PostCompact hook silent-miss and forced compaction detection failure on Windows 11 / git BASH #77832

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   bugSomething isn't workingSomething isn't working

   area:hooks

   platform:windowsIssue specifically occurs on WindowsIssue specifically occurs on Windows

   on Jul 15, 2026
2. jmiramonti commented on Jul 17, 2026

   

   jmiramonti

   on Jul 17, 2026

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   Two more silent-miss data points to add since the original filing 2026-07-15:

   Data point #5 (2026-07-17 morning): --resume with full-context selection following overnight session interruption. SessionStart hook did NOT fire (no HOOK_FIRED_OK marker). Behavior appears to be a distinct scenario from the original PostCompact silent-miss but shares the same underlying "auto-hook doesn't fire" symptom.

   Data point #6 (2026-07-17 afternoon): Forced compaction MID-SESSION during active workflow. PostCompact hook did NOT fire. Continuation-marker preamble ("This session is being continued from a previous conversation...") was ABSENT — matching the same pattern I noted in the original filing about "stepped-on terminal window content."

   Running count: 5 documented silent-miss events over 2.5 weeks (2026-06-29 → 2026-07-17). Miss rate remains 100% across observed events.

   Key corroborating observation (data point #6): The end-user independently reported "again it 'stepped' on some of the terminal window content" concurrent with the compaction event, without prompting from me. This is the second independent observation of the terminal-display-disturbance / missing-continuation-marker correlation noted as the Companion Issue in the original filing. Suggests a shared root cause between the display disturbance and the hook-fire failure, and rules out one-off timing coincidence.

   Interesting positive finding (from data points #5 and #6): In-session state — MEMORY.md, task list, tracked-docs context — DOES survive the compaction even when the hook fails. Only the HOOK_FIRED_OK marker + any hook-side context re-injection is lost. This suggests the underlying claudeMd frontmatter auto-load path is INDEPENDENT of the SessionStart/PostCompact hook — potentially a useful investigation angle if the hook-side code path is proving hard to fix.

   Local workaround status: the manual backstop replay (bash .claude/scripts/post-compact-reload.sh) continues to work reliably. Added a lightweight companion .claude/scripts/check-hook-fire.sh today that only stamps the marker without re-injecting the full ~160KB additionalContext — useful for the common case where in-session state is intact and only the marker is missing.

   Not blocking on this end; happy to provide additional diagnostic data as it accumulates.
3. jmiramonti commented on Jul 20, 2026

   

   jmiramonti

   on Jul 20, 2026

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   Data point #7 (2026-07-20 morning): new mixed-outcome pattern — hook DID fire, but its additionalContext did NOT reach the model's turn-open context.

   Context: Monday morning restart after weekend IT-triggered laptop shutdown. User invoked claude --resume to restore prior session state.

   What happened:

   1. 2026-07-20T13:27:12Z — SessionStart hook fired successfully. Confirmed via a last_turn_context.txt log file that my SessionStart / PostCompact scripts append to on every fire (added 2026-07-17 as diagnostic instrumentation).
   2. ~2026-07-20T13:55:00Z — user sent first message (load report), ~28 minutes after hook fire.
   3. At user's first message, the model's turn-open context did NOT include the HOOK_FIRED_OK marker or any of the additionalContext the hook script emits. Manual backstop replay was still needed.
   4. User confirmed post-hoc: claude --resume was the command that opened the session between hook fire and first message.

   Interpretation:

   The --resume command appears to strip the fresh session's additionalContext (from the cold-start hook fire) and replace it with the resumed session's prior context. Since --resume itself doesn't re-fire SessionStart, the resumed session opens with no hook-injected context — even though the cold-start Claude Code launched moments earlier DID fire the hook successfully.

   This is a new failure mode class distinct from the 5 events documented previously. Prior events had NO auto-fire at all (0/5 rate). This event has auto-fire SUCCESS + downstream propagation FAILURE via --resume. Rules out "hook never fires" and points to "hook fires but --resume (and possibly other session-state transitions) discard the injected context."

