feat(context): preserve verbatim tail-anchor across compaction

[vellum-apollo-bot]

What

Force-keep the last assistant text from the compactable region by splicing it verbatim into the post-compaction summary message as a tag-wrapped <verbatim_tail>…</verbatim_tail> block.

Why

When _maybeCompact runs in the middle of a long agent work span, the LLM-produced summary doesn't reliably preserve the model's most recent self-narration ("Next step: …", "About to …"). The post-compaction model then falls back to a "where am I?" recovery shape — re-checking NOW.md / scratch/ / signals/ — and drifts off the active thread.

This was the structural root cause of the May 7 surface-action conversation drift: while iterating on PR #29895, compaction fired at 20:58:31 UTC; the assistant's prior turn had stated "next step: file the SSE followup as promised", but that statement landed in the compactable region and was summarized away. The post-compaction model fell into a heartbeat-shaped recovery (ls signals/, ls scratch/, cat HEARTBEAT.md, scan PRs) and drifted onto unrelated bot feedback on PRs #6227 and #29900.

How

In assistant/src/context/window-manager.ts:

• New extractTailAssistantText(messages, maxChars=1500): walks compactableMessages backward, returns the trimmed text of the most recent assistant message that has at least one non-empty text block. Skips tool_use / tool_result / image / unknown blocks. When the tail exceeds maxChars, clamps from the START with a [...truncated] prefix so the END (where "next step" lines land) wins. Returns null when nothing eligible is found — the caller treats that as "no anchor to splice".
• New appendTailAnchorToSummary(summary, tailText): idempotent splice. Tag-wrapped block (<verbatim_tail>) is structurally distinct from any ## section the LLM might produce, so it survives clampSummaryAtSectionBoundary (which only runs on the LLM summary, before the splice). If the existing summary already contains a <verbatim_tail> block (e.g. carried forward as existingSummary from a prior compaction), it's replaced rather than stacked.
• In _maybeCompact: extract tailAnchorText from compactableMessages, splice via appendTailAnchorToSummary when non-null, fall through otherwise. The spliced summary flows into both createContextSummaryMessage (in-context user-role message) and the summaryText returned in the result — so updateConversationContextWindow (DB persistence) and the context_compacted client event both see the anchored tail.

Tests

3 new test groups in assistant/src/__tests__/context-window-manager.test.ts, all passing:

• describe("extractTailAssistantText") — 5 tests: returns last assistant text; null when none; skips tool_use-only assistants; clamps long text preserving END; ignores whitespace-only.
• describe("appendTailAnchorToSummary") — 2 tests: appends tag-wrapped block; idempotent (replaces prior tail).
• describe("compaction tail-anchor") — 3 integration tests via ContextWindowManager.maybeCompact: splices the last compactable assistant text into the summary message + summaryText; omits gracefully when compactable region has only tool_use assistants; clamps long tails preserving the END marker.

Full suite: 61/61 pass on bun test src/__tests__/context-window-manager.test.ts.

Notes

• This is the structural fix for the compaction drift class. A complementary lighter prompt-side fix (adding "Immediate next action: …" guidance to SUMMARY_PROMPT_FALLBACK and prompts/compact.md) is worth considering as a follow-up — they're orthogonal: the prompt change asks the LLM nicely, the verbatim splice guarantees it.
• 1500 chars (~375 tokens) is a soft default. Most assistant turns fit comfortably; long ones get the END preserved which is where the "next step" sentence almost always lives.
• The anchor is also surfaced verbatim through summaryText, so observability/debug consumers (DB row, context_compacted event) match what's in-context.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[awlevin]

Oh nice catch if true