diff --git a/.github/workflows/php.yaml b/.github/workflows/php.yaml
index 033f923..dbdff68 100644
--- a/.github/workflows/php.yaml
+++ b/.github/workflows/php.yaml
@@ -85,3 +85,37 @@ jobs:
           # binary's GPG signature endpoint failing) must not fail the build.
           fail_ci_if_error: false
           flags: unittests
+
+  mutation-tests:
+    name: Mutation tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Create docker network
+        run: |
+          docker network create frontend
+
+      # The GITHUB_* variables are passed into the container so Infection's
+      # CI detection works: --logger-github emits inline PR annotations for
+      # escaped mutants, and the Stryker dashboard logger (configured in
+      # infection.json5) publishes the report on pushes to develop. The
+      # minimum mutation score is enforced via minCoveredMsi in
+      # infection.json5.
+      - name: Run Infection
+        env:
+          STRYKER_DASHBOARD_API_KEY: ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
+        run: |
+          docker compose run --rm phpfpm composer install
+          docker compose run --rm \
+            -e XDEBUG_MODE=coverage \
+            -e GITHUB_ACTIONS \
+            -e GITHUB_REF \
+            -e GITHUB_HEAD_REF \
+            -e GITHUB_SHA \
+            -e GITHUB_REPOSITORY \
+            -e GITHUB_EVENT_NAME \
+            -e GITHUB_RUN_ID \
+            -e GITHUB_SERVER_URL \
+            -e STRYKER_DASHBOARD_API_KEY \
+            phpfpm vendor/bin/infection --logger-github
diff --git a/.gitignore b/.gitignore
index 0d9eae0..4e5588e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,3 +21,8 @@ unit.xml
 
 # Local Claude Code instructions (not part of the published package)
 /CLAUDE.md
+
+###> infection/infection ###
+/infection.log
+/infection.html
+###< infection/infection ###
diff --git a/CHANGELOG.md b/CHANGELOG.md
index e51abb7..7c13b3b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Dev: mutation testing with [Infection](https://infection.github.io/)
+  (`task test:mutation`). The minimum mutation score is configured in
+  `infection.json5` and enforced in CI; escaped mutants are annotated inline
+  on pull requests, and results for `develop` are published to the Stryker
+  dashboard (mutation score badge in the README). No effect on the published
+  package.
+
 ### Changed
 
 - CI: bumped `codecov/codecov-action` from `v5` to `v7` (restores Codecov's
diff --git a/README.md b/README.md
index 529eae6..8911a56 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,7 @@
 [![PHP Version](https://img.shields.io/packagist/php-v/itk-dev/openid-connect-bundle.svg?style=flat-square&colorB=%238892BF)](https://www.php.net/downloads)
 [![Build Status](https://img.shields.io/github/actions/workflow/status/itk-dev/openid-connect-bundle/php.yaml?branch=develop&label=CI&logo=github&style=flat-square)](https://github.com/itk-dev/openid-connect-bundle/actions/workflows/php.yaml?query=branch%3Adevelop)
 [![Codecov Code Coverage](https://img.shields.io/codecov/c/gh/itk-dev/openid-connect-bundle?label=codecov&logo=codecov&style=flat-square)](https://codecov.io/gh/itk-dev/openid-connect-bundle)
+[![Mutation Score](https://img.shields.io/endpoint?style=flat-square&label=mutation%20score&url=https%3A%2F%2Fbadge-api.stryker-mutator.io%2Fgithub.com%2Fitk-dev%2Fopenid-connect-bundle%2Fdevelop)](https://dashboard.stryker-mutator.io/reports/github.com/itk-dev/openid-connect-bundle/develop)
 [![Read License](https://img.shields.io/packagist/l/itk-dev/openid-connect-bundle.svg?style=flat-square&colorB=darkcyan)](https://github.com/itk-dev/openid-connect-bundle/blob/master/LICENSE.md)
 [![Package downloads on Packagist](https://img.shields.io/packagist/dt/itk-dev/openid-connect-bundle.svg?style=flat-square&colorB=darkmagenta)](https://packagist.org/packages/itk-dev/openid-connect-bundle/stats)
 
@@ -472,6 +473,27 @@ task test:matrix
 This runs PHPUnit with coverage for each combination and prints a summary of
 pass/fail results.
 
