Fix custom header schema freshness (#358)

Co-authored-by: Paul Carleton <paulcarletonjr@gmail.com>

src/scenarios/client/draft-result-fields.test.ts

@@ -349,7 +349,11 @@ describe('http-custom-headers mock results (2026-07-28)', () => {
         method: 'tools/list',
         params: {}
       });
-      expect(list.body.result).toMatchObject(CACHEABLE_FIELDS);
+      expect(list.body.result).toMatchObject({
+        resultType: 'complete',
+        cacheScope: 'private'
+      });
+      expect(list.body.result.ttlMs).toBeGreaterThan(0);
 
       const call = await post(serverUrl, {
         jsonrpc: '2.0',

src/scenarios/client/http-custom-headers.test.ts

@@ -18,8 +18,8 @@ async function post(
   serverUrl: string,
   body: object,
   headers: Record<string, string> = {}
-): Promise<void> {
-  await fetch(serverUrl, {
+): Promise<{ status: number; body: any }> {
+  const response = await fetch(serverUrl, {
     method: 'POST',
     headers: {
       'Content-Type': 'application/json',
@@ -28,6 +28,7 @@ async function post(
     },
     body: JSON.stringify(body)
   });
+  return { status: response.status, body: await response.json() };
 }
 
 async function postJson(serverUrl: string, body: object): Promise<any> {
@@ -162,6 +163,83 @@ describe('HttpCustomHeadersScenario (SEP-2243) check IDs', () => {
       await scenario.stop();
     }
   });
+
+  it('serves a fresh tools/list TTL before requiring schema-derived custom headers', async () => {
+    const scenario = new HttpCustomHeadersScenario();
+    const { serverUrl } = await scenario.start(testScenarioContext());
+    try {
+      const toolsList = await post(serverUrl, {
+        jsonrpc: '2.0',
+        id: 1,
+        method: 'tools/list'
+      });
+      const schemaIsFresh = toolsList.body.result.ttlMs > 0;
+
+      const nonAscii = 'Hello, 世界';
+      const nonAsciiB64 = Buffer.from(nonAscii, 'utf-8').toString('base64');
+      await post(
+        serverUrl,
+        {
+          jsonrpc: '2.0',
+          id: 2,
+          method: 'tools/call',
+          params: {
+            name: 'test_custom_headers',
+            arguments: {
+              region: 'us-west1',
+              priority: 42,
+              non_ascii_val: nonAscii,
+              query: 'SELECT 1'
+            }
+          }
+        },
+        schemaIsFresh
+          ? {
+              'Mcp-Method': 'tools/call',
+              'Mcp-Name': 'test_custom_headers',
+              'Mcp-Param-Region': 'us-west1',
+              'Mcp-Param-Priority': '42',
+              'Mcp-Param-NonAscii': `=?base64?${nonAsciiB64}?=`
+            }
+          : {}
+      );
+      await post(
+        serverUrl,
+        {
+          jsonrpc: '2.0',
+          id: 3,
+          method: 'tools/call',
+          params: {
+            name: 'test_custom_headers_null',
+            arguments: {
+              region: 'us-east1',
+              priority: 1,
+              verbose: null,
+              query: 'SELECT 1'
+            }
+          }
+        },
+        schemaIsFresh
+          ? {
+              'Mcp-Method': 'tools/call',
+              'Mcp-Name': 'test_custom_headers_null',
+              'Mcp-Param-Region': 'us-east1',
+              'Mcp-Param-Priority': '1'
+            }
+          : {}
+      );
+
+      expect(toolsList.body.result.ttlMs).toBeGreaterThan(0);
+      const checks = scenario.getChecks();
+      for (const id of CUSTOM_HEADERS_DECLARED_CHECK_IDS) {
+        const statuses = statusesFor(checks, id);
+        expect(statuses.length, id).toBeGreaterThan(0);
+        expect(statuses, id).not.toContain('FAILURE');
+      }
+    } finally {
+      await scenario.stop();
+    }
+  });
 });
 
 describe('HttpInvalidToolHeadersScenario (SEP-2243) check IDs', () => {

src/scenarios/client/http-custom-headers.ts

@@ -278,7 +278,7 @@ export class HttpCustomHeadersScenario extends BaseHttpScenario {
       id: request.id,
       result: {
         resultType: 'complete',
-        ttlMs: 0,
+        ttlMs: 300000,
         cacheScope: 'private',
         tools: [
           {