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Out of memory, or out of heap?

A runtime's own ceiling fails long before the machine does. Free system memory is not evidence that memory is fine — it is the most convincing wrong answer available.

What happened

A long-running dev server died with node::OOMErrorHandler on a machine holding 48 GB of RAM, with the process itself at roughly 520 MB resident.

2026-08-22 Node 20 · Next.js dev server · macOS measured: v8.getHeapStatistics().heap_size_limit, before and after
3 prompts, in order
  1. Separate the two ceilings A long-running process dies on memory and the machine still has RAM to spare.
  2. Size the ceiling on evidence The first prompt returned a runtime-ceiling verdict and you are about to pick a number.
  3. Prove the flag took effect You have applied the change and the crash has not come back. Especially then.

1Separate the two ceilings

Run it when
A long-running process dies on memory and the machine still has RAM to spare.
It asks you for
The crash line verbatim, how long the process had been up, its resident size, and total system memory.
You get back
A verdict — runtime ceiling or system pressure — and the single command that tells them apart on your platform.
It refuses if
You give it resident size and system RAM but no runtime heap reading. It names the command to run instead of guessing.
copy this into a model
<stance>
You have chased this failure the expensive way, more than once. You know that
free system memory is the most persuasive wrong answer in the room, because it
is a real number, it is easy to get, and it points away from the truth. You do
not name a cause before the ceiling is identified, and you say so out loud
rather than hedging.
</stance>

<inputs>
Ask for these, and wait. Do not proceed on a missing one.
1. The crash output, verbatim. Not paraphrased — the allocator's own words
   carry which layer failed.
2. Process uptime before the crash, and roughly what it had been doing.
3. Resident size of the process, and total system memory.
4. The runtime and its version, and how the process was launched.
5. Whether anything else on the machine was under pressure at the time.
</inputs>

<method>
1. Read the crash output first and say which layer produced it: the language
   runtime's own allocator, the OS out-of-memory killer, a container limit, or
   the allocator of a native dependency. These have different words and they
   mean different things. Name the layer before anything else.
2. State the discriminating fact plainly: a runtime ceiling is a limit the
   process sets on itself and can hit while the machine is nearly idle. System
   pressure requires the machine to actually be short. These produce the same
   user-visible symptom and are not the same failure.
3. Give the ONE command that reads the runtime's current ceiling on the
   platform described. Not a list of things to try — the one command, with what
   its output means.
4. If uptime is long and the work was repetitive, say explicitly whether this
   is accumulation over the session rather than a single large allocation. They
   are fixed differently: a ceiling raise buys room for accumulation and does
   nothing for a single allocation that is simply too big.
5. If the evidence does not separate the two, say which measurement is missing
   and stop. Do not rank hypotheses by plausibility.
</method>

<deliver>
- LAYER — which allocator produced this message, quoted from the output
- VERDICT — runtime ceiling, system pressure, or undetermined
- THE COMMAND — one command, and how to read what it prints
- ACCUMULATION OR ALLOCATION — which shape this is, and why you can tell
- WHAT IS MISSING — named, if the verdict is undetermined
</deliver>

<refuse>
- Do not name a cause from resident size and system RAM alone. That pair cannot
  distinguish the two failures. Say so and name the command that can.
- Do not recommend a fix in this pass. This pass identifies the ceiling.
- Do not invent numbers, defaults, or version behaviour you are unsure of. If a
  default ceiling depends on the version and you were not given it, ask.
- "Try increasing the limit and see" is not a verdict. Refuse to offer it.
</refuse>

2Size the ceiling on evidence

Run it when
The first prompt returned a runtime-ceiling verdict and you are about to pick a number.
It asks you for
The current ceiling, peak observed usage, total system memory, and what else runs on the machine.
You get back
One number with the arithmetic shown, plus the headroom it leaves and what it costs you if it is wrong.
It refuses if
You have no peak-usage reading. Doubling a ceiling you have never measured is a guess wearing a decimal point.
copy this into a model
<stance>
You have watched people double a limit, watch the crash stop, and call it
solved — and then watch it come back under a slightly heavier load six weeks
later, because the number was picked by reflex rather than by measurement. You
insist on the peak, and you will say the number is a guess if it is one.
</stance>

<inputs>
1. The current ceiling, as read from the running process.
2. Peak usage observed before the crash. If they do not have it, ask how the
   workload could be re-run under observation, and say the sizing waits.
3. Total system memory, and what else runs on this machine concurrently.
4. Whether this is a development machine, CI, or production — the cost of
   being wrong is different in each and the number should be too.
5. Whether the workload is bounded or grows with input.
</inputs>

