feat: add conformance tests for SEP-1699 SSE polling (#47)

examples/clients/typescript/sse-retry-test.ts

@@ -0,0 +1,89 @@
+#!/usr/bin/env node
+
+/**
+ * SSE Retry Test Client
+ *
+ * Tests that the MCP client respects the SSE retry field when reconnecting.
+ * This client connects to a test server that closes the SSE stream mid-tool-call,
+ * then waits for the client to reconnect and sends the tool result.
+ */
+
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
+import { CallToolResultSchema } from '@modelcontextprotocol/sdk/types.js';
+
+async function main(): Promise<void> {
+  const serverUrl = process.argv[2];
+
+  if (!serverUrl) {
+    console.error('Usage: sse-retry-test <server-url>');
+    process.exit(1);
+  }
+
+  console.log(`Connecting to MCP server at: ${serverUrl}`);
+  console.log('This test validates SSE retry field compliance (SEP-1699)');
+
+  try {
+    const client = new Client(
+      {
+        name: 'sse-retry-test-client',
+        version: '1.0.0'
+      },
+      {
+        capabilities: {}
+      }
+    );
+
+    const transport = new StreamableHTTPClientTransport(new URL(serverUrl), {
+      reconnectionOptions: {
+        initialReconnectionDelay: 1000,
+        maxReconnectionDelay: 10000,
+        reconnectionDelayGrowFactor: 1.5,
+        maxRetries: 3
+      }
+    });
+
+    transport.onerror = (error) => {
+      console.log(`Transport error: ${error.message}`);
+    };
+
+    transport.onclose = () => {
+      console.log('Transport closed');
+    };
+
+    console.log('Initiating connection...');
+    await client.connect(transport);
+    console.log('Connected to MCP server');
+
+    console.log('Calling test_reconnection tool...');
+    console.log(
+      'Server will close SSE stream mid-call and send result after reconnection'
+    );
+
+    const result = await client.request(
+      {
+        method: 'tools/call',
+        params: {
+          name: 'test_reconnection',
+          arguments: {}
+        }
+      },
+      CallToolResultSchema
+    );
+
+    console.log('Tool call completed:', JSON.stringify(result, null, 2));
+
+    await transport.close();
+    console.log('Connection closed successfully');
+
+    process.exit(0);
+  } catch (error) {
+    console.error('Test failed:', error);
+    process.exit(1);
+  }
+}
+
+main().catch((error) => {
+  console.error('Unhandled error:', error);
+  process.exit(1);
+});

examples/servers/typescript/everything-server.ts

@@ -12,7 +12,12 @@ import {
   McpServer,
   ResourceTemplate
 } from '@modelcontextprotocol/sdk/server/mcp.js';
-import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
+import {
+  StreamableHTTPServerTransport,
+  EventStore,
+  EventId,
+  StreamId
+} from '@modelcontextprotocol/sdk/server/streamableHttp.js';
 import {
   ElicitResultSchema,
   ListToolsRequestSchema,
@@ -33,6 +38,41 @@ const watchedResourceContent = 'Watched resource content';
 const transports: { [sessionId: string]: StreamableHTTPServerTransport } = {};
 const servers: { [sessionId: string]: McpServer } = {};
 
+// In-memory event store for SEP-1699 resumability
+const eventStoreData = new Map<
+  string,
+  { eventId: string; message: any; streamId: string }
+>();
+
+function createEventStore(): EventStore {
+  return {
+    async storeEvent(streamId: StreamId, message: any): Promise<EventId> {
+      const eventId = `${streamId}::${Date.now()}_${randomUUID()}`;
+      eventStoreData.set(eventId, { eventId, message, streamId });
+      return eventId;
+    },
+    async replayEventsAfter(
+      lastEventId: EventId,
+      { send }: { send: (eventId: EventId, message: any) => Promise<void> }
+    ): Promise<StreamId> {
+      const streamId = lastEventId.split('::')[0];
+      const eventsToReplay: Array<[string, { message: any }]> = [];
+      for (const [eventId, data] of eventStoreData.entries()) {
+        if (data.streamId === streamId && eventId > lastEventId) {
+          eventsToReplay.push([eventId, data]);
+        }
+      }
+      eventsToReplay.sort(([a], [b]) => a.localeCompare(b));
+      for (const [eventId, { message }] of eventsToReplay) {
+        if (Object.keys(message).length > 0) {
+          await send(eventId, message);
+        }
+      }
+      return streamId;
+    }
+  };
+}
+
 // Sample base64 encoded 1x1 red PNG pixel for testing
 const TEST_IMAGE_BASE64 =
   'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8DwHwAFBQIAX8jx0gAAAABJRU5ErkJggg==';
@@ -309,6 +349,46 @@ function createMcpServer() {
     }
   );
 
+  // SEP-1699: Reconnection test tool - closes SSE stream mid-call to test client reconnection
+  mcpServer.registerTool(
+    'test_reconnection',
+    {
+      description:
+        'Tests SSE stream disconnection and client reconnection (SEP-1699). Server will close the stream mid-call and send the result after client reconnects.',
+      inputSchema: {}
+    },
+    async (_args, { sessionId, requestId }) => {
+      const sleep = (ms: number) =>
+        new Promise((resolve) => setTimeout(resolve, ms));
+
+      console.log(`[${sessionId}] Starting test_reconnection tool...`);
+
+      // Get the transport for this session
+      const transport = sessionId ? transports[sessionId] : undefined;
+      if (transport && requestId) {
+        // Close the SSE stream to trigger client reconnection
+        console.log(
+          `[${sessionId}] Closing SSE stream to trigger client polling...`
+        );
+        transport.closeSSEStream(requestId);
+      }
+
+      // Wait for client to reconnect (should respect retry field)
+      await sleep(100);
+
+      console.log(`[${sessionId}] test_reconnection tool complete`);
+
+      return {
+        content: [
+          {
+            type: 'text',
+            text: 'Reconnection test completed successfully. If you received this, the client properly reconnected after stream closure.'
+          }
+        ]
+      };
+    }
+  );
+
   // Sampling tool - requests LLM completion from client
   mcpServer.registerTool(
     'test_sampling',
@@ -1003,6 +1083,8 @@ app.post('/mcp', async (req, res) => {
 
       transport = new StreamableHTTPServerTransport({
         sessionIdGenerator: () => randomUUID(),
+        eventStore: createEventStore(),
+        retryInterval: 5000, // 5 second retry interval for SEP-1699
         onsessioninitialized: (newSessionId) => {
           transports[newSessionId] = transport;
           servers[newSessionId] = mcpServer;

