fix(claude-sdk-oauth): ignore volatile top hooks in continuity hashes

[codeg-dev]

Summary

Follow-on to #791. Top-of-turn hook injections (omo-memory notice, goal-continuation, mindy-team context blocks, senpi-task usage) convert to user-role bodies. Hashing those bodies makes a rewrite or prepend look like sent_stream_diverged, so decideFromBinding flattens to a cold seed and burns the Anthropic prompt cache even when the real conversation prefix is intact.

• isTransmittedMessage excludes those hooks. Detection uses customType / provenance when present, and content signatures after convertToLlm strips customType.
• Genuine user rewrites still report sent_stream_diverged (fail-closed).
• A user message that only starts with "Continue working toward the active thread goal" (the claude-sdk-oauth: a content-less user message forces a full-history re-send every time it disappears #790 fixture, no untrusted_objective tag) stays hash-significant.

This is complementary to #1075 (rolling cache checkpoints after serialize). #1075 cannot help if continuity flattens the session first.

Evidence

• Stock RED: notice rewrite or prepend -> flatten / sent_stream_diverged.
• GREEN: bun x vitest --run packages/coding-agent/test/suite/regressions/claude-sdk-oauth-volatile-hook-continuity.test.ts packages/coding-agent/test/suite/regressions/790-claude-sdk-oauth-empty-user-continuity.test.ts (9 passed).

Test plan

• New regression: hook rewrite/prepend reattach; real user rewrite still diverges
• Existing claude-sdk-oauth: a content-less user message forces a full-history re-send every time it disappears #790 empty-user continuity still passes

---

Summary by cubic

Prevents false session divergence in claude-sdk-oauth by excluding volatile top-of-turn hook content from continuity hashes. Previously hashing converted hook bodies triggered sent_stream_diverged and a flatten; now hooks are hashed by kind while staying on the wire, so only real user edits diverge.

• convertToLlm stamps hook customType into request-local provenance; content signatures keep compatibility with already-converted hooks.
• Covers memory notices, goal continuations, mindy-team context blocks, and senpi-task, monitor, wake, terminal, directive, and skill-pointer hooks.
• Genuine user rewrites and structural prepends still diverge. Regression tests added.

^{Written for commit 52b4843. Summary will update on new commits.}

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[codeg-dev]

P1 accepted. isTransmittedMessage is restored to the previous contract (content-less user messages only). Hook rewrite is neutralized inside sentMessageHashes as { role, volatileHook: kind } so the slot stays in messages.slice(from) and a goal-continuation-only turn still reaches the SDK.

Structural prepend still reports sent_stream_diverged (fail-closed). Hook content rewrite reattaches.

Pushed on this branch (3fd8bde). Tests: claude-sdk-oauth-volatile-hook-continuity + #790 still 9/9.

[code-yeongyu]

Thanks @codeg-dev — the core idea still matters and is not superseded by today's main (6dd662c4b: #1318 unconfirmed bindings, #1311 content-blocks, #1312 is_error, #1310 SDK 0.3.259, #1309 OAuth slot). Hashing converted hook bodies in sentMessageHashes can still false-flatten a rewrite (omo-memory notices especially). isTransmittedMessage is the wrong seam for this; commit 3fd8bde already got that right — keep hooks in the transmitted list so buildDeltaPromptBlocks(messages.slice(from)) can send a hook-only turn.

Remaining work before this can merge:

1. Rebase onto main (6dd662c4b). session-sync.ts should apply clean around sentMessageHashes; do not touch appendContent / content-blocks.ts. Resolve changes.md by prepending a new dated entry at the top (do not let it land next to the 2026-08-21 settings-cache section).
2. Make detection work on the live list: convertToLlm drops customType (messages.ts custom arm). Either stamp customType into __piContextProvenance at convert time, or add content signatures for monitor/wake/terminal. As written, five of nine VOLATILE_HOOK_CUSTOM_TYPES never match session-stream.ts hashes.
3. Re-scope kinds: goal-continuation is append-only (messages.ts); hashing it by kind is optional and fail-open. Keep memory-notice / RULES / senpi-task prefixes if those still rewrite in place.
4. Replace the tautological continuity fixtures. Assert sentMessageHashes on convertToLlm output (or provenance-stamped user messages). Do not plant customType on already-user-role objects. Set sdkSessionIdConfirmed: true on reattach bindings (fix(claude-sdk-oauth): never resume an SDK session id that init never confirmed #1318). Keep a real user rewrite and a structural prepend as fail-closed cases. Keep claude-sdk-oauth: a content-less user message forces a full-history re-send every time it disappears #790 empty-user coverage.
5. Add a packages/coding-agent/CHANGELOG.md [Unreleased] ### Fixed bullet (runtime source; no no-changelog label). Tracker entry already has the four canonical sections if it names session-sync.ts.
6. After rebase, CI must actually run (required: Check and test, Changelog gate, Review claim gate).

Suggested tests (not run in this triage):
bunx vitest run packages/coding-agent/test/suite/regressions/claude-sdk-oauth-volatile-hook-continuity.test.ts packages/coding-agent/test/suite/regressions/790-claude-sdk-oauth-empty-user-continuity.test.ts
plus whatever session-sync / session-continuity regressions main now has. Changelog: bun scripts/check-pr-changelog.mjs with CHANGELOG_GATE_BASE set to the rebase base.

[codeg-dev]

Updated the branch against current main and addressed the remaining continuity review points.

• convertToLlm now preserves custom-message type in request-local provenance, so the live converted list identifies monitor/wake/terminal/directive hooks without planting customType on user-role fixtures.
• Rewrite-in-place hooks stay on the wire and hash by kind; append-only goal-continuation remains content-significant.
• Reattach fixtures now set sdkSessionIdConfirmed: true; structural prepend and real user rewrites remain fail-closed.
• Added the coding-agent [Unreleased] changelog entry and updated both applicable changes.md trackers.

Local verification:

• 4 continuity test files: 32 passed
• npm run check: passed
• changelog gate against current rebase base: passed
• hermetic anthropic-format mock-loop smoke: passed