diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 820692030a..2fddcee352 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -185,10 +185,17 @@ export async function handleChatCore({
 
   // Translate request (pass reqLogger for intermediate logging)
   let translatedBody = body;
+  const isClaudePassthrough = sourceFormat === FORMATS.CLAUDE && targetFormat === FORMATS.CLAUDE;
   try {
     if (nativeCodexPassthrough) {
       translatedBody = { ...body, _nativeCodexPassthrough: true };
       log?.debug?.("FORMAT", "native codex passthrough enabled");
+    } else if (isClaudePassthrough) {
+      // Claude-to-Claude passthrough: forward body completely untouched.
+      // No translation, no field stripping, no thinking normalization.
+      // We are just a gateway -- do not interfere with the request in any way.
+      translatedBody = { ...body };
+      log?.debug?.("FORMAT", "claude->claude passthrough -- forwarding untouched");
     } else {
       translatedBody = { ...body };
 
diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index ce3f353f60..5b75f54685 100644
--- a/open-sse/translator/index.ts
+++ b/open-sse/translator/index.ts
@@ -131,7 +131,7 @@ export function translateRequest(
   }
 
   // Final step: prepare request for Claude format endpoints
-  if (targetFormat === FORMATS.CLAUDE) {
+  if (targetFormat === FORMATS.CLAUDE && sourceFormat !== FORMATS.CLAUDE) {
     result = prepareClaudeRequest(result, provider);
   }