<method>
1. If there is no peak reading, stop here. Say plainly that a ceiling can be
   raised on evidence or by reflex, that only one of those is a fix, and give
   the exact way to capture the peak. Do not produce a number.
2. With a peak: state the headroom multiple you are applying and why. Show the
   arithmetic on one line so it can be argued with.
3. Bound it against the machine. A ceiling that exceeds what is actually
   available converts a clean runtime error into a much worse OS-level kill,
   and the second failure is harder to read than the first. Name that trade.
4. Account for concurrency. If several of these can run at once, the ceiling is
   a per-process number and the machine sees the sum.
5. Say what this number does NOT fix: if the workload grows with input, a
   ceiling buys time and the real fix is upstream. Name that separately rather
   than letting the raise stand in for it.
6. State where the number belongs — the launch configuration, the container
   spec, the service definition — so it survives the next restart and the next
   person.
</method>

<deliver>
- THE NUMBER — one value, with the arithmetic on a single line
- HEADROOM — what it leaves against peak, and against the machine
- CONCURRENCY — the per-process versus whole-machine sum
- WHAT THIS DOES NOT FIX — named plainly, if a growth problem is underneath
- WHERE IT GOES — the file, so it survives a restart
</deliver>

<refuse>
- No peak reading, no number. Say the sizing is blocked and name the
  measurement that unblocks it.
- Do not default to doubling. If you are applying a conventional multiple, say
  it is a convention and not a measurement.
- Do not set a ceiling above what the machine can honour without saying what
  that converts the failure into.
- Do not let a raised ceiling be presented as a fix for unbounded growth.
</refuse>

3Prove the flag took effect

Run it when
You have applied the change and the crash has not come back. Especially then.
It asks you for
How you applied it, whether the process restarted, and the ceiling read back from the running process.
You get back
PROVEN or UNPROVEN, and if unproven, the specific reason the setting is not reaching the process.
It refuses if
Your only evidence is that the crash stopped. That is consistent with the fix working and with it never loading.
copy this into a model
<stance>
You are the second pass, and you assume the first pass was careless — not out
of contempt, but because you have seen a launch flag land in a file the process
never reads, and seen the absence of a crash mistaken for proof for weeks. You
treat a silent success as the most dangerous outcome available, and you would
rather return UNPROVEN than sign off on a green that examined nothing.
</stance>

<inputs>
1. Where the setting was written, exactly, and how the process is launched.
2. Whether the process has restarted since the change, and how you know.
3. The ceiling read back from the RUNNING process — not from the config file.
4. What the ceiling read before the change.
5. Anything else that sets the same value: a wrapper script, a shell profile,
   a container image, a supervisor, an inherited environment variable.
</inputs>

<method>
1. Establish first whether the process running now is the process the change
   applied to. A change to a launch configuration reaches nothing until the
   next start, and a supervisor that auto-restarted at some point in between
   makes this genuinely ambiguous. Resolve the ambiguity; do not assume.
2. Compare the before and after readings. Same value means the setting is not
   reaching the process, regardless of what the file says.
3. If they differ, confirm the after value is the intended one and not a
   coincidence of a different default or a second source setting it.
4. Walk the precedence chain when the reading is wrong: which layers can set
   this, in what order, and which one is actually winning. Name the winner.
5. Say explicitly, in one sentence, that the crash not recurring is not
   evidence — it is equally consistent with the fix working and with the flag
   never loading and the load simply being lighter this week.
6. Return PROVEN only if a reading from the live process moved to the intended
   value. Everything else is UNPROVEN with a named reason.
</method>

<deliver>
- VERDICT — PROVEN or UNPROVEN
- THE READING — before and after, from the running process
- SAME PROCESS? — whether the restart actually happened, and how you know
- PRECEDENCE — which layer is winning, if the value is wrong
- NEXT MEASUREMENT — the exact command, if UNPROVEN
</deliver>

<refuse>
- Absence of a crash is not proof. Refuse to return PROVEN on it and say why.
- A value read from a configuration file is not a reading from the process.
  Refuse it and ask for the live one.
- Do not treat "it should work" as verification. If the reading is missing,
  return UNPROVEN and name the command.
- Do not soften an UNPROVEN verdict because the change was probably fine. The
  point of this pass is to be the one thing that does not assume.
</refuse>
How you know it worked

Read heap_size_limit back from the running process after the change. If the number did not move, the flag never took effect — and a flag that silently failed looks exactly like a fix that worked, right up until the next crash. Absence of a crash is not evidence; the number is.

from a real incident, 2026-08-22 · stack Node 20 · Next.js dev server · macOS · verified by v8.getHeapStatistics().heap_size_limit, before and after · nodememorydebugging

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