mutation-testing

Name: mutation-testing
Author: ThibautBaissac

635stars79forks

How to install mutation-testing?

thibautbaissac/rails_ai_agents/mutation-testing

$npx -y skills add ThibautBaissac/rails_ai_agents --skill mutation-testing

Installs into the current project.

Summary

Run mutant, read mutation reports, fix alive mutations, and verify coverage. Use when running mutation testing, responding to alive mutations, or improving test quality. Triggers: "mutation testing", "mutant", "alive mutation", "mutation coverage". WHEN NOT: Writing tests from sc

Files · 1

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SKILL.md

1	# Mutation Testing with Mutant
2
3	## When to Activate
4
5	- The user asks to run mutation testing.
6	- The user has alive mutations to fix.
7	- The user asks to verify mutation coverage on a subject.
8
9	## When Not to Use
10
11	- The task does not involve mutation testing.
12
13	## Inputs
14
15	- Alive mutation output to act on (paste the `evil:` block).
16	- Optional: subject expression to scope the run.
17
18	## Outputs (Fixed Order)
19
20	1. Mutation results (alive count, coverage percentage).
21	2. Clear action for each alive mutation: add test or simplify code.
22
23	---
24
25	## Reading Mutation Output
26
27	An alive mutation looks like:
28
29	```text
30	evil:YourClass#method:YourClass#method:lib/your_class.rb:42:abc12
31	@@ -1,3 +1,3 @@
32	def method
33	- @value >= threshold
34	+ @value > threshold
35	end
36	```
37
38	- `evil` means no test killed this mutation.
39	- The diff shows original (`-`) and mutated (`+`) code.
40
41	---
42
43	## Reporting Format (BLUF)
44
45	Lead with the verdict — bottom line up front:
46
47	If unkillable:
48
49	> Unkillable. Both forms are equivalent because \[reason\].
50	> Add to ignore list.
51
52	If killable:
53
54	> Killable.
55	>
56	> Option A — add test:
57	>
58	> ```diff
59	> (test diff)
60	> ```
61	>
62	> Option B — simplify code:
63	>
64	> ```diff
65	> (source diff)
66	> ```
67
68	Always present both options so the user can choose. Include evidence: if
69	you cannot think of a test that would kill the mutation, say so — that is
70	valuable signal toward unkillable.
71
72	---
73
74	## Usage
75
76	### 1. Run Mutant
77
78	```bash
79	bundle exec mutant run --fail-fast
80	```
81
82	When the subject is known, scope the run to avoid testing unrelated subjects:
83
84	```bash
85	bundle exec mutant run --fail-fast 'Entities::CreateService#call'
86	```
87
88	If the command succeeds, coverage is 100% — done.
89	If it fails, find the `evil:` line in the output — it has the subject
90	name, file path, and line number. The diff block immediately after shows
91	the original and mutated code.
92
93	### 2. Investigate
94
95	Read the source file and existing spec file for the subject.
96	Ask: "Is the mutated code acceptable for all valid inputs?"
97
98	### 3. Decide and Act
99
100	- Add a test when the mutated code is wrong but tests do not prove it.
101	- Simplify the code when the mutated code is correct for all valid inputs.
102
103	Do not change both code and tests in the same commit. If both need
104	changing, commit the test first, then simplify the code in a second commit.
105
106	### 4. Re-run and Verify
107
108	Re-run mutant (step 1) until 100%. If the same mutation survives after
109	2 attempts, evaluate whether it is unkillable.
110
111	```bash
112	bundle exec rspec # full suite must pass
113	```
114
115	### 5. Commit
116
117	Follow the project's conventional commits format. The commit body must
118	explain which mutation survived, why the test kills it or why the
119	simplification is correct.
120
121	---
122
123	## Fixing Alive Mutations
124
125	### When to Add a Test
126
127	Add a test when the mutated code is wrong but the existing tests do not
128	prove it. The test must pass with the original code and fail with the
129	mutated code. Common reasons a test is missing:
130
131	- Only one value of a boolean/flag is tested — add the other.
132	- A collection has one element, so `next` and `break` behave the same —
133	add a test with multiple elements.
134	- Two objects return the same value in test data, hiding which one the
135	code actually uses — add test data where they differ.
136	- A method has a default parameter, and all tests pass the argument
137	explicitly — add a test that omits the argument.
138
139	### When to Simplify Code
140
141	Simplify when the mutated code is correct for all valid inputs. Apply the
142	mutation's change directly to the source code — do not restructure or
143	rewrite, just accept the mutated form.
144
145	Mutant encodes the principle of least power: use the most constrained
146	primitive that satisfies the requirement. When Ruby offers multiple methods
147	that overlap in behavior but differ in power, mutant replaces the more
148	powerful one with the less powerful one. If tests still pass, you did not
149	need the extra power — accept the simpler form.
150
151	Before accepting a simplification, verify the mutation preserves behavior
152	across the method's full input domain and all call sites.
153
154	### Simplification Trap: Syntax Rewriting
155
156	Do not rewrite code to eliminate a mutation axis without first ensuring the
157	expression has test coverage. Changing syntax may make the mutation
158	disappear without proving the code is correct. The correct sequence:
159
160	1. Add test coverage for the expression.
161	2. Apply the simplification mutant suggests.

