Skill pairing: scope by session_id and only drop the canonical payload#124
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cboos merged 1 commit intodev/pair-skill-user-messagefrom Apr 24, 2026
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CodeRabbit on PR #121 caught two failure modes in `_pair_skill_tool_uses` that the original commit (dc1770a) didn't defend against. Both are session-/payload-precision concerns that surface in combined transcripts or with malformed/error tool_results. ## #1 — Cross-session pairing collision The lookup was keyed on `tool_use_id` alone. `RenderingContext.messages` spans combined transcripts (multiple sessions), but Anthropic tool_use ids are only session-unique. A stray collision would let session B's slash body fold into session A's Skill tool_use (or vice versa) silently. Fix: key the lookup by `(render_session_id, source_tool_use_id)` and match the same compound on the consuming side. `render_session_id` is fork-aware so within-session branches stay distinct too. ## #2 — Real error tool_results silently dropped The original code dropped EVERY tool_result with the matching `tool_use_id`, on the assumption that only the canonical `"Launching skill: <name>"` result carries that id. A real error result ("Skill 'X' not found", `is_error=True`) would have the same id and get swallowed too — hiding the failure from the user entirely. Fix: only drop tool_results that are (a) in the same session, (b) `is_error=False`, AND (c) whose payload starts with `"Launching skill:"`. New `_is_launching_skill_payload` helper handles both string- and list-shaped `ToolResultContent.content`. ## Tests (3 new in TestSkillPairing) - `test_same_tool_use_id_across_sessions_does_not_cross_pair` — combined transcript with two sessions sharing `tool_use_id="toolu_DUP"` but different bodies; asserts each Skill keeps its own session's body. - `test_error_tool_result_with_same_id_is_preserved` — a Skill failure flow where the error result shares the tool_use_id; asserts the error survives while the canonical "Launching skill:" result is still dropped. - `test_non_launching_skill_result_with_same_id_is_preserved` — divergent payload sharing the id (e.g. malformed transcript) survives. All three fail on the pre-fix code (cross-pair, swallow error, swallow divergent payload) and pass after. Refs #93 / #121. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Follow-up to #121 addressing CodeRabbit's review (#121 (review)).
Superseded — fix consolidated directly onto #121's branch (
dev/pair-skill-user-message) ata584d7a. This stacked PR is no longer needed.Why this exists
CodeRabbit flagged two failure modes in
_pair_skill_tool_usesthat theoriginal commit (dc1770a) didn't defend against:
Cross-session pairing collision — the lookup was keyed on
tool_use_idalone.RenderingContext.messagesspans combinedtranscripts (multiple sessions), but Anthropic tool_use ids are only
session-unique. A stray collision would let session B's slash body
fold into session A's Skill tool_use silently.
Real error tool_results silently dropped — the original code
dropped EVERY tool_result with the matching
tool_use_id, on theassumption that only the canonical "Launching skill: " result
carries that id. A real error result (
is_error=True) sharing the idwould get swallowed too.
What changes
(render_session_id, source_tool_use_id), not baretool_use_id._is_launching_skill_payload(output)helper checksToolResultContent.contentfor the"Launching skill:"prefix (handles both string- and list-shaped content).is_error=False, payload prefix matches.Tests (3 new)
test_same_tool_use_id_across_sessions_does_not_cross_pairtest_error_tool_result_with_same_id_is_preservedtest_non_launching_skill_result_with_same_id_is_preservedAll three fail on the pre-fix code and pass after.
Refs #93 / #121.