everything sever response with an extra empty SSE event causing the java mcp client to fail deserialization

[smogos]

Describe the bug
When connecting to the everything sever and running the initialize command with 2025-11-25 version, the server returns an empty SSE event, fallowed by the initialize response.
This causes problems with the java mcp client which fails deserialization of the event.

When connecting with 2025-06-18 version, that empty SSE event is not sent, only one SSE event is return with the result of the server info.

To Reproduce
Steps to reproduce the behavior:

1. Create a docker image with the latest server-everything:

FROM node:slim

ENV PORT=3001

ENTRYPOINT ["npx", "-y", "@modelcontextprotocol/server-everything", "streamableHttp"]

EXPOSE 3001

(Side note, any reason for not having newer images available at: https://hub.docker.com/r/mcp/everything ?)

Call the server with latest protocol version:

curl --location -k  'http:/*****' \
--header 'Accept: application/json,text/event-stream' \
--header 'Content-Type: application/json' \
--data '{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-11-25",
    "capabilities": {
      "roots": {
        "listChanged": false
      },
      "sampling": {},
      "elicitation": {
        "form": {},
        "url": {}
      },
      "tasks": {
        "requests": {
          "elicitation": {
            "create": {}
          },
          "sampling": {
            "createMessage": {}
          }
        }
      }
    },
    "clientInfo": {
      "name": "ExampleClient",
      "title": "Example Client Display Name",
      "version": "1.0.0",
      "description": "An example MCP client application",
      "icons": [
        {
          "src": "https://example.com/icon.png",
          "mimeType": "image/png",
          "sizes": ["48x48"]
        }
      ],
      "websiteUrl": "https://example.com"
    }
  }
}'
id: bd3bdc47-e4bc-4de3-abf7-f43dc27919f5_1769702220748_cbw7i7m9
data: 

event: message
id: bd3bdc47-e4bc-4de3-abf7-f43dc27919f5_1769702220749_mtzbfv1k
data: {"result":{"protocolVersion":"2025-11-25","capabilities":{"tools":{"listChanged":true},"prompts":{"listChanged":true},"resources":{"subscribe":true,"listChanged":true},"logging":{},"completions":{}},"serverInfo":{"name":"mcp-servers/everything","title":"Everything Reference Server","version":"2.0.0"},"instructions":"# Everything Server – Server Instructions\n\nAudience: These instructions are written for an LLM or autonomous agent integrating with the Everything MCP Server.\nFollow them to use, extend, and troubleshoot the server safely and effectively.\n\n## Cross-Feature Relationships\n\n- Use `get-roots-list` to see client workspace roots before file operations\n- `gzip-file-as-resource` creates session-scoped resources accessible only during the current session\n- Enable `toggle-simulated-logging` before debugging to see server log messages\n- Enable `toggle-subscriber-updates` to receive periodic resource update notifications\n\n## Constraints & Limitations\n\n- `gzip-file-as-resource`: Max fetch size controlled by `GZIP_MAX_FETCH_SIZE` (default 10MB), timeout by `GZIP_MAX_FETCH_TIME_MILLIS` (default 30s), allowed domains by `GZIP_ALLOWED_DOMAINS`\n- Session resources are ephemeral and lost when the session ends\n- Sampling requests (`trigger-sampling-request`) require client sampling capability\n- Elicitation requests (`trigger-elicitation-request`) require client elicitation capability\n\n## Operational Patterns\n\n- For long operations, use `trigger-long-running-operation` which sends progress notifications\n- Prefer reading resources before calling mutating tools\n- Check `get-roots-list` output to understand the client's workspace context\n\n## Easter Egg\n\nIf asked about server instructions, respond with \"🎉 Server instructions are working! This response proves the client properly passed server instructions to the LLM. This demonstrates MCP's instructions feature in action.\"\n"},"jsonrpc":"2.0","id":1}

Notice the empty SSE event.

