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intercept arktype's standard-schema jsonSchema.input for Gemini sanitizer

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The previous sanitizer proxied `schema.toJsonSchema()`, but fastmcp 3.x uses
`xsschema.toJsonSchema()` which reads `schema["~standard"].jsonSchema.input(...)`
directly when the StandardJSONSchemaV1 extension is present (arktype 2.x).
Our proxy was never invoked, so the sanitizer was a silent no-op.

Proxy the entire `~standard` → `jsonSchema` → `input` chain so the transform
runs regardless of which path xsschema picks. Also add case 1 (add `type:"string"`
to enum-only schemas) — arktype 2.x emits `{enum:["A","B"]}` without a type
field, which is the exact form Gemini rejects with
"only allowed for STRING type".

Verified locally: wrapped schema now emits `{type:"string", enum:[...]}` and
drops `$schema`; validation still works.
This commit is contained in:
Colin McDonnell
2026-04-16 23:18:05 +00:00
committed by pullfrog[bot]
parent c608051b79
commit 6533ffddae
1 changed files with 91 additions and 29 deletions
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mcp/geminiSanitizer.ts
+91 -29
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@@ -5,23 +5,30 @@ import type { ToolContext } from "./server.ts";
// ── gemini schema sanitizer ────────────────────────────────────────────────────
//
// gemini's generateContent API expects an OpenAPI 3.0 Schema subset, not full
// JSON Schema. arktype emits constructs that gemini rejects with errors like:
// - "any_of[0].enum: only allowed for STRING type"
// - "parameters.type schema didn't specify the schema type field"
// JSON Schema. arktype 2.x emits constructs that gemini rejects with errors like:
// - "parameters.<field>.enum: only allowed for STRING type"
// - "functionDeclaration parameters.<field> schema didn't specify the schema type field"
// - "anyOf must be the only field in a schema node"
//
// specific transforms applied here:
// 1. collapse `{anyOf: [{enum:["a"]}, {enum:["b"]}]}` (arktype's string-enum
// encoding) into the direct form gemini wants: `{type:"string", enum:[...]}`.
// also handles `{const:"a"}` variants defensively (arktype 2.x may emit these).
// 2. when `anyOf` / `oneOf` can't be collapsed but has sibling fields
// (e.g. `type`, `description`, `items`), strip those siblings — gemini
// rejects `anyOf` alongside any peer keywords. see opencode #14659.
// 3. drop `$schema` metadata and rename `$defs` -> `definitions` (draft-07
// compatibility; gemini doesn't understand `$defs`).
// transforms applied here:
// 1. add `type: "string"` to enum-only schemas. arktype emits string literal
// unions as `{enum: ["a","b"]}` without a `type` field — gemini requires
// the type declaration for any non-object schema.
// 2. collapse `{anyOf: [{enum:["a"]}, {enum:["b"]}]}` (older arktype form)
// into `{type:"string", enum:[...]}`. also handles `{const:"a"}` branches.
// 3. when `anyOf` / `oneOf` can't be collapsed, strip sibling fields (`type`,
// `description`, `items`, etc.) — gemini rejects `anyOf` alongside any
// peer keywords. see opencode #14659.
// 4. drop `$schema` metadata and rename `$defs` → `definitions` (draft-07
// compatibility; gemini doesn't understand either).
//
// gated to gemini-routed traffic via `isGeminiRouted()` so other providers
// continue to see the original (untransformed) schema.
// gating: `isGeminiRouted()` detects gemini-targeted traffic so other
// providers continue to see the original (untransformed) schema.
//
// delivery: fastmcp (3.x) uses `xsschema.toJsonSchema()` which reads
// `schema["~standard"].jsonSchema.input({target:"draft-07"})` when present
// (arktype 2.x exposes this). we proxy the whole `~standard` chain so our
// transform runs regardless of which path xsschema takes.
function parseStringEnumBranch(item: unknown): { values: string[] } | null {
if (!item || typeof item !== "object") return null;
@@ -57,7 +64,18 @@ export function sanitizeForGemini(schema: unknown): unknown {
const source = schema as Record<string, unknown>;
