| name | unison-development |
| description | Write, test, update, and repair Unison code using Unison MCP tools when available. Use when working with Unison language files (.u extension), UCM operations, Unison projects, or when `update-definitions` returns `sourceCodeUpdates` for affected definitions that no longer typecheck. An extension to the software-development skill. |
Unison Development
Extends software-development. Use the Unison MCP tools for every operation when they are available in the session. If they are missing, stop and say so rather than falling back to ad hoc UCM commands — the command line bypasses the safeguards below.
Why Unison is different
Unison stores code in the UCM codebase, not in files or Git. Two consequences drive everything else:
- The CLI and
scratch.u files are not the source of truth, so editing or running code outside the MCP tools desyncs your work from the codebase. Drive everything through the MCP tools directly — the one exception is branch creation.
- Git never holds Unison code. A git commit won't capture your changes and
commit-commands don't apply, so don't reach for them.
Work in a branch, and use fully qualified names when writing code so references resolve unambiguously.
Branch first
Before any code change, create a branch with the MCP server tool — working outside a branch mutates shared state. Use a descriptive name like extract-domain-service or fix-login-bug.
Workflow
- Explore:
view-definitions, search-definitions-by-name, list-project-definitions to understand existing code before writing.
- Typecheck:
mcp__unison__typecheck-code to validate before updating.
- Update:
mcp__unison__update-definitions to apply changes to the codebase.
- Test:
mcp__unison__run-tests to verify.
When update-definitions reports broken dependents
If the call returns sourceCodeUpdates (affected definitions that no longer typecheck), use the unison-update skill — it owns the repair loop. The risk it guards: any affected definition omitted from the next update is removed from the codebase.
Modifying abilities
Changing an ability breaks its dependents, so repair them in the same update. View the ability and its default handler, use list-definition-dependents to find every caller, and include the ability and all dependents in one update-definitions call.
Done when
- Code typechecks via the MCP tools.
- Tests pass via
mcp__unison__run-tests.
- Fully qualified names are used throughout.
