Under the hoodTier: how much it should matter to you
1Useful: my rating, 1 to 5
1Signal: worth watching, 1 to 5
SDKArea: what it touches
Internal ChangesKind: in v2.1.282,
Internal ChangesSection of the release
What
When Claude Code runs under the SDK (the programming interface for driving Claude Code from other software) or with --print, it occasionally checks a sample of its output messages against the expected output format. Building the description of that format used to happen right away, every time a sampled message was written.
Now the format description is built later, on a timer that starts after the first result message is written, and only while the session is idle. Sampled checks run only once it exists. If building it fails, the failure is logged and checking stays off until the next attempt.
Why
This takes a costly step out of the path that writes output, which should keep SDK and --print sessions from slowing down on it. The trade-off is that messages written before the first idle result are no longer checked.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up --print, on Error reference.| `Error: --agents takes a JSON object, or a file path only with --print (-p)` | [Command-line errors](#invalid-agents-configuration) |errorssee the edit
Added sinceA small documentation edit on CLI reference touched a line naming --print after this was published.| `--bg`, `--background` | Start the session as a [background agent](/docs/en/agent-view) and return immediately. Prints the session ID and management commands. Combine with `--exec` to run a shell command as a background job instead of a …cli-referencesee the edit
How sure we are
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agreesA small documentation edit on CLI reference touched a line naming --print after this was published.
