fix(providers): detect rejected reasoning history

src/server/providers/auto-config.test.ts

@@ -5,7 +5,8 @@
  * winning combo for each backend based on actual observed behavior.
  */
 
-import { describe, it, expect } from 'vitest'
+import { afterEach, describe, it, expect, vi } from 'vitest'
+import { autoConfig } from './auto-config.js'
 
 /**
  * Simulated probe results based on real data collected from each backend.
@@ -197,3 +198,51 @@ describe('Context window detection', () => {
     })
   })
 })
+
+describe('Reasoning message compatibility', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals()
+  })
+
+  it('disables reasoning messages when the API rejects assistant reasoning', async () => {
+    const fetchMock = vi.fn(async (_input: string | URL, init?: RequestInit) => {
+      const body = JSON.parse(String(init?.body)) as { messages?: Array<Record<string, unknown>> }
+      if (body.messages?.some((message) => message['reasoning'] === 'probe')) {
+        return new Response(JSON.stringify({ error: 'reasoning is not allowed' }), { status: 422 })
+      }
+      return new Response(JSON.stringify({ choices: [{ message: { content: 'hi' } }] }), { status: 200 })
+    })
+    vi.stubGlobal('fetch', fetchMock)
+
+    const result = await autoConfig({
+      url: 'https://provider.example/v1',
+      backend: 'unknown',
+      models: [{ id: 'deepseek-v4-flash' }],
+    })
+
+    expect(result.models[0]?.sendReasoningInMessages).toBe(false)
+    expect(
+      fetchMock.mock.calls.some(([, init]) => {
+        const body = JSON.parse(String(init?.body)) as { messages?: Array<Record<string, unknown>> }
+        return body.messages?.some((message) => message['reasoning'] === 'probe')
+      }),
+    ).toBe(true)
+  })
+
+  it('leaves reasoning messages unchanged when probing cannot reach the API', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => {
+        throw new TypeError('network unavailable')
+      }),
+    )
+
+    const result = await autoConfig({
+      url: 'https://provider.example/v1',
+      backend: 'unknown',
+      models: [{ id: 'deepseek-v4-flash' }],
+    })
+
+    expect(result.models[0]?.sendReasoningInMessages).toBeUndefined()
+  })
+})

src/server/providers/auto-config.ts

@@ -17,6 +17,8 @@ export interface ModelProbeResult {
   supportsVision: boolean
   thinkingConfig: Record<string, unknown> | null
   nonThinkingConfig: Record<string, unknown> | null
+  /** Set to false when the provider rejects reasoning in assistant history. */
+  sendReasoningInMessages?: boolean
 }
 
 export interface AutoConfigInput {
@@ -281,6 +283,38 @@ async function probeCombos(
   return null
 }
 
+async function probeReasoningInMessages(
+  baseUrl: string,
+  apiKey: string | undefined,
+  model: string,
+): Promise<boolean | undefined> {
+  const headers: Record<string, string> = { 'Content-Type': 'application/json' }
+  if (apiKey) headers['Authorization'] = `Bearer ${apiKey}`
+
+  try {
+    const response = await fetch(`${ensureVersionPrefix(baseUrl)}/chat/completions`, {
+      method: 'POST',
+      headers,
+      body: JSON.stringify({
+        model,
+        messages: [
+          { role: 'user', content: 'say hi in one word' },
+          { role: 'assistant', content: 'hi', reasoning: 'probe' },
+          { role: 'user', content: 'say hi in one word' },
+        ],
+        max_tokens: 1,
+      }),
+      signal: AbortSignal.timeout(15_000),
+    })
+
+    return response.ok
+  } catch {
+    // A transport failure is inconclusive; do not disable a provider setting
+    // based on an unavailable endpoint.
+    return undefined
+  }
+}
+
 // ============================================================================
 // Main entry point
 // ============================================================================
@@ -305,13 +339,18 @@ export async function autoConfig(input: AutoConfigInput): Promise<AutoConfigOutp
       probeCombos(baseUrl, apiKey, model.id, NON_THINKING_COMBOS),
     ])
 
+    const sendReasoningInMessages = thinkingConfig
+      ? await probeReasoningInMessages(baseUrl, apiKey, model.id)
+      : undefined
+
     results.push({
       id: model.id,
       contextWindow,
       contextSource,
       supportsVision,
       thinkingConfig,
       nonThinkingConfig,
+      ...(sendReasoningInMessages !== undefined ? { sendReasoningInMessages } : {}),
     })
   }
 

web/src/components/shared/ProviderModal.test.tsx

@@ -522,4 +522,57 @@ describe('ProviderModal - thinkingLevel persistence', () => {
     expect(savedData.authAdapter).toBeUndefined()
     expect(savedData.transportAdapter).toBeUndefined()
   })
+
+  it('disables reasoning messages when auto-config detects a rejected history field', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async (input: RequestInfo | URL) => {
+        const url = String(input)
+        if (url.includes('/api/providers/auto-config')) {
+          return new Response(
+            JSON.stringify({
+              models: [
+                {
+                  id: 'test-model',
+                  contextWindow: 200000,
+                  supportsVision: false,
+                  thinkingConfig: { reasoning_effort: 'high' },
+                  nonThinkingConfig: null,
+                  sendReasoningInMessages: false,
+                },
+              ],
+            }),
+            { status: 200 },
+          )
+        }
+        return new Response(JSON.stringify({ presets: [] }), { status: 200 })
+      }),
+    )
+
+    await new Promise<void>((resolve) => {
+      root.render(
+        <ProviderModal
+          isOpen={true}
+          onClose={vi.fn()}
+          onSave={onSaveMock as (provider: ProviderFormData) => void}
+          initialStep={2}
+          editProvider={makeEditProvider()}
+          editModelId="test-model"
+        />,
+      )
+      setTimeout(resolve, 200)
+    })
+
+    const autoConfigButton = Array.from(container.querySelectorAll('button')).find(
+      (button) => button.textContent?.trim() === 'Auto-config',
+    ) as HTMLButtonElement | undefined
+    autoConfigButton?.click()
+    await new Promise((resolve) => setTimeout(resolve, 50))
+
+    const saveButton = container.querySelector('[data-testid="provider-modal-save"]') as HTMLButtonElement | null
+    saveButton?.click()
+
+    const savedData: ProviderFormData = onSaveMock.mock.calls[0]![0]!
+    expect(savedData.sendReasoningInMessages).toBe(false)
+  })
 })

web/src/components/shared/ProviderModal.tsx

@@ -844,6 +844,7 @@ export function ProviderModal({
           supportsVision: boolean
           thinkingConfig: Record<string, unknown> | null
           nonThinkingConfig: Record<string, unknown> | null
+          sendReasoningInMessages?: boolean
         }>
       }
       for (const m of data.models) {
@@ -861,6 +862,9 @@ export function ProviderModal({
           config.nonThinkingEnabled = true
           config.nonThinkingQueryParams = JSON.stringify(m.nonThinkingConfig)
         }
+        if (m.sendReasoningInMessages === false) {
+          setSendReasoningInMessages(false)
+        }
         updateModelConfig(m.id, config)
         setAutoConfigState((prev) => ({
           ...prev,