diff --git a/apps/sim/app/api/guardrails/validate/route.test.ts b/apps/sim/app/api/guardrails/validate/route.test.ts
new file mode 100644
index 00000000000..5c4f91b9109
--- /dev/null
+++ b/apps/sim/app/api/guardrails/validate/route.test.ts
@@ -0,0 +1,159 @@
+/**
+ * @vitest-environment node
+ */
+import { createMockRequest, hybridAuthMockFns, workflowAuthzMockFns } from '@sim/testing'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+const { mockAuthorizeCredentialUse, mockCheckActorUsageLimits, mockValidateHallucination } =
+  vi.hoisted(() => ({
+    mockAuthorizeCredentialUse: vi.fn(),
+    mockCheckActorUsageLimits: vi.fn(),
+    mockValidateHallucination: vi.fn(),
+  }))
+
+vi.mock('@/lib/auth/credential-access', () => ({
+  authorizeCredentialUse: mockAuthorizeCredentialUse,
+}))
+
+vi.mock('@/lib/billing/calculations/usage-monitor', () => ({
+  checkActorUsageLimits: mockCheckActorUsageLimits,
+}))
+
+vi.mock('@/lib/guardrails/validate_hallucination', () => ({
+  validateHallucination: mockValidateHallucination,
+}))
+
+vi.mock('@/lib/guardrails/validate_json', () => ({
+  validateJson: vi.fn(() => ({ passed: true })),
+}))
+
+vi.mock('@/lib/guardrails/validate_pii', () => ({
+  validatePII: vi.fn(() => ({ passed: true })),
+}))
+
+vi.mock('@/lib/guardrails/validate_regex', () => ({
+  validateRegex: vi.fn(() => ({ passed: true })),
+}))
+
+vi.mock('@/ee/access-control/utils/permission-check', () => ({
+  assertPermissionsAllowed: vi.fn(),
+  ModelNotAllowedError: class ModelNotAllowedError extends Error {},
+  ProviderNotAllowedError: class ProviderNotAllowedError extends Error {},
+}))
+
+import { POST } from '@/app/api/guardrails/validate/route'
+
+describe('POST /api/guardrails/validate', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    hybridAuthMockFns.mockCheckSessionOrInternalAuth.mockResolvedValue({
+      success: true,
+      userId: 'user-1',
+      authType: 'session',
+    })
+    workflowAuthzMockFns.mockAuthorizeWorkflowByWorkspacePermission.mockResolvedValue({
+      allowed: true,
+      workflow: { id: 'wf-1', workspaceId: 'ws-1' },
+    })
+    mockCheckActorUsageLimits.mockResolvedValue({ isExceeded: false })
+    mockValidateHallucination.mockResolvedValue({ passed: true, score: 8 })
+  })
+
+  it('rejects a vertexCredential the caller does not have access to before calling validateHallucination', async () => {
+    mockAuthorizeCredentialUse.mockResolvedValue({
+      ok: false,
+      error: 'You do not have access to this credential.',
+    })
+
+    const res = await POST(
+      createMockRequest('POST', {
+        validationType: 'hallucination',
+        input: 'test input',
+        knowledgeBaseId: 'kb-1',
+        model: 'vertex/gemini-2.5-pro',
+        workflowId: 'wf-1',
+        vertexCredential: 'someone-elses-account-id',
+      })
+    )
+
+    expect(res.status).toBe(401)
+    expect(mockAuthorizeCredentialUse).toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({
+        credentialId: 'someone-elses-account-id',
+        workflowId: 'wf-1',
+        requireWorkflowIdForInternal: false,
+      })
+    )
+    expect(mockValidateHallucination).not.toHaveBeenCalled()
+  })
+
+  it('proceeds with hallucination validation when the caller has access to the vertexCredential', async () => {
+    mockAuthorizeCredentialUse.mockResolvedValue({ ok: true })
+
+    const res = await POST(
+      createMockRequest('POST', {
+        validationType: 'hallucination',
+        input: 'test input',
+        knowledgeBaseId: 'kb-1',
+        model: 'vertex/gemini-2.5-pro',
+        workflowId: 'wf-1',
+        vertexCredential: 'my-own-account-id',