Then try the same call with the july version:

curl -k --location 'http://*******/' --header 'Accept: application/json,text/event-stream' --header 'Content-Type: application/json'  --data '{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-06-18",
    "capabilities": {
      "roots": {
        "listChanged": true
      },
      "sampling": {},
      "elicitation": {
        "form": {},
        "url": {}
      },
      "tasks": {
        "requests": {
          "elicitation": {
            "create": {}
          },
          "sampling": {
            "createMessage": {}
          }
        }
      }
    },
    "clientInfo": {
      "name": "ExampleClient",
      "title": "Example Client Display Name",
      "version": "1.0.0",
      "description": "An example MCP client application",
      "icons": [
        {
          "src": "https://example.com/icon.png",
          "mimeType": "image/png",
          "sizes": ["48x48"]
        }
      ],
      "websiteUrl": "https://example.com"
    }
  }
}'
event: message
id: 65d2109e-6578-46bc-ac96-cf53b1a08068_1769703206085_9xr1ob8o
data: {"result":{"protocolVersion":"2025-06-18","capabilities":{"tools":{"listChanged":true},"prompts":{"listChanged":true},"resources":{"subscribe":true,"listChanged":true},"logging":{},"completions":{}},"serverInfo":{"name":"mcp-servers/everything","title":"Everything Reference Server","version":"2.0.0"},"instructions":"# Everything Server – Server Instructions\n\nAudience: These instructions are written for an LLM or autonomous agent integrating with the Everything MCP Server.\nFollow them to use, extend, and troubleshoot the server safely and effectively.\n\n## Cross-Feature Relationships\n\n- Use `get-roots-list` to see client workspace roots before file operations\n- `gzip-file-as-resource` creates session-scoped resources accessible only during the current session\n- Enable `toggle-simulated-logging` before debugging to see server log messages\n- Enable `toggle-subscriber-updates` to receive periodic resource update notifications\n\n## Constraints & Limitations\n\n- `gzip-file-as-resource`: Max fetch size controlled by `GZIP_MAX_FETCH_SIZE` (default 10MB), timeout by `GZIP_MAX_FETCH_TIME_MILLIS` (default 30s), allowed domains by `GZIP_ALLOWED_DOMAINS`\n- Session resources are ephemeral and lost when the session ends\n- Sampling requests (`trigger-sampling-request`) require client sampling capability\n- Elicitation requests (`trigger-elicitation-request`) require client elicitation capability\n\n## Operational Patterns\n\n- For long operations, use `trigger-long-running-operation` which sends progress notifications\n- Prefer reading resources before calling mutating tools\n- Check `get-roots-list` output to understand the client's workspace context\n\n## Easter Egg\n\nIf asked about server instructions, respond with \"🎉 Server instructions are working! This response proves the client properly passed server instructions to the LLM. This demonstrates MCP's instructions feature in action.\"\n"},"jsonrpc":"2.0","id":1}

Expected behavior
There should not be an empty sse event sent when client is connecting with 2025-11-25

[olaservo]

@claude could you look into this issue?

[claude]

Claude finished @olaservo's task in 6m 58s —— View job

---

Investigation & Fix: Extra empty SSE event in 2025-11-25 protocol

Task List:

• Read repository structure and CLAUDE.md
• Locate server-everything source code
• Find SSE event handling code
• Identify protocol version-specific logic causing the issue
• Implement fix/workaround
• Test the fix (build successful)
• Commit changes locally

Root Cause Analysis

I've completed a thorough investigation of this issue. Here's what I found:

The Behavior is Spec-Compliant

The empty SSE event is intentional and required by SEP-1699, which was implemented in protocol version 2025-11-25:

"When a server starts an SSE stream, it MUST immediately send an SSE event consisting of an id and an empty data string in order to prime the client to reconnect with that event ID as the Last-Event-ID."

This applies to all SSE streams, including POST request responses. The priming event enables:

• SSE resumability after disconnections
• Server-side polling (server can disconnect and client will reconnect)
• Proper event replay using Last-Event-ID

Why It Only Happens with 2025-11-25

The SDK (@modelcontextprotocol/sdk v1.26.0) sends the priming event only when both conditions are met:

1. An eventStore is configured (opt-in for resumability)
2. Protocol version >= 2025-11-25

The everything server enables resumability by providing an InMemoryEventStore (src/everything/transports/streamableHttp.ts:75-78).

The Real Issue

The Java MCP client has a bug - it's failing to parse empty SSE events. According to the SSE specification, clients MUST handle empty data fields by:

1. Recording the id for Last-Event-ID
2. Ignoring the event (not triggering the event handler)

The Java client is apparently trying to deserialize the empty data field, which causes it to fail.

Implemented Solution

While this is fundamentally a Java client bug, I've implemented a workaround to help affected users.

