Anthropic API 400 Error: No Low Surrogate in String — Invalid JSON Request Body Validation Failure

What this error means

API Error: 400 The request body is not valid JSON: no low surrogate in string: line 1 column 195621 (char 195620) is a Claude Code failure pattern reported for developers trying to fix 400 error caused by invalid unicode surrogate pair in large json request body sent to anthropic/claude api. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub issue #60394 on anthropics/claude-code (2026-05-19). Claude Code sends oversized request body with malformed UTF-16 surrogate (no low surrogate after high surrogate). API rejects with 400 invalid_request_error. Affects users processing large context windows with non-ASCII characters. Category: API-level error → ‘Anthropic API’.

Common causes

• GitHub issue #60394 on anthropics/claude-code (2026-05-19). Claude Code sends oversized request body with malformed UTF-16 surrogate (no low surrogate after high surrogate). API rejects with 400 invalid_request_error. Affects users processing large context windows with non-ASCII characters. Category: API-level error → ‘Anthropic API’.

Quick fixes

1. Confirm the exact error signature matches API Error: 400 The request body is not valid JSON: no low surrogate in string: line 1 column 195621 (char 195620).
2. Check the Claude Code account, local tool state, and provider configuration involved in the failing workflow.
3. Compare the failing environment with a known working setup, then change one configuration value at a time.

Platform/tool-specific checks

• Verify the command, editor, extension, or API client that produced the error.
• Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
• Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

1. Capture the exact error message and the command, editor action, or request that triggered it.
2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
3. Review the source evidence below and compare it with your environment.
4. Apply one change at a time and rerun the smallest failing action.
5. Keep the working fix documented for the team or deployment environment.

How to prevent it

• Keep provider/tool configuration documented.
• Record non-secret diagnostics such as tool version, provider name, model name, and command path.
• Add a lightweight check before CI or production workflows depend on the tool.

Sources checked

• https://github.com/anthropics/claude-code/issues/60394

Evidence note: GitHub issue #60394 on anthropics/claude-code (2026-05-19). Claude Code sends oversized request body with malformed UTF-16 surrogate (no low surrogate after high surrogate). API rejects with 400 invalid_request_error. Affects users processing large context windows with non-ASCII characters. Category: API-level error → ‘Anthropic API’.

Related errors

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FAQ

What should I check first?

Start with the exact API Error: 400 The request body is not valid JSON: no low surrogate in string: line 1 column 195621 (char 195620) text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Claude Code workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without API Error: 400 The request body is not valid JSON: no low surrogate in string: line 1 column 195621 (char 195620).