feat(sdk): add custom OpenAI-compatible provider branch to getModelForRequest

When ModelRequestParams.customProvider.baseUrl is set, return an
OpenAICompatibleChatLanguageModel pointed at that endpoint and flag
the result with isCustomProvider: true. Bypasses both the Codebuff
backend and the ChatGPT OAuth direct path. No metadataExtractor —
direct calls don't flow through Codebuff cost accounting.

Mirrors the existing ChatGPT-OAuth-direct branch pattern. Trailing
slashes on baseUrl are trimmed. apiKey defaults to "codebuff" when
absent (most local runtimes ignore it).

Adds 5 unit tests covering the new branch, regression-tested against
existing model-provider-free-mode tests.

Part of issue #678.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: vraj00222

sdk/src/impl/__tests__/model-provider-custom.test.ts

@@ -0,0 +1,70 @@
+import { describe, expect, test, afterEach, mock } from 'bun:test'
+
+describe('getModelForRequest with customProvider', () => {
+  afterEach(() => {
+    mock.restore()
+  })
+
+  test('returns isCustomProvider: true when customProvider.baseUrl is set', async () => {
+    const { getModelForRequest } = await import('../model-provider')
+
+    const result = await getModelForRequest({
+      apiKey: 'cb-test-key',
+      model: 'gemma2:9b',
+      customProvider: { baseUrl: 'http://localhost:11434/v1', apiKey: 'ollama' },
+    })
+
+    expect(result.isCustomProvider).toBe(true)
+    expect(result.isChatGptOAuth).toBe(false)
+    expect(result.model).toBeDefined()
+    expect((result.model as any).modelId).toBe('gemma2:9b')
+  })
+
+  test('does not return isCustomProvider when baseUrl is missing', async () => {
+    const { getModelForRequest } = await import('../model-provider')
+
+    const result = await getModelForRequest({
+      apiKey: 'cb-test-key',
+      model: 'anthropic/claude-sonnet-4',
+    })
+
+    expect(result.isCustomProvider).toBe(false)
+  })
+
+  test('customProvider takes precedence over ChatGPT OAuth eligibility', async () => {
+    const { getModelForRequest } = await import('../model-provider')
+
+    const result = await getModelForRequest({
+      apiKey: 'cb-test-key',
+      model: 'openai/gpt-5.3',
+      customProvider: { baseUrl: 'http://localhost:11434/v1' },
+    })
+
+    expect(result.isCustomProvider).toBe(true)
+    expect(result.isChatGptOAuth).toBe(false)
+  })
+
+  test('trims trailing slash from baseUrl (constructs cleanly)', async () => {
+    const { getModelForRequest } = await import('../model-provider')
+
+    const result = await getModelForRequest({
+      apiKey: 'cb-test-key',
+      model: 'gemma2:9b',
+      customProvider: { baseUrl: 'http://localhost:11434/v1/' },
+    })
+
+    expect(result.isCustomProvider).toBe(true)
+  })
+
+  test('omitting apiKey is allowed', async () => {
+    const { getModelForRequest } = await import('../model-provider')
+
+    const result = await getModelForRequest({
+      apiKey: 'cb-test-key',
+      model: 'gemma2:9b',
+      customProvider: { baseUrl: 'http://localhost:11434/v1' },
+    })
+
+    expect(result.isCustomProvider).toBe(true)
+  })
+})

sdk/src/impl/model-provider.ts

@@ -86,6 +86,8 @@ export interface ModelRequestParams {
   skipChatGptOAuth?: boolean
   /** Cost mode (e.g. 'free') — affects fallback behavior for OAuth routes */
   costMode?: string
+  /** When set, route this request directly to the OpenAI-compatible endpoint and bypass Codebuff/OAuth. */
+  customProvider?: { baseUrl: string; apiKey?: string }
 }
 
 /**
@@ -96,6 +98,8 @@ export interface ModelResult {
   model: LanguageModel
   /** Whether this model uses ChatGPT OAuth direct (affects cost tracking) */
   isChatGptOAuth: boolean
+  /** Whether this model uses a custom OpenAI-compatible endpoint (affects cost tracking + metadata) */
+  isCustomProvider: boolean
 }
 
 // Usage accounting type for OpenRouter/Codebuff backend responses
@@ -115,7 +119,21 @@ type OpenRouterUsageAccounting = {
  * This function is async because it may need to refresh the OAuth token.
  */
 export async function getModelForRequest(params: ModelRequestParams): Promise<ModelResult> {
-  const { apiKey, model, skipChatGptOAuth, costMode } = params
+  const { apiKey, model, skipChatGptOAuth, costMode, customProvider } = params
+
+  // 1) Custom OpenAI-compatible endpoint wins — explicit per-agent / client / env override.
+  //    Bypasses Codebuff backend AND ChatGPT OAuth.
+  if (customProvider?.baseUrl) {
+    return {
+      model: createCustomProviderModel({
+        model,
+        baseUrl: customProvider.baseUrl,
+        apiKey: customProvider.apiKey,
+      }),
+      isChatGptOAuth: false,
+      isCustomProvider: true,
+    }
+  }
 
   // Check if we should use ChatGPT OAuth direct
   // Only attempt for allowlisted models; non-allowlisted models silently fall through to backend.
@@ -140,6 +158,7 @@ export async function getModelForRequest(params: ModelRequestParams): Promise<Mo
         return {
           model: createOpenAIOAuthModel(model, chatGptOAuthCredentials.accessToken),
           isChatGptOAuth: true,
+          isCustomProvider: false,
         }
       }
 
@@ -156,6 +175,7 @@ export async function getModelForRequest(params: ModelRequestParams): Promise<Mo
   return {
     model: createCodebuffBackendModel(apiKey, model),
     isChatGptOAuth: false,
+    isCustomProvider: false,
   }
 }
 
@@ -256,3 +276,32 @@ function createCodebuffBackendModel(
     supportsStructuredOutputs: true,
   })
 }
+
+/**
+ * Create an OpenAI-compatible model pointed at a user-supplied base URL.
+ * Used for local providers (Ollama, LM Studio) and self-hosted endpoints.
+ *
+ * No metadata extractor — direct calls don't flow through Codebuff's usage
+ * accounting. No codebuff_metadata is sent (handled by the caller).
+ */
+function createCustomProviderModel(params: {
+  model: string
+  baseUrl: string
+  apiKey?: string
+}): LanguageModel {
+  const { model, baseUrl, apiKey } = params
+  const trimmedBase = baseUrl.replace(/\/+$/, '')
+
+  return new OpenAICompatibleChatLanguageModel(model, {
+    provider: 'custom',
+    url: ({ path: endpoint }) => `${trimmedBase}${endpoint}`,
+    headers: () => ({
+      Authorization: `Bearer ${apiKey ?? 'codebuff'}`,
+      'Content-Type': 'application/json',
+      'user-agent': `ai-sdk/openai-compatible/${VERSION}/codebuff-custom-provider`,
+    }),
+    fetch: undefined,
+    includeUsage: undefined,
+    supportsStructuredOutputs: true,
+  })
+}