[BUG] Agent SDK crashes with "Unterminated string in JSON" when tool output contains U+2028/U+2029 characters

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

[AGENT SDK BUG - not CLI]

The Agent SDK crashes with "Unterminated string in JSON" when a tool returns content containing Unicode Line Separator (U+2028) or Paragraph Separator (U+2029) characters. These characters commonly appear in web-scraped content and cause ProcessTransport.readMessages to fail when parsing JSONL from the CLI.

What Should Happen?

The SDK should properly handle U+2028 and U+2029 characters in tool outputs without crashing. These are valid Unicode characters that appear in real-world web content.

Error Messages/Logs

SyntaxError: Unterminated string in JSON at position 872 (line 1 column 873)
      at JSON.parse (<anonymous>)
      at ProcessTransport.readMessages (sdk.mjs:13316:32)
      at async Query.readMessages (sdk.mjs:13583:24)

Steps to Reproduce

1. Create a Bash skill/tool that runs:

```bash
printf '{"success": true, "data": {"markdown": "hello'
printf '\xe2\x80\xa8'
printf 'world"}}\n'
exit 0

1. Agent calls this tool
2. SDK crashes with: Unterminated string in JSON at position 136

Root cause: U+2028 and U+2029 act as line terminators in JavaScript. When the SDK reads JSONL line-by-line, these characters split the JSON mid-string, causing parse failure.

Workaround: Sanitize tool outputs: sed $'s/\xe2\x80\xa8//g' | sed $'s/\xe2\x80\xa9//g'

Environment:

• @anthropic-ai/claude-agent-sdk@0.1.76
• Node v24, Linux (Debian 12)

Key fixes:

• Added bash language hint to code blocks
• Fixed list indentation (3 spaces for nested code block)
• Bolded section headers
• Backticks around package name

Claude Model

Not sure / Multiple models

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

N/A - Using Agent SDK directly (@anthropic-ai/claude-agent-sdk@0.1.76), not Claude Code CLI

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

Why this happens in practice

U+2028 (0xE2 0x80 0xA8) and U+2029 (0xE2 0x80 0xA9) commonly appear in web-scraped content. They're invisible in most editors but act as line terminators in JavaScript, breaking JSONL line-by-line parsing.

## Technical details

The SDK's ProcessTransport reads CLI output line-by-line. When a JSON string contains U+2028/U+2029, the line gets split mid-string:

Expected: {"content": "hello\u2028world"}
Parsed as two lines:
Line 1: {"content": "hello ← Unterminated string
Line 2: world"}

## Suggested fix

Escape or strip U+2028/U+2029 when serializing tool outputs to JSONL, or use a JSON-aware line parser instead of splitting on newlines.