diff --git a/src/proxy/middleware/response/handle-streamed-response.ts b/src/proxy/middleware/response/handle-streamed-response.ts
index fc6afa3..56ced3f 100644
--- a/src/proxy/middleware/response/handle-streamed-response.ts
+++ b/src/proxy/middleware/response/handle-streamed-response.ts
@@ -81,7 +81,8 @@ export const handleStreamedResponse: RawResponseBodyHandler = async (
   // Transformer converts server-sent events from one vendor's API message
   // format to another.
   const transformer = new SSEMessageTransformer({
-    inputFormat: req.outboundApi,
+    inputFormat: req.outboundApi, // The format of the upstream service's events
+    outputFormat: req.inboundApi, // The format the client requested
     inputApiVersion: String(req.headers["anthropic-version"]),
     logger: req.log,
     requestId: String(req.id),
diff --git a/src/proxy/middleware/response/streaming/sse-message-transformer.ts b/src/proxy/middleware/response/streaming/sse-message-transformer.ts
index 90c0313..800b286 100644
--- a/src/proxy/middleware/response/streaming/sse-message-transformer.ts
+++ b/src/proxy/middleware/response/streaming/sse-message-transformer.ts
@@ -20,6 +20,7 @@ type SSEMessageTransformerOptions = TransformOptions & {
   requestId: string;
   inputFormat: APIFormat;
   inputApiVersion?: string;
+  outputFormat?: APIFormat;
   logger: typeof logger;
 };
 
@@ -47,7 +48,8 @@ export class SSEMessageTransformer extends Transform {
     this.msgCount = 0;
     this.transformFn = getTransformer(
       options.inputFormat,
-      options.inputApiVersion
+      options.inputApiVersion,
+      options.outputFormat
     );
     this.inputFormat = options.inputFormat;
     this.fallbackId = options.requestId;