fix(antigravity): strip orphan tool_use/tool_result pairs before upstream send (#6026)

[diegosouzapw]

Closes #6026.

Bug

Antigravity (Claude-opus-4.6 on Vertex) via AgentBridge/MITM returned 400:

messages.0.content.1: unexpected tool_use_id found in tool_result blocks: toolu_vrtx_... Each tool_result block must have a corresponding tool_use block in the previous message.

The client shipped a history whose first message already carried a tool_result with no preceding tool_use (an orphan).

Root cause

open-sse/services/contextManager.ts::fixToolPairs() already strips exactly this orphan — but AntigravityExecutor overrides BaseExecutor.execute(), and the base executor's tool-pair guard (base.ts ~L1099, fixToolPairs/fixToolAdjacency/stripTrailingAssistantOrphanToolUse) lives in the base execute(). So the Antigravity path never ran it — the orphan reached Vertex/Anthropic unmodified.

Fix

Add sanitizeAntigravityToolMessages() that mirrors the exact sequence the base executor uses for Claude-bound providers (fixToolPairs → fixToolAdjacency → re-fixToolPairs → stripTrailingAssistantOrphanToolUse), and call it in AntigravityExecutor.execute() on the upstream body before tool cloaking (which only remaps names). Idempotent on clean histories.

Validation (Hard Rule #18 — TDD)

• New: tests/unit/antigravity-orphan-toolresult-6026.test.ts (3) — reproduces the orphan-tool_result-at-messages[0] payload and asserts it's stripped; asserts a valid tool_use→tool_result pair is preserved; asserts a no-op on bodies without a messages array. Fails on the pre-fix path (no sanitization ran).
• typecheck:core ✓ · lint ✓ (0 errors) · check:cycles ✓ · check:docs-sync ✓ · existing antigravity suites (43) ✓

Note

Reproduced from the issue's error + confirmed root cause in code (executor override bypasses the base guard). Applied the proven base-executor sequence rather than a bespoke fix to avoid over/under-stripping.

Not merged — opened for review, pointed at release/v3.8.44.

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[diegosouzapw]

Closing as superseded — the underlying issue #6026 (orphan Antigravity tool_result) was fixed and merged into the release via #6115 (strip orphan tool_result on Antigravity MITM path, commit 29e04b7), which lands the canonical translator-side fix plus a dedicated regression test. This PR's executor-side approach is no longer needed. 🙏