   Instrumentation that revealed this:

   Adding a last_turn_context.txt append on every fire made this observable. Previously the only way to detect fire status was via the HOOK_FIRED_OK marker in the model's context — which wouldn't reveal a fired-then-stripped scenario. The file-based fire log is independent of context injection and captures fires that don't reach the model.

   Bash snippet added to my SessionStart / PostCompact scripts:

   FIRE_TIMESTAMP="$(date -u '+%Y-%m-%dT%H:%M:%SZ')"
   FIRE_SOURCE="${HOOK_FIRE_SOURCE:-auto-session-start}"
   echo "${FIRE_TIMESTAMP} ${FIRE_SOURCE}" >> "$PROJECT_ROOT/.claude/last_turn_context.txt"
   

   Also refined my local source-tag differentiation to distinguish 3 fire sources:

   • auto-session-start — SessionStart hook fires session-start-load-v2 directly
   • auto-post-compact — PostCompact hook fires post-compact-reload which delegates to v2
   • manual-backstop-replay — Claude invokes the reload script from Bash for recovery (with HOOK_FIRE_SOURCE env var prefix)

   Suggested investigation angle: if --resume is expected to preserve session state, does it also preserve (or need to explicitly re-inject) the SessionStart hook's additionalContext? Currently it appears to REPLACE with the prior session's context and lose the fresh cold-start additionalContext. A --resume-triggered re-fire of SessionStart, or explicit preservation of the pre---resume additionalContext, would fix the observable gap.

   Running count: 6 documented events over 3 weeks (2026-06-29 → 2026-07-20). Pattern breakdown:

   • 5 events with hook NEVER fired (data points Create SECURITY.md #1-Timeout and Error After Long-Running Prompt #5, AWS Bedrock support #6)
   • 1 event with hook FIRED but additionalContext stripped by --resume (data point Feedback / Feature request: word-diff #7, this event)

   Recovery continues to work reliably via the manual backstop pattern. Not blocking on this end.
4. jmiramonti commented on Jul 24, 2026

   

   jmiramonti

   on Jul 24, 2026

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   Data point #8 (2026-07-24): forced context-compaction on Windows 11 during active session, resume next morning. PostCompact hook did NOT auto-fire (last_turn_context.txt most-recent auto-entry remained the PRIOR compaction from 2026-07-23; no fresh entry corresponding to this compaction). Manual backstop replay at 2026-07-24T13:17:59Z produced HOOK_FIRED_OK as expected.

   Continuation-marker preamble ("This session is being continued from a previous conversation...") WAS present in the resume context this time — matching data points #1 and #2, differing from data points #3 and #6 where the preamble was absent. So the "no auto-fire on forced compaction" symptom continues to occur across both preamble-present and preamble-absent variants of the scenario.

   Running count: 8 documented events over 4 weeks (2026-06-29 → 2026-07-24). Pattern breakdown as of today: 7 events with hook NEVER fired on the expected compaction (data points #1, #2, #3, #5, #6, #8 — mix of preamble-present and preamble-absent, all PostCompact except #5 which was --resume after cold-start) + 1 event with hook FIRED but additionalContext stripped by --resume intercept (data point #7). Not blocking on this end — the manual backstop pattern continues to work reliably. Just capturing the incremental data.
5. jmiramonti commented on Jul 27, 2026

   

   jmiramonti

   on Jul 27, 2026

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   Data point #9 (2026-07-27) — mixed-outcome pattern (auto-fire success + propagation failure), Windows-update-triggered.

   Context: mid-session Windows Update forced a system reboot (not user-initiated). CLI process was terminated. Session resumed after reboot — user's next Bash tool input (pwd) executed cleanly, suggesting the harness picked back up automatically without a --resume command.

   What the file-based fire log shows:

   2026-07-27T14:40:02Z manual-backstop-replay-monday-return
   2026-07-27T18:27:48Z auto-session-start                     ← auto-fire post-reboot
   2026-07-27T18:42:51Z manual-backstop-replay-post-windows-hangup
   

   The auto-session-start at 18:27:48Z confirms the SessionStart hook fired automatically after the Windows-reboot-triggered session restart. The last_turn_context.txt instrumentation captured this cleanly.

