feat(checks): add empty-body rule

Warns when a skill file has valid frontmatter but no body content. A body-less skill gives Claude nothing to act on at runtime, making the skill silently useless. Also registers the rule in the SARIF catalog.

Author: protosphinx

src/checks.ts

@@ -32,6 +32,7 @@ export function runChecks(
     diagnostics.push(...checkToolsOverloaded(v));
     diagnostics.push(...checkDescriptionLength(v));
     diagnostics.push(...checkNameDrift(v));
+    diagnostics.push(...checkEmptyBody(v));
   }
 
   diagnostics.push(...checkCollisions(validated));
@@ -144,6 +145,20 @@ function checkNameDrift(v: ValidatedSkill): Diagnostic[] {
   return [];
 }
 
+function checkEmptyBody(v: ValidatedSkill): Diagnostic[] {
+  if (v.body.trim().length === 0) {
+    return [
+      {
+        severity: "warn",
+        rule: "empty-body",
+        message: "skill body is empty; add instructions for Claude to follow",
+        file: v.file,
+      },
+    ];
+  }
+  return [];
+}
+
 function tokenize(s: string): Set<string> {
   return new Set(
     s

src/sarif.ts

@@ -91,6 +91,14 @@ export const RULES: readonly SarifRule[] = [
     helpUri: HELP_BASE,
     defaultLevel: "warning",
   },
+  {
+    id: "empty-body",
+    name: "emptyBody",
+    shortDescription:
+      "Skill body is empty; the file has frontmatter but no instructions for Claude.",
+    helpUri: HELP_BASE,
+    defaultLevel: "warning",
+  },
 ];
 
 const SEVERITY_TO_LEVEL: Record<Severity, SarifLevel> = {

test/checks.test.ts

@@ -201,4 +201,37 @@ describe("runChecks", () => {
     const d = ds.find((d) => d.rule === "tools-overloaded");
     expect(d?.message).toContain("11");
   });
+
+  it("warns when skill body is empty", () => {
+    const s = mkSkill("/test/foo/foo.md", {
+      name: "foo",
+      description: "do the foo thing",
+    }, "");
+    const ds = runChecks([s], config);
+    expect(ds.some((d) => d.rule === "empty-body")).toBe(true);
+  });
+
+  it("warns when skill body is whitespace only", () => {
+    const s = mkSkill("/test/foo/foo.md", {
+      name: "foo",
+      description: "do the foo thing",
+    }, "   \n\n   ");
+    const ds = runChecks([s], config);
+    expect(ds.some((d) => d.rule === "empty-body")).toBe(true);
+  });
+
+  it("does not warn on empty-body when body has content", () => {
+    const s = mkSkill("/test/foo/foo.md", {
+      name: "foo",
+      description: "do the foo thing",
+    }, "Use Read to read a file, then summarize it.");
+    const ds = runChecks([s], config);
+    expect(ds.find((d) => d.rule === "empty-body")).toBeUndefined();
+  });
+
+  it("does not fire empty-body when frontmatter is invalid", () => {
+    const s = mkSkill("/test/foo/foo.md", { name: "foo" }, "");
+    const ds = runChecks([s], config);
+    expect(ds.find((d) => d.rule === "empty-body")).toBeUndefined();
+  });
 });