Output format·Security & Testing

Mutate what I changed

Run Stryker mutation testing on the source files changed in the current branch (vs origin/master) across vitest packages.

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Install this skill Read the source first Free Written by n8n-io · unverified publisher
Context cost
1.6k tokensestimated from the bundle, loaded when it triggers
Bundle
1 file · 6.4 kBtext throughout, nothing executable
Licence
Source-availablefree to use
Last change
no release on file
Servers it uses
Noneruns standalone

What it does

Run Stryker mutation testing on the source files changed in the current branch (vs origin/master) across vitest packages. One command for "did my work hold up under mutation?" before pushing. Triages which files dropped below threshold and offers to invoke n8n:mutant-fix on them. Use when the user says /mutant-diff, "mutate what I changed", "check my changes", or has just finished writing a feature and wants pre-merge feedback.

Installed, it changes the agent in these ways.

What this skill changes about the agent is not written down here yet. The listing was collected from its source, and the description is in its own SKILL.md.

Output format

Produces one artefact, exactly shaped.

securitytestingvitest

The skill itself

This is the whole product. A skill is instructions the model reads, so there is nothing behind the listing you cannot see first — the front matter loads with every session, and the body below it loads when the skill triggers.

SKILL.md6.4 kB · 127 lines
--- description: Run Stryker mutation testing on the source files changed in the current branch (vs origin/master) across vitest packages. One command for "did my work hold up under mutation?" before pushing. Triages which files dropped below threshold and offers to invoke n8n:mutant-fix on them. Use when the user says /mutant-diff, "mutate what I changed", "check my changes", or has just finished writing a feature and wants pre-merge feedback. ---
5# Mutate what I changed
6
7Closes the local dev loop. Single command to run Stryker against every changed source file the current branch touched (vs origin/master) in a mutation-eligible package, then point at any reds that need strengthening.
8
9## When to use
10
11- User says /mutant-diff, "mutate the files I changed", "check my changes", "did my tests stick"
12- Mid-feature: dev wants pre-merge feedback before pushing
13- Pre-PR: cheaper than waiting for the nightly cron
14
15**Don't** use:
16- For a single specific file (/n8n:mutant-score <path> is faster)
17- After the user already ran /n8n:mutant-fix (which calls mutant-score internally for verification — running both again is wasted compute)
18
19Changed files in **jest** packages (nodes-base, cli, db, …) and in @n8n/expression-runtime are skipped automatically — see step 1.
20
21## Inputs
22
23- **Default base**: origin/master. Override with --base <ref> if comparing against another branch (e.g. --base HEAD~5).
24- **Default scope**: packages/**/src/**/*.ts, narrowed to mutation-eligible (vitest) packages in step 1.
25
26## Steps
27
28### 1. Identify changed, eligible source files
29
30```bash
31git diff --name-only origin/master...HEAD -- 'packages/**/src/**/*.ts'
32```
33
34(... is correct — three-dot means "since the branch diverged from base," which is what we want.)
35
36If git fetch hasn't been run recently, suggest the user git fetch origin master first; otherwise the base ref is stale.
37
38Filter out:
39- **/*.d.ts (declarations, no behaviour)
40- **/*.stories.ts (Storybook scaffolding)
41- index.ts files (barrels)
42- interfaces.ts, types.ts, constants.ts (low-value, same filter the picker uses)
43- **files in non-eligible packages**: for each changed file, find its package (nearest ancestor dir with a package.json). Keep the file only if that package has a vitest.config.* **and** is not @n8n/expression-runtime (it's the isolated-vm engine — blocked on DEVP-257). Drop jest packages with a one-line note: skipped (jest): packages/cli/src/....
44
45### 2. Surface the plan to the user
46
47Print the filtered list before running anything. Each Stryker run is 1–5 minutes; the user should confirm if there are many.
48
49```
50Found N changed source files to mutate (J skipped: jest / expression-runtime):
51 - packages/workflow/src/foo.ts
52 - packages/@n8n/crdt/src/bar.ts
53 ...
54
55Estimated runtime: ~M-K minutes (M minutes minimum if every Stryker run is fast).
56Proceed? (skill default: yes if N ≤ 3, ask if N > 3)
57```
58
59If the filtered list is empty: report "No mutation-eligible source files changed vs $base — nothing to mutate." and stop. Exit cleanly.
60
61If N > 8: refuse and ask the user to narrow scope (a different base ref, or invoke per-file). Running 8+ mutations sequentially is a 30+ minute session that should be a deliberate choice.
62
63### 3. Run mutation testing per file
64
65For each file in the plan, invoke pnpm mutate <repo-relative-path> (the package is inferred from the path). Capture the score per file from the ✓ / ✗ summary line mutate.mjs prints to stderr — don't re-read summary.json (it's overwritten per run, and lives in that file's own package).
66
67After each run completes, print one line:
68
69```
70✓ packages/workflow/src/foo.ts 95.12% (39/41 killed) GREEN
71✗ packages/@n8n/crdt/src/bar.ts 54.83% (17/31 killed) RED — 13 survivors, top: ConditionalExpression, EqualityOperator
72```
73
74If a Stryker run hard-fails (exit 3, no summary.json), print ! <file> Stryker failed — see stderr and continue to the next file. Don't abort the whole batch.
75
76### 4. Summary table
77
78After all files have been mutated, print one compact table:
79
80```
81=== Mutation results: N files, M green, K red, J failed ===
82| File | Score | Verdict | Survivors |
83|---------------------------------|---------|---------|-----------|
84| packages/workflow/src/foo.ts | 95.12% | GREEN | 2 |
85| packages/@n8n/crdt/src/bar.ts | 54.83% | RED | 13 |
86| packages/workflow/src/baz.ts | n/a | FAILED | - |
87```
88
89### 5. Offer the strengthen step on the worst red file
90
91If any file came back red:
92
93> The lowest-score red file is packages/@n8n/crdt/src/bar.ts (54.83%, 13 survivors). Run /n8n:mutant-fix <file> to triage them and write assertion changes? (suggesting; don't auto-invoke)
94
95Only suggest one file at a time — n8n:mutant-fix caps at 5 survivors per invocation, and re-running this skill after edits is cheap.
96
97If everything is green: report it and stop. No follow-up needed.
98
99## Output shape
100
101Three deliverable sections per invocation:
102
1031. **Plan** (before running) — list of eligible files (+ skipped count), estimated runtime
1042. **Per-file progress** (during) — one line per file as it completes
1053. **Summary table + recommendation** (after) — compact view
106
107Don't dump full summary.json payloads — each mutate run writes one to <package>/reports/mutation/ (overwritten per run). The user can read the latest one if they want detail.
108
109## Constraints
110
111- **Vitest packages only.** Jest packages and @n8n/expression-runtime are filtered out in step 1, not mutated.
112- **Max 8 files per invocation.** Above that, ask user to narrow.
113- **Don't auto-invoke /n8n:mutant-fix.** Suggest, don't act. Same reasoning as the other skills: each pass should be a deliberate human-approved step.
114- **No commits.** Edits land in working tree; user reviews.
115- **No fabricated scores.** If a Stryker run fails, mark FAILED in the table — never guess a value.
116
117## Common follow-ups
118
119- "strengthen them all" → loop the user through /n8n:mutant-fix, one file at a time
120- "what changed?" → git diff origin/master...HEAD -- <file> for the file in question
121- "ignore <pattern>" → re-run with the user's exclude added to the filter
122
123## Related
124
125- n8n:mutant-score — single-file version of this skill
126- n8n:mutant-fix — the natural next step when reds show up
127
In the file
SKILL.md951 words
Files1
LicenceSource-available
Why you can read it

