fix(mobile): recognize merge-into-tail compaction envelope (P0 erased/unanswered chat)

[IgorGanapolsky]

Root cause

Igor reported Hermes Mobile "erasing inputs and not answering" on thread "Why we made zero dollars? #7" (screenshot showed a raw wall of [PRIOR CONTEXT — for reference only...] / [END OF PRIOR CONTEXT — COMPACTION SUMMARY BELOW] / [CONTEXT COMPACTION — REFERENCE ONLY] text with an empty composer).

Traced to the gateway agent (~/.hermes/hermes-agent, agent/context_compressor.py): when compaction needs to insert a summary but head/tail role alternation would collide, it uses a merge-into-tail path (_MERGED_PRIOR_CONTEXT_HEADER / _MERGED_SUMMARY_DELIMITER) that wraps the REAL preserved turn (often the user's actual next message, or an empty tail message) like this:

[PRIOR CONTEXT — for reference only; not a new message]
<the real preserved turn>

[END OF PRIOR CONTEXT — COMPACTION SUMMARY BELOW]

[CONTEXT COMPACTION — REFERENCE ONLY] Earlier turns were compacted...
<summary body>

--- END OF CONTEXT SUMMARY — respond to the message below, not the summary above ---

hermes-mobile/src/utils/chatCompactionHandoff.ts only recognized the standalone [CONTEXT COMPACTION — REFERENCE ONLY] prefix (added for #123/#164/#186). It never learned this merge-into-tail envelope, so isContextCompactionHandoff / splitCompactionHandoff both fell through and the raw scaffold rendered unfiltered in the chat transcript — with the real turn buried and invisible inside it. That is the "erased input" illusion, and (per the compressor's own prompt engineering comments, #11475/#14521) is also a plausible contributor to weak-model non-answers when this format leaks into the prompt unexpectedly.

Fix

chatCompactionHandoff.ts:

• isContextCompactionHandoff now also matches the [PRIOR CONTEXT — for reference only; not a new message] header.
• splitCompactionHandoff extracts the real preserved turn (between the header and the delimiter) as remainder, and hides everything from the delimiter onward as summary — mirroring the Python side's own _is_context_summary_content / _strip_summary_prefix handling of the same envelope.
• A malformed/truncated envelope (missing delimiter) is hidden rather than shown raw.
• An envelope wrapping empty real content is dropped entirely — never rendered as a scary empty wall.

This flows through the existing prepareMessagesForDisplay → stripCompactionHandoffsFromMessages pipeline already wired into ChatScreen.tsx, and reuses the exact SSOT filtering pattern already shipped for the #777 cron-delivery-scaffolding filter. The existing "Start fresh chat" CTA (lastTurnIsCompactionStall / shouldAutoOfferFreshOnCompactionStall, already on main) now also sees through this envelope correctly.

Scope note

Investigation also found PR #795 (merged same day) already fixed partial-history reconciliation, and PR #777 already established the scaffold-hiding pattern this PR extends. No ChatScreen.tsx / chatMessageDisplay.ts edits were needed — the fix is fully contained in the mobile display SSOT (chatCompactionHandoff.ts), per the mission's "filter on mobile display SSOT" instruction. No Chrome/computer-use was used — CLI/git/Jest only.

Verification

• Focused: 13/13 new chatCompactionHandoff cases (merge-into-tail detect / extract / empty-drop / malformed-hide / prepareMessagesForDisplay).
• Related: 5 suites / 78 tests green (chatMessageDisplay, sessionTokenGuards, emptyStreamReplyRecovery, streamAssistantText, chatCompactionHandoff).
• Full: 223 suites / 1969 tests green (1 pre-existing unrelated skip).
• tsc --noEmit clean.
• Pre-commit hook (typecheck + related-file Jest, 54 suites / 668 tests) passed.

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Greptile Summary

This PR updates the mobile chat compaction filter to handle merge-into-tail envelopes. The main changes are:

• Recognizes [PRIOR CONTEXT] compaction envelopes in chatCompactionHandoff.ts.
• Extracts the real preserved turn while hiding the compaction scaffold.
• Drops empty or malformed merge envelopes instead of rendering raw markers.
• Adds tests for detection, extraction, empty-drop, malformed-hide, and display pipeline behavior.

Confidence Score: 5/5

Safe to merge with low risk.

The change is narrowly scoped to the mobile display-side compaction filter and includes focused tests for the new envelope formats. No verified correctness or security issues were found in the reviewed paths.

No files require special attention.

T-Rex Logs

What T-Rex did

• Ran the Jest test suite and confirmed all 13 tests passed with exit code 0.
• Ran TypeScript typechecking with noEmit and confirmed it completed successfully with exit code 0.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
hermes-mobile/src/utils/chatCompactionHandoff.ts	Adds merge-into-tail envelope detection and splitting while preserving real chat content and hiding scaffold text.
hermes-mobile/src/tests/chatCompactionHandoff.test.ts	Adds regression tests for merge-into-tail compaction handoff display behavior.

_{Reviews (1): Last reviewed commit: "fix(mobile): recognize merge-into-tail c..." | Re-trigger Greptile}