examples/servers/typescript/package.json

@@ -18,7 +18,8 @@
     "@modelcontextprotocol/sdk": "^1.20.1",
     "@types/cors": "^2.8.19",
     "cors": "^2.8.5",
-    "express": "^5.1.0"
+    "express": "^5.1.0",
+    "zod-to-json-schema": "^3.25.0"
   },
   "devDependencies": {
     "tsx": "^4.7.0",

package-lock.json

@@ -11,6 +11,7 @@
       "dependencies": {
         "@modelcontextprotocol/sdk": "^1.23.0-beta.0",
         "commander": "^14.0.2",
+        "eventsource-parser": "^3.0.6",
         "express": "^5.1.0",
         "zod": "^3.25.76"
       },

package.json

@@ -47,6 +47,7 @@
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.23.0-beta.0",
     "commander": "^14.0.2",
+    "eventsource-parser": "^3.0.6",
     "express": "^5.1.0",
     "zod": "^3.25.76"
   }

src/index.ts

@@ -204,11 +204,14 @@ program
     'Suite to run: "active" (default, excludes pending), "all", or "pending"',
     'active'
   )
+  .option('--verbose', 'Show verbose output (JSON instead of pretty print)')
   .action(async (options) => {
     try {
       // Validate options with Zod
       const validated = ServerOptionsSchema.parse(options);
 
+      const verbose = options.verbose ?? false;
+
       // If a single scenario is specified, run just that one
       if (validated.scenario) {
         const result = await runServerConformanceTest(
@@ -218,7 +221,8 @@ program
 
         const { failed } = printServerResults(
           result.checks,
-          result.scenarioDescription
+          result.scenarioDescription,
+          verbose
         );
         process.exit(failed > 0 ? 1 : 0);
       } else {

src/runner/server.ts

@@ -2,7 +2,7 @@ import { promises as fs } from 'fs';
 import path from 'path';
 import { ConformanceCheck } from '../types';
 import { getClientScenario } from '../scenarios';
-import { ensureResultsDir, createResultDir } from './utils';
+import { ensureResultsDir, createResultDir, formatPrettyChecks } from './utils';
 
 /**
  * Format markdown-style text for terminal output using ANSI codes
@@ -54,7 +54,8 @@ export async function runServerConformanceTest(
 
 export function printServerResults(
   checks: ConformanceCheck[],
-  scenarioDescription: string
+  scenarioDescription: string,
+  verbose: boolean = false
 ): {
   passed: number;
   failed: number;
@@ -68,7 +69,11 @@ export function printServerResults(
   const failed = checks.filter((c) => c.status === 'FAILURE').length;
   const warnings = checks.filter((c) => c.status === 'WARNING').length;
 
-  console.log(`Checks:\n${JSON.stringify(checks, null, 2)}`);
+  if (verbose) {
+    console.log(JSON.stringify(checks, null, 2));
+  } else {
+    console.log(`Checks:\n${formatPrettyChecks(checks)}`);
+  }
 
