diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index f0ecea2dc8..3d33b7f900 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -856,6 +856,11 @@ export class BaseExecutor {
         // CLI fingerprint ordering — always-on for native Claude OAuth, opt-in
         // for other providers. Header + body field order is itself a fingerprint.
         let finalHeaders = headers;
+        // Strip internal sentinel fields set by remapToolNamesInRequest before
+        // serializing — Anthropic rejects unknown top-level fields (issue #2260).
+        delete (transformedBody as Record<string, unknown>)[
+          "_claudeCodeRequiresLowercaseToolNames"
+        ];
         let bodyString = JSON.stringify(transformedBody);
 
         const shouldFingerprint =
diff --git a/open-sse/services/claudeCodeCompatible.ts b/open-sse/services/claudeCodeCompatible.ts
index f48912d1f9..2b67bc7092 100644
--- a/open-sse/services/claudeCodeCompatible.ts
+++ b/open-sse/services/claudeCodeCompatible.ts
@@ -354,7 +354,8 @@ export async function buildAndSignClaudeCodeRequest(
     obfuscateInBody(body);
   }
 
-  // Step 7: Serialize with CCH placeholder
+  // Step 7: Serialize with CCH placeholder (strip internal sentinel fields)
+  delete (body as Record<string, unknown>)["_claudeCodeRequiresLowercaseToolNames"];
   const serialized = JSON.stringify(body);
 
   // Step 8: Sign with xxHash64