ruff/biome/bash-format: two JSON documents on stdout when one run both rewrites and reports findings

[kyle-sexton]

Context

lib/hook-utils.sh:75-78 documents the invariant: Claude Code parses the hook's whole stdout as a single JSON document, so a run that has both a user-channel message and agent-channel context "must compose them here rather than print twice." Three formatter hooks violate it on the rewrite-plus-findings path:

• plugins/ruff-format/hooks/ruff-format.sh: hook::emit_system_message (rewrite disclosure, ~line 243) prints one JSON document; when the verify pass then returns findings (RC -eq 1), hook::ctx_flush (~line 272) prints a second.
• plugins/biome-format/hooks/biome-format.sh: same shape, disclosure at line 221, hook::ctx_flush at line 252 (and 282).
• plugins/bash-format/hooks/bash-format.sh: disclosure at line 240, then hook::emit_channels at line 300/302 prints a second document on the same run.

Any single run that both rewrites the file and reports findings puts two JSON documents on stdout. #3401 (a7f9537) named this exact violation "pre-existing ... in the same file" while fixing powershell-format; these three siblings still carry it. No test in ruff/biome/bash covers rewrite-plus-findings composition: jq -s length single-document assertions exist only in powershell-format.test.sh:280,435, typos-format.test.sh:322, and bash-format.test.sh:613 (the notice-plus-findings path, which already routes through hook::emit_channels).

Verified against the code on 2026-08-27.

Proposed work

• Compose the rewrite disclosure into the single end-of-run document (the hook::emit_channels path powershell now uses) in all three hooks, instead of emitting it immediately.
• Add a rewrite-plus-findings case to each suite asserting stdout parses as exactly one JSON document (jq -s 'length == 1') carrying both channels.

Acceptance criteria

• A run that rewrites and reports findings emits exactly one JSON document carrying both systemMessage and additionalContext, in each of ruff-format, biome-format, bash-format.
• A rewrite-only run and a findings-only run each still emit exactly one document.
• Each suite has a non-vacuous jq -s composition assertion for the rewrite-plus-findings path.

References

• lib/hook-utils.sh:75-78 (the invariant)
• plugins/ruff-format/hooks/ruff-format.sh:243,272 · plugins/biome-format/hooks/biome-format.sh:221,252 · plugins/bash-format/hooks/bash-format.sh:240,300-302
• fix(powershell-format): release the _ps_before snapshot and disclose a rewrite on the tool-break arm #3401 / commit a7f9537 (powershell fix that named the sibling violations)
• Related architecture finding: the disclosure protocol is hand-maintained per plugin; a deepening candidate consolidating it is filed separately.
• Found by the /architecture:improve deepening scan (2026-08-27); claim reproduced against the code before filing.