   console.log(`\nTest Results:`);
   console.log(

src/scenarios/client/sse-retry.ts

@@ -0,0 +1,555 @@
+/**
+ * SSE Retry conformance test scenarios for MCP clients (SEP-1699)
+ *
+ * Tests that clients properly respect the SSE retry field by:
+ * - Waiting the specified milliseconds before reconnecting
+ * - Sending Last-Event-ID header on reconnection
+ * - Treating graceful stream closure as reconnectable
+ */
+
+import http from 'http';
+import { Scenario, ScenarioUrls, ConformanceCheck } from '../../types.js';
+
+export class SSERetryScenario implements Scenario {
+  name = 'sse-retry';
+  description =
+    'Tests that client respects SSE retry field timing and reconnects properly (SEP-1699)';
+
+  private server: http.Server | null = null;
+  private checks: ConformanceCheck[] = [];
+  private port: number = 0;
+
+  // Timing tracking
+  private toolStreamCloseTime: number | null = null;
+  private getReconnectionTime: number | null = null;
+  private getConnectionCount: number = 0;
+  private lastEventIds: (string | undefined)[] = [];
+  private retryValue: number = 500; // 500ms
+  private eventIdCounter: number = 0;
+  private sessionId: string = `session-${Date.now()}`;
+
+  // Pending tool call to respond to after reconnection
+  private pendingToolCallId: number | string | null = null;
+  private getResponseStream: http.ServerResponse | null = null;
+
+  // Tolerances for timing validation
+  private readonly EARLY_TOLERANCE = 50; // Allow 50ms early for scheduler variance
+  private readonly LATE_TOLERANCE = 200; // Allow 200ms late for network/event loop
+  private readonly VERY_LATE_MULTIPLIER = 2; // If >2x retry value, client is likely ignoring it
+
+  async start(): Promise<ScenarioUrls> {
+    return new Promise((resolve, reject) => {
+      this.server = http.createServer((req, res) => {
+        this.handleRequest(req, res);
+      });
+
+      this.server.on('error', reject);
+
+      this.server.listen(0, () => {
+        const address = this.server!.address();
+        if (address && typeof address === 'object') {
+          this.port = address.port;
+          resolve({
+            serverUrl: `http://localhost:${this.port}`
+          });
+        } else {
+          reject(new Error('Failed to get server address'));
+        }
+      });
+    });
+  }
+
+  async stop(): Promise<void> {
+    return new Promise((resolve, reject) => {
+      if (this.server) {
+        this.server.close((err) => {
+          if (err) {
+            reject(err);
+          } else {
+            this.server = null;
+            resolve();
+          }
+        });
+      } else {
+        resolve();
+      }
+    });
+  }
+
+  getChecks(): ConformanceCheck[] {
+    // Generate checks based on observed behavior
+    this.generateChecks();
+    return this.checks;
+  }
+
+  private handleRequest(
+    req: http.IncomingMessage,
+    res: http.ServerResponse
+  ): void {
+    if (req.method === 'GET') {
+      // Track GET reconnection timing and Last-Event-ID
+      this.getConnectionCount++;
+      this.getReconnectionTime = performance.now();
+
+      const lastEventId = req.headers['last-event-id'] as string | undefined;
+      const description = lastEventId
+        ? `Received GET request for ${req.url} (Last-Event-ID: ${lastEventId})`
+        : `Received GET request for ${req.url}`;
+      this.checks.push({
+        id: 'incoming-request',
+        name: 'IncomingRequest',
+        description,
+        status: 'INFO',
+        timestamp: new Date().toISOString(),
+        details: {
+          method: 'GET',
+          url: req.url,
+          headers: req.headers,
+          connectionCount: this.getConnectionCount
+        }
+      });
+
+      if (lastEventId) {
+        this.lastEventIds.push(lastEventId);
+      }
+
+      // Handle GET SSE stream request (reconnection)
+      this.handleGetSSEStream(req, res);
+    } else if (req.method === 'POST') {
+      // Handle POST JSON-RPC requests
+      this.handlePostRequest(req, res);
+    } else {
+      res.writeHead(405);
+      res.end('Method Not Allowed');
+    }
+  }
+
+  private handleGetSSEStream(
+    _req: http.IncomingMessage,
+    res: http.ServerResponse
+  ): void {
+    // Set SSE headers
+    res.writeHead(200, {
+      'Content-Type': 'text/event-stream',
+      'Cache-Control': 'no-cache',
+      Connection: 'keep-alive',
+      'mcp-session-id': this.sessionId
+    });
+
+    // Generate event ID
+    this.eventIdCounter++;
+    const eventId = `event-${this.eventIdCounter}`;
+
+    // Send priming event with ID and retry field
+    const primingContent = `id: ${eventId}\nretry: ${this.retryValue}\ndata: \n\n`;
+    res.write(primingContent);
+
+    this.checks.push({
+      id: 'outgoing-sse-event',
+      name: 'OutgoingSseEvent',
+      description: `Sent SSE priming event on GET stream (id: ${eventId}, retry: ${this.retryValue}ms)`,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        eventId,
+        retryMs: this.retryValue,
+        eventType: 'priming',
+        raw: primingContent
+      }
+    });
+
+    // Store the GET stream to send pending tool response
+    this.getResponseStream = res;
+
+    // If we have a pending tool call, send the response now
+    if (this.pendingToolCallId !== null) {
+      const toolResponse = {
+        jsonrpc: '2.0',
+        id: this.pendingToolCallId,
+        result: {
+          content: [
+            {
+              type: 'text',
+              text: 'Reconnection test completed successfully'
+            }
+          ]
+        }
+      };
+
+      const responseEventId = `event-${++this.eventIdCounter}`;
+      const responseContent = `event: message\nid: ${responseEventId}\ndata: ${JSON.stringify(toolResponse)}\n\n`;
+      res.write(responseContent);
+
+      this.checks.push({
+        id: 'outgoing-sse-event',
+        name: 'OutgoingSseEvent',
+        description: `Sent tool response on GET stream after reconnection (id: ${responseEventId})`,
+        status: 'INFO',
+        timestamp: new Date().toISOString(),
+        details: {
+          eventId: responseEventId,
+          eventType: 'message',
+          jsonrpcId: this.pendingToolCallId,
+          body: toolResponse,
+          raw: responseContent
+        }
+      });
+
+      this.pendingToolCallId = null;
+    }
+  }
+
+  private handlePostRequest(
+    req: http.IncomingMessage,
+    res: http.ServerResponse
+  ): void {
+    let body = '';
+
+    req.on('data', (chunk) => {
+      body += chunk.toString();
+    });
+
+    req.on('end', () => {
+      try {
+        const request = JSON.parse(body);
+
+        this.checks.push({
+          id: 'incoming-request',
+          name: 'IncomingRequest',
+          description: `Received POST request for ${req.url} (method: ${request.method})`,
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          details: {
+            method: 'POST',
+            url: req.url,
+            jsonrpcMethod: request.method,
+            jsonrpcId: request.id
+          }
+        });
+
+        if (request.method === 'initialize') {
+          this.handleInitialize(req, res, request);
+        } else if (request.method === 'tools/list') {
+          this.handleToolsList(res, request);
+        } else if (request.method === 'tools/call') {
+          this.handleToolsCall(res, request);
+        } else if (request.id === undefined) {
+          // Notifications (no id) - return 202 Accepted
+          res.writeHead(202);
+          res.end();
+        } else {
+          // For other requests, send a simple JSON response
+          res.writeHead(200, {
+            'Content-Type': 'application/json',
+            'mcp-session-id': this.sessionId
+          });
+          res.end(
+            JSON.stringify({
+              jsonrpc: '2.0',
+              id: request.id,
+              result: {}
+            })
+          );
+        }
+      } catch (error) {
+        res.writeHead(400, { 'Content-Type': 'application/json' });
+        res.end(
+          JSON.stringify({
+            jsonrpc: '2.0',
+            error: {
+              code: -32700,
+              message: `Parse error: ${error}`
+            }
+          })
+        );
+      }
+    });
+  }
+
+  private handleInitialize(
+    _req: http.IncomingMessage,
+    res: http.ServerResponse,
+    request: any
+  ): void {
+    res.writeHead(200, {
+      'Content-Type': 'application/json',
+      'mcp-session-id': this.sessionId
+    });
+
+    const response = {
+      jsonrpc: '2.0',
+      id: request.id,
+      result: {
+        protocolVersion: '2025-03-26',
+        serverInfo: {
