fix(providers): refuse an empty Responses input before sending instead of retrying a guaranteed 400

apps/sim/providers/openai/core.response-status.test.ts

@@ -11,6 +11,8 @@ import {
 } from '@sim/testing/mocks/providers-conversation-history.mock'
 import { providersUtilsMock, providersUtilsMockFns } from '@sim/testing/mocks/providers-utils.mock'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { classifyFailure } from '@/lib/core/errors/failure-log'
+import { isNonRetryableExecutionError } from '@/lib/execution/non-retryable-error'
 import { executeResponsesProviderRequest } from '@/providers/openai/core'
 import type { ProviderRequest, ProviderResponse } from '@/providers/types'
 
@@ -113,6 +115,41 @@ describe('OpenAI non-streaming response status handling', () => {
     expect(fetchMock).not.toHaveBeenCalled()
   })
 
+  it('refuses a request with no input items before sending, as a nonretryable author failure', async () => {
+    const fetchMock = vi.fn().mockResolvedValue(
+      jsonResponse(
+        {
+          error: {
+            message:
+              'One of input or previous_response_id or prompt or conversation must be provided.',
+          },
+        },
+        400
+      )
+    )
+    const error = await run(fetchMock, {
+      systemPrompt: '',
+      messages: [
+        { role: 'system', content: '' },
+        { role: 'user', content: '' },
+      ],
+    }).catch((caught: unknown) => caught)
+
+    expect(error).toMatchObject({ name: 'EmptyProviderInputError' })
+    expect(isNonRetryableExecutionError(error)).toBe(true)
+    expect(classifyFailure(error)).toBe('user')
+    expect(fetchMock).not.toHaveBeenCalled()
+  })
+
+  it('still sends a request that carries only a system prompt', async () => {
+    const fetchMock = vi.fn().mockResolvedValue(jsonResponse(COMPLETED_RESPONSE))
+    await run(fetchMock, { systemPrompt: 'You are terse.', messages: [] })
+
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+    const body = JSON.parse(fetchMock.mock.calls[0][1].body)
+    expect(body.input).toEqual([{ role: 'system', content: 'You are terse.' }])
+  })
+
   it('lets an estimated oversized request reach the provider and propagates its actual context rejection', async () => {
     mockConversationContext.mockReturnValue({ agentConversation: {} })
     const fetchMock = vi.fn().mockResolvedValue({

apps/sim/providers/openai/core.ts

@@ -30,7 +30,7 @@ import { createStreamingExecution } from '@/providers/streaming-execution'
 import { isAbortError, parseToolArguments } from '@/providers/streaming-tool-loop-shared'
 import { adaptOpenAIChatToolSchema } from '@/providers/tool-schema-adapter'
 import type { Message, ProviderRequest, ProviderResponse, TimeSegment } from '@/providers/types'
-import { ProviderError } from '@/providers/types'
+import { EmptyProviderInputError, ProviderError } from '@/providers/types'
 import {
   enforceStrictSchema,
   prepareToolExecution,
@@ -188,6 +188,7 @@ export async function executeResponsesProviderRequest(
   }
 
   const initialInput = buildResponsesInputFromMessages(allMessages, config.providerId)
+  if (initialInput.length === 0) throw new EmptyProviderInputError()
 
   /**
    * `stream` is always explicit: OpenAI defaults it to false, but Responses-compatible

apps/sim/providers/types.ts

@@ -1,4 +1,5 @@
 import type { BillingAttributionSnapshot } from '@/lib/billing/core/attribution'
+import { UserFailure } from '@/lib/core/errors/user-failure'
 import type { CustomBlockInputFieldType } from '@/blocks/custom/build-config'
 import type { ProviderTimingSegment, StreamingExecution, UserFile } from '@/executor/types'
 
@@ -294,4 +295,20 @@ export class ProviderError extends Error {
   }
 }
 
+/**
+ * A request with no input items to send: every system and user message was absent or resolved
+ * to empty. The provider would reject it on every attempt, so it is the author's to fix and is
+ * never retried or handed to a fallback model.
+ */
+export class EmptyProviderInputError extends UserFailure {
+  readonly retryable = false
+
+  constructor() {
+    super(
+      'The Agent has no messages to send. Add a system prompt or user message, or check that the value it references (for example <start.input>) is not empty.'
+    )
+    this.name = 'EmptyProviderInputError'
+  }
+}
+
 export const providers: Record<string, ProviderConfig> = {}