diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 7ae08c60..40462f77 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -56,3 +56,28 @@ jobs:
           report_type: test_results
           files: junit-unit.xml,junit-e2e.xml
           disable_search: true
+
+  mutation:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    name: Mutation Testing
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.4'
+          coverage: pcov
+          ini-values: zend.assertions=1
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Install fixture dependencies
+        run: composer install --prefer-dist --no-progress --working-dir=tests/Fixtures
+
+      - name: Mutate changed lines
+        run: composer mutation
diff --git a/.gitignore b/.gitignore
index f4fc7879..b92deb61 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,4 +3,5 @@
 /composer.lock
 /.phpunit.cache/
 /.deptrac.cache
+/.infection/
 /phpcs.xml
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 0cdf03fe..5a10738b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -67,6 +67,22 @@ Each incomplete statement needs its own method; two in one method confuse parser
 `//hover:marker` puts the cursor on the last member access or function call on that line.
 Use it for complete code such as `$this->method()`.
 
+### Mutation testing
+
+`composer mutation` runs [Infection](https://infection.github.io/) on the lines you changed in `src/`.
+It fails if a mutant escapes, which means neither the unit tests nor PHPStan caught it.
+CI runs the same check on pull requests.
+
+It compares your working tree against `origin/main`, so commit and fetch before running it.
+If you work from a fork, `origin` needs to be this repository.
+
+When a mutant escapes, add a test that catches it.
+If the mutation doesn't change behavior, so no test could catch it, rewrite the code so the mutation isn't possible.
+If you can't, ask in the pull request about ignoring that `Class::method` in `infection.json5`.
+Don't ignore by line number.
+
+Name tests after the behavior they check, not the mutant they catch.
+
 ## Code Reviews
 
 Reviewing code should focus on things not covered by CI or automated tooling:
diff --git a/composer.json b/composer.json
index 6ec1a2de..dcc00583 100644
--- a/composer.json
+++ b/composer.json
@@ -24,6 +24,7 @@
         "brianium/paratest": "^7",
         "composer/composer": "^2.10",
         "deptrac/deptrac": "^4.7",
+        "infection/infection": "0.35.9",
         "phpstan/phpstan": "^2.3",
         "phpstan/phpstan-deprecation-rules": "^2.0",
         "phpstan/phpstan-strict-rules": "^2.0",
@@ -50,6 +51,10 @@
         "phpstan": "phpstan analyze",
         "deptrac": "deptrac analyse --no-progress",
         "layer-coverage": "deptrac debug:unassigned",
+        "mutation": [
+            "Composer\\Config::disableProcessTimeout",
+            "infection --git-diff-lines --git-diff-base=origin/main"
+        ],
         "test": [
             "@phpstan",
             "@deptrac",
@@ -65,7 +70,8 @@
     "config": {
         "sort-packages": true,
         "allow-plugins": {
-            "dealerdirect/phpcodesniffer-composer-installer": false
+            "dealerdirect/phpcodesniffer-composer-installer": false,
+            "infection/extension-installer": false
         }
     }
 }
diff --git a/infection.json5 b/infection.json5
new file mode 100644
index 00000000..1b54fec6
--- /dev/null
+++ b/infection.json5
@@ -0,0 +1,15 @@
+{
+    "$schema": "vendor/infection/infection/resources/schema.json",
+    "source": {
+        "directories": ["src"],
+    },
+    "tmpDir": ".infection",
+    "logs": {
+        "text": ".infection/infection.log",
+    },
+    "minMsi": 100,
+    "minCoveredMsi": 100,
+    "ignoreMsiWithNoMutations": true,
+    "staticAnalysisTool": "phpstan",
+    "testFrameworkExtraArgs": "--testsuite=unit",
+}