API error: lone surrogates in JSON body cause 'no low surrogate in string' rejection

[codeg-old]

Bug Description

When tools read non-UTF-8 files or capture terminal output containing invalid Unicode, lone surrogate characters (U+D800–U+DFFF) can enter the conversation context. JavaScript's JSON.stringify() permits lone surrogates per ECMA-262, but Anthropic's API uses Rust serde_json which strictly enforces RFC 8259 and rejects the request:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error",
"message":"The request body is not valid JSON: no low surrogate in string: line 1 column 88449 (char 88448)"}}

Reproduction

1. Create a file containing a lone surrogate: printf 'Hello \xed\xa0\x80 World' > /tmp/test.txt
2. Ask OpenCode to read that file
3. The next API request fails with the above error

Expected Behavior

OpenCode should sanitize lone surrogates (replace with U+FFFD) before sending API requests, so invalid Unicode in tool outputs never causes JSON parse failures.

Context

This is a well-known issue across the Claude ecosystem:

• [BUG] Session broken -> API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"The request body is not valid JSON: no low surrogate in string: line 1 column 410559 (char 410558)"}} anthropics/claude-code#1709 (original report)
• JSON Parsing Error: Invalid Low Surrogate in Request Body anthropics/claude-code#1832, tui: ability to view active lsps (or errors if failed to start) #1939, Is there a repository map or RAG feature? #2108, Incorrect token usage #5440, The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175 (duplicates)

Microsoft Playwright solved the same problem using String.prototype.toWellFormed().

Environment

• OpenCode v1.2.6
• Bun 1.3.9
• macOS

[codeg-old]

⚠️ This comment was authored with AI assistance (Claude Sonnet 4.6 / OpenCode).

Update: This issue has already been resolved in the codebase.

After reviewing the upstream source code, I can confirm that sanitize-surrogates.ts was added in commit 9da720414 (2026-02-18) and fully handles this case:

• fixLoneSurrogateCodeUnits() — handles literal lone surrogates (U+D800–U+DFFF in the string)
• fixJsonSurrogateEscapes() — handles JSON-escaped lone surrogates (\uD800 ASCII escape sequences)
• sanitizeSurrogates() — orchestrates both, called from provider.ts before every API request

The provider.ts integration at line ~1208:
```ts
opts.body = sanitizeSurrogates(opts.body)
```

This covers both the literal-surrogate and JSON-escaped-surrogate cases that triggered the Anthropic API 400 errors. The issue is already fixed; this can be closed.

[SunCreation]

PR submitted: #17758 — adds sanitizeJsonSurrogates() in the fetch wrapper to handle both literal and JSON-escaped lone surrogates. Tested and verified on v1.2.27 with Korean web search MCP tools.

[github-actions]

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