+          name: 'sse-retry-test-server',
+          version: '1.0.0'
+        },
+        capabilities: {
+          tools: {}
+        }
+      }
+    };
+
+    res.end(JSON.stringify(response));
+
+    this.checks.push({
+      id: 'outgoing-response',
+      name: 'OutgoingResponse',
+      description: `Sent initialize response`,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        jsonrpcId: request.id,
+        body: response
+      }
+    });
+  }
+
+  private handleToolsList(res: http.ServerResponse, request: any): void {
+    res.writeHead(200, {
+      'Content-Type': 'application/json',
+      'mcp-session-id': this.sessionId
+    });
+
+    const response = {
+      jsonrpc: '2.0',
+      id: request.id,
+      result: {
+        tools: [
+          {
+            name: 'test_reconnection',
+            description:
+              'A tool that triggers SSE stream closure to test client reconnection behavior',
+            inputSchema: {
+              type: 'object',
+              properties: {},
+              required: []
+            }
+          }
+        ]
+      }
+    };
+
+    res.end(JSON.stringify(response));
+
+    this.checks.push({
+      id: 'outgoing-response',
+      name: 'OutgoingResponse',
+      description: `Sent tools/list response`,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        jsonrpcId: request.id,
+        body: response
+      }
+    });
+  }
+
+  private handleToolsCall(res: http.ServerResponse, request: any): void {
+    // Store the request ID so we can respond after reconnection
+    this.pendingToolCallId = request.id;
+
+    // Start SSE stream
+    res.writeHead(200, {
+      'Content-Type': 'text/event-stream',
+      'Cache-Control': 'no-cache',
+      Connection: 'keep-alive',
+      'mcp-session-id': this.sessionId
+    });
+
+    // Send priming event with retry field
+    this.eventIdCounter++;
+    const primingEventId = `event-${this.eventIdCounter}`;
+    const primingContent = `id: ${primingEventId}\nretry: ${this.retryValue}\ndata: \n\n`;
+    res.write(primingContent);
+
+    this.checks.push({
+      id: 'outgoing-sse-event',
+      name: 'OutgoingSseEvent',
+      description: `Sent SSE priming event for tools/call (id: ${primingEventId}, retry: ${this.retryValue}ms)`,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        eventId: primingEventId,
+        retryMs: this.retryValue,
+        eventType: 'priming',
+        raw: primingContent
+      }
+    });
+
+    // Close the stream after a short delay to trigger reconnection
+    setTimeout(() => {
+      this.toolStreamCloseTime = performance.now();
+      this.checks.push({
+        id: 'outgoing-stream-close',
+        name: 'OutgoingStreamClose',
+        description:
+          'Closed tools/call SSE stream to trigger client reconnection',
+        status: 'INFO',
+        timestamp: new Date().toISOString(),
+        details: {
+          retryMs: this.retryValue,
+          pendingToolCallId: this.pendingToolCallId
+        }
+      });
+      res.end();
+    }, 50);
+  }
+
+  private generateChecks(): void {
+    // Check 1: Client should have reconnected via GET after tool call stream close
+    if (this.getConnectionCount < 1) {
+      this.checks.push({
+        id: 'client-sse-graceful-reconnect',
+        name: 'ClientGracefulReconnect',
+        description:
+          'Client reconnects via GET after SSE stream is closed gracefully',
+        status: 'FAILURE',
+        timestamp: new Date().toISOString(),
+        errorMessage: `Client did not attempt GET reconnection after stream closure. Client should treat graceful stream close as reconnectable.`,
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          getConnectionCount: this.getConnectionCount,
+          toolStreamCloseTime: this.toolStreamCloseTime,
+          retryValue: this.retryValue
+        }
+      });
+      return;
+    }
+
+    // Client did reconnect - SUCCESS for graceful reconnection
+    this.checks.push({
+      id: 'client-sse-graceful-reconnect',
+      name: 'ClientGracefulReconnect',
+      description:
+        'Client reconnects via GET after SSE stream is closed gracefully',
+      status: 'SUCCESS',
+      timestamp: new Date().toISOString(),
+      specReferences: [
+        {
+          id: 'SEP-1699',
+          url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+        }
+      ],
+      details: {
+        getConnectionCount: this.getConnectionCount
+      }
+    });
+
+    // Check 2: Client MUST respect retry field timing
+    if (
+      this.toolStreamCloseTime !== null &&
+      this.getReconnectionTime !== null
+    ) {
+      const actualDelay = this.getReconnectionTime - this.toolStreamCloseTime;
+      const minExpected = this.retryValue - this.EARLY_TOLERANCE;
+      const maxExpected = this.retryValue + this.LATE_TOLERANCE;
+
+      const tooEarly = actualDelay < minExpected;
+      const slightlyLate = actualDelay > maxExpected;
+      const veryLate =
+        actualDelay > this.retryValue * this.VERY_LATE_MULTIPLIER;
+      const withinTolerance = !tooEarly && !slightlyLate;
+
+      let status: 'SUCCESS' | 'FAILURE' | 'WARNING' = 'SUCCESS';
+      let errorMessage: string | undefined;
+
+      if (tooEarly) {
+        // Client reconnected too soon - MUST violation
+        status = 'FAILURE';
+        errorMessage = `Client reconnected too early (${actualDelay.toFixed(0)}ms instead of ${this.retryValue}ms). Client MUST respect the retry field and wait the specified time.`;
+      } else if (veryLate) {
+        // Client reconnected way too late - likely ignoring retry field entirely
+        status = 'FAILURE';
+        errorMessage = `Client reconnected very late (${actualDelay.toFixed(0)}ms instead of ${this.retryValue}ms). Client appears to be ignoring the retry field and using its own backoff strategy.`;
+      } else if (slightlyLate) {
+        // Client reconnected slightly late - not a spec violation but suspicious
+        status = 'WARNING';
+        errorMessage = `Client reconnected slightly late (${actualDelay.toFixed(0)}ms instead of ${this.retryValue}ms). This is acceptable but may indicate network delays.`;
+      }
+
+      this.checks.push({
+        id: 'client-sse-retry-timing',
+        name: 'ClientRespectsRetryField',
+        description:
+          'Client MUST respect the retry field, waiting the given number of milliseconds before attempting to reconnect',
+        status,
+        timestamp: new Date().toISOString(),
+        errorMessage,
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          expectedRetryMs: this.retryValue,
+          actualDelayMs: Math.round(actualDelay),
+          minAcceptableMs: minExpected,
+          maxAcceptableMs: maxExpected,
+          veryLateThresholdMs: this.retryValue * this.VERY_LATE_MULTIPLIER,
+          earlyToleranceMs: this.EARLY_TOLERANCE,
+          lateToleranceMs: this.LATE_TOLERANCE,
+          withinTolerance,
+          tooEarly,
+          slightlyLate,
+          veryLate,
+          getConnectionCount: this.getConnectionCount
+        }
+      });
+    } else {
+      this.checks.push({
+        id: 'client-sse-retry-timing',
+        name: 'ClientRespectsRetryField',
+        description: 'Client MUST respect the retry field timing',
+        status: 'WARNING',
+        timestamp: new Date().toISOString(),
+        errorMessage:
+          'Could not measure timing - tool stream close time or GET reconnection time not recorded',
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          toolStreamCloseTime: this.toolStreamCloseTime,
+          getReconnectionTime: this.getReconnectionTime
+        }
+      });
+    }
+
+    // Check 3: Client SHOULD send Last-Event-ID header on reconnection
+    const hasLastEventId =
+      this.lastEventIds.length > 0 && this.lastEventIds[0] !== undefined;
+
+    this.checks.push({
+      id: 'client-sse-last-event-id',
+      name: 'ClientSendsLastEventId',
+      description:
+        'Client SHOULD send Last-Event-ID header on reconnection for resumability',
+      status: hasLastEventId ? 'SUCCESS' : 'WARNING',
+      timestamp: new Date().toISOString(),
+      specReferences: [
+        {
+          id: 'SEP-1699',
+          url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+        }
+      ],
+      details: {
+        hasLastEventId,
+        lastEventIds: this.lastEventIds,
+        getConnectionCount: this.getConnectionCount
+      },
+      errorMessage: !hasLastEventId
+        ? 'Client did not send Last-Event-ID header on reconnection. This is a SHOULD requirement for resumability.'
+        : undefined
+    });
+  }
+}