// case 1: collapsible string-enum union → `{type:"string", enum:[...]}`
// case 1: enum-only string union → add `type: "string"`.
// arktype emits `type: "'A' | 'B'"` as `{enum: ["A","B"]}` without a type.
if (Array.isArray(source.enum) && typeof source.type !== "string") {
const allStrings = source.enum.every((v) => typeof v === "string");
if (allStrings) {
const result: Record<string, unknown> = { type: "string", enum: source.enum };
if (typeof source.description === "string") result.description = source.description;
return result;
}
}
// case 2: collapsible string-enum union (older arktype form)
for (const unionKey of ["anyOf", "oneOf"] as const) {
const branches = source[unionKey];
if (Array.isArray(branches) && branches.length > 0) {
@@ -70,7 +88,7 @@ export function sanitizeForGemini(schema: unknown): unknown {
}
}
// case 2: non-collapsible anyOf/oneOf → strip sibling fields (gemini rule)
// case 3: non-collapsible anyOf/oneOf → strip sibling fields (gemini rule)
if (Array.isArray(source.anyOf) || Array.isArray(source.oneOf)) {
const result: Record<string, unknown> = {};
if (Array.isArray(source.anyOf)) result.anyOf = source.anyOf.map(sanitizeForGemini);
@@ -78,7 +96,7 @@ export function sanitizeForGemini(schema: unknown): unknown {
return result;
}
// case 3: generic pass — drop $schema, rename $defs, recurse
// case 4: generic pass — drop $schema, rename $defs, recurse
const sanitized: Record<string, unknown> = {};
for (const [key, value] of Object.entries(source)) {
if (key === "$schema") continue;
@@ -91,19 +109,63 @@ export function sanitizeForGemini(schema: unknown): unknown {
return sanitized;
}
/**
* Wraps a StandardSchemaV1 so its `toJsonSchema()` output is sanitized
* for gemini. other methods on the schema are passed through unchanged.
*/
export function wrapSchemaForGemini(schema: StandardSchemaV1<any>): StandardSchemaV1<any> {
const originalToJsonSchema = (schema as any).toJsonSchema?.bind(schema);
if (!originalToJsonSchema) return schema;
return new Proxy(schema, {
get(target, prop) {
if (prop === "toJsonSchema") {
return () => sanitizeForGemini(originalToJsonSchema());
// ── delivery mechanism ─────────────────────────────────────────────────────────
//
// fastmcp 3.x resolves the JSON schema via xsschema, which takes two paths:
// path A: `schema["~standard"].jsonSchema.input({target:"draft-07"})` when
// the StandardJSONSchemaV1 extension is present (arktype 2.x).
// path B: `schema.toJsonSchema()` via a vendor-dispatched function (older
// arktype, other vendors).
//
// we proxy both entry points so the transform runs regardless of which path
// xsschema picks.
function wrapJsonSchemaProducer<T extends object>(producer: T): T {
return new Proxy(producer, {
get(target, prop, receiver) {
const value = Reflect.get(target, prop, receiver);
if ((prop === "input" || prop === "output") && typeof value === "function") {
const fn = value as (...args: unknown[]) => unknown;
return (...args: unknown[]) => sanitizeForGemini(fn.apply(target, args));
}
return (target as any)[prop];
return value;
},
});
}
function wrapStandard<T extends object>(standard: T): T {
return new Proxy(standard, {
get(target, prop, receiver) {
if (prop === "jsonSchema") {
const value = Reflect.get(target, prop, receiver);
if (value && typeof value === "object") {
return wrapJsonSchemaProducer(value as object);
}
return value;
}
return Reflect.get(target, prop, receiver);
},
});
}
export function wrapSchemaForGemini(schema: StandardSchemaV1<any>): StandardSchemaV1<any> {
return new Proxy(schema, {
get(target, prop, receiver) {
if (prop === "~standard") {
const value = Reflect.get(target, prop, receiver);
if (value && typeof value === "object") {
return wrapStandard(value as object);
}
return value;
}
if (prop === "toJsonSchema") {
const method = Reflect.get(target, prop, receiver);
if (typeof method === "function") {
return () => sanitizeForGemini((method as (...args: unknown[]) => unknown).call(target));
}
return method;
}
return Reflect.get(target, prop, receiver);
},
}) as StandardSchemaV1<any>;
}