diff --git a/apps/gateway/src/lib/cf-validate-json-schema-object.ts b/apps/gateway/src/lib/cf-validate-json-schema.ts
similarity index 74%
rename from apps/gateway/src/lib/cf-validate-json-schema-object.ts
rename to apps/gateway/src/lib/cf-validate-json-schema.ts
index 87f71e94..5e89b402 100644
--- a/apps/gateway/src/lib/cf-validate-json-schema-object.ts
+++ b/apps/gateway/src/lib/cf-validate-json-schema.ts
@@ -1,10 +1,10 @@
 import type { ContentfulStatusCode } from 'hono/utils/http-status'
 import { Validator } from '@agentic/json-schema'
-import { HttpError } from '@agentic/platform-core'
+import { assert, HttpError } from '@agentic/platform-core'
 import plur from 'plur'
 
 /**
- * Validates `data` against the provided JSON schema object.
+ * Validates `data` against the provided JSON schema.
  *
  * This method uses a fork of `@cfworker/json-schema`. It does not use `ajv`
  * because `ajv` is not supported on CF workers due to its dynamic code
@@ -14,9 +14,7 @@ import plur from 'plur'
  * not running on CF workers, consider using `validateJsonSchemaObject` from
  * `@agentic/platform-openapi-utils`.
  */
-export function cfValidateJsonSchemaObject<
-  T extends Record<string, any> = Record<string, any>
->({
+export function cfValidateJsonSchema<T = unknown>({
   schema,
   data,
   coerce = false,
@@ -25,16 +23,29 @@ export function cfValidateJsonSchemaObject<
   errorStatusCode = 400
 }: {
   schema: any
-  data: Record<string, unknown>
+  data: unknown
   coerce?: boolean
   strictAdditionalProperties?: boolean
   errorMessage?: string
   errorStatusCode?: ContentfulStatusCode
 }): T {
-  // Special-case check for required fields to give better error messages.
-  if (schema.required && Array.isArray(schema.required)) {
+  assert(schema, 400, '`schema` is required')
+  const isSchemaObject =
+    typeof schema === 'object' &&
+    !Array.isArray(schema) &&
+    schema.type === 'object'
+  const isDataObject = typeof data === 'object' && !Array.isArray(data)
+  if (isSchemaObject && !isDataObject) {
+    throw new HttpError({
+      statusCode: 400,
+      message: `${errorMessage ? errorMessage + ': ' : ''}Data must be an object according to its schema.`
+    })
+  }
+
+  // Special-case check for required fields to give better error messages
+  if (isSchemaObject && Array.isArray(schema.required)) {
     const missingRequiredFields: string[] = schema.required.filter(
-      (field: string) => (data as T)[field] === undefined
+      (field: string) => (data as Record<string, unknown>)[field] === undefined
     )
 
     if (missingRequiredFields.length > 0) {
@@ -48,11 +59,12 @@ export function cfValidateJsonSchemaObject<
   // Special-case check for additional top-level fields to give better error
   // messages.
   if (
+    isSchemaObject &&
     schema.properties &&
     (schema.additionalProperties === false ||
       (schema.additionalProperties === undefined && strictAdditionalProperties))
   ) {
-    const extraProperties = Object.keys(data).filter(
+    const extraProperties = Object.keys(data as Record<string, unknown>).filter(
       (key) => !schema.properties[key]
     )
 
diff --git a/apps/gateway/src/lib/create-http-response-from-mcp-tool-call-response.ts b/apps/gateway/src/lib/create-http-response-from-mcp-tool-call-response.ts
index 2715d254..eaa5d7fa 100644
--- a/apps/gateway/src/lib/create-http-response-from-mcp-tool-call-response.ts
+++ b/apps/gateway/src/lib/create-http-response-from-mcp-tool-call-response.ts
@@ -2,7 +2,7 @@ import type { AdminDeployment, Tool } from '@agentic/platform-types'
 import { assert } from '@agentic/platform-core'
 
 import type { GatewayHonoContext, McpToolCallResponse } from './types'
-import { cfValidateJsonSchemaObject } from './cf-validate-json-schema-object'
+import { cfValidateJsonSchema } from './cf-validate-json-schema'
 
 export async function createHttpResponseFromMcpToolCallResponse(
   _ctx: GatewayHonoContext,
@@ -35,7 +35,7 @@ export async function createHttpResponseFromMcpToolCallResponse(
     )
 