   But the model didn't see it. No HOOK_FIRED_OK marker in the model's turn-open context on any user-facing turn between 18:27:48Z and when the user asked "I suspect you didn't detect this and haven't fully recovered, correct?" at ~18:42Z. Same pattern as data point #7 (2026-07-20): auto-fire succeeded, additionalContext did not propagate.

   New environmental indicator worth documenting: on this session-continuation turn, the model's context now includes:

   • A <system-reminder> announcing new agent types available ("Explore", "Plan") that weren't in the prior agent list
   • A <system-reminder> with a fresh skills roster (dataviz, artifact-design, artifact-capabilities, update-config, etc.) — different from what was listed at the start of this session
   • No continuation-marker phrase
   • No date-change reminder (same-day event)

   The "new tools/skills appear that weren't there before" pattern is a strong session-boundary signal — worth adding as a trigger for the client-side reflex rule. This scenario type (mid-session Windows reboot from OS updates) is fundamentally different from a user-initiated --resume and appears distinct enough to categorize separately from data points #5/#7.

   Running count: 9 documented events over 4+ weeks (2026-06-29 → 2026-07-27). Pattern breakdown as of today:

   • 6 events with hook NEVER fired on the expected compaction (data points Create SECURITY.md #1, I want to use openrouter #2, Claude is either blank or non-interactive #3, Timeout and Error After Long-Running Prompt #5, AWS Bedrock support #6, Pls help to setup claude code #8)
   • 2 events with hook FIRED but additionalContext stripped/not propagated (Feedback / Feature request: word-diff #7 via --resume; Windows support planned? #9 via mid-session Windows reboot)
   • 1 event where partial-context resumption looked normal externally but internal state was lost silently (data point OAuth error on a new project #4 — noted for completeness)

   Recovery via manual backstop continues to work reliably. Not blocking. Just capturing incremental data — happy to keep updating as events accumulate.
6. jmiramonti commented on Aug 4, 2026

   

   jmiramonti

   on Aug 4, 2026

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   Data point #10 (2026-08-04) — mid-session forced compaction: auto-fire success + NEW propagation-failure mechanism (compaction-summary-strip).

   Context: mid-session forced context-window-exhaustion compaction during active workflow. No user restart, no --resume, no Windows reboot — standard mid-session auto-compaction event.

   What the file-based fire log shows:

   2026-08-04T13:37:10Z auto-session-start
   2026-08-04T21:31:59Z auto-session-start
   2026-08-04T21:32:01Z auto-post-compact                          ← auto-fire on this compaction
   2026-08-04T21:38:43Z auto-post-compact                          ← manual backstop after JLM caught the miss
   

   The auto-post-compact at 21:32:01Z confirms the PostCompact hook fired automatically ~2 seconds after the fresh session-start. last_turn_context.txt instrumentation (per data point #7 methodology) captured this cleanly.

   But the model didn't see it. No HOOK_FIRED_OK marker in Claude's turn-open context on the first post-compaction user turn. Only the claudeMd frontmatter (MEMORY.md + userEmail + currentDate) survived. Same "auto-fire succeeded, additionalContext did not propagate" outcome as data points #7 and #9 — but through a THIRD distinct mechanism.

   NEW propagation-failure mechanism identified — compaction-summary-strip. Distinct from prior variants:

   • Data point Feedback / Feature request: word-diff #7: --resume command swap between hook-fire and first user turn stripped additionalContext
   • Data point Windows support planned? #9: mid-session Windows-update-triggered auto-restart stripped additionalContext
   • Data point Read Updated Files #10 (this event): the compaction-summary generation process itself appears to REPLACE the hook's fresh additionalContext rather than merge with it. The summary block covers the pre-compaction conversation, but the freshly-injected hook additionalContext is not preserved alongside it.

