test(result): pin MAX_LIST_ITEMS, EVIDENCE_KEYS, MIME map, TRUNCATED_KEY, steering note

src/mcp/tools/result.test.ts

@@ -0,0 +1,39 @@
+import { expect, test } from 'bun:test';
+import {
+  EVIDENCE_KEYS,
+  MAX_LIST_ITEMS,
+  mimeTypeFor,
+  steeringNote,
+  TRUNCATED_KEY,
+} from './result.js';
+
+test('MAX_LIST_ITEMS is 20', () => {
+  expect(MAX_LIST_ITEMS).toBe(20);
+});
+
+test('EVIDENCE_KEYS contains exactly screenshot and evidence', () => {
+  expect(EVIDENCE_KEYS).toEqual(new Set(['screenshot', 'evidence']));
+});
+
+test('mimeTypeFor returns the expected MIME for each known extension', () => {
+  expect(mimeTypeFor('/tmp/x.png')).toBe('image/png');
+  expect(mimeTypeFor('/tmp/x.jpg')).toBe('image/jpeg');
+  expect(mimeTypeFor('/tmp/x.jpeg')).toBe('image/jpeg');
+  expect(mimeTypeFor('/tmp/x.mp4')).toBe('video/mp4');
+  expect(mimeTypeFor('/tmp/x.log')).toBe('text/plain');
+});
+
+test('mimeTypeFor returns undefined for an unknown extension', () => {
+  expect(mimeTypeFor('/tmp/x.txt')).toBeUndefined();
+});
+
+test('TRUNCATED_KEY is "truncated"', () => {
+  expect(TRUNCATED_KEY).toBe('truncated');
+});
+
+test('steeringNote contains the literal phrases for the capped fields', () => {
+  const note = steeringNote(['steps', 'bugs']);
+  expect(note).toContain('Truncated steps, bugs to the first 20 items');
+  expect(note).toContain('see "truncated" for the real totals');
+  expect(note).toContain('fields=["steps", "bugs"]');
+});

src/mcp/tools/result.ts

@@ -26,15 +26,15 @@ function isRecord(value: unknown): value is Record<string, unknown> {
 }
 
 /** Cap on a top-level array field before it gets truncated for the response. */
-const MAX_LIST_ITEMS = 20;
+export const MAX_LIST_ITEMS = 20;
 
 /** Keys whose string value — when an absolute path — is evidence worth linking. */
-const EVIDENCE_KEYS = new Set(['screenshot', 'evidence']);
+export const EVIDENCE_KEYS = new Set(['screenshot', 'evidence']);
 
 type ResourceLinkContent = Extract<CallToolResult['content'][number], { type: 'resource_link' }>;
 
 /** Guess a MIME type from an evidence path's extension; omit when unknown. */
-function mimeTypeFor(path: string): string | undefined {
+export function mimeTypeFor(path: string): string | undefined {
   if (path.endsWith('.png')) return 'image/png';
   if (path.endsWith('.jpg') || path.endsWith('.jpeg')) return 'image/jpeg';
   if (path.endsWith('.mp4')) return 'video/mp4';
@@ -117,7 +117,7 @@ function capLists(data: Record<string, unknown>): {
 }
 
 /** Steering text appended when one or more lists got capped. */
-function steeringNote(truncated: string[]): string {
+export function steeringNote(truncated: string[]): string {
   const fields = truncated.map((f) => `"${f}"`).join(', ');
   return (
     `Truncated ${truncated.join(', ')} to the first ${MAX_LIST_ITEMS} items (see "${TRUNCATED_KEY}" ` +