diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 64d8bf4..07460ae 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -30,3 +30,27 @@ jobs:
           size=$(wc -c < target/release/polygo); echo "release binary: $size bytes"
           [ "$size" -lt 15728640 ]
           time target/release/polygo --help > /dev/null
+
+  # GitHub loads action/action.yml before a step runs, and a manifest it cannot parse
+  # fails every job that uses the Action with a YAML error instead of a polygo message.
+  # Nothing else here loads it, so this job does: it uses the Action from this checkout.
+  action:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - name: A project with a broken translation
+        run: |
+          mkdir -p fixture/locales
+          printf '{\n  "greet": "Hi {{name}}"\n}\n' > fixture/locales/en.json
+          printf '{\n  "greet": "Hallo {{nome}}"\n}\n' > fixture/locales/de.json
+      - id: check
+        uses: ./action
+        continue-on-error: true
+        with:
+          mode: check
+          path: fixture/locales
+      - name: The Action ran and found the placeholder
+        run: |
+          [ "${{ steps.check.outcome }}" = failure ] || { echo "the Action should have failed on the error"; exit 1; }
+          [ "${{ steps.check.outputs.errors }}" = 1 ] || { echo "expected 1 error, got '${{ steps.check.outputs.errors }}'"; exit 1; }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 128d567..90975bd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 All notable changes to polygo. Versions follow [SemVer](https://semver.org); dates are ISO.
 
+## Unreleased
+
+- The GitHub Action's manifest could not be parsed at all: one input description held an unquoted `(default: …)`, which YAML reads as a mapping, so every workflow using `Na5co/polygo/action@v0` failed at "Set up job" with a YAML error before polygo ran. Found by using the Action on a real pull request. Quoted, plus two guards: a test that lints every plain scalar in the manifest for the same trap, and a CI job that uses the Action from this checkout on a fixture and asserts it finds the planted error.
+
 ## 0.1.8 — 2026-09-22
 
 - `syntax` (error): a string file that does not parse — a comma dropped from a JSON catalog, an unclosed `<string>`, a file in the wrong encoding — with the line the parser stopped at. It used to be invisible: the detector could not read the file, so the locale disappeared from the run and `check` reported the project as fine. Found while testing the reviewer on a pull request that corrupted a locale file and got a clean review. Every locale file the `locale_path` template matches is scanned, including locales the configuration never lists; when the broken file is one the project must read, the run says so and stops instead of failing with a stack of parser context.
diff --git a/action/action.yml b/action/action.yml
index f9e148a..38d339d 100644
--- a/action/action.yml
+++ b/action/action.yml
@@ -23,7 +23,7 @@ inputs:
     description: polygo release to install (tag), or "latest"
     default: latest
   locales:
-    description: Comma-separated locales to translate (default: all targets in polygo.toml)
+    description: "Comma-separated locales to translate (default: all targets in polygo.toml)"
     default: ""
   args:
     description: Extra arguments for `polygo translate` (e.g. --no-context)
diff --git a/tests/action.rs b/tests/action.rs
index 74e2fae..0fed110 100644
--- a/tests/action.rs
+++ b/tests/action.rs
@@ -238,3 +238,45 @@ fn action_yml_has_a_check_mode() {
         assert!(yml.contains(needle), "action.yml lacks {needle}");
     }
 }
+
+/// The Action's manifest is YAML GitHub has to load before anything runs, and a broken one
+/// fails every job that uses it with a parser error rather than a polygo message. polygo's
+/// own CI never loads it, so it is linted here instead: a plain scalar holding `: ` (a
+/// description with a parenthetical, say) is read as a mapping and breaks the file.
+#[test]
+fn action_manifest_is_loadable_yaml() {
+    let text = fs::read_to_string(
+        std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("action/action.yml"),
+    )
+    .unwrap();
+    let mut bad = Vec::new();
+    for (n, line) in text.lines().enumerate() {
+        let Some((key, value)) = line.split_once(": ") else {
+            continue;
+        };
+        let key = key.trim();
+        if !matches!(key, "description" | "default" | "value" | "name" | "title") {
+            continue;
+        }
+        let value = value.trim();
+        let quoted = (value.starts_with('"') && value.ends_with('"'))
+            || (value.starts_with('\'') && value.ends_with('\''))
+            || value.starts_with("${{")
+            || value.starts_with('|');
+        if !quoted && (value.contains(": ") || value.ends_with(':')) {
+            bad.push(format!("{}: {line}", n + 1));
+        }
+    }
+    assert!(
+        bad.is_empty(),
+        "action.yml has {} value(s) YAML reads as a mapping; quote them:\n{}",
+        bad.len(),
+        bad.join("\n")
+    );
+    // The inputs the README and the tests rely on are all there.
+    for input in [
+        "mode", "path", "strict", "sarif", "review", "version", "token",
+    ] {
+        assert!(text.contains(&format!("\n  {input}:")), "no {input} input");
+    }
+}
diff --git a/tests/release_layout.rs b/tests/release_layout.rs
index 50dce70..d8980a0 100644
--- a/tests/release_layout.rs
+++ b/tests/release_layout.rs
@@ -60,6 +60,9 @@ fn release_assets_match_every_consumer() {
         "{ci}"
     );
     assert!(ci.contains("cargo test --workspace"), "{ci}");
+    // A job that actually loads action/action.yml: a manifest GitHub cannot parse fails
+    // every job that uses the Action, and nothing else in CI reads it.
+    assert!(ci.contains("uses: ./action"), "{ci}");
     // The formula is generated, not checked in.
     assert!(!std::path::Path::new(concat!(env!("CARGO_MANIFEST_DIR"), "/packaging")).exists());
 }