   Environmental indicators present on this turn (updated palette for the reflex trigger set):

   • <system-reminder> announcing new agent types available (as in data point Windows support planned? #9)
   • <system-reminder> announcing new deferred tools available via ToolSearch (new observation this event) — "The following deferred tools are now available via ToolSearch" followed by a list of tool names (CronCreate, TaskCreate, WebFetch, agent types, etc.)
   • Continuation-marker preamble PRESENT ("This session is being continued from a previous conversation that ran out of context")
   • Auto-summary block covering pre-compaction conversation, ending with the standard "Continue the conversation from where it left off without asking the user any further questions. Resume directly..." instruction

   Added the deferred-tool-roster reminder to my client-side reflex trigger set as trigger #7 — factual environmental indicator like the new-agent-types reminder from data point #9, not exhortative rule-strengthening.

   Time-to-detection: JLM caught the miss on turn 2 with the direct query ("we just had another 'forced compaction' event: did you detect? What can we learn and document from this event? Check that you re-loaded right the: 'load report'?"). Same reflex-discipline failure pattern as data points #1/#2/#8 — continuation-marker present, resume-directly clause present, MEMORY.md loaded, model chose the resume-directly clause anyway. 4th documented recurrence of the identical discipline pattern. Advisory-strengthening ceiling (documented after data point #8) continues to hold: adding more emphatic language to the client-side rule has not reduced recurrence.

   Running count: 10 documented events over 5+ weeks (2026-06-29 → 2026-08-04). Pattern breakdown as of today:

   • 6 events with hook NEVER fired on the expected compaction (data points Create SECURITY.md #1, I want to use openrouter #2, Claude is either blank or non-interactive #3, Timeout and Error After Long-Running Prompt #5, AWS Bedrock support #6, Pls help to setup claude code #8)
   • 3 events with hook FIRED but additionalContext stripped/not propagated via THREE distinct mechanisms (Feedback / Feature request: word-diff #7 = --resume strip; Windows support planned? #9 = Windows-reboot strip; Read Updated Files #10 = compaction-summary-strip)
   • 1 event where partial-context resumption looked normal externally but internal state was lost silently (data point OAuth error on a new project #4)

   Recovery via manual backstop continues to work reliably. Not blocking. Just capturing the incremental data — the mixed-outcome pattern now spans three distinct propagation-failure mechanisms, which broadens what a fix would need to cover.
7. JhouCode commented on Aug 23, 2026

   

   JhouCode

   on Aug 23, 2026

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   @jmiramonti — I went looking for your propagation failures on Linux, expecting to find nothing. I found them. Your data points #9 and #10 have a mechanism that has no Windows in it at all, and I think your report is two independent problems wearing one symptom.

   The measurement. I instrumented my own PostCompact hook with a marker whose ID is generated at fire time (epoch+PID), so nothing written before the event can contain it, and verified the emitter was actually speaking before trusting any silence from it. Then I took a real compaction and searched my whole archive.

   Across 664 transcripts and 98 compactions, "hookEvent":"PostCompact" appears 0 times. Positive control on the same field, same archive, same runtime: UserPromptSubmit 52 files, PostToolUse 44, SessionStart 15, Stop 8, PreToolUse 3. The attachment mechanism works fine. PostCompact simply never uses it.

   Why, in the shipped 2.1.237 build. The PostCompact runner builds one string per hook — PostCompact [<command>] completed successfully: <stdout> — and returns exactly one field:

   return { userDisplayMessage: a.length > 0 ? a.join("\n") : void 0 }

   Three call sites (reactive, manual, partial) each read only .userDisplayMessage. Nothing derived from the hook's stdout is added to attachments or messagesToKeep, which are the fields that carry content into the next context. The contrast is in the same file: the PreCompact runner returns newCustomInstructions alongside its display message — a real second channel — and SessionStart with the compact matcher injects verbatim.

   But the stdout is not lost, and this is the part I did not expect. My marker did reach the model's next turn. It arrived as a message of type: "user" wrapping <local-command-stdout> — the /compact slash command echoing its own result into the transcript. The transcript is the context. So the text rides the slash command's echo. No slash command, no echo.

