Inside Claude Code’s 1400-line agent loop

The core agent logic in query.ts of Claude Code v2.1.88 is a 1400-line while(true) loop. Unlike textbook examples, this code handles real-world failures like network drops, context overflows, and script stalls.

The system is a single-threaded async generator. The single-threaded design avoids race conditions when executing tools. Nine conditions govern the loop, including API error retries, token limit handling, and sub-agent management.

A tiered compression system manages context. Aggressive prompt caching saves up to 90% of tokens per session. All events are logged to JSONL files in real-time.