diff --git a/apps/sim/lib/mcp/application/execute-tool.test.ts b/apps/sim/lib/mcp/application/execute-tool.test.ts
index 4de6ad29275..d4870e2a3f8 100644
--- a/apps/sim/lib/mcp/application/execute-tool.test.ts
+++ b/apps/sim/lib/mcp/application/execute-tool.test.ts
@@ -36,7 +36,7 @@ vi.mock('@/ee/access-control/utils/permission-check', () => permissionCheckMock)
 vi.mock('@/lib/mcp/service', () => mcpServiceMock)
 vi.mock('@/lib/core/telemetry', () => telemetryMock)
 
-import { executeMcpToolUseCase } from '@/lib/mcp/application/execute-tool'
+import { executeMcpToolUseCase, validateToolArguments } from '@/lib/mcp/application/execute-tool'
 
 const mocks = {
   assertPermissionsAllowed: permissionCheckMockFns.mockAssertPermissionsAllowed,
@@ -391,3 +391,33 @@ describe('executeMcpToolUseCase', () => {
     expect(mocks.executeTool).not.toHaveBeenCalled()
   })
 })
+
+describe('validateToolArguments', () => {
+  const tool = {
+    name: 'search',
+    serverId: 'mcp-server-1',
+    serverName: 'Search',
+    inputSchema: {
+      type: 'object' as const,
+      required: ['query'],
+      additionalProperties: false,
+      properties: {
+        query: { type: 'string' },
+        limit: { type: 'integer' },
+      },
+    },
+  }
+
+  it('names the invalid argument so the caller can correct the call', () => {
+    expect(() => validateToolArguments(tool, { query: 'sim', limit: 'ten' })).toThrow(
+      'Invalid MCP operation arguments: /limit must be integer'
+    )
+    expect(() => validateToolArguments(tool, { query: 'sim', extra: true })).toThrow(
+      "Invalid MCP operation arguments: must NOT have additional properties 'extra'"
+    )
+  })
+
+  it('accepts arguments the schema allows', () => {
+    expect(() => validateToolArguments(tool, { query: 'sim', limit: 5 })).not.toThrow()
+  })
+})
diff --git a/apps/sim/lib/mcp/application/execute-tool.ts b/apps/sim/lib/mcp/application/execute-tool.ts
index 647917c7968..538fa445a8b 100644
--- a/apps/sim/lib/mcp/application/execute-tool.ts
+++ b/apps/sim/lib/mcp/application/execute-tool.ts
@@ -1,5 +1,6 @@
 import { createLogger } from '@sim/logger'
 import { getErrorMessage } from '@sim/utils/errors'
+import type { ErrorObject } from 'ajv'
 import { defineAuthorizedWorkspaceUseCase } from '@/lib/core/application'
 import { OrchestrationError } from '@/lib/core/orchestration/types'
 import { SIM_VIA_HEADER, serializeCallChain } from '@/lib/execution/call-chain'
@@ -94,8 +95,25 @@ export function coerceToolArguments(
 
 /** Validates the complete discovered JSON Schema, including nested constraints and additional properties. */
 export function validateToolArguments(tool: McpTool, args: Record<string, unknown>): void {
-  if (!compileMcpToolSchema(tool.inputSchema)(args))
-    throw new OrchestrationError('validation', 'Invalid MCP operation arguments')
+  const validate = compileMcpToolSchema(tool.inputSchema)
+  if (validate(args)) return
+  // Ajv stops at the first failure (no allErrors: unbounded work on untrusted input).
+  const first = validate.errors?.[0]
+  throw new OrchestrationError(
+    'validation',
+    first
+      ? `Invalid MCP operation arguments: ${describeArgumentError(first)}`
+      : 'Invalid MCP operation arguments'
+  )
+}
+
+/** Names the failing argument so the caller can correct it; the schema and arguments are not secret. */
+function describeArgumentError(error: ErrorObject): string {
+  const extra =
+    error.keyword === 'additionalProperties' && typeof error.params.additionalProperty === 'string'
+      ? ` '${error.params.additionalProperty}'`
+      : ''
+  return [error.instancePath, `${error.message ?? 'is invalid'}${extra}`].filter(Boolean).join(' ')
 }
 
 export function transformToolResult(result: McpToolResult): ExecuteMcpToolResult {
diff --git a/apps/sim/lib/mcp/client.ts b/apps/sim/lib/mcp/client.ts
index 3f843362398..5e577fa94ce 100644
--- a/apps/sim/lib/mcp/client.ts
+++ b/apps/sim/lib/mcp/client.ts
@@ -239,12 +239,11 @@ export class McpClient {
         logger.info(`[${this.config.name}] Registered tools/list_changed notification handler`)
       }
 
