JSONL is easy to underestimate. One object per line sounds like a format you can parse with a short loop. A live coding-agent transcript adds the cases that make that loop unsafe: a partial final append, an unusually large record, duplicate events, provider-specific envelopes, and old completion markers that later activity has invalidated.
Here is the defensive design we use before inferring session state.
Bound I/O before JSON parsing
Do not read an unbounded transcript into one string. Use a streaming reader and a documented line policy. Agent Island v1.7.1 currently uses different released safeguards on each platform: the macOS Claude reader has a 64 MiB backstop, while the Windows reader skips a line above 1,000,000 characters before parsing.
Those are not one shared limit. They are platform-specific policies with the same goal: one malformed or pathological record should not take down the rest of the scan.
