[JS-banana]

Problem

Since #652 (first released in v0.10.0), every Antigravity request fails with a 400 as soon as a session loads a tool whose TypeBox schema contains a literal union — subagent_check's evidence parameter being the in-tree instance (Type.Union([Type.Literal("transcript"), ...]), serialized as anyOf branches carrying "const"). Cloud Code Assist validates function declarations against OpenAPI 3.0, which has no const, and protojson rejects the whole request:

Error: Cloud Code Assist API error (400): Invalid JSON payload received.
Unknown name "const" at
'request.tools[0].function_declarations[8].parameters.properties[2].value.any_of[0]': Cannot find field.

This affects every google-antigravity user on v0.10.0–v1.1.0 with the subagents extension loaded (i.e. the default surface). Setup's role/browser literal unions trigger the same 400 when that extension's tools are registered. No issue exists for it yet; the verbatim error above is included so it is searchable.

Value

Restores Antigravity usability for all affected users; the fix is contained in the provider's existing schema sanitizer rather than requiring schema changes in every extension that uses literal unions.

Approach

sanitizeSchemaForCca already strips OpenAPI-rejected keywords on the wire path shared by the Gemini and Claude routes (buildTools); this adds a rewrite of const: X into a single-value enum: [X], merging with any sibling enum without duplicates. enum is proven accepted: enum-carrying parameters (subagent_spawn.agent_type, intercom.action) pass in the exact payloads that 400 on const. The advertised value set is unchanged, so the model-visible semantics are identical.

Relates to #465 (same sanitizer, adjacent failure class — does not address the $ref drop described there).

Validation

• node --test --experimental-strip-types tests/extensions/ai-providers/antigravity.test.ts: all pass, including new cases for anyOf-branch literals, enum merging, and a property named const.
• Live A/B against google-antigravity/gemini-3.8-flash-high with pi -ne -a -e ./extensions/ai-providers/index.ts -e ./extensions/subagents/index.ts -p "Reply with exactly: OK": before (main@ea4dfb99) reproduces the verbatim 400; after, the model replies OK.
• bun run check: clean (biome format/lint, tsc --noEmit).
• Full suite (node scripts/run-tests.mjs): 2 failures in tests/extensions/code-mode/index.test.ts that fail identically on clean main (pre-existing, unrelated).

Impact

Model-visible context/tools: subagent_check.evidence is now advertised as anyOf of singleton enum branches instead of const literals — the same value set. No user-visible, config, persistence, or lifecycle change. Out-of-tree tools carrying const schemas stop 400ing the session.