fix(ai): send const as enum in Gemini tool schemas

packages/ai/src/api/google-shared.ts

@@ -371,11 +371,37 @@ function sanitizeForOpenApi(schema: unknown): unknown {
 	return result;
 }
 
+/** Schema keywords whose values map names to subschemas, so their keys are not schema keywords. */
+const JSON_SCHEMA_NAME_MAPS = new Set(["properties", "patternProperties", "$defs", "definitions", "dependentSchemas"]);
+
+/**
+ * Rewrite string and number `const` values to single-value `enum`s.
+ * Gemini ignores `const` in `parametersJsonSchema` but honors `enum`.
+ */
+function rewriteConstAsEnum(schema: unknown, isNameMap = false): unknown {
+	if (Array.isArray(schema)) return schema.map((item) => rewriteConstAsEnum(item));
+	if (typeof schema !== "object" || schema === null) return schema;
+
+	const result: Record<string, unknown> = {};
+	for (const [key, value] of Object.entries(schema)) {
+		result[key] = isNameMap ? rewriteConstAsEnum(value) : rewriteConstAsEnum(value, JSON_SCHEMA_NAME_MAPS.has(key));
+	}
+	if (!isNameMap && "const" in result && result.enum === undefined) {
+		const value = result.const;
+		if (typeof value === "string" || typeof value === "number") {
+			delete result.const;
+			result.enum = [value];
+		}
+	}
+	return result;
+}
+
 /**
  * Convert tools to Gemini function declarations format.
  *
  * By default uses `parametersJsonSchema` which supports full JSON Schema (including
- * anyOf, oneOf, const, etc.). Set `useParameters` to true to use the legacy `parameters`
+ * anyOf, oneOf, etc.). Gemini ignores `const` there, so string and number `const`
+ * values are sent as single-value `enum`s. Set `useParameters` to true to use the legacy `parameters`
  * field instead (OpenAPI 3.03 Schema). This is needed for Cloud Code Assist with Claude
  * models, where the API translates `parameters` into Anthropic's `input_schema`.
  */
@@ -395,7 +421,7 @@ export function convertTools(
 					description: tool.description,
 					...(useParameters
 						? { parameters: sanitizeForOpenApi(parameters as unknown) }
-						: { parametersJsonSchema: parameters }),
+						: { parametersJsonSchema: rewriteConstAsEnum(parameters) }),
 				};
 			}),
 		},

packages/ai/test/google-shared-convert-tools.test.ts

@@ -201,3 +201,48 @@ describe("google-shared convertTools", () => {
 		expect(convertTools([], true)).toBeUndefined();
 	});
 });
+
+describe("google-shared convertTools const handling", () => {
+	it("rewrites string and number const values to single-value enums in parametersJsonSchema", () => {
+		const parameters = {
+			type: "object",
+			properties: {
+				origin: { type: "string", const: "zq-src-7" },
+				mode: {
+					anyOf: [
+						{ type: "string", const: "a" },
+						{ type: "string", const: "b" },
+					],
+				},
+				level: { oneOf: [{ const: 1 }, { const: 2 }] },
+				flag: { type: "boolean", const: true },
+				list: { type: "array", items: { const: "x" } },
+				const: { type: "string" },
+			},
+			required: ["origin"],
+		};
+		const tools = [makeTool(parameters)];
+
+		const result = convertTools(tools);
+		const decl = result?.[0]?.functionDeclarations?.[0];
+
+		expect(decl?.parametersJsonSchema).toEqual({
+			type: "object",
+			properties: {
+				origin: { type: "string", enum: ["zq-src-7"] },
+				mode: {
+					anyOf: [
+						{ type: "string", enum: ["a"] },
+						{ type: "string", enum: ["b"] },
+					],
+				},
+				level: { oneOf: [{ enum: [1] }, { enum: [2] }] },
+				flag: { type: "boolean", const: true },
+				list: { type: "array", items: { enum: ["x"] } },
+				const: { type: "string" },
+			},
+			required: ["origin"],
+		});
+		expect(parameters.properties.origin).toEqual({ type: "string", const: "zq-src-7" });
+	});
+});