diff --git a/src/scenarios/server/sse-multiple-streams.test.ts b/src/scenarios/server/sse-multiple-streams.test.ts
new file mode 100644
index 00000000..967f5d8a
--- /dev/null
+++ b/src/scenarios/server/sse-multiple-streams.test.ts
@@ -0,0 +1,183 @@
+import { describe, test, expect } from 'vitest';
+import http from 'http';
+import { testContext } from '../../connection/testing';
+import { ServerSSEMultipleStreamsScenario } from './sse-multiple-streams';
+import type { ConformanceCheck } from '../../types';
+
+/**
+ * Regression coverage for issue #412: the stateful scenario must send the
+ * protocol version negotiated during initialize on its raw follow-up POSTs,
+ * not a hard-coded historical revision. The fixture server below stores the
+ * version it returned from initialize and rejects (400) any later request
+ * whose MCP-Protocol-Version header does not match it.
+ */
+
+const SESSION_ID = 'test-session-412';
+
+interface VersionEnforcingServer {
+  url: string;
+  negotiatedVersion: () => string | undefined;
+  close: () => Promise<void>;
+}
+
+async function startVersionEnforcingServer(): Promise<VersionEnforcingServer> {
+  let negotiated: string | undefined;
+
+  const server = http.createServer((req, res) => {
+    if (req.method !== 'POST') {
+      res.writeHead(405).end();
+      return;
+    }
+    let raw = '';
+    req.setEncoding('utf8');
+    req.on('data', (c) => (raw += c));
+    req.on('end', () => {
+      let body: {
+        id?: number | string;
+        method?: string;
+        params?: { protocolVersion?: string };
+      } = {};
+      try {
+        body = JSON.parse(raw);
+      } catch {
+        // Treat unparseable bodies as empty requests.
+      }
+      const respond = (status: number, payload?: object) => {
+        res.writeHead(status, { 'Content-Type': 'application/json' });
+        res.end(payload ? JSON.stringify(payload) : undefined);
+      };
+
+      if (body.method === 'initialize') {
+        negotiated = body.params?.protocolVersion;
+        res.setHeader('mcp-session-id', SESSION_ID);
+        respond(200, {
+          jsonrpc: '2.0',
+          id: body.id,
+          result: {
+            protocolVersion: negotiated,
+            capabilities: {},
+            serverInfo: { name: 'version-enforcing-server', version: '1.0.0' }
+          }
+        });
+        return;
+      }
+
+      if (req.headers['mcp-protocol-version'] !== negotiated) {
+        respond(400, {
+          jsonrpc: '2.0',
+          id: body.id ?? null,
+          error: { code: -32000, message: 'Protocol version mismatch' }
+        });
+        return;
+      }
+
+      if (body.method === 'notifications/initialized') {
+        respond(202);
+        return;
+      }
+      if (body.method === 'tools/list') {
+        respond(200, { jsonrpc: '2.0', id: body.id, result: { tools: [] } });
+        return;
+      }
+      respond(404, {
+        jsonrpc: '2.0',
+        id: body.id ?? null,
+        error: { code: -32601, message: 'Not found' }
+      });
+    });
+  });
+
+  await new Promise<void>((resolve) => server.listen(0, resolve));
+  const address = server.address();
+  const port = typeof address === 'object' && address ? address.port : 0;
+  return {
+    url: `http://localhost:${port}/mcp`,
+    negotiatedVersion: () => negotiated,
+    close: () =>
+      new Promise<void>((resolve) => {
+        server.closeAllConnections?.();
+        server.close(() => resolve());
+      })
+  };
+}
+
+const findAll = (checks: ConformanceCheck[], id: string) =>
+  checks.filter((c) => c.id === id);
+
+describe('server-sse-multiple-streams — negotiated protocol version', () => {
+  test('passes against a server that rejects non-negotiated versions', async () => {
+    const srv = await startVersionEnforcingServer();
+
+    const scenario = new ServerSSEMultipleStreamsScenario();
+    let checks: ConformanceCheck[];
+    try {
+      checks = await scenario.run(testContext(srv.url));
+    } finally {
+      await srv.close();
+    }
+
+    // The scenario negotiated a current version, not the old hard-coded pin.
+    expect(srv.negotiatedVersion()).toBeDefined();
+    expect(srv.negotiatedVersion()).not.toBe('2025-03-26');
+
+    const accepted = findAll(checks, 'server-accepts-multiple-post-streams')[0];
+    expect(accepted?.status).toBe('SUCCESS');
+    expect(accepted?.details).toMatchObject({
+      numStreamsAttempted: 3,
+      numStreamsAccepted: 3
+    });
+    expect(checks.every((c) => c.status !== 'FAILURE')).toBe(true);
+  });
+
+  test('fixture server rejects a supported but non-negotiated version', async () => {
+    const srv = await startVersionEnforcingServer();
+
+    try {
+      const init = await fetch(srv.url, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Accept: 'text/event-stream, application/json'
+        },
+        body: JSON.stringify({
+          jsonrpc: '2.0',
+          id: 1,
+          method: 'initialize',
+          params: {
+            protocolVersion: '2025-11-25',
+            capabilities: {},
+            clientInfo: { name: 'test', version: '1.0.0' }
+          }
+        })
+      });
+      expect(init.status).toBe(200);
+
+      const listWith = (version: string) =>
+        fetch(srv.url, {
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            Accept: 'text/event-stream, application/json',
+            'mcp-session-id': SESSION_ID,
+            'mcp-protocol-version': version
+          },
+          body: JSON.stringify({
+            jsonrpc: '2.0',
+            id: 2,
+            method: 'tools/list',
+            params: {}
+          })
+        });
+
+      // The old hard-coded pin is a valid MCP version but not the one
+      // negotiated above, so this fixture must reject it.
+      const rejected = await listWith('2025-03-26');
+      expect(rejected.status).toBe(400);
+
+      const acceptedRes = await listWith('2025-11-25');
+      expect(acceptedRes.status).toBe(200);
+    } finally {
+      await srv.close();
+    }
+  });
+});
diff --git a/src/scenarios/server/sse-multiple-streams.ts b/src/scenarios/server/sse-multiple-streams.ts
index 23f7e797..d2eb4931 100644
--- a/src/scenarios/server/sse-multiple-streams.ts
+++ b/src/scenarios/server/sse-multiple-streams.ts
@@ -37,6 +37,7 @@ export class ServerSSEMultipleStreamsScenario implements ClientScenario {
     const stateless = specVersion === DRAFT_PROTOCOL_VERSION;
 
     let sessionId: string | undefined;
+    let negotiatedProtocolVersion: string | undefined;
     let client: Client | undefined;
     let transport: StreamableHTTPClientTransport | undefined;
 
@@ -59,8 +60,9 @@ export class ServerSSEMultipleStreamsScenario implements ClientScenario {
         transport = new StreamableHTTPClientTransport(new URL(serverUrl));
         await client.connect(transport);
 
-        // Extract session ID from transport
+        // Extract session ID and negotiated protocol version from transport
         sessionId = (transport as unknown as { sessionId?: string }).sessionId;
+        negotiatedProtocolVersion = transport.protocolVersion;
 
         if (!sessionId) {
           checks.push({
@@ -96,7 +98,7 @@ export class ServerSSEMultipleStreamsScenario implements ClientScenario {
             'Content-Type': 'application/json',
             Accept: 'text/event-stream, application/json',
             'mcp-session-id': sessionId!,
-            'mcp-protocol-version': '2025-03-26'
+            'mcp-protocol-version': negotiatedProtocolVersion ?? specVersion
           };
       const requestParams = stateless
         ? {