-      const serverVersion = this.client.getServerVersion()
       logger.info(`Successfully connected to MCP server: ${this.config.name}`, {
         ...diagnostics,
         durationMs: Date.now() - startedAt,
         outcome: 'connected' satisfies ConnectionOutcome,
-        protocolVersion: serverVersion,
+        protocolVersion: this.getNegotiatedVersion(),
       })
     } catch (error) {
       this.isConnected = false
@@ -543,9 +542,9 @@ export class McpClient {
     }
   }
 
+  /** The protocol version agreed during initialize, which the transport records. */
   getNegotiatedVersion(): string | undefined {
-    const serverVersion = this.client.getServerVersion()
-    return typeof serverVersion === 'string' ? serverVersion : undefined
+    return this.transport.protocolVersion
   }
 
   getSessionId(): string | undefined {
diff --git a/apps/sim/lib/mcp/tool-schema.ts b/apps/sim/lib/mcp/tool-schema.ts
index 302f42e0992..43d65244f44 100644
--- a/apps/sim/lib/mcp/tool-schema.ts
+++ b/apps/sim/lib/mcp/tool-schema.ts
@@ -13,7 +13,6 @@ export function compileMcpToolSchema(schema: McpToolSchema): ValidateFunction {
     const validate = new Validator({
       strict: false,
       validateFormats: false,
-      allErrors: false,
     }).compile(schema)
     if ('$async' in validate && validate.$async)
       throw new Error('Asynchronous schemas are not supported')
diff --git a/apps/sim/tools/params.test.ts b/apps/sim/tools/params.test.ts
index 443106f4aa6..e00166e6e40 100644
--- a/apps/sim/tools/params.test.ts
+++ b/apps/sim/tools/params.test.ts
@@ -1,3 +1,4 @@
+import Ajv from 'ajv'
 import { afterAll, beforeEach, describe, expect, it, type MockInstance, vi } from 'vitest'
 import type { ExecutorDelegationOrigin } from '@/executor/types'
 import { mergeToolParameters } from '@/tools/merge-params'
@@ -186,11 +187,42 @@ describe('Tool Parameters Utils', () => {
       const { schema } = await createLLMToolSchema(legacyJsonTool, {})
 
       expect(schema.properties.payload).toEqual({
-        type: 'object',
         description: 'Legacy JSON array payload',
       })
     })
 
+    it('lets a json or any param take an array as well as an object', async () => {
+      const untypedTool = {
+        ...mockToolConfig,
+        id: 'untyped_tool',
+        params: {
+          orgIds: {
+            type: 'json',
+            required: true,
+            visibility: 'user-or-llm' as ParameterVisibility,
+            description: 'Organization IDs, e.g. [129410, 1034157]',
+          },
+          value: {
+            type: 'any',
+            required: true,
+            visibility: 'user-or-llm' as ParameterVisibility,
+          },
+        },
+      }
+
+      const { schema: llmSchema } = await createLLMToolSchema(untypedTool, {})
+      const userSchema = createUserToolSchema(untypedTool, { surface: 'copilot' })
+
+      for (const schema of [llmSchema, userSchema]) {
+        const validate = new Ajv({ strict: false }).compile(schema)
+        expect(validate({ orgIds: [129410, 1034157], value: ['a'] })).toBe(true)
+        expect(validate({ orgIds: { id: 129410 }, value: { a: 1 } })).toBe(true)
+        expect(schema.properties.orgIds.description).toBe(
+          'Organization IDs, e.g. [129410, 1034157]'
+        )
+      }
+    })
+
     it.concurrent('should create schema excluding user-provided parameters', async () => {
       const userProvidedParams = {
         apiKey: 'user-provided-key',
diff --git a/apps/sim/tools/params.ts b/apps/sim/tools/params.ts
index 8ff780bcc11..dc71edd7135 100644
--- a/apps/sim/tools/params.ts
+++ b/apps/sim/tools/params.ts
@@ -35,7 +35,8 @@ type ToolParamDefinition = ToolConfig['params'][string]
 type ToolInputBlockConfig = Pick<AppBlockConfig, 'type' | 'subBlocks' | 'tools'>
 
 interface SchemaProperty {
-  type: string
+  /** Absent for a `json` or `any` param, which accepts any JSON value. */
+  type?: string
   description?: string
   items?: ToolParameterItemSchema
   properties?: Record<string, SchemaProperty>
@@ -162,13 +163,13 @@ function buildParameterSchema(
       : buildFileReferenceParameterSchema(param)
   }
 
-  let schemaType = param.type
-  if (schemaType === 'json' || schemaType === 'any') {
-    schemaType = 'object'
-  }
+  // `json` and `any` params take arrays as often as objects, so they carry no
+  // `type`: an `object` claim would contradict their descriptions and make a
+  // validating runtime reject a correct array argument.
+  const isUntyped = param.type === 'json' || param.type === 'any'
 
   const propertySchema: SchemaProperty = {
-    type: schemaType,
+    ...(isUntyped ? {} : { type: param.type }),
     description: param.description || '',
     ...(param.minItems !== undefined ? { minItems: param.minItems } : {}),
     ...(param.maxItems !== undefined ? { maxItems: param.maxItems } : {}),