src/scenarios/index.ts

@@ -2,6 +2,7 @@ import { Scenario, ClientScenario } from '../types';
 import { InitializeScenario } from './client/initialize';
 import { ToolsCallScenario } from './client/tools_call';
 import { ElicitationClientDefaultsScenario } from './client/elicitation-defaults';
+import { SSERetryScenario } from './client/sse-retry';
 
 // Import all new server test scenarios
 import { ServerInitializeScenario } from './server/lifecycle';
@@ -29,6 +30,8 @@ import { JsonSchema2020_12Scenario } from './server/json-schema-2020-12';
 
 import { ElicitationDefaultsScenario } from './server/elicitation-defaults';
 import { ElicitationEnumsScenario } from './server/elicitation-enums';
+import { ServerSSEPollingScenario } from './server/sse-polling';
+import { ServerSSEMultipleStreamsScenario } from './server/sse-multiple-streams';
 
 import {
   ResourcesListScenario,
@@ -63,7 +66,11 @@ const pendingClientScenariosList: ClientScenario[] = [
   // On hold until elicitation schema types are fixed
   // https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1863
   new ToolsCallElicitationScenario(),
-  new ElicitationDefaultsScenario()
+  new ElicitationDefaultsScenario(),
+
+  // On hold until server-side SSE improvements are made
+  // https://github.com/modelcontextprotocol/typescript-sdk/pull/1129
+  new ServerSSEPollingScenario()
 ];
 
 // All client scenarios
@@ -94,6 +101,10 @@ const allClientScenariosList: ClientScenario[] = [
   // Elicitation scenarios (SEP-1034)
   new ElicitationDefaultsScenario(),
 
+  // SSE Polling scenarios (SEP-1699)
+  new ServerSSEPollingScenario(),
+  new ServerSSEMultipleStreamsScenario(),
+
   // Elicitation scenarios (SEP-1330) - pending
   new ElicitationEnumsScenario(),
 
@@ -132,6 +143,7 @@ const scenariosList: Scenario[] = [
   new InitializeScenario(),
   new ToolsCallScenario(),
   new ElicitationClientDefaultsScenario(),
+  new SSERetryScenario(),
   ...authScenariosList
 ];
 

src/scenarios/server/all-scenarios.test.ts

@@ -125,9 +125,9 @@ describe('Server Scenarios', () => {
         throw new Error(`Scenario failed with checks:\n  ${failureMessages}`);
       }
 
-      // All checks should be SUCCESS
+      // All checks should be non-FAILURE (SUCCESS, WARNING, or INFO are acceptable)
-      const successes = checks.filter((c) => c.status === 'SUCCESS');
+      const nonFailures = checks.filter((c) => c.status !== 'FAILURE');
-      expect(successes.length).toBe(checks.length);
+      expect(nonFailures.length).toBe(checks.length);
     }, 10000); // 10 second timeout per scenario
   }
 });

src/scenarios/server/sse-multiple-streams.ts

@@ -0,0 +1,268 @@
+/**
+ * SSE Multiple Streams conformance test scenarios for MCP servers (SEP-1699)
+ *
+ * Tests that servers properly support multiple concurrent SSE streams:
+ * - Accepting multiple POST requests that return SSE streams simultaneously
+ * - Each POST request gets its own stream with unique stream ID
+ *
+ * Note: The standalone GET stream (without Last-Event-ID) is limited to one per session.
+ * Multiple concurrent streams are achieved via POST requests, each getting their own stream.
+ */
+
+import { ClientScenario, ConformanceCheck } from '../../types.js';
+import { EventSourceParserStream } from 'eventsource-parser/stream';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
+
+export class ServerSSEMultipleStreamsScenario implements ClientScenario {
+  name = 'server-sse-multiple-streams';
+  description =
+    'Test server supports multiple concurrent POST SSE streams (SEP-1699)';
+
+  async run(serverUrl: string): Promise<ConformanceCheck[]> {
+    const checks: ConformanceCheck[] = [];
+
+    let sessionId: string | undefined;
+    let client: Client | undefined;
+    let transport: StreamableHTTPClientTransport | undefined;
+
+    try {
+      // Step 1: Initialize session with the server
+      client = new Client(
+        {
+          name: 'conformance-test-client',
+          version: '1.0.0'
+        },
+        {
+          capabilities: {
+            sampling: {},
+            elicitation: {}
+          }
+        }
+      );
+
+      transport = new StreamableHTTPClientTransport(new URL(serverUrl));
+      await client.connect(transport);
+
+      // Extract session ID from transport
+      sessionId = (transport as unknown as { sessionId?: string }).sessionId;
+
+      if (!sessionId) {
+        checks.push({
+          id: 'server-sse-multiple-streams-session',
+          name: 'ServerSSEMultipleStreamsSession',
+          description: 'Server provides session ID for multiple streams test',
+          status: 'WARNING',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            message:
+              'Server did not provide session ID - multiple streams test may not work correctly'
+          }
+        });
+        return checks;
+      }
+
+      // Step 2: Open multiple POST SSE streams concurrently
+      // Each POST request gets its own stream with unique streamId
+      // Spec says: "The client MAY remain connected to multiple SSE streams simultaneously"
+      const streamResponses: Response[] = [];
+      const numStreams = 3;
+
+      // Launch all POST requests concurrently
+      const postPromises = [];
+      for (let i = 0; i < numStreams; i++) {
+        const promise = fetch(serverUrl, {
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            Accept: 'text/event-stream, application/json',
+            'mcp-session-id': sessionId,
+            'mcp-protocol-version': '2025-03-26'