Nothing in a skill executes. The client loads the text and the model follows it, so a skill can be audited the way a runbook is — by reading it.

What it costs in context

Skills are not billed by the call. They are paid for in context: every token the instructions occupy is a token your code, your diff and your conversation cannot use. Here is what this one takes and when it takes it.

≈110
always loaded
The name and description, so the model knows the skill exists and when to reach for it.
1,490
on trigger
The instruction body, read only when the skill fires.
0.80%
of a 200k window
Ten skills this size would take about 8% of the window before you open a file.
050k100k150k200k context window

1.6k tokens, estimated from the bundle at four bytes to the token, held for the rest of the session once it triggers. Middling. Fine to keep on in a project where you use it weekly, worth unloading in one where you never do.

Servers bill, skills cost

A server charges by the month. A skill charges once per session, in context, and then keeps charging it for as long as the session lives.

Before and after

The same question, put to the same model twice: once as it comes, and once with these instructions loaded.

No worked example has been published for this skill yet.

Adoption
Installsnone yet
Ratingno reviews yet

The procedure it runs

The procedure has not been published here. It is in the skill’s own SKILL.md, which its author has not sent to the marketplace yet.

Prose, not code

These steps are written for a model to follow, not executed by a runtime. It can still be told to skip one, and it will say so when it does.

Servers it uses

None. This skill calls no MCP servers at all.

Everything it needs is in the instructions, so it works in a project with nothing connected — the model reads the file and changes how it works with what it can already reach.

It writes no files and reaches no network. All it changes is how the model reasons and writes.

What it asks for
Writes filesno
Network accessno

Read from the allowed-tools line of this skill’s own SKILL.md. A skill grants no permissions of its own — it can only ask for tools your client already has.

What it will not do

Every skill is narrow, and the useful ones say where they stop. These are the jobs this one is the wrong tool for.

What this skill is not for has not been published here. Nothing is implied by that: it is a section the author has not filled in.

What is in the bundle

1 file, 6.4 kB on disk. A bundle is text throughout: the instructions the model reads, plus the templates it fills in.

  • SKILL.md6.4 kB
What is not in it

No dependencies and nothing executable: a skill is text the agent reads, so the bundle is 1 file you can review in full before installing. The Source-available licence covers the templates and examples as well as the instructions.

Install

Installing copies the bundle into your project. Nothing runs at install time — the files sit on disk until the model reads them.

# Mutate what I changed · 1.6k tokens when loaded npx mcprush@latest skill add n8n-io/mutate-what-i-changed

Writes to .claude/skills/mutate-what-i-changed/ in the current project. Add --global to put it in your home directory instead, for every project.

Which clients pick it up on their own

A skill is a folder of text. A client with a skills folder reads it without being told; everywhere else the same text works, it is just handed to the model rather than found.

Claude Code.claude/skills/
Claude Desktop
ChatGPT
Cursor.cursor/skills/
VS Code.github/skills/
Codex CLI.agents/skills/
Gemini CLI.gemini/skills/
Grok.grok/skills/
Zed.agents/skills/
Windsurf.windsurf/skills/
Agent SDK.claude/skills/
HTTP / API
This release
Versionnot versioned
Publishedno release date on file
PriceFree
Referencen8n-io/mutate-what-i-changed

Versions

Its author publishes no version number, so there is nothing here to pin to: what you install is the folder as it stands today. Instructions change more often than APIs do — a skill can be rewritten entirely without anything it depends on moving.

v
  • No earlier releases have been published to the marketplace.
Pinning

Nothing to pin to: this skill carries no version number of its own. What you install is what the folder holds on the day you install it.

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