fix(client): accept the draft protocol version the json-schema-ref-no-deref mock advertises (#398)

The scenario's hand-rolled server/discover handler advertises
supportedVersions: ['2026-07-28'], but every other method is delegated
to StreamableHTTPServerTransport from the pinned SDK (^1.29.0), whose
version whitelist tops out at 2025-11-25. A SEP-2575 client that honors
the negotiated version then gets HTTP 400 on tools/list, and the
sep-2106-no-network-ref-deref check can never run.

Extend the existing server/discover workaround to the version check:
rewrite an incoming MCP-Protocol-Version: 2026-07-28 header to the
SDK's LATEST_PROTOCOL_VERSION before delegating (patching rawHeaders
too, since the SDK's Node adapter rebuilds its web-standard Request
from rawHeaders). Add a stateless-draft-client regression test.

Fixes #397


Claude-Session: https://claude.ai/code/session_01N4TdyLB13XSsPwNScYHf4C

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]

src/scenarios/client/json-schema-ref-deref.test.ts

@@ -8,6 +8,8 @@ import {
 } from './auth/test_helpers/testClient';
 import { JsonSchemaRefDerefScenario } from './json-schema-ref-deref';
 import { getScenario } from '../index';
+import { sendStatelessRequest } from '../../connection/stateless';
+import { DRAFT_PROTOCOL_VERSION } from '../../types';
 
 /**
  * SEP-2106: implementations MUST NOT automatically dereference $ref values
@@ -71,6 +73,36 @@ async function dereferencingClient(serverUrl: string): Promise<void> {
   await transport.close();
 }
 
+/**
+ * SEP-2575 stateless client: probe `server/discover` first, then honor the
+ * negotiated protocol version on subsequent requests. Regression client for
+ * issue #397 — the scenario's hand-rolled `server/discover` advertises the
+ * draft version, so the pinned SDK transport must not reject that version's
+ * MCP-Protocol-Version header on the follow-up tools/list.
+ */
+async function statelessDraftClient(serverUrl: string): Promise<void> {
+  const discover = await sendStatelessRequest(serverUrl, 'server/discover');
+  const supportedVersions = (
+    discover.body?.result as { supportedVersions?: string[] } | undefined
+  )?.supportedVersions;
+  if (!supportedVersions?.includes(DRAFT_PROTOCOL_VERSION)) {
+    throw new Error(
+      `server/discover did not advertise ${DRAFT_PROTOCOL_VERSION}: ` +
+        JSON.stringify(supportedVersions)
+    );
+  }
+
+  // sendStatelessRequest sends MCP-Protocol-Version: <draft> by default —
+  // exactly what a client that honors the negotiated version would do.
+  const tools = await sendStatelessRequest(serverUrl, 'tools/list');
+  if (tools.status !== 200 || tools.body?.result === undefined) {
+    throw new Error(
+      `tools/list with the negotiated draft version failed: HTTP ${tools.status} ` +
+        JSON.stringify(tools.body ?? tools.text)
+    );
+  }
+}
+
 describe('json-schema-ref-no-deref (SEP-2106)', () => {
   test('scenario is registered', () => {
     expect(getScenario('json-schema-ref-no-deref')).toBeDefined();
@@ -83,6 +115,13 @@ describe('json-schema-ref-no-deref (SEP-2106)', () => {
     );
   });
 
+  test('stateless draft client passes: negotiated version reaches tools/list (issue #397)', async () => {
+    await runClientAgainstScenario(
+      new InlineClientRunner(statelessDraftClient),
+      'json-schema-ref-no-deref'
+    );
+  });
+
   test('dereferencing client fails: canary fetch is detected', async () => {
     await runClientAgainstScenario(
       new InlineClientRunner(dereferencingClient),

src/scenarios/client/json-schema-ref-deref.ts

@@ -1,7 +1,10 @@
 import type { ScenarioContext } from '../../mock-server';
 import { Server } from '@modelcontextprotocol/sdk/server/index.js';
 import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
-import { ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js';
+import {
+  ListToolsRequestSchema,
+  LATEST_PROTOCOL_VERSION as SDK_LATEST_PROTOCOL_VERSION
+} from '@modelcontextprotocol/sdk/types.js';
 import type { Scenario, ConformanceCheck } from '../../types';
 import express, { Request, Response } from 'express';
 import { ScenarioUrls, DRAFT_PROTOCOL_VERSION } from '../../types';
@@ -132,6 +135,21 @@ The scenario advertises a tool whose inputSchema contains a \`$ref\` pointing at
           }
         });
       }
+      // Second half of the same workaround: the pinned SDK transport
+      // whitelists MCP-Protocol-Version headers and would reject the draft
+      // version that the server/discover response above advertises with an
+      // HTTP 400. Rewrite it to the newest version the SDK understands so a
+      // client that honors the negotiated version can reach tools/list.
+      if (req.headers['mcp-protocol-version'] === DRAFT_PROTOCOL_VERSION) {
+        req.headers['mcp-protocol-version'] = SDK_LATEST_PROTOCOL_VERSION;
+        // The SDK's Node adapter rebuilds its web-standard Request from
+        // rawHeaders, not the parsed headers object, so patch those too.
+        for (let i = 0; i < req.rawHeaders.length; i += 2) {
+          if (req.rawHeaders[i].toLowerCase() === 'mcp-protocol-version') {
+            req.rawHeaders[i + 1] = SDK_LATEST_PROTOCOL_VERSION;
+          }
+        }
+      }
       try {
         // Stateless: fresh server and transport per request
         const server = createMcpServer(this.canaryUrl(), () => {