definition
• ~ (modified) — updated existing

Verify you see ~ for modifications!

7. Final Typecheck

Copymcp__unison__typecheck-code with {"filePath": "/path/to/scratch.u"}

8. UPDATE MODE: Handling Typecheck Errors

If UCM adds this comment after update:

Copy-- The definitions below no longer typecheck with the changes above.
-- Please fix the errors and try `update` again.

CRITICAL:

• DO NOT delete functions from scratch.u — they will be removed from codebase
• Repair broken code, typechecking as you go
• Ask user to verify via UCM output
• After successful update, you may remove code from scratch.u

Success Criteria

• All code typechecks successfully
• Tests written before implementation
• Fully qualified names in scratch.u
• Modified functions show ~ not +
• Comprehensive test coverage

Modifying Abilities

When modifying abilities it is easier to modify the ability first, ask the user to update in the UCM which will result in an update branch, fix the code in the update branch, then ask the user to update.