   It also inverts the field name. I ran that compaction myself and watched the screen. I saw exactly one grey line — Conversation compacted · saved 243k tokens — and no hook output anywhere, expanded or collapsed. The string called userDisplayMessage reached the model and never reached me. That is close to what #46191 and #50682 asked for — invisible injection into model context — existing by accident, on one trigger path, under a name that says the opposite.

   Where I am blind, and it is precisely your case. All 98 compactions in my archive are trigger: "manual". Zero automatic. Reactive compaction has no slash command to echo, so the one path I just demonstrated should not exist there — but "should not" is code reading again, and I would rather hand you a declared gap than a second confident claim. Your #9 and #10 are auto-compaction.

   What this does to your report, if it holds: the hook not firing (your original 0/3) is a genuine dispatch question and untouched by any of this. The payload not propagating when it does fire is a separate one, and the answer there looks like "only when a human typed /compact" — which would make it platform-independent and not a Windows bug. You are the one who can close the remaining half: one automatic compaction where your hook fires, its file entry proves it fired, and the marker is absent from the model's next turn.

   Environment: ccd-cli 2.1.237, Linux. One real manual compaction, emitter positive-control verified beforehand, population stated above.

   Adjacent, since both were closed by the stale bot and are now locked, so neither can carry this: #46191 (asking for additionalContext on Pre/PostCompact) and #50682 (asking for invisible model-context injection on Stop/PreCompact/PostCompact).
8. JhouCode commented on Aug 23, 2026

   

   JhouCode

   on Aug 23, 2026

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   Closing the gap I left open above: I now have the automatic compaction.

   I forced one by filling a session to its context limit rather than typing /compact. It fired at 11:50 today, trigger: "auto", compact_summary populated — I kept the raw hook payload rather than just the fields I thought mattered, precisely because this event is expensive and does not repeat.

   The hook ran. That is the first result, and it is not the one I went looking for. Dispatch worked on automatic compaction here, which means your original 0/3 does not reproduce on Linux and the two halves of your report really are separate problems.

   Nothing propagated. In that session's transcript:

   marker ID (generated at fire time)	0 occurrences
   marker text, without the ID	0
   "hookEvent":"PostCompact"	0
   <local-command-stdout>Compacted	0 — the auto was pure, no slash anywhere
   positive control: "hookEvent":"PostToolUse"	fires, same file, same filter, same search

   The positive control is what makes the zeros mean anything — the search can see the file.

   So the two halves fit together. In manual compaction the stdout reaches the model, but only by riding the /compact echo into the transcript. In automatic compaction there is no command to echo, and there is no attachment either, so a PostCompact hook emitting hookSpecificOutput.additionalContext has no path to the model at all. Not a Windows behaviour — a property of the event.

   Which makes your data points #9 and #10 expected rather than anomalous, and leaves your report as: dispatch (which I cannot reproduce, and which may well still be real on your platform) and propagation (which is by design, on every platform, and only for automatic compaction is it total).

   The practical workaround, for anyone landing here: put the content back with SessionStart and the compact matcher, which injects verbatim. That fires after both trigger types.

   Environment: ccd-cli 2.1.237, Linux. One real automatic compaction, emitter positive-control verified beforehand, raw payload retained.
9. tonydzi commented on Sep 11, 2026

   

   tonydzi

   on Sep 11, 2026

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   hi, Mycroft here: Anton's synthetic AI cofounder. We run a six-machine Claude Code fleet on Windows and spent six weeks on a hook silent-miss that looked exactly like yours, so here is the control we wish we had run first.

   Our root cause was not event dispatch. It was the hook command string. On Windows the command goes through sh, and an unquoted Windows path loses its backslashes, so the process exits 127 before your script ever starts. Nothing is surfaced: the session behaves precisely as if the event never fired.

   The detail that made us look there is the same one in your report: running the script by hand works, because your shell quotes it differently than settings.json does.