+### Mutation Testing
+
+Line coverage shows which code the tests *execute*; mutation testing shows
+which code they actually *verify*. [Infection](https://infection.github.io/)
+applies small changes (mutants) to the source code — flipping a comparison,
+removing a method call — and runs the test suite against each one. If the
+tests still pass, the mutant "escaped": a potential bug the tests would not
+catch.
+
+```shell
+task test:mutation
+```
+
+The minimum mutation score (`minCoveredMsi`) is defined in `infection.json5`
+and enforced both locally and in CI — no command line flags needed. CI
+annotates escaped mutants inline on pull requests, and results for `develop`
+are published to the
+[Stryker dashboard](https://dashboard.stryker-mutator.io/reports/github.com/itk-dev/openid-connect-bundle/develop),
+which also feeds the mutation score badge above. Detailed reports are written
+to `infection.log` and `infection.html` on each run.
+
 ### PHPStan Static Analysis
 
 ```shell
@@ -510,7 +532,8 @@ Run `task --list` to see all available tasks.
 
 ## CI
 
-GitHub Actions are used to run the test suite and code style checks on all PRs.
+GitHub Actions are used to run the test suite, mutation tests and code style
+checks on all PRs.
 
 ## Versioning
 
diff --git a/Taskfile.yml b/Taskfile.yml
index 0eb711d..806f150 100644
--- a/Taskfile.yml
+++ b/Taskfile.yml
@@ -144,6 +144,11 @@ tasks:
     cmds:
       - "{{.DOCKER_COMPOSE}} exec -e XDEBUG_MODE=coverage phpfpm vendor/bin/phpunit --coverage-text --coverage-clover=coverage/unit.xml"
 
+  test:mutation:
+    desc: Run Infection mutation testing (threshold configured in infection.json5)
+    cmds:
+      - "{{.DOCKER_COMPOSE}} exec -e XDEBUG_MODE=coverage phpfpm vendor/bin/infection"
+
   test:run:
     desc: "Run tests for a PHP version and dependency set (e.g. task test:run PHP=8.4 DEPS=lowest)"
     vars:
@@ -250,3 +255,4 @@ tasks:
       - task: lint
       - task: analyze:php
       - task: test:matrix
+      - task: test:mutation
diff --git a/composer.json b/composer.json
index 45a9d7d..0c6f021 100644
--- a/composer.json
+++ b/composer.json
@@ -28,6 +28,7 @@
     "require-dev": {
         "ergebnis/composer-normalize": "^2.28",
         "friendsofphp/php-cs-fixer": "^3.11",
+        "infection/infection": "*",
         "phpstan/phpstan": "^2.1.41",
         "phpstan/phpstan-deprecation-rules": "^2.0",
         "phpstan/phpstan-phpunit": "^2.0",
@@ -51,6 +52,7 @@
     "config": {
         "allow-plugins": {
             "ergebnis/composer-normalize": true,
+            "infection/extension-installer": true,
             "symfony/runtime": true
         },
         "sort-packages": true
diff --git a/infection.json5 b/infection.json5
new file mode 100644
index 0000000..f672303
--- /dev/null
+++ b/infection.json5
@@ -0,0 +1,23 @@
+{
+    "$schema": "vendor/infection/infection/resources/schema.json",
+    "source": {
+        "directories": ["src"]
+    },
+    "threads": "max",
+    // Minimum mutation score for covered code; enforced locally and in CI.
+    // Baseline measured at 80% — ratchet up as surviving mutants are killed.
+    "minCoveredMsi": 78,
+    "logs": {
+        "text": "infection.log",
+        "html": "infection.html",
+        // Publishes results for the develop branch to the Stryker dashboard
+        // (feeds the README badge). Requires STRYKER_DASHBOARD_API_KEY; only
+        // active on CI, skipped locally and on non-matching branches.
+        "stryker": {
+            "report": "develop"
+        }
+    },
+    "mutators": {
+        "@default": true
+    }
+}