requests old version of Anthropic API due to breaking SSE changes

src/proxy/middleware/request/transform-outbound-payload.ts

@@ -99,6 +99,13 @@ function openaiToAnthropic(body: any, req: Request) {
     throw result.error;
   }
 
+  // Anthropic has started versioning their API, indicated by an HTTP header
+  // `anthropic-version`. The new June 2023 version is not backwards compatible
+  // with our OpenAI-to-Anthropic transformations so we need to explicitly
+  // request the older version for now. 2023-01-01 will be removed in September.
+  // https://docs.anthropic.com/claude/reference/versioning
+  req.headers["anthropic-version"] = "2023-01-01";
+
   const { messages, ...rest } = result.data;
   const prompt =
     result.data.messages