Long tool-use sessions could get stuck with repeated 400 errors when a tool result was not adjacent to its tool_use (for example a notification flushed between step.begin and the first tool.call, or an interrupted or nested step delaying the result). Micro compaction exposed this latent misordering by busting the prompt cache and forcing a full revalidation.
Repair the adjacency at projection time so every tool_use is immediately followed by its tool_result(s), and default the experimental micro compaction flag off.
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