Add automatic prefill for Deepseek - works the same way as with Claude

src/proxy/deepseek.ts

@@ -1,4 +1,4 @@
-import { Router } from "express";
+import { Request, Router } from "express";
 import { createPreprocessorMiddleware } from "./middleware/request";
 import { ipLimiter } from "./rate-limit";
 import { createQueuedProxyMiddleware } from "./middleware/request/proxy-middleware-factory";
@@ -22,20 +22,40 @@ const deepseekResponseHandler: ProxyResHandlerWithBody = async (
 
 const deepseekProxy = createQueuedProxyMiddleware({
   mutations: [addKey, finalizeBody],
-  target: "https://api.deepseek.com",
+  target: "https://api.deepseek.com/beta",
   blockingResponseHandler: deepseekResponseHandler,
 });
 
 const deepseekRouter = Router();
 
+// combines all the assistant messages at the end of the context and adds the
+// beta 'prefix' option, makes prefills work the same way they work for Claude
+function enablePrefill(req: Request) {
+  // If you want to disable
+  if (process.env.NO_DEEPSEEK_PREFILL) return
+  
+  const msgs = req.body.messages;
+  if (msgs.at(-1)?.role !== 'assistant') return;
+
+  let i = msgs.length - 1;
+  let content = '';
+  
+  while (i >= 0 && msgs[i].role === 'assistant') {
+    // maybe we should also add a newline between messages? no for now.
+    content = msgs[i--].content + content;
+  }
+  
+  msgs.splice(i + 1, msgs.length, { role: 'assistant', content, prefix: true });
+}
+
+
 deepseekRouter.post(
   "/v1/chat/completions",
   ipLimiter,
-  createPreprocessorMiddleware({ 
-    inApi: "openai",
-    outApi: "openai",
-    service: "deepseek"
-  }),
+  createPreprocessorMiddleware(
+    { inApi: "openai", outApi: "openai", service: "deepseek" },
+    { afterTransform: [ enablePrefill ] }
+  ),
   deepseekProxy
 );