U+2028 LINE SEPARATOR in MCP tool results causes JSON parsing failures.

[bmaurer]

A U+2028 LINE SEPARATOR in MCP tool results causes the claude SDK to fail with

Error: CLI output was not valid JSON. This may indicate an error during startup. Output: {"type":"user","message":{"role":"user","content":[{"tool_use_id":"toolu_01F1kfdYcsZrBCXmE8y7bn6k","type":"tool_result","content":[{"type":"text","text":"before

Minimal repro below:

#!/usr/bin/env npx tsx
/**
 * U+2028 LINE SEPARATOR in MCP tool results causes JSON parsing failures.
 *
 * - JSON.stringify does NOT escape U+2028
 * - Node's readline treats U+2028 as a line terminator
 * - The JSON line gets split, causing JSON.parse to fail
 */

import { query, createSdkMcpServer, tool } from "@anthropic-ai/claude-agent-sdk";
import { z } from "zod";

const mcpServer = createSdkMcpServer({
  name: "test",
  version: "1.0.0",
  tools: [
    tool("get_text", "Get text", { id: z.string() }, async () => ({
      content: [{ type: "text", text: "before\u2028after" }],
    })),
  ],
});

(async () => {
  const q = query({
    prompt: "Call get_text with id 'x'",
    options: {
      model: "haiku",
      mcpServers: { test: mcpServer },
      allowedTools: ["mcp__test__get_text"],
      tools: [],
    },
  });

  for await (const msg of q) {
    if (msg.type === "result") break;
  }
})();