Fix skills frontmatter descriptions being YAML-typed instead of preserved

yaml.safe_load uses YAML 1.1, where bare words like `yes`/`no`/`on`/`off`
and numeric-looking scalars resolve to non-str Python values. Stringifying
that coerced value (e.g. `description: yes` -> "True") lost the author's
original text. Recover it via the existing line-based parser instead.

fastmcp_slim/fastmcp/server/providers/skills/_common.py

@@ -104,7 +104,13 @@ def parse_frontmatter(content: str) -> tuple[dict[str, Any], str]:
     # YAML may type `description: true` / dates as non-str, but SkillInfo.description
     # is a str field that callers truthiness-check.
     if "description" in parsed and parsed["description"] is not None:
-        parsed["description"] = str(parsed["description"])
+        if not isinstance(parsed["description"], str):
+            # YAML 1.1 coerces bare words like `yes` to non-str; recover the
+            # original text via the line-based parser instead.
+            raw = _parse_frontmatter_line_based(frontmatter_text).get("description")
+            parsed["description"] = (
+                raw if isinstance(raw, str) else str(parsed["description"])
+            )
 
     return parsed, remaining
 

tests/server/providers/test_skills_provider.py

@@ -152,7 +152,7 @@ Body
         assert frontmatter["enabled"] is True
         assert isinstance(frontmatter["enabled"], bool)
 
-    def test_frontmatter_description_bool_coerced_to_str(self):
+    def test_frontmatter_description_bool_recovers_original_text(self):
         content = """---
 description: true
 ---
@@ -160,9 +160,41 @@ description: true
 Body
 """
         frontmatter, body = parse_frontmatter(content)
-        assert frontmatter["description"] == "True"
+        assert frontmatter["description"] == "true"
         assert isinstance(frontmatter["description"], str)
 
+    def test_frontmatter_description_yes_not_coerced_to_true(self):
+        # FastMCP #4416 regression: `yes` bare word previously coerced to True.
+        content = """---
+description: yes
+---
+
+Body
+"""
+        frontmatter, body = parse_frontmatter(content)
+        assert frontmatter["description"] == "yes"
+
+    def test_frontmatter_description_no_not_coerced_to_false(self):
+        content = """---
+description: no
+---
+
+Body
+"""
+        frontmatter, body = parse_frontmatter(content)
+        assert frontmatter["description"] == "no"
+
+    def test_frontmatter_description_numeric_scalar_preserves_original_text(self):
+        # YAML resolves `1.10` to float 1.1, dropping the trailing zero the author wrote.
+        content = """---
+description: 1.10
+---
+
+Body
+"""
+        frontmatter, body = parse_frontmatter(content)
+        assert frontmatter["description"] == "1.10"
+
 
 class TestSkillProvider:
     """Tests for SkillProvider - single skill folder."""