+      })
+    )
+
+    expect(res.status).toBe(200)
+    const json = await res.json()
+    expect(json.output.passed).toBe(true)
+    expect(mockAuthorizeCredentialUse).toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({ credentialId: 'my-own-account-id' })
+    )
+    expect(mockValidateHallucination).toHaveBeenCalled()
+  })
+
+  it('does not gate on vertexCredential for non-hallucination validation types', async () => {
+    const res = await POST(
+      createMockRequest('POST', {
+        validationType: 'json',
+        input: '{"a":1}',
+      })
+    )
+
+    expect(res.status).toBe(200)
+    expect(mockAuthorizeCredentialUse).not.toHaveBeenCalled()
+  })
+
+  it('does not gate hallucination validation when no vertexCredential is supplied', async () => {
+    const res = await POST(
+      createMockRequest('POST', {
+        validationType: 'hallucination',
+        input: 'test input',
+        knowledgeBaseId: 'kb-1',
+        model: 'gpt-4o',
+        workflowId: 'wf-1',
+      })
+    )
+
+    expect(res.status).toBe(200)
+    expect(mockAuthorizeCredentialUse).not.toHaveBeenCalled()
+    expect(mockValidateHallucination).toHaveBeenCalled()
+  })
+
+  it('does not gate on a leftover vertexCredential when the resolved model is not vertex', async () => {
+    const res = await POST(
+      createMockRequest('POST', {
+        validationType: 'hallucination',
+        input: 'test input',
+        knowledgeBaseId: 'kb-1',
+        model: 'gpt-4o',
+        workflowId: 'wf-1',
+        vertexCredential: 'someone-elses-account-id',
+      })
+    )
+
+    expect(res.status).toBe(200)
+    expect(mockAuthorizeCredentialUse).not.toHaveBeenCalled()
+    expect(mockValidateHallucination).toHaveBeenCalled()
+  })
+})
diff --git a/apps/sim/app/api/guardrails/validate/route.ts b/apps/sim/app/api/guardrails/validate/route.ts
index ca50b20b2d2..f4dea3d3367 100644
--- a/apps/sim/app/api/guardrails/validate/route.ts
+++ b/apps/sim/app/api/guardrails/validate/route.ts
@@ -3,6 +3,7 @@ import { authorizeWorkflowByWorkspacePermission } from '@sim/platform-authz/work
 import { type NextRequest, NextResponse } from 'next/server'
 import { guardrailsValidateContract } from '@/lib/api/contracts'
 import { parseRequest } from '@/lib/api/server'
+import { authorizeCredentialUse } from '@/lib/auth/credential-access'
 import { checkSessionOrInternalAuth } from '@/lib/auth/hybrid'
 import { checkActorUsageLimits } from '@/lib/billing/calculations/usage-monitor'
 import { generateRequestId } from '@/lib/core/utils/request'
@@ -16,6 +17,7 @@ import {
   ModelNotAllowedError,
   ProviderNotAllowedError,
 } from '@/ee/access-control/utils/permission-check'
+import { getProviderFromModel } from '@/providers/utils'
 
 const logger = createLogger('GuardrailsValidateAPI')
 
@@ -187,6 +189,24 @@ export const POST = withRouteHandler(async (request: NextRequest) => {
           { status: 402 }
         )
       }
+
+      if (vertexCredential && getProviderFromModel(model) === 'vertex') {
+        const vertexCredAccess = await authorizeCredentialUse(request, {
+          credentialId: vertexCredential,
+          workflowId,
+          requireWorkflowIdForInternal: false,
+        })
+        if (!vertexCredAccess.ok) {
+          logger.warn(`[${requestId}] Vertex credential access denied`, {
+            error: vertexCredAccess.error,
+            credentialId: vertexCredential,
+          })
+          return NextResponse.json(
+            { error: vertexCredAccess.error || 'Unauthorized' },
+            { status: 401 }
+          )
+        }
+      }
     }
 
     const inputStr = convertInputToString(input)