SIGKILL / crash when MCP client sends `elicitation` capability in initialize payload (protocol version 2025-11-25)

[swarthyplacebo]

Summary

@fangjunjie/ssh-mcp-server v1.8.3 crashes with SIGKILL when the MCP initialize request includes the elicitation capability, which is standard in MCP protocol version 2025-11-25. This affects all modern MCP clients using the current spec, including Claude Desktop, Anthropic's Antigravity (Gemini CLI), and any client that negotiates 2025-11-25.

Affected Version

@fangjunjie/ssh-mcp-server v1.8.3 (latest as of 2026-06-04)

Reproduction

Send a 2025-11-25-spec initialize payload to the server:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-11-25",
    "capabilities": {
      "roots": { "listChanged": true },
      "sampling": {},
      "elicitation": {}
    },
    "clientInfo": { "name": "test-client", "version": "1.0" }
  }
}

Result: Process exits with SIGKILL immediately. No error output, no graceful shutdown.

Expected: Server responds with its own capabilities and negotiates the protocol version normally (ignoring or acknowledging elicitation as an unknown/unsupported capability).

Impact

Any MCP client implementing protocol version 2025-11-25 will be unable to connect to this server. Claude Desktop and Gemini CLI (Antigravity) both send elicitation: {} in their initialize payloads as of mid-2025.

Workaround Applied

We deployed a thin Node.js shim that intercepts the initialize payload, strips the elicitation field, and forwards the modified payload to the binary:

// elicitation-shim.js
const { spawn } = require('child_process');
const readline = require('readline');

const server = spawn('/usr/local/bin/ssh-mcp-server', process.argv.slice(2), {
  stdio: ['pipe', 'pipe', 'inherit']
});

const rl = readline.createInterface({ input: process.stdin });

rl.on('line', (line) => {
  try {
    const msg = JSON.parse(line);
    if (msg.method === 'initialize' && msg.params?.capabilities?.elicitation) {
      delete msg.params.capabilities.elicitation;
    }
    server.stdin.write(JSON.stringify(msg) + '\n');
  } catch {
    server.stdin.write(line + '\n');
  }
});

server.stdout.on('data', (data) => process.stdout.write(data));
server.on('exit', (code) => process.exit(code ?? 0));

This restores connectivity but requires maintaining a custom Docker image. The fix should be in the server itself.

Suggested Fix

In the initialize handler, treat unrecognized capability keys (including elicitation) as no-ops rather than allowing them to cause a crash. The server should respond with only the capabilities it supports and ignore unknown ones per the MCP spec.

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Co-authored by Claude Sonnet 4.6