+          },
+          body: JSON.stringify({
+            jsonrpc: '2.0',
+            id: 1000 + i, // Different request IDs for each stream
+            method: 'tools/list',
+            params: {}
+          })
+        });
+        postPromises.push(promise);
+      }
+
+      // Wait for all responses
+      const responses = await Promise.all(postPromises);
+      streamResponses.push(...responses);
+
+      // Check that all streams were accepted (HTTP 200)
+      const allAccepted = streamResponses.every((r) => r.ok);
+      const statuses = streamResponses.map((r) => r.status);
+      const contentTypes = streamResponses.map((r) =>
+        r.headers.get('content-type')
+      );
+
+      // Count how many returned SSE streams vs JSON
+      const sseStreams = contentTypes.filter((ct) =>
+        ct?.includes('text/event-stream')
+      ).length;
+
+      checks.push({
+        id: 'server-accepts-multiple-post-streams',
+        name: 'ServerAcceptsMultiplePostStreams',
+        description:
+          'Server allows multiple concurrent POST requests (each may return SSE or JSON)',
+        status: allAccepted ? 'SUCCESS' : 'FAILURE',
+        timestamp: new Date().toISOString(),
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          numStreamsAttempted: numStreams,
+          numStreamsAccepted: statuses.filter((s) => s === 200).length,
+          numSseStreams: sseStreams,
+          statuses,
+          contentTypes
+        },
+        errorMessage: !allAccepted
+          ? `Server rejected some requests. Statuses: ${statuses.join(', ')}`
+          : undefined
+      });
+
+      // Step 3: Verify SSE streams are functional by reading events
+      // Only test streams that returned SSE content-type
+      const eventResults = await Promise.all(
+        streamResponses.map(async (response, index) => {
+          const contentType = response.headers.get('content-type');
+
+          // Skip non-SSE responses (JSON responses are also valid)
+          if (!contentType?.includes('text/event-stream')) {
+            return { index, type: 'json', skipped: true };
+          }
+
+          if (!response.ok || !response.body) {
+            return { index, type: 'sse', error: 'Stream not available' };
+          }
+
+          try {
+            const reader = response.body
+              .pipeThrough(new TextDecoderStream())
+              .pipeThrough(new EventSourceParserStream())
+              .getReader();
+
+            // Wait for one event with timeout
+            const timeoutPromise = new Promise<null>((resolve) =>
+              setTimeout(() => resolve(null), 2000)
+            );
+
+            const eventPromise = reader.read().then(({ value }) => value);
+
+            const event = await Promise.race([eventPromise, timeoutPromise]);
+
+            // Cancel reader
+            await reader.cancel();
+
+            return { index, type: 'sse', event };
+          } catch (error) {
+            return {
+              index,
+              type: 'sse',
+              error: error instanceof Error ? error.message : String(error)
+            };
+          }
+        })
+      );
+
+      // Count functional SSE streams (received event or timed out waiting - both are valid)
+      const sseResults = eventResults.filter((r) => r.type === 'sse');
+      const functionalSseStreams = sseResults.filter(
+        (r) => !('error' in r)
+      ).length;
+
+      // If server returned SSE streams, they should be functional
+      if (sseStreams > 0) {
+        checks.push({
+          id: 'server-sse-streams-functional',
+          name: 'ServerSSEStreamsFunctional',
+          description: 'Multiple POST SSE streams should be functional',
+          status:
+            functionalSseStreams === sseStreams
+              ? 'SUCCESS'
+              : functionalSseStreams > 0
+                ? 'WARNING'
+                : 'FAILURE',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            numSseStreams: sseStreams,
+            functionalSseStreams,
+            results: eventResults
+          },
+          errorMessage:
+            functionalSseStreams < sseStreams
+              ? `Only ${functionalSseStreams}/${sseStreams} SSE streams were functional`
+              : undefined
+        });
+      } else {
+        // Server returned JSON for all requests - this is valid but worth noting
+        checks.push({
+          id: 'server-sse-streams-functional',
+          name: 'ServerSSEStreamsFunctional',
+          description: 'Server returned JSON responses (SSE streams optional)',
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            numSseStreams: 0,
+            message:
+              'Server returned JSON for all requests - SSE streaming is optional',
+            results: eventResults
+          }
+        });
+      }
+    } catch (error) {
+      checks.push({
+        id: 'server-sse-multiple-streams-error',
+        name: 'ServerSSEMultipleStreamsTest',
+        description: 'Test server multiple SSE streams behavior',
+        status: 'FAILURE',
+        timestamp: new Date().toISOString(),
+        errorMessage: `Error: ${error instanceof Error ? error.message : String(error)}`,
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ]
+      });
+    } finally {
+      // Clean up
+      if (client) {
+        try {
+          await client.close();
+        } catch {
+          // Ignore cleanup errors
+        }
+      }
+    }
+
+    return checks;
+  }
+}

