Claude.AI hangs with 408 timeout when MCP tool response JSON contains U+2028 (LINE SEPARATOR) or U+2029 (PARAGRAPH SEPARATOR)

Summary

Claude.AI times out with 408 (Waited 300.0 seconds) for certain MCP tool calls, even though the MCP server logs show a successful and fast 200 response.
This appears to happen when upstream JSON data includes raw Unicode line/paragraph separators in string fields:

• U+2028 LINE SEPARATOR
• U+2029 PARAGRAPH SEPARATOR

Environment

• Client: Claude.AI (web)
• Transport: MCP Streamable HTTP via POST /mcp
• Server runtime: Node.js + TypeScript
• MCP SDK: @modelcontextprotocol/sdk ^1.29.0
• Hosting: Railway
• Backend API source: KEXP public API

Actual Behavior

• Claude.AI tool call hangs and eventually returns 408 timeout.
• Server logs show:
  • Request received
  • Tool executed
  • 200 response returned quickly
• Issue is reproducible with one specific record containing unusual Unicode content, while adjacent records work normally.

Expected Behavior

• Tool call should succeed and return parsed response.
• If parsing fails client-side, client should fail fast with a parse error, not hang until timeout.

Repro (Minimal)

Create a minimal MCP tool that returns JSON containing raw U+2028 in a string value.

Example server snippet

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';

const server = new McpServer({ name: 'unicode-repro', version: '1.0.0' });

server.tool('repro_u2028', 'Returns JSON with U+2028 in a string', {}, async () => {
  const payload = {
    comment: 'Line one.\u2028Line two.'
  };

  return {
    content: [
      {
        type: 'text',
        text: JSON.stringify(payload)
      }
    ]
  };
});

const transport = new StdioServerTransport();
await server.connect(transport);

Repro steps

1. Connect this MCP server to Claude.AI.
2. Invoke the repro_u2028 tool.
3. Observe timeout behavior.
4. Change the payload so separators are escaped in transport text (for example replacing raw U+2028 with escaped sequence form before parse/forward).
5. Re-run and observe successful response.

Why this seems important

U+2028 and U+2029 are valid in JSON text, but historically treated as line terminators in JavaScript contexts.
A parser/transport boundary handling bug could cause a stall instead of a deterministic parse error.

Server-Side Workaround Confirmed

Sanitizing response text before JSON.parse resolved the issue in production for the affected records.

const text = await response.text();
const sanitized = text
  .replace(/\u2028/g, '\\u2028')
  .replace(/\u2029/g, '\\u2029');
const data = JSON.parse(sanitized);

Request

Please investigate Claude.AI MCP response parsing/transport handling for payloads containing U+2028 and U+2029, and return a deterministic error or successful parse instead of timeout behavior.

Start at the Streamable HTTP POST /mcp response path and JSON parsing boundary, using the supplied repro_u2028 tool with raw U+2028 and U+2029. Trace the response handling and add coverage for both separators. Done means a successful parsed tool response or an immediate deterministic parse error instead of a timeout.