Deepseek Responses Proxy

DeepSeek Model Proxying for OpenAI-Compatible Endpoints

When implementing OpenAI-compatible API endpoints that support multiple upstream providers (e.g., Cursor API and DeepSeek API), model-specific routing must happen before the default handler is invoked.

The Pattern

1. Check for DeepSeek models early in the request handler, before calling resolveCursorModel() or similar default routing logic
2. Proxy directly to DeepSeek API when a DeepSeek model is detected
3. Fall through to Cursor API for non-DeepSeek models

Implementation

// In handleOpenAiRoute or similar handler
const requestedModel = typeof (body as { model?: unknown })?.model === "string" 
  ? (body as { model: string }).model 
  : "composer-2.5";

// Check for DeepSeek models FIRST
if (isDeepSeekModel(requestedModel)) {
  if (route.kind === "chat") {
    return proxyDeepSeekCompletion(env, deps, body, auth.api2agentKey);
  }
  if (route.kind === "responses") {
    return proxyDeepSeekResponses(env, deps, body, auth.api2agentKey);
  }
}

// Then proceed with Cursor API for non-DeepSeek models
const cursorModel = resolveCursorModel(requestedModel);
// ... rest of Cursor API handling

Why This Matters

• DeepSeek API uses different endpoints: /chat/completions for chat, /responses for responses API
• Cursor API doesn't recognize DeepSeek model names: Passing deepseek-v4-flash to Cursor API will fail
• Authentication passthrough: The user's API key should be forwarded directly to DeepSeek, not exchanged through Cursor

Files Involved

• worker/deepseek.ts: Contains proxyDeepSeekCompletion() and proxyDeepSeekResponses() functions
• worker/index.ts: Main request handler that routes to appropriate proxy based on model

Key Functions

// worker/deepseek.ts
export function isDeepSeekModel(model: string): boolean {
  return model.startsWith("deepseek-");
}

export async function proxyDeepSeekResponses(
  env: Env,
  deps: Deps,
  body: unknown,
  apiKey: string
): Promise<Response> {
  const baseUrl = env.DEEPSEEK_API_BASE || "https://api.deepseek.com";
  const isStream = typeof body === "object" && body !== null && 
    (body as Record<string, unknown>).stream === true;

  const response = await deps.fetch(`${baseUrl}/responses`, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Authorization": `Bearer ${apiKey}`,
      "Accept": isStream ? "text/event-stream" : "application/json"
    },
    body: JSON.stringify(body)
  });

  // Handle streaming vs buffered responses
  if (isStream) {
    return withCors(new Response(response.body, {
      status: response.status,
      headers: { "content-type": "text/event-stream" }
    }));
  }

  const data = await response.json();
  return withCors(new Response(JSON.stringify(data), {
    headers: { "content-type": "application/json; charset=utf-8" }
  }));
}