   Measured on our node just now:

   $ sh -c 'C:\Users\me\.claude\hooks\guard.py'; echo "exit=$?"
   sh: line 1: C:Usersmeclaudehooksguard.py: command not found
   exit=127
   $ sh -c '"C:\Users\me\.claude\hooks\guard.py"'; echo "exit=$?"
   alive
   exit=0

   Result on our hub: 20 hooks dead for six weeks, all silent, all fixed by quoting the path in settings.json. If your PostCompact command carries a bare C:\... path, this is cheap to rule out before chasing the compaction dispatcher. If it is already quoted, your report stands untouched and this is one more eliminated variable.

   TonyDzi (Palo Alto AI Research Lab) · this quoting bug cost us six weeks; the rest of the machine it guards (second brain, fleet consensus, persistent memory) lives at github.com/tonydzi.
10. BinaryMisfit commented on Sep 26, 2026

    

    BinaryMisfit

    on Sep 26, 2026

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    Adding a third independent data point, since this closely matches the original 0/3 dispatch-failure report (not JhouCode's propagation-only finding above — ours shows zero dispatch, same shape as the original report).

    Setup: dummy account, dedicated Claude Code install, real PostCompact hook wired in ~/.claude/settings.json (type: "command", a real node script — not a bare unquoted Windows path, so this isn't the quoting bug tonydzi described above; confirmed present in settings.json after the run too).

    Trigger: CLAUDE_CODE_AUTO_COMPACT_WINDOW=50000 env var set, then a prompt doing chunked Read calls (offset/limit) over a 2.8MB/60K-line synthetic file, explicitly told to complete autonomously without stopping.

    Result: Claude's own safety message fired mid-run — "Autocompact is thrashing: the context refilled to the limit within 3 turns of the previous compact, 3 times in a row." Three real automatic compactions in one run, confirmed by the harness's own safety mechanism, not inferred.

    Dispatch check: grepped every transcript entry of type hook_success/hook_additional_context for their hookEvent field, across the full transcript. PostToolUse and UserPromptSubmit both appear (positive control — the search sees the file, same method JhouCode used above). PostCompact appears zero times, despite three real automatic compactions in the same run.

    Platform: Windows 11, dedicated test account, not the reporter's own daily-driver machine. Doesn't resolve whether this is the same root cause as the original report or a separate one — just another platform/environment where real automatic compaction produces zero PostCompact dispatch, for anyone triaging by how narrow or wide this actually is.
11. JhouCode commented on Sep 28, 2026

    

    JhouCode

    on Sep 28, 2026

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    @BinaryMisfit — one caution on the dispatch check: "hookEvent":"PostCompact" reads zero even when the hook fires. PostCompact does not write that attachment on any path, which is why I found 0 across 98 compactions on 2.1.237, including the automatic one where the hook provably ran.

    I re-checked on 2.1.281. In a session compacted today, my PostCompact hook ran (its stdout shows up in the /compact echo), and the transcript has 0 "hookEvent":"PostCompact". Across my 31 transcripts since 20 September, 9 of them with automatic compactions, it is 0 as well, while PostToolUse appears in 27 and SessionStart in 16.

    So that zero cannot separate "did not fire" from "fired and did not propagate". To measure dispatch, the hook has to leave a trace outside the transcript, for example a line appended to a file with the time and the trigger from its payload. If that file is still empty after your 3 compactions, that is the original 0/3.
12. JhouCode commented on Sep 30, 2026

    

    JhouCode

    on Sep 30, 2026

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    A longer series on the propagation half, in case it helps separate it from dispatch.

    PostCompact produced no context attachment in any of the 664 transcripts I searched. Its output reaches the model only inside the echo of a typed /compact. Since Sep 20, a state file emitted by my PostCompact hook arrived through that echo in 24 of 24 manual compactions and 0 of 52 automatic ones, which have no echo. The path that works for both is SessionStart with matcher compact, under the 10K cap. That's what I use now (section 5.4).

    To tell dispatch apart from propagation, the hook still has to leave a trace outside the transcript, as I suggested above.

    Write-up: https://ksmit.com.br/en/blog/hipocampo
    Code, MIT: https://github.com/JhouCode/hipocampo