feat(langchain): register stream transformers on middleware (#37591)

Adds a `transformers` attribute to `AgentMiddleware` so middleware can
declare scope-aware `StreamTransformer` factories alongside their
`tools` and lifecycle hooks. `create_agent` merges middleware-registered
factories with any caller-supplied ones at compile time.

## API

```python
class MyMiddleware(AgentMiddleware):
    transformers = (MyTransformer,)  # factory: (scope,) -> StreamTransformer
```

When the agent compiles, the final transformer order on the run mux is:

1. Built-in ``ToolCallTransformer``
2. Middleware-registered factories, in middleware order
3. Caller-supplied ``transformers=`` from ``create_agent``

This ordering keeps the built-in tool-call projection in front of any
consumer transformers and gives caller-supplied entries the final word.

libs/langchain_v1/langchain/agents/factory.py

@@ -81,6 +81,7 @@ if TYPE_CHECKING:
     from langgraph.graph.state import CompiledStateGraph
     from langgraph.runtime import Runtime
     from langgraph.store.base import BaseStore
+    from langgraph.stream._mux import TransformerFactory
     from langgraph.types import Checkpointer
 
     from langchain.agents.middleware.types import ToolCallWrapper
@@ -708,7 +709,7 @@ def create_agent(
     debug: bool = False,
     name: str | None = None,
     cache: BaseCache[Any] | None = None,
-    transformers: Sequence[Callable[[tuple[str, ...]], Any]] | None = None,
+    transformers: Sequence[TransformerFactory] | None = None,
 ) -> CompiledStateGraph[
     AgentState[ResponseT], ContextT, _InputAgentState, _OutputAgentState[ResponseT]
 ]:
@@ -806,9 +807,11 @@ def create_agent(
         cache: An optional `BaseCache` instance to enable caching of graph execution.
         transformers: Optional sequence of scope-aware `StreamTransformer`
             factories to register on the compiled graph in addition to
-            the agent defaults. Each factory is invoked per-scope