diff --git a/src/anthropic/lib/tools/agent_toolset.py b/src/anthropic/lib/tools/agent_toolset.py
index d771996a..083e09aa 100644
--- a/src/anthropic/lib/tools/agent_toolset.py
+++ b/src/anthropic/lib/tools/agent_toolset.py
@@ -544,7 +544,7 @@ async def write(file_path: str, content: str) -> str:
             target.write_text(content)
         except OSError as e:
             raise _fs_error("write", file_path, e) from e
-        return f"wrote {len(content)} bytes to {file_path}"
+        return f"wrote {len(content.encode(\"utf-8\"))} bytes to {file_path}"
 
     return write