diff --git a/.gitea/workflows/pr.yaml b/.gitea/workflows/pr.yaml
new file mode 100644
index 0000000..f51ec9a
--- /dev/null
+++ b/.gitea/workflows/pr.yaml
@@ -0,0 +1,17 @@
+name: pr
+on:
+  pull_request:
+jobs:
+  contract:
+    runs-on: fritzlab
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: validate composite contract without registry operations
+        run: |
+          python3 -m venv /tmp/image-push-tests
+          /tmp/image-push-tests/bin/pip install --quiet -r tests/requirements.txt
+          /tmp/image-push-tests/bin/python -m unittest discover -s tests
+          git diff --check
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c18dd8d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+__pycache__/
diff --git a/README.md b/README.md
index ecd0450..c947caf 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,7 @@ local Docker daemon, image-push uploads it.
 | `org` | yes | — | Gitea org for package API (`fritzlab`, `dns`). |
 | `name` | yes | — | Package name as registered in the registry. |
 | `latest` | no | `true` | Also push a `:latest` tag. |
+| `prune` | no | `true` | Prune old numeric registry tags. Set `false` for retained or independently pinned artifacts. |
 | `keep` | no | `3` | Numeric tags to retain. Older are deleted. |
 
 ## Behavior
@@ -38,6 +39,16 @@ local Docker daemon, image-push uploads it.
 1. `docker login code.fritzlab.net` as `ci-bot`.
 2. `docker push <image>:<tag>`.
 3. If `latest=true`, also `docker push <image>:latest`.
-4. Prune: list numeric tags from Gitea package API, keep the newest `keep`,
+4. If `prune=true`, list numeric tags from Gitea package API, keep the newest `keep`,
    delete the rest. Failures here do not fail the workflow
    (`continue-on-error: true`).
+
+`prune` accepts only the strings `true` and `false`; invalid input fails before
+registry login or push. With `false`, remote package listing and deletion are
+skipped. Image upload and local Docker cleanup remain unchanged. The action does
+not discover deployment pins; owners choosing pruning must account for that
+retention contract.
+
+Run `python3 -m unittest discover -s tests` with the pinned test dependencies in
+`tests/requirements.txt`. Tests inspect the composite action and execute only
+its credential-free validation step; no registry or package deletion runs.
diff --git a/action.yaml b/action.yaml
index 0fbd486..17d2d36 100644
--- a/action.yaml
+++ b/action.yaml
@@ -21,6 +21,10 @@ inputs:
     description: Also push a :latest tag
     required: false
     default: 'true'
+  prune:
+    description: Prune old numeric registry tags; false retains every remote tag
+    required: false
+    default: 'true'
   keep:
     description: Numeric tags to retain after prune; older ones are deleted
     required: false
@@ -28,6 +32,16 @@ inputs:
 runs:
   using: composite
   steps:
+    - name: Validate pruning policy
+      shell: bash
+      env:
+        PRUNE: ${{ inputs.prune }}
+      run: |
+        case "$PRUNE" in
+          true|false) ;;
+          *) echo "prune must be true or false" >&2; exit 1 ;;
+        esac
+
     - name: Log in to registry
       uses: docker/login-action@v3
       with:
@@ -54,6 +68,7 @@ runs:
         fi
 
     - name: Prune old tags
+      if: ${{ inputs.prune == 'true' }}
       continue-on-error: true
       shell: bash
       env:
diff --git a/tests/requirements.txt b/tests/requirements.txt
new file mode 100644
index 0000000..f62ce0c
--- /dev/null
+++ b/tests/requirements.txt
@@ -0,0 +1 @@
+PyYAML==6.0.3
diff --git a/tests/test_action.py b/tests/test_action.py
new file mode 100644
index 0000000..a068032
--- /dev/null
+++ b/tests/test_action.py
@@ -0,0 +1,38 @@
+import os
+from pathlib import Path
+import subprocess
+import unittest
+
+import yaml
+
+
+class PruningContract(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.action = yaml.safe_load((Path(__file__).resolve().parents[1] / "action.yaml").read_text())
+        cls.steps = cls.action["runs"]["steps"]
+
+    def test_default_preserves_existing_callers_and_only_prune_is_conditional(self):
+        self.assertEqual(self.action["inputs"]["prune"]["default"], "true")
+        gated = [step for step in self.steps if "if" in step]
+        self.assertEqual(len(gated), 1)
+        self.assertEqual(gated[0]["name"], "Prune old tags")
+        self.assertEqual(gated[0]["if"], "${{ inputs.prune == 'true' }}")
+        remote = [step for step in self.steps if "/packages/" in step.get("run", "")]
+        self.assertEqual(remote, gated)
+        self.assertIn("docker push", next(step for step in self.steps if step["name"] == "Push")["run"])
+        self.assertNotIn("if", next(step for step in self.steps if step["name"] == "Untag local image"))
+
+    def test_invalid_policy_stops_before_credential_step(self):
+        validation = self.steps[0]
+        self.assertEqual(validation["name"], "Validate pruning policy")
+        self.assertEqual(validation["env"], {"PRUNE": "${{ inputs.prune }}"})
+        self.assertNotIn("uses", validation)
+        for value, success in (("true", True), ("false", True), ("False", False), ("", False), ("yes", False), ("false\ntrue", False)):
+            with self.subTest(value=value):
+                result = subprocess.run(["bash", "-c", validation["run"]], env={"PATH": os.defpath, "PRUNE": value}, capture_output=True)
+                self.assertEqual(result.returncode == 0, success)
+
+
+if __name__ == "__main__":
+    unittest.main()