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;
  }
})();

[dyoung522]

We're seeing a related issue with the Anthropic Messages streaming API (not MCP/claude-agent-sdk). During SSE streaming of claude-opus-4-6 responses, JSON.parse() on data: lines occasionally throws:

Bad control character in string literal in JSON at position N (line 1 column N+1)

The position varies across occurrences (122, 131, 134, 156), suggesting it's wherever a control character lands in that particular SSE chunk. This happens intermittently during coding-heavy sessions where the model is generating code blocks.

Environment:

• @anthropic-ai/sdk 0.73.0
• Node.js 22.x
• Running via OpenClaw gateway (streaming enabled)

Root cause appears the same as this issue — the SDK's streaming parser (core/streaming.mjs line 32: yield JSON.parse(sse.data)) doesn't sanitize control characters before parsing. A guard that strips/escapes C0 control characters (U+0000–U+001F) and Unicode line/paragraph separators (U+2028, U+2029) from SSE data lines before JSON.parse() would fix both this issue and ours.

Impact: Non-blocking — the session recovers on the next turn, but the current response is lost and the user sees a raw JSON parse error.

[s1van]

U+0085 (NEL) causes the same failure in Claude.ai's remote MCP connector

Same root cause as this issue, different character and component.

Character: U+0085 (NEL / NEXT LINE), a C1 control. JSON.stringify doesn't escape it. UTF-8: 0xC2 0x85.

Symptom: MCP tool responses with raw U+0085 in JSON string values hang indefinitely through Claude.ai's remote MCP connector (Streamable HTTP). No error, no timeout.

Reproduction (same server, same 10KB payload, same tunnel):

Client	Result
Claude.ai remote MCP (Streamable HTTP)	❌ Hangs
ChatGPT remote MCP (same server)	✅ Works
Claude local MCP (stdio)	✅ Works
curl direct	✅ Works

Source: PDF bookmark titles from pdfcpu — e.g. "Summary\u0085Book Outline" (6 occurrences in one response).

Workaround: Server-side escaping after JSON.stringify:

JSON.stringify(payload).replace(/\u0085/g, '\\u0085');

Note: This issue covers the SDK's stdio readline transport. Our bug is in Claude.ai's remote MCP connector (Streamable HTTP) — different component, same root cause.