src/scenarios/server/sse-polling.ts

@@ -0,0 +1,601 @@
+/**
+ * SSE Polling conformance test scenarios for MCP servers (SEP-1699)
+ *
+ * Tests that servers properly implement SSE polling behavior including:
+ * - Sending priming events with event ID and empty data on POST SSE streams
+ * - Sending retry field in priming events when configured
+ * - Closing SSE stream mid-operation and resuming after client reconnects
+ * - Replaying events when client reconnects with Last-Event-ID
+ */
+
+import { ClientScenario, ConformanceCheck } from '../../types.js';
+import { EventSourceParserStream } from 'eventsource-parser/stream';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
+
+function createLoggingFetch(checks: ConformanceCheck[]) {
+  return async (url: string, options: RequestInit): Promise<Response> => {
+    const method = options.method || 'GET';
+    let description = `Sending ${method} request`;
+    if (options.body) {
+      try {
+        const body = JSON.parse(options.body as string);
+        if (body.method) {
+          description = `Sending ${method} ${body.method}`;
+        }
+      } catch {
+        // Not JSON
+      }
+    }
+
+    checks.push({
+      id: 'outgoing-request',
+      name: 'OutgoingRequest',
+      description,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        method,
+        url,
+        headers: options.headers,
+        body: options.body ? JSON.parse(options.body as string) : undefined
+      }
+    });
+
+    const response = await fetch(url, options);
+
+    const responseHeaders: Record<string, string> = {};
+    response.headers.forEach((value, key) => {
+      responseHeaders[key] = value;
+    });
+
+    checks.push({
+      id: 'incoming-response',
+      name: 'IncomingResponse',
+      description: `Received ${response.status} response for ${method}`,
+      status: 'INFO',
+      timestamp: new Date().toISOString(),
+      details: {
+        statusCode: response.status,
+        headers: responseHeaders
+      }
+    });
+
+    return response;
+  };
+}
+
+export class ServerSSEPollingScenario implements ClientScenario {
+  name = 'server-sse-polling';
+  description =
+    'Test server SSE polling via test_reconnection tool that closes stream mid-call (SEP-1699)';
+
+  async run(serverUrl: string): Promise<ConformanceCheck[]> {
+    const checks: ConformanceCheck[] = [];
+
+    let sessionId: string | undefined;
+    let client: Client | undefined;
+    let transport: StreamableHTTPClientTransport | undefined;
+
+    try {
+      // Step 1: Initialize session with the server
+      client = new Client(
+        {
+          name: 'conformance-test-client',
+          version: '1.0.0'
+        },
+        {
+          capabilities: {
+            sampling: {},
+            elicitation: {}
+          }
+        }
+      );
+
+      transport = new StreamableHTTPClientTransport(new URL(serverUrl));
+      await client.connect(transport);
+
+      // Extract session ID from transport (accessing internal state)
+      sessionId = (transport as unknown as { sessionId?: string }).sessionId;
+
+      if (!sessionId) {
+        checks.push({
+          id: 'server-sse-polling-session',
+          name: 'ServerSSEPollingSession',
+          description: 'Server provides session ID for SSE polling tests',
+          status: 'WARNING',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            message:
+              'Server did not provide session ID - SSE polling tests may not work correctly'
+          }
+        });
+      }
+
+      // Step 2: Call test_reconnection tool via raw fetch to observe SSE behavior
+      // This tool should close the stream mid-call, requiring reconnection
+      const loggingFetch = createLoggingFetch(checks);
+
+      const postResponse = await loggingFetch(serverUrl, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Accept: 'text/event-stream, application/json',
+          ...(sessionId && { 'mcp-session-id': sessionId }),
+          'mcp-protocol-version': '2025-03-26'
+        },
+        body: JSON.stringify({
+          jsonrpc: '2.0',
+          id: 1,
+          method: 'tools/call',
+          params: {
+            name: 'test_reconnection',
+            arguments: {}
+          }
+        })
+      });
+
+      if (!postResponse.ok) {
+        // Check if tool doesn't exist (method not found or similar)
+        if (postResponse.status === 400 || postResponse.status === 404) {
+          checks.push({
+            id: 'server-sse-test-reconnection-tool',
+            name: 'ServerTestReconnectionTool',
+            description:
+              'Server implements test_reconnection tool for SSE polling tests',
+            status: 'WARNING',
+            timestamp: new Date().toISOString(),
+            errorMessage: `Server does not implement test_reconnection tool (HTTP ${postResponse.status}). This tool is recommended for testing SSE polling behavior.`,
+            specReferences: [
+              {
+                id: 'SEP-1699',
+                url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+              }
+            ]
+          });
+          return checks;
+        }
+
+        checks.push({
+          id: 'server-sse-post-request',
+          name: 'ServerSSEPostRequest',
+          description: 'Server accepts POST request with SSE stream response',
+          status: 'FAILURE',
+          timestamp: new Date().toISOString(),
+          errorMessage: `Server returned HTTP ${postResponse.status}`,
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ]
+        });
+        return checks;
+      }
+
+      // Check if server returned SSE stream
+      const contentType = postResponse.headers.get('content-type');
+      if (!contentType?.includes('text/event-stream')) {
+        checks.push({
+          id: 'server-sse-content-type',
+          name: 'ServerSSEContentType',
+          description: 'Server returns text/event-stream for POST request',
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            contentType,
+            message:
+              'Server returned JSON instead of SSE stream - priming event tests not applicable'
+          }
+        });
+        return checks;
+      }
+
+      // Step 3: Parse SSE stream for priming event and tool response
+      let hasEventId = false;
+      let hasPrimingEvent = false;
+      let primingEventIsFirst = false;
+      let hasRetryField = false;
+      let retryValue: number | undefined;
+      let lastEventId: string | undefined;
+      let eventCount = 0;
+      let receivedToolResponse = false;
+
+      if (!postResponse.body) {
+        checks.push({
+          id: 'server-sse-polling-stream',
+          name: 'ServerSSEPollingStream',
+          description: 'Server provides SSE response body',
+          status: 'FAILURE',
+          timestamp: new Date().toISOString(),
+          errorMessage: 'Response body is null',
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ]
+        });
+        return checks;
+      }
+
+      const reader = postResponse.body
+        .pipeThrough(new TextDecoderStream())
+        .pipeThrough(
+          new EventSourceParserStream({
+            onRetry: (retryMs: number) => {
+              hasRetryField = true;
+              retryValue = retryMs;
+            }
+          })
+        )
+        .getReader();
+
+      // Read events with timeout - expect stream to close before we get the response
+      const timeout = setTimeout(() => {
+        reader.cancel();
+      }, 10000);
+
+      try {
+        while (true) {
+          const { value: event, done } = await reader.read();
+
+          if (done) {
+            break;
+          }
+
+          eventCount++;
+
+          // Track the last event ID for reconnection
+          if (event.id) {
+            hasEventId = true;
+            lastEventId = event.id;
+
+            // Check if this is a priming event (empty or minimal data)
+            const isPriming =
+              event.data === '' ||
+              event.data === '{}' ||
+              event.data.trim() === '';
+            if (isPriming) {
+              hasPrimingEvent = true;
+              // Check if priming event is the first event
+              if (eventCount === 1) {
+                primingEventIsFirst = true;
+              }
+            }
+
+            // Log the SSE event
+            checks.push({
+              id: 'incoming-sse-event',
+              name: 'IncomingSseEvent',
+              description: isPriming
+                ? `Received SSE priming event (id: ${event.id})`
+                : `Received SSE event (id: ${event.id})`,
+              status: 'INFO',
+              timestamp: new Date().toISOString(),
+              details: {
+                eventId: event.id,
+                eventType: event.event || 'message',
+                isPriming,
+                hasRetryField,
+                retryValue,
+                data: event.data
+              }
+            });
+          }
+
+          // Check if this is the tool response
+          if (event.data) {
+            try {
+              const parsed = JSON.parse(event.data);
+              if (parsed.id === 1 && parsed.result) {
+                receivedToolResponse = true;
+                const isError = parsed.result?.isError === true;
+                checks.push({
+                  id: 'incoming-sse-event',
+                  name: 'IncomingSseEvent',
+                  description: `Received tool response on POST stream`,
+                  status: isError ? 'FAILURE' : 'INFO',
+                  timestamp: new Date().toISOString(),
+                  details: {
+                    eventId: event.id,
+                    body: parsed
+                  },
+                  ...(isError && { errorMessage: `Tool call failed` })
+                });
