diff --git a/src/agents/tracing/processors.py b/src/agents/tracing/processors.py
index 1f8cfa9d4e..89042fe133 100644
--- a/src/agents/tracing/processors.py
+++ b/src/agents/tracing/processors.py
@@ -283,6 +283,8 @@ def _truncate_span_field_value(self, value: Any) -> Any:
 
         return self._truncate_json_value_for_limit(sanitized_value, max_bytes)
 
+    _TRUNCATED_PREVIEW_KEY = "__truncated_preview__"
+
     def _truncate_json_value_for_limit(self, value: Any, max_bytes: int) -> Any:
         if self._value_json_size_bytes(value) <= max_bytes:
             return value
@@ -291,6 +293,14 @@ def _truncate_json_value_for_limit(self, value: Any, max_bytes: int) -> Any:
             return self._truncate_string_for_json_limit(value, max_bytes)
 
         if isinstance(value, dict):
+            # Avoid re-truncating our own truncated preview dicts, which
+            # would cause infinite recursion (the preview dict contains bool
+            # values that get wrapped into new preview dicts, and so on).
+            # Use a sentinel key to reliably identify SDK-generated previews
+            # without matching arbitrary user data that happens to use the
+            # same key names.
+            if value.get(self._TRUNCATED_PREVIEW_KEY) is True and "original_type" in value and "preview" in value:
+                return value
             return self._truncate_mapping_for_json_limit(value, max_bytes)
 
         if isinstance(value, list):
@@ -356,6 +366,7 @@ def _truncated_preview(self, value: Any) -> dict[str, Any]:
             preview = f"<{type_name} bytes={len(value)} truncated>"
 
         return {
+            self._TRUNCATED_PREVIEW_KEY: True,
             "truncated": True,
             "original_type": type_name,
             "preview": preview,