fix: cache test and lint build artifacts

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- save the GitHub cache after testing and linting so their build artifacts can be reused, preserving caching after test or lint failures and skipping saves on an exact cache hit
+
 ## v1.6.1 - 2026-10-02
 
 ### Changed

README.md

@@ -33,6 +33,9 @@ however if you encounter an issue when using a different runner, please still op
 
 ## Caching `.lake` directory with GitHub's `actions\cache`
 By default, `lean-action` uses [`actions\cache`](https://github.com/actions/cache) to cache the `.lake` directory and speed up builds.
+The cache is saved after building, testing, and linting, so it includes artifacts produced by all three steps.
+A test or lint failure still allows completed build artifacts to be cached; setup or build failures do not.
+Saving is skipped when the exact cache key was restored.
 
 > [!NOTE]
 GitHub caching is distinct from Mathlib caching with `lake exe cache get`

action.yml

@@ -261,6 +261,7 @@ runs:
       working-directory: ${{ inputs.lake-package-directory }}
 
     - uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
+      id: cache-restore
       if: ${{ inputs.use-github-cache == 'true' }}
       with:
         path: ${{ inputs.lake-package-directory }}/.lake
@@ -302,12 +303,6 @@ runs:
       shell: bash
       working-directory: ${{ inputs.lake-package-directory }}
 
-    - uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
-      if: ${{ inputs.use-github-cache == 'true' }}
-      with:
-        path: ${{ inputs.lake-package-directory }}/.lake
-        key: lake-${{ runner.os }}-${{ runner.arch}}-${{ hashFiles(format('{0}/lean-toolchain', inputs.lake-package-directory)) }}-${{ hashFiles(format('{0}/lake-manifest.json', inputs.lake-package-directory)) }}-${{ github.sha }}
-
     - name: test ${{ github.repository }}
       id: test
       if: ${{ steps.config.outputs.run-lake-test == 'true'}}
@@ -331,6 +326,17 @@ runs:
       shell: bash
       working-directory: ${{ inputs.lake-package-directory }}
 
+    - uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
+      # Keep successful build artifacts even when tests or linting fail.
+      # Failures before those steps must still prevent saving the cache.
+      if: >-
+        ${{ !cancelled() && inputs.use-github-cache == 'true' &&
+            steps.cache-restore.outputs.cache-hit != 'true' &&
+            (success() || steps.test.outcome == 'failure' || steps.lint.outcome == 'failure') }}
+      with:
+        path: ${{ inputs.lake-package-directory }}/.lake
+        key: lake-${{ runner.os }}-${{ runner.arch}}-${{ hashFiles(format('{0}/lean-toolchain', inputs.lake-package-directory)) }}-${{ hashFiles(format('{0}/lake-manifest.json', inputs.lake-package-directory)) }}-${{ github.sha }}
+
     - name: check reservoir eligibility
       if: ${{ inputs.check-reservoir-eligibility == 'true' }}
       # Passes in the private status, number of stars, and license id of the repository to check_reservoir_eligibility.sh script