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mutation-testing

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View on GitHub

How to install mutation-testing?

thibautbaissac/rails_ai_agents/mutation-testing

$npx -y skills add ThibautBaissac/rails_ai_agents --skill mutation-testing

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Mutation Testing with Mutant
2
3	## When to Activate
4
5	- The user asks to run mutation testing.
6	- The user has alive mutations to fix.
7	- The user asks to verify mutation coverage on a subject.
8
9	## When Not to Use
10
11	- The task does not involve mutation testing.
12
13	## Inputs
14
15	- Alive mutation output to act on (paste the `evil:` block).
16	- Optional: subject expression to scope the run.
17
18	## Outputs (Fixed Order)
19
20	1. Mutation results (alive count, coverage percentage).
21	2. Clear action for each alive mutation: add test or simplify code.
22
23	---
24
25	## Reading Mutation Output
26
27	An alive mutation looks like:
28
29	```text
30	evil:YourClass#method:YourClass#method:lib/your_class.rb:42:abc12
31	@@ -1,3 +1,3 @@
32	def method
33	- @value >= threshold
34	+ @value > threshold
35	end
36	```
37
38	- `evil` means no test killed this mutation.
39	- The diff shows original (`-`) and mutated (`+`) code.
40
41	---
42
43	## Reporting Format (BLUF)
44
45	Lead with the verdict — bottom line up front:
46
47	If unkillable:
48
49	> Unkillable. Both forms are equivalent because \[reason\].
50	> Add to ignore list.
51
52	If killable:
53
54	> Killable.
55	>
56	> Option A — add test:
57	>
58	> ```diff
59	> (test diff)
60	> ```
61	>
62	> Option B — simplify code:
63	>
64	> ```diff
65	> (source diff)
66	> ```
67
68	Always present both options so the user can choose. Include evidence: if
69	you cannot think of a test that would kill the mutation, say so — that is
70	valuable signal toward unkillable.
71
72	---
73
74	## Usage
75
76	### 1. Run Mutant
77
78	```bash
79	bundle exec mutant run --fail-fast
80	```
81
82	When the subject is known, scope the run to avoid testing unrelated subjects:
83
84	```bash
85	bundle exec mutant run --fail-fast 'Entities::CreateService#call'
86	```
87
88	If the command succeeds, coverage is 100% — done.
89	If it fails, find the `evil:` line in the output — it has the subject
90	name, file path, and line number. The diff block immediately after shows
91	the original and mutated code.
92
93	### 2. Investigate
94
95	Read the source file and existing spec file for the subject.
96	Ask: "Is the mutated code acceptable for all valid inputs?"
97
98	### 3. Decide and Act
99
100	- Add a test when the mutated code is wrong but tests do not prove it.
101	- Simplify the code when the mutated code is correct for all valid inputs.
102
103	Do not change both code and tests in the same commit. If both need
104	changing, commit the test first, then simplify the code in a second commit.
105
106	### 4. Re-run and Verify
107
108	Re-run mutant (step 1) until 100%. If the same mutation survives after
109	2 attempts, evaluate whether it is unkillable.
110
111	```bash
112	bundle exec rspec # full suite must pass
113	```
114
115	### 5. Commit
116
117	Follow the project's conventional commits format. The commit body must
118	explain which mutation survived, why the test kills it or why the
119	simplification is correct.
120
121	---
122
123	## Fixing Alive Mutations
124
125	### When to Add a Test
126
127	Add a test when the mutated code is wrong but the existing tests do not
128	prove it. The test must pass with the original code and fail with the
129	mutated code. Common reasons a test is missing:
130
131	- Only one value of a boolean/flag is tested — add the other.
132	- A collection has one element, so `next` and `break` behave the same —
133	add a test with multiple elements.
134	- Two objects return the same value in test data, hiding which one the
135	code actually uses — add test data where they differ.
136	- A method has a default parameter, and all tests pass the argument
137	explicitly — add a test that omits the argument.
138
139	### When to Simplify Code
140
141	Simplify when the mutated code is correct for all valid inputs. Apply the
142	mutation's change directly to the source code — do not restructure or
143	rewrite, just accept the mutated form.
144
145	Mutant encodes the principle of least power: use the most constrained
146	primitive that satisfies the requirement. When Ruby offers multiple methods
147	that overlap in behavior but differ in power, mutant replaces the more
148	powerful one with the less powerful one. If tests still pass, you did not
149	need the extra power — accept the simpler form.
150
151	Before accepting a simplification, verify the mutation preserves behavior
152	across the method's full input domain and all call sites.
153
154	### Simplification Trap: Syntax Rewriting
155
156	Do not rewrite code to eliminate a mutation axis without first ensuring the
157	expression has test coverage. Changing syntax may make the mutation
158	disappear without proving the code is correct. The correct sequence:
159
160	1. Add test coverage for the expression.
161	2. Apply the simplification mutant suggests.

People who liked this also viewed

6 picks

Type