     // Validate tool response against the tool's output schema.
-    const toolCallResponseContent = cfValidateJsonSchemaObject({
+    const toolCallResponseContent = cfValidateJsonSchema({
       schema: tool.outputSchema,
       data: toolCallResponse.structuredContent as Record<string, unknown>,
       coerce: false,
diff --git a/apps/gateway/src/lib/get-tool-args-from-request.ts b/apps/gateway/src/lib/get-tool-args-from-request.ts
index 2b561861..d95c3c29 100644
--- a/apps/gateway/src/lib/get-tool-args-from-request.ts
+++ b/apps/gateway/src/lib/get-tool-args-from-request.ts
@@ -2,7 +2,7 @@ import type { AdminDeployment, Tool } from '@agentic/platform-types'
 import { assert } from '@agentic/platform-core'
 
 import type { GatewayHonoContext } from './types'
-import { cfValidateJsonSchemaObject } from './cf-validate-json-schema-object'
+import { cfValidateJsonSchema } from './cf-validate-json-schema'
 
 export async function getToolArgsFromRequest(
   ctx: GatewayHonoContext,
@@ -48,7 +48,7 @@ export async function getToolArgsFromRequest(
   }
 
   // Validate incoming request params against the tool's input schema.
-  const incomingRequestArgs = cfValidateJsonSchemaObject({
+  const incomingRequestArgs = cfValidateJsonSchema<Record<string, any>>({
     schema: tool.inputSchema,
     data: incomingRequestArgsRaw,
     errorMessage: `Invalid request parameters for tool "${tool.name}"`,
diff --git a/packages/fixtures/valid/basic-mcp/src/index.ts b/packages/fixtures/valid/basic-mcp/src/index.ts
index 6b4f1fee..2c344292 100644
--- a/packages/fixtures/valid/basic-mcp/src/index.ts
+++ b/packages/fixtures/valid/basic-mcp/src/index.ts
@@ -22,6 +22,18 @@ server.addTool({
   }
 })
 
+server.addTool({
+  name: 'add2',
+  description: 'TODO',
+  parameters: z.object({
+    a: z.number(),
+    b: z.number()
+  }),
+  execute: async (args) => {
+    return String(args.a + args.b)
+  }
+})
+
 await server.start({
   transportType: 'httpStream',
   httpStream: {
diff --git a/packages/openapi-utils/src/validate-json-schema-object.ts b/packages/openapi-utils/src/validate-json-schema-object.ts
index b99c11d9..2b449378 100644
--- a/packages/openapi-utils/src/validate-json-schema-object.ts
+++ b/packages/openapi-utils/src/validate-json-schema-object.ts
@@ -1,4 +1,4 @@
-import { hashObject, HttpError } from '@agentic/platform-core'
+import { assert, hashObject, HttpError } from '@agentic/platform-core'
 import { betterAjvErrors } from '@apideck/better-ajv-errors'
 import Ajv, { type ValidateFunction } from 'ajv'
 import addFormats from 'ajv-formats'
@@ -19,7 +19,7 @@ const globalAjv = new Ajv({
 addFormats(globalAjv)
 
 /**
- * Validates `data` against the provided JSON schema object.
+ * Validates `data` against the provided JSON schema.
  *
  * This method uses `ajv` and is therefore not compatible with CF workers due
  * to its use of code generation and evaluation.
@@ -29,9 +29,7 @@ addFormats(globalAjv)
  *
  * @see https://github.com/ajv-validator/ajv/issues/2318
  */
-export function validateJsonSchemaObject<
-  T extends Record<string, unknown> = Record<string, unknown>
->({
+export function validateJsonSchema<T = unknown>({
   schema,
   data,
   ajv = globalAjv,
@@ -42,10 +40,23 @@ export function validateJsonSchemaObject<
   ajv?: Ajv
   errorMessage?: string
 }): T {
+  assert(schema, 400, '`schema` is required')
+  const isSchemaObject =
+    typeof schema === 'object' &&
+    !Array.isArray(schema) &&
+    schema.type === 'object'
+  const isDataObject = typeof data === 'object' && !Array.isArray(data)
+  if (isSchemaObject && !isDataObject) {
+    throw new HttpError({
+      statusCode: 400,
+      message: `${errorMessage ? errorMessage + ': ' : ''}Data must be an object according to its schema.`
+    })
+  }
+
   // Special-case check for required fields to give better error messages
-  if (Array.isArray(schema.required)) {
+  if (isSchemaObject && Array.isArray(schema.required)) {
     const missingRequiredFields: string[] = schema.required.filter(
-      (field: string) => (data as T)[field] === undefined
+      (field: string) => (data as Record<string, unknown>)[field] === undefined
     )
 
     if (missingRequiredFields.length > 0) {