[JS-banana]

Stacked on #905 — the first commit of this branch is that PR's; only 7e18c1f9 is new here. If #905 merges first, this PR's diff collapses to the hardening commit.

Problem

sanitizeSchemaForCca is a denylist snapshot of oh-my-pi's older fields.ts. Two gaps remain after the const fix (#905): (1) nine Google-dialect rejected keywords are missing from the denylist ($id, $anchor, contains, minContains, maxContains, additionalItems, contentEncoding, contentMediaType, contentSchema), so tools carrying them still 400; (2) the denylist shape fails hard — any keyword outside the list reaches the wire, and one rejection kills every request in the session. That failure class has now recurred twice on this sanitizer (#465, #905); out-of-tree schemas (MCP tools, third-party extensions) make recurrence a matter of time, not chance.

Value

Aligns the sanitizer with the reference implementation for the same endpoint (oh-my-pi packages/ai/src/utils/schema, MIT — shipped and exercised daily against Cloud Code Assist) and converts the boundary from fail-hard to fail-soft: a novel schema keyword now degrades the advertised schema instead of 400ing the session.

Approach

Four changes to sanitizeSchemaForCca, ported from omp's current pipeline where noted (hand-ported into our single-file sanitizer — no omp dependency, and none of omp's fork-level machinery):

• Collapse all-literal combinators (anyOf/oneOf/allOf where every variant carries const, e.g. TypeBox Type.Union([Type.Literal(...)])) into a flat enum with a single type, instead of anyOf of singleton enums (omp normalize.ts "allHaveConst"). For allOf the collapse widens an intersection to a union, which only broadens what the model may pick — sound for an advertised schema.
• Infer a bare const node's type from its value (string/number/boolean).
• Sync the nine missing denylist entries from omp fields.ts (mechanism unchanged: liftable constraints still spill into description).
• Add a final allowlist pass: only keys omp's Google/CCA dialect ships through this endpoint today reach the wire (type, title, description, enum, items, properties, required, anyOf, oneOf, allOf); type arrays are scalarized (a null-only array yields no type). Anything else is dropped — fail-soft, mirroring omp's validateAndFallback intent.

Intentionally not ported: omp's $ref/$defs dereferencing, 2020-12 dialect upgrade, nullable extraction, property ordering, and the whole-profile machinery (~2.5k lines); also omp's parametersJsonSchema route for Gemini models — both routes here stay on parameters, verified live.

Validation

• node --test --experimental-strip-types tests/extensions/ai-providers/antigravity.test.ts: 68 pass, covering combinator collapse (anyOf/oneOf/allOf, with and without explicit types), mixed-type unions staying unions, bare-const inference, the synced denylist entries, the gate (drops if/then/nullable, keeps title per omp parity), type-array scalarization, and properties named const/anyOf; plus a buildRequestBody end-to-end assertion that a literal union reaches the wire as a flat enum.
• Live against google-antigravity/gemini-3.8-flash-high with subagents+workflows+setup loaded: reply OK; a model-driven subagent_check round-trip succeeds through the collapsed enum.
• bun run check: clean (biome format/lint, tsc --noEmit).
• Full suite (node scripts/run-tests.mjs): the same 2 pre-existing tests/extensions/code-mode/index.test.ts failures as clean main.

Impact

• Model-visible context/tools: literal unions are advertised as one flat enum (same value set, fewer tokens).
• Wire behavior change for out-of-tree tools: keywords omp does not ship through this endpoint (if/then/else/not, nullable, custom x-*) are now dropped from the advertised schema instead of being sent; previously they were sent and 400ed whenever a tool actually carried them. In-tree tools use none of these today (verified by grep), so there is no in-tree regression.
• No config, persistence, or lifecycle change. Reverting this PR does not reopen the const 400 fixed by fix(ai-providers): rewrite const schema literals as enum for Cloud Code Assist #905.