fix(matrix): omit reply fallback on split message tails

[nepenth]

Summary

Omit Matrix reply-fallback m.in_reply_to on split-message tails.

Main attaches reply fallback to every chunk and test_long_response_split_preserves_thread_context currently requires that. Long replies then produce N quoted bubbles in clients that render fallback. This PR keeps m.thread on every chunk when threaded, and puts reply fallback only on the first chunk by default.

Behavior

• Default reply_to_mode: first: first chunk may carry m.in_reply_to; later chunks keep m.thread only.
• reply_to_mode: all: fallback on every chunk (old behavior).
• reply_to_mode: off (YAML off / false): no fallback on text or media. Thread relations still apply.
• Invalid values fall back to first.
• E2EE key-share retry of a chunk preserves that chunk's relation payload.

Tests

python -m pytest tests/gateway/test_matrix.py -q

141 passed on this head. The previous all-chunk in_reply_to assertion is rewritten.

Non-goals

• Changing chunk size / 16k default.
• Changing auto-thread policy.
• Tool activity or approval cards.

Security

No new network or credentials. Relation metadata only.

[tonydwb]

Code Review Summary

Verdict: Approved

Looks Good

• Fixes Matrix reply fallback behavior on split message tails by omitting the fallback when no prior reply context exists
• Test coverage added
• Focused, well-scoped fix
• No security concerns

[teknium1]

Thanks for the focused Matrix fix. Current main repeats m.in_reply_to because send() applies identical relation metadata to every truncated chunk (plugins/platforms/matrix/adapter.py:1591-1597), and the thread helper unconditionally supplies that fallback (plugins/platforms/matrix/adapter.py:4010-4022). The PR limits the fallback to the first chunk while preserving m.thread on tails, and adds threaded plus plain-reply split coverage in tests/gateway/test_matrix.py.

Automated hermes-sweeper review.

[nepenth]

Current head 3198393e8 remains limited to split-reply fallback relations. tests/gateway/test_matrix.py — 253 passed; Ruff, Python compilation, and git diff --check pass. All 36 current-head GitHub checks are complete with no failures. Ready for maintainer review.

[nepenth]

Rebased onto current main (6cdba3c98a41).

Multi-chunk Matrix sends now attach m.in_reply_to reply fallback only on the first chunk; later chunks keep m.thread when present so clients do not re-quote the original mid-answer. Updated long-split tests (including fixtures sized from adapter.max_message_length) assert first-only fallback.

Verification: 4 focused long-reply/thread tests passed.

[nepenth]

Status — rebuilt on current main (66666f6e2eca)

Single-commit recut. Current head 9a54a2c561de554befc06786dcc5de8746d3088d.

Focused local result on this head: 3 passed (tests/gateway/test_matrix.py -k 'long_response_split or include_reply_fallback or first_chunk').

Main still applies reply-fallback m.in_reply_to to every split chunk and the existing long-split test asserts that. This PR keeps the relation on the first chunk only and rewrites that test.

[nepenth]

Updated on upstream main 966637323e6f90864e069dbc12755934c2c86387; current head 004e78c8e6e20f45187b1005f9f5dd611a6b9685.

Current main still attaches reply fallback to every split chunk. This rebase keeps first-chunk-only fallback by default, honors reply_to_mode off/all, and leaves threaded tails as bare thread relations. Verified: Matrix 128 passed.

The rewritten PR description is the current scope and verification reference. Local suite results are not a claim of full-repository or live-homeserver validation; current-head CI is tracked separately by the checks below.

[nepenth]

Rebased onto current main (36221c6d32).

Behavior unchanged in intent: split Matrix sends attach reply-fallback m.in_reply_to only on the first chunk by default (reply_to_mode: first). Later chunks keep m.thread when present. off suppresses fallback on text and media. The previous all-chunk in_reply_to assertion in test_long_response_split_preserves_thread_context is rewritten to match.

Verification: python -m pytest tests/gateway/test_matrix.py -q — 141 passed.