diff --git a/src/agents/result.py b/src/agents/result.py
index 5e27634f7..365d00079 100644
--- a/src/agents/result.py
+++ b/src/agents/result.py
@@ -8,6 +8,9 @@
 from dataclasses import InitVar, dataclass, field
 from typing import Any, Literal, TypeVar, cast
 
+from pydantic import GetCoreSchemaHandler
+from pydantic_core import core_schema
+
 from .agent import Agent
 from .agent_output import AgentOutputSchemaBase
 from .exceptions import (
@@ -124,6 +127,16 @@ class RunResultBase(abc.ABC):
     _trace_state: TraceState | None = field(default=None, init=False, repr=False)
     """Serialized trace metadata captured during the run."""
 
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls,
+        _source_type: Any,
+        _handler: GetCoreSchemaHandler,
+    ) -> core_schema.CoreSchema:
+        # RunResult objects are runtime values; schema generation should treat them as instances
+        # instead of recursively traversing internal dataclass annotations.
+        return core_schema.is_instance_schema(cls)
+
     @property
     @abc.abstractmethod
     def last_agent(self) -> Agent[Any]:
diff --git a/tests/test_result_cast.py b/tests/test_result_cast.py
index 63e4d2e8f..8cbbe038b 100644
--- a/tests/test_result_cast.py
+++ b/tests/test_result_cast.py
@@ -7,7 +7,7 @@
 
 import pytest
 from openai.types.responses import ResponseOutputMessage, ResponseOutputText
-from pydantic import BaseModel
+from pydantic import BaseModel, ConfigDict
 
 from agents import (
     Agent,
@@ -45,6 +45,16 @@ class Foo(BaseModel):
     bar: int
 
 
+def test_run_result_streaming_supports_pydantic_model_rebuild() -> None:
+    class StreamingRunContainer(BaseModel):
+        query_id: str
+        run_stream: RunResultStreaming | None
+
+        model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    StreamingRunContainer.model_rebuild()
+
+
 def _create_message(text: str) -> ResponseOutputMessage:
     return ResponseOutputMessage(
         id="msg",