fix(agent-core): mark truncated skill descriptions with an ellipsis

.changeset/fix-skill-listing-truncation.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix truncated skill descriptions missing an ellipsis in the model's skill listing.

packages/agent-core/src/skill/registry.ts

@@ -183,6 +183,18 @@ function formatModelSkill(skill: SkillDefinition): readonly string[] {
   return lines;
 }
 
+const graphemeSegmenter = new Intl.Segmenter(undefined, { granularity: 'grapheme' });
+
 function truncate(value: string, max: number): string {
-  return value.length > max ? value.slice(0, max) : value;
+  if (value.length <= max) return value;
+  // Reserve one code unit for the trailing ellipsis and walk whole grapheme
+  // clusters so we never split a surrogate pair or combining sequence.
+  let length = 0;
+  let result = '';
+  for (const { segment } of graphemeSegmenter.segment(value)) {
+    if (length + segment.length > max - 1) break;
+    result += segment;
+    length += segment.length;
+  }
+  return `${result}…`;
 }

packages/agent-core/test/skill/registry.test.ts

@@ -73,7 +73,9 @@ describe('skill registry prompt rendering', () => {
   });
 
   it('end-to-end: a project skill that shadows other scopes renders once under Project', () => {
-    const registry = makeRegistry([makeSkill('foo', 'project', 'project version', '/tmp/proj/foo/SKILL.md')]);
+    const registry = makeRegistry([
+      makeSkill('foo', 'project', 'project version', '/tmp/proj/foo/SKILL.md'),
+    ]);
 
     const rendered = registry.getKimiSkillsDescription();
 
@@ -96,6 +98,39 @@ describe('skill registry prompt rendering', () => {
   });
 });
 
+describe('getModelSkillListing description truncation', () => {
+  it('keeps descriptions at or below the 250-char limit unchanged', () => {
+    const description = 'a'.repeat(250);
+    const rendered = makeRegistry([makeSkill('demo', 'user', description)]).getModelSkillListing();
+
+    expect(rendered).toContain(`- demo: ${description}`);
+    expect(rendered).not.toContain('…');
+  });
+
+  it('appends an ellipsis and stays within the limit when a description is truncated', () => {
+    const description = 'a'.repeat(300);
+    const rendered = makeRegistry([makeSkill('demo', 'user', description)]).getModelSkillListing();
+
+    expect(rendered).toContain(`- demo: ${'a'.repeat(249)}…`);
+    expect(rendered).not.toContain('a'.repeat(250));
+  });
+
+  it('does not split a grapheme cluster at the truncation boundary', () => {
+    // The 250-char budget cuts at code-unit 249; the emoji spans 248-249, so a
+    // naive slice would leave a dangling surrogate. Grapheme-safe truncation
+    // must drop the whole emoji instead.
+    const description = `${'a'.repeat(248)}😀${'b'.repeat(100)}`;
+    const rendered = makeRegistry([makeSkill('demo', 'user', description)]).getModelSkillListing();
+
+    expect(rendered).toContain(`- demo: ${'a'.repeat(248)}…`);
+    expect(rendered).not.toContain('😀');
+    // no lone high or low surrogate should remain in the rendered output
+    expect(rendered).not.toMatch(
+      /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/,
+    );
+  });
+});
+
 function makeRegistry(skills: readonly SkillDefinition[]): SessionSkillRegistry {
   const registry = new SessionSkillRegistry();
   for (const skill of skills) registry.register(skill);