diff --git a/packages/ai/src/utils/validation.ts b/packages/ai/src/utils/validation.ts
index f6a65714b..6057c0896 100644
--- a/packages/ai/src/utils/validation.ts
+++ b/packages/ai/src/utils/validation.ts
@@ -299,6 +299,14 @@ function normalizeOptionalNullsForSchema(schema: unknown, value: unknown): { val
 		return { value: changed ? nextValue : value, changed };
 	}
 
+	// Coerce string → number/integer when the schema branch declares those types.
+	// This fixes anyOf:[{type:"number"},{type:"null"}] (i.e. Optional<number>) where
+	// AJV reports an "anyOf" error rather than a "type" error, bypassing
+	// coerceArgsFromErrors which only handles keyword:"type" errors.
+	if ((schemaObject.type === "number" || schemaObject.type === "integer") && typeof value === "string") {
+		return tryParseNumberString(value, [schemaObject.type as string]);
+	}
+
 	if (schemaObject.type !== "object") return { value, changed: false };
 	if (typeof value !== "object" || value === null) return { value, changed: false };
 	if (Array.isArray(value)) return { value, changed: false };
diff --git a/packages/ai/test/tool-argument-coercion.test.ts b/packages/ai/test/tool-argument-coercion.test.ts
index e375fd18a..c87f1d856 100644
--- a/packages/ai/test/tool-argument-coercion.test.ts
+++ b/packages/ai/test/tool-argument-coercion.test.ts
@@ -325,4 +325,37 @@ describe("Tool argument coercion", () => {
 
 		expect(() => validateToolArguments(tool, toolCall)).toThrow('Validation failed for tool "t6"');
 	});
+
+	it("coerces numeric string for Optional<number> (anyOf:[number,null])", () => {
+		const tool: Tool = {
+			name: "t14",
+			description: "",
+			parameters: Type.Object({ tick_size: Type.Optional(Type.Number()) }),
+		};
+		const toolCall: ToolCall = {
+			type: "toolCall",
+			id: "call-14",
+			name: "t14",
+			arguments: { tick_size: "1.0" },
+		};
+		const result = validateToolArguments(tool, toolCall);
+		expect(result.tick_size).toBe(1);
+		expect(typeof result.tick_size).toBe("number");
+	});
+
+	it("leaves Optional<number> as undefined when absent", () => {
+		const tool: Tool = {
+			name: "t15",
+			description: "",
+			parameters: Type.Object({ tick_size: Type.Optional(Type.Number()) }),
+		};
+		const toolCall: ToolCall = {
+			type: "toolCall",
+			id: "call-15",
+			name: "t15",
+			arguments: {},
+		};
+		const result = validateToolArguments(tool, toolCall);
+		expect(result.tick_size).toBeUndefined();
+	});
 });