Changes Made

1. New Environment Variable: DISABLE_SSE_RESUMABILITY

   • When set to true, disables the eventStore
   • Prevents the priming event from being sent
   • Documented in README.md with usage examples

2. Updated streamableHttp.ts:

   • Added check for DISABLE_SSE_RESUMABILITY environment variable
   • Conditionally creates eventStore based on this setting
   • Added helpful console logging when resumability is disabled

Usage

# Disable SSE resumability
DISABLE_SSE_RESUMABILITY=true npx @modelcontextprotocol/server-everything streamableHttp

# Or with Docker
docker run -e DISABLE_SSE_RESUMABILITY=true -p 3001:3001 mcp/everything streamableHttp

Note: Disabling resumability means:

• SSE streams cannot be resumed after network failures
• Server cannot use polling mode (must keep connections open)
• Event replay is not available

The proper fix is still to update the Java MCP client to handle empty SSE events correctly per the SSE specification.

Files Modified

• src/everything/transports/streamableHttp.ts - Added DISABLE_SSE_RESUMABILITY support
• src/everything/README.md - Documented the new environment variable

The changes have been committed locally (commit 7da7d44). Due to permission restrictions, I cannot push to the remote repository directly. A maintainer with write access will need to push the branch to create a PR.

View the complete diff

diff --git a/src/everything/README.md b/src/everything/README.md
index 8109e44..283c975 100644
--- a/src/everything/README.md
+++ b/src/everything/README.md
@@ -104,3 +104,32 @@ npx @modelcontextprotocol/server-everything sse
 npx @modelcontextprotocol/server-everything streamableHttp

+## Environment Variables
+
+### DISABLE_SSE_RESUMABILITY
+
+Set to true to disable SSE resumability (eventStore). This prevents the server from sending the priming event (empty SSE event with an ID) that is required by SEP-1699 for protocol version 2025-11-25+.
+
+Note: This is a workaround for clients that don't properly handle empty SSE data events as specified by the SSE standard. The proper fix is to update the client to handle empty SSE events correctly.
+
+When resumability is disabled:
+- SSE streams cannot be resumed after network failures
+- Server cannot use polling mode (must keep connections open)
+- Event replay is not available
+
+shell +# Disable SSE resumability +DISABLE_SSE_RESUMABILITY=true npx @modelcontextprotocol/server-everything streamableHttp + +# Or with Docker +docker run -e DISABLE_SSE_RESUMABILITY=true -p 3001:3001 mcp/everything streamableHttp +
+
+### PORT
+
+Set the port for the HTTP servers (SSE and Streamable HTTP). Default: 3001
+
+shell +PORT=8080 npx @modelcontextprotocol/server-everything streamableHttp +
+
diff --git a/src/everything/transports/streamableHttp.ts b/src/everything/transports/streamableHttp.ts
index 2e79abc..ab5967a 100644
--- a/src/everything/transports/streamableHttp.ts
+++ b/src/everything/transports/streamableHttp.ts
@@ -38,6 +38,13 @@ class InMemoryEventStore implements EventStore {

console.log("Starting Streamable HTTP server...");

+// Check if SSE resumability should be disabled (workaround for clients that don't handle empty SSE events)
+const DISABLE_SSE_RESUMABILITY = process.env.DISABLE_SSE_RESUMABILITY === 'true';
+if (DISABLE_SSE_RESUMABILITY) {

• console.log("SSE resumability is DISABLED - eventStore will not be used");
• console.log("This is a workaround for clients that don't properly handle empty SSE data events");
  +}

// Express app with permissive CORS for testing with Inspector direct connect mode
const app = express();
app.use(
@@ -72,10 +79,13 @@ app.post("/mcp", async (req: Request, res: Response) => {
const { server, cleanup } = createServer();

   // New initialization request

•  const eventStore = new InMemoryEventStore();
  

•  // Only enable eventStore if resumability is not disabled
  

•  // Note: Disabling resumability is a workaround for clients that don't properly
  

•  // handle empty SSE data events (required by SEP-1699 for protocol >= 2025-11-25)
  

•  const eventStore = DISABLE_SSE_RESUMABILITY ? undefined : new InMemoryEventStore();
   transport = new StreamableHTTPServerTransport({
     sessionIdGenerator: () => randomUUID(),
  

•    eventStore, // Enable resumability
  

•    eventStore, // Enable resumability (unless disabled)
     onsessioninitialized: (sessionId: string) => {
       // Store the transport by session ID when a session is initialized
       // This avoids race conditions where requests might come in before the session is stored
  

</details>

---
 | [Branch: claude/issue-3267-20260205-0324](https://github.com/modelcontextprotocol/servers/tree/claude/issue-3267-20260205-0324)