diff --git a/.github/mutmut-coverage.rc b/.github/mutmut-coverage.rc
new file mode 100644
index 00000000000..c607df68853
--- /dev/null
+++ b/.github/mutmut-coverage.rc
@@ -0,0 +1,5 @@
+# mutmut's gather_coverage() looks covered lines up by absolute path, so the
+# repo's `relative_files = true` makes every lookup miss and mutmut generates
+# zero mutants. Point COVERAGE_RCFILE here for mutation runs only.
+[run]
+relative_files = false
diff --git a/.github/workflows/mutation-test.yml b/.github/workflows/mutation-test.yml
index 602c26a3e98..6c278b152a1 100644
--- a/.github/workflows/mutation-test.yml
+++ b/.github/workflows/mutation-test.yml
@@ -87,11 +87,20 @@ jobs:
         run: |
           uv pip uninstall pytest-retry || true
 
+      # Ends before the job's own deadline so a run that outlasts the budget is
+      # still followed by the report and upload steps. mutmut writes each
+      # mutant's result to mutants/mutmut-stats.json as it goes, so an
+      # interrupted run still scores the mutants it finished; a cancelled job
+      # skips those steps and publishes nothing at all.
       - name: Run mutmut
+        timeout-minutes: 300
         env:
           # Make the mutants/ sandbox win over site-packages on sys.path so the
           # trampolined files are imported instead of the installed copy.
           PYTHONPATH: ${{ github.workspace }}/mutants
+          # Without this mutmut finds no covered lines and generates 0 mutants.
+          # See the file itself for why.
+          COVERAGE_RCFILE: ${{ github.workspace }}/.github/mutmut-coverage.rc
         run: |
           set -o pipefail
           mkdir -p mutants
diff --git a/.gitignore b/.gitignore
index 201e02f2189..deb0acae56e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,8 @@
 tests/e2e/.fixtures/
 .venv-typecheck
 .venv_policy_test
+.venv-mutmut
+mutants/
 .env
 .claude
 CLAUDE.local.md