fix(coding-agent): persist kernel stderr to disk and bound the in-memory tail - #1947
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fix(coding-agent): persist kernel stderr to disk and bound the in-memory tail#1947snimu wants to merge 2 commits into
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xeophon
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Sep 1, 2026
kernelStderr accumulated the kernel's stderr in memory for its lifetime while every reader shows at most the last 1 KiB. Hand the spawned kernel an fd onto kernel-stderr.log in the session artifact directory (rotated once per spawn) so the full pre-ready stderr stays inspectable, read failure tails from that file, and bound the in-memory diagnostics tail to 8 KiB (ENG-5832).
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…up failures stderrTail() loaded the whole log to slice 1KB off the end; a kernel that spews to stderr until the 30s ready timeout can leave a log far too large to buffer in the host, so the failure report now does a positional read of the last 1KB only.
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When a Python kernel failed to start, the failure was often undiagnosable: its stderr lived only in an in-memory string that died with the process, and the startup error showed just the last kilobyte of it. Nothing stayed on disk to inspect after the fact — and on the other end, a long-lived chatty kernel grew that same string without bound for its whole lifetime, even though every reader only ever showed the tail.
The fix is spawn-shaped because the kernel's stderr is startup-shaped: the runtime dup2's fd 2 into its protocol pipe before it reports ready (repl.py
_setup_fds), so the process-level stderr only ever carries pre-ready bytes — interpreter, venv, and import failures; everything after ready already flows through the protocol into the bounded per-execution stderr. So at spawn the manager now openskernel-stderr.login the session's artifact directory, rotates it to.oldwhen it exceeds 5 MiB (once per spawn — pre-ready output is startup-bounded), and hands the fd straight to the child as its stderr slot. The OS writes the bytes: no host-side relay, decoding, buffering, or flush to get wrong, and the file is complete the moment the process exits.Startup failure messages read their 1 KiB tail from that file, plus any host-side
[kernel]diagnostics from the in-memory string, now bounded to an 8 KiB tail. Sessions without an artifact directory (or whose log open fails) fall back to the previous pipe-plus-memory-tail behavior. The log persists across kernel restarts within the rotation cap.Validation: two pins — a kernel that writes CR progress output and a split multi-byte character with no trailing newline before dying produces a log byte-identical to what it wrote, with that tail in the startup error; and a log pre-filled past 5 MiB rotates to
.oldat the next spawn, with the new incarnation's stderr starting a fresh file. repl-kernel test suites and rootnpm run checkgreen.Linear: RES-1246 https://linear.app/primeintellect/issue/RES-1246/persist-python-kernel-stderr-to-disk-and-bound-the-in-memory-buffer