+              }
+            } catch {
+              // Not JSON, ignore
+            }
+          }
+        }
+      } finally {
+        clearTimeout(timeout);
+      }
+
+      // Log stream closure
+      checks.push({
+        id: 'stream-closed',
+        name: 'StreamClosed',
+        description: `POST SSE stream closed after ${eventCount} event(s)`,
+        status: 'INFO',
+        timestamp: new Date().toISOString(),
+        details: {
+          eventCount,
+          lastEventId,
+          receivedToolResponse
+        }
+      });
+
+      // Check 1: Server SHOULD send priming event with ID on POST SSE stream
+      let primingStatus: 'SUCCESS' | 'WARNING' = 'SUCCESS';
+      let primingErrorMessage: string | undefined;
+
+      if (!hasPrimingEvent) {
+        primingStatus = 'WARNING';
+        primingErrorMessage =
+          'Server did not send priming event with id and empty data on POST SSE stream. This is recommended for resumability.';
+      } else if (!primingEventIsFirst) {
+        primingStatus = 'WARNING';
+        primingErrorMessage =
+          'Priming event was not sent first. It should be sent immediately when the SSE stream is established.';
+      }
+
+      checks.push({
+        id: 'server-sse-priming-event',
+        name: 'ServerSendsPrimingEvent',
+        description:
+          'Server SHOULD send priming event with id and empty data on POST SSE streams',
+        status: primingStatus,
+        timestamp: new Date().toISOString(),
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          hasPrimingEvent,
+          primingEventIsFirst,
+          hasEventId,
+          lastEventId,
+          eventCount
+        },
+        errorMessage: primingErrorMessage
+      });
+
+      // Check 2: Server SHOULD send retry field in priming event
+      checks.push({
+        id: 'server-sse-retry-field',
+        name: 'ServerSendsRetryField',
+        description:
+          'Server SHOULD send retry field to control client reconnection timing',
+        status: hasRetryField ? 'SUCCESS' : 'WARNING',
+        timestamp: new Date().toISOString(),
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ],
+        details: {
+          hasRetryField,
+          retryValue
+        },
+        errorMessage: !hasRetryField
+          ? 'Server did not send retry field. This is recommended for controlling client reconnection timing.'
+          : undefined
+      });
+
+      // Step 4: If tool response wasn't received, reconnect with Last-Event-ID
+      if (!receivedToolResponse && lastEventId && sessionId) {
+        // Make a GET request with Last-Event-ID to get the tool response
+        const getResponse = await loggingFetch(serverUrl, {
+          method: 'GET',
+          headers: {
+            Accept: 'text/event-stream',
+            'mcp-session-id': sessionId,
+            'mcp-protocol-version': '2025-03-26',
+            'last-event-id': lastEventId
+          }
+        });
+
+        if (getResponse.ok && getResponse.body) {
+          const reconnectReader = getResponse.body
+            .pipeThrough(new TextDecoderStream())
+            .pipeThrough(new EventSourceParserStream())
+            .getReader();
+
+          const reconnectTimeout = setTimeout(() => {
+            reconnectReader.cancel();
+          }, 5000);
+
+          try {
+            while (true) {
+              const { value: event, done } = await reconnectReader.read();
+              if (done) break;
+
+              // Log each event received on GET stream
+              checks.push({
+                id: 'incoming-sse-event',
+                name: 'IncomingSseEvent',
+                description: `Received SSE event on GET reconnection stream (id: ${event.id || 'none'})`,
+                status: 'INFO',
+                timestamp: new Date().toISOString(),
+                details: {
+                  eventId: event.id,
+                  eventType: event.event || 'message',
+                  data: event.data
+                }
+              });
+
+              // Check if this is the tool response
+              if (event.data) {
+                try {
+                  const parsed = JSON.parse(event.data);
+                  if (parsed.id === 1 && parsed.result) {
+                    receivedToolResponse = true;
+                    break;
+                  }
+                } catch {
+                  // Not JSON, ignore
+                }
+              }
+            }
+          } finally {
+            clearTimeout(reconnectTimeout);
+          }
+
+          checks.push({
+            id: 'server-sse-disconnect-resume',
+            name: 'ServerDisconnectResume',
+            description:
+              'Server closes SSE stream mid-call and resumes after client reconnects with Last-Event-ID',
+            status: receivedToolResponse ? 'SUCCESS' : 'WARNING',
+            timestamp: new Date().toISOString(),
+            specReferences: [
+              {
+                id: 'SEP-1699',
+                url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+              }
+            ],
+            details: {
+              lastEventIdUsed: lastEventId,
+              receivedToolResponse,
+              message: receivedToolResponse
+                ? 'Successfully received tool response after reconnection'
+                : 'Tool response not received after reconnection'
+            },
+            errorMessage: !receivedToolResponse
+              ? 'Server did not send tool response after client reconnected with Last-Event-ID'
+              : undefined
+          });
+        } else {
+          // Check if server doesn't support standalone GET streams
+          if (getResponse.status === 405) {
+            checks.push({
+              id: 'server-sse-disconnect-resume',
+              name: 'ServerDisconnectResume',
+              description:
+                'Server supports GET reconnection with Last-Event-ID',
+              status: 'INFO',
+              timestamp: new Date().toISOString(),
+              specReferences: [
+                {
+                  id: 'SEP-1699',
+                  url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+                }
+              ],
+              details: {
+                statusCode: getResponse.status,
+                message:
+                  'Server does not support standalone GET SSE endpoint (405 Method Not Allowed)'
+              }
+            });
+          } else {
+            checks.push({
+              id: 'server-sse-disconnect-resume',
+              name: 'ServerDisconnectResume',
+              description:
+                'Server supports GET reconnection with Last-Event-ID',
+              status: 'WARNING',
+              timestamp: new Date().toISOString(),
+              specReferences: [
+                {
+                  id: 'SEP-1699',
+                  url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+                }
+              ],
+              details: {
+                statusCode: getResponse.status,
+                lastEventIdUsed: lastEventId,
+                message: `Server returned ${getResponse.status} for GET request with Last-Event-ID`
+              },
+              errorMessage: `Server did not accept reconnection with Last-Event-ID (HTTP ${getResponse.status})`
+            });
+          }
+        }
+      } else if (receivedToolResponse) {
+        // Tool response was received on the initial POST stream (server didn't disconnect)
+        checks.push({
+          id: 'server-sse-disconnect-resume',
+          name: 'ServerDisconnectResume',
+          description:
+            'Server closes SSE stream mid-call and resumes after reconnection',
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            receivedToolResponse: true,
+            message:
+              'Tool response received on initial POST stream - server did not disconnect mid-call. The test_reconnection tool should close the stream before sending the result.'
+          }
+        });
+      } else {
+        checks.push({
+          id: 'server-sse-disconnect-resume',
+          name: 'ServerDisconnectResume',
+          description:
+            'Server closes SSE stream mid-call and resumes after reconnection',
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          specReferences: [
+            {
+              id: 'SEP-1699',
+              url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+            }
+          ],
+          details: {
+            lastEventId,
+            sessionId,
+            message:
+              'Could not test disconnect/resume - no last event ID or session ID available'
+          }
+        });
+      }
+    } catch (error) {
+      checks.push({
+        id: 'server-sse-polling-error',
+        name: 'ServerSSEPollingTest',
+        description: 'Test server SSE polling behavior',
+        status: 'FAILURE',
+        timestamp: new Date().toISOString(),
+        errorMessage: `Error: ${error instanceof Error ? error.message : String(error)}`,
+        specReferences: [
+          {
+            id: 'SEP-1699',
+            url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1699'
+          }
+        ]
+      });
+    } finally {
+      // Clean up
+      if (client) {
+        try {
+          await client.close();
+        } catch {
+          // Ignore cleanup errors
+        }
+      }
+    }
+
+    return checks;
+  }
+}

vitest.config.ts

@@ -5,7 +5,7 @@ export default defineConfig({
     globals: true,
     environment: 'node',
     include: ['**/*.test.ts'],
-    exclude: ['node_modules', 'dist'],
+    exclude: ['**/node_modules/**', 'dist'],
     // Run test files sequentially to avoid port conflicts
     fileParallelism: false,
     // Increase timeout for server tests in CI