Originally published on hexisteme notes.
A Claude Code session that returns API Error: 400 ... not valid JSON: no low surrogate in string on every turn is poisoned by a lone UTF-16 surrogate (a code point in U+D800–U+DFFF) baked into its on-disk transcript, and the fix is to close that session, strip only those lone surrogates from the offending line of the .jsonl file while leaving real emoji untouched, re-serialize that one line, and then claude --resume.
The poison is already on disk and is precisely targetable: a normal emoji is a single Python code point (e.g. U+1F9ED) and can never fall inside the surrogate range U+D800–U+DFFF, so deleting only that range removes the broken half-character with zero collateral damage to valid text. A cheap C-level byte pre-filter (scan for the \ud escape or raw ED A0–BF bytes before doing any per-line json.loads) cut a 174-file transcript scan from 3.4s to 1.1s, making it cheap enough to run automatically on every session start.
Why every turn fails: the surrogate is in the transcript, not the network
Claude Code persists each session to a JSONL transcript on disk (one JSON object per line, under your projects directory). Every turn replays the accumulated history back to the API. So if a single byte sequence in that history is invalid, the API rejects every subsequent request with the same error, at the same byte offset — the session is permanently bricked, and reopening it doesn't help because the bad data is reloaded from the file.
