Mutation Testing

Mutation Testing Skill

This skill runs mutation testing end-to-end: generates a coverage report, groups functions, shells out to claude -p to write tests, verifies mutations are killed, and creates draft PRs with Linear issues.

Prerequisites

• A running nREPL connected to the Metabase dev environment
• LINEAR_API_KEY environment variable set with a valid Linear personal API key
• gh CLI authenticated with GitHub
• claude CLI available on PATH

Reference Files

• dev/src/dev/coverage.clj — mutation testing engine (generates reports, runs mutations)
• dev/src/dev/mutation_testing.clj — orchestration, Linear API, PR helpers

Invocation

The argument is a Clojure namespace, optionally followed by a base branch:

/mutation-testing metabase.lib.order-by
/mutation-testing metabase.lib.order-by --base-branch release-x.52.x
/mutation-testing metabase.lib.order-by --project-id abc-123
/mutation-testing metabase.lib.order-by --base-branch release-x.52.x --project-id abc-123

• --base-branch: defaults to the value in config (set via set-config!), or "master" if not configured.
• --project-id: if provided, issues are added to this existing Linear project instead of creating a new one.

Steps

1. Parse arguments

Parse $ARGUMENTS to extract:

• namespace (required) — the first positional argument
• --base-branch (optional) — if present, the next argument is the base branch name
• --project-id (optional) — if present, the next argument is an existing Linear project ID

2. Load and configure

(require '[dev.mutation-testing :as mut-test] :reload)
(require '[dev.coverage :as cov] :reload)

If this is the first invocation (or the REPL was restarted), set up the Linear team:

(mut-test/list-teams!)
(mut-test/set-config! {:team-id "<id>"})

3. Run

(mut-test/run! '<target-ns>)
;; Or with options:
(mut-test/run! '<target-ns> {:base-branch "release-x.52.x"})
(mut-test/run! '<target-ns> {:project-id "abc-123"})
(mut-test/run! '<target-ns> {:base-branch "release-x.52.x" :project-id "abc-123"})

Pass the opts map with :base-branch and/or :project-id if those flags were provided in the arguments.

This will:

1. Generate a baseline mutation testing report
2. Create a Linear project for the namespace
3. Group functions by coverage relationships
4. For each group: create branch → invoke Claude to write tests → verify → retry if needed → commit & push → create Linear issue → create draft PR
5. Print a summary with PR links

4. Handle failures

If a group fails mid-processing, run! catches the error and continues to the next group. To retry a single group manually:

;; Get the parsed namespace info
(def parsed (mut-test/parse-namespace '<target-ns>))

;; Run coverage to get the data
(def coverage-results (cov/test-namespace (:target-ns parsed) [(:test-ns parsed)]))

;; Group and find the one you want
(def groups (mut-test/group-functions coverage-results))

;; Process just that group
(mut-test/process-group! parsed (nth groups <index>))

5. Verify the prompt (dry run)

To inspect what Claude will see without invoking it:

(def parsed (mut-test/parse-namespace '<target-ns>))
(def coverage-results (cov/test-namespace (:target-ns parsed) [(:test-ns parsed)]))
(def groups (mut-test/group-functions coverage-results))
(println (mut-test/build-test-prompt
           (merge (select-keys parsed [:target-ns :test-ns :source-path :test-path])
                  (select-keys (first groups) [:fn-names :mutations]))))

Configuration

• Team ID: Set once per REPL session via (mut-test/list-teams!) and (mut-test/set-config! {:team-id "..."})
• Project ID: Set automatically when run! calls create-project-for-namespace!
• Base Branch: Defaults to "master". Override via (mut-test/set-config! {:base-branch "release-x.52.x"}) or pass {:base-branch "..."} as the second arg to run!
• Project ID: If set (via set-config! or run! opts), run! reuses the existing Linear project instead of creating a new one