diff --git a/.changeset/wise-koalas-climb.md b/.changeset/wise-koalas-climb.md
new file mode 100644
index 000000000..1514c507e
--- /dev/null
+++ b/.changeset/wise-koalas-climb.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3435
+---
+**`/gsd-debug` command metadata now parses as valid YAML** — external YAML tooling and command-contract validation no longer fail on the unquoted argument hint.
diff --git a/commands/gsd/debug.md b/commands/gsd/debug.md
index 1fefac827..0aa94b3cf 100644
--- a/commands/gsd/debug.md
+++ b/commands/gsd/debug.md
@@ -1,7 +1,7 @@
 ---
 name: gsd:debug
 description: Systematic debugging with persistent state across context resets
-argument-hint: [list | status <slug> | continue <slug> | --diagnose] [issue description]
+argument-hint: "[list | status <slug> | continue <slug> | --diagnose] [issue description]"
 allowed-tools:
   - Read
   - Write
diff --git a/tests/bug-3431-debug-command-yaml.test.cjs b/tests/bug-3431-debug-command-yaml.test.cjs
new file mode 100644
index 000000000..66f59e4f8
--- /dev/null
+++ b/tests/bug-3431-debug-command-yaml.test.cjs
@@ -0,0 +1,28 @@
+// allow-test-rule: source-text-is-the-product
+// Command markdown frontmatter is the deployed contract; this regression test
+// verifies the real YAML surface that external parsers consume.
+
+'use strict';
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const { parseFrontmatter } = require('./helpers.cjs');
+
+const DEBUG_COMMAND_PATH = path.join(__dirname, '..', 'commands', 'gsd', 'debug.md');
+
+function readFrontmatter(filePath) {
+  return parseFrontmatter(fs.readFileSync(filePath, 'utf8'));
+}
+
+test('#3431: commands/gsd/debug.md frontmatter parses as YAML and preserves argument-hint', () => {
+  const frontmatter = readFrontmatter(DEBUG_COMMAND_PATH);
+
+  assert.equal(frontmatter.name, 'gsd:debug');
+  assert.equal(
+    frontmatter['argument-hint'],
+    '[list | status <slug> | continue <slug> | --diagnose] [issue description]',
+    'argument-hint should remain user-visible text after YAML parsing'
+  );
+});