diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d45167d..c7a47eb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,11 +25,24 @@ jobs:
           key: ${{ runner.os }}-platformio-native-${{ hashFiles('platformio.ini') }}
 
       - name: Install PlatformIO
-        run: pip install --upgrade platformio
+        run: pip install --upgrade platformio gcovr
 
       - name: Run native unit tests
         run: pio test -e native
 
+      - name: Coverage report
+        run: |
+          output=$(gcovr --root . --filter 'src/' --exclude 'src/main\.cpp' --print-summary --fail-under-line 90)
+          status=$?
+          {
+            echo '### Coverage (src/, excluding main.cpp)'
+            echo '```'
+            echo "$output"
+            echo '```'
+          } >> "$GITHUB_STEP_SUMMARY"
+          echo "$output"
+          exit $status
+
   kb2040-build:
     runs-on: ubuntu-latest
     steps:
diff --git a/CLAUDE.md b/CLAUDE.md
index abc7d07..30a786b 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -6,6 +6,7 @@
 - All new logic should have unit tests in `test/` to prevent regressions.
 - Tests run on the native PlatformIO environment (no hardware required) — keep them that way. Do not introduce test dependencies that require Arduino or physical hardware.
 - When fixing a bug, add a test that would have caught it.
+- CI enforces a minimum line-coverage threshold (via `gcovr`, currently 90%) on `src/`, excluding `main.cpp` (Arduino-only, can't run on native). Keep new logic covered well enough to not drop below that bar — don't lower the threshold in `.github/workflows/ci.yml` just to unblock a PR; that's a deliberate call for a human to make.
 
 **Bug fixes and regression test**
 - Any time you fix a bug, that bugfix should be covered by a new regression test.
diff --git a/platformio.ini b/platformio.ini
index df15e20..9cc2b93 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -29,3 +29,5 @@ platform = native
 test_build_src = yes
 build_src_filter = +<*> -<main.cpp>
 test_framework = unity
+build_flags = --coverage
+extra_scripts = pre:scripts/native_coverage_linkflags.py
diff --git a/scripts/native_coverage_linkflags.py b/scripts/native_coverage_linkflags.py
new file mode 100644
index 0000000..13f4326
--- /dev/null
+++ b/scripts/native_coverage_linkflags.py
@@ -0,0 +1,5 @@
+Import("env")
+
+# PlatformIO's `build_flags` only reach the compile step for --coverage; the
+# link step needs it too so the gcov/profiling runtime gets linked in.
+env.Append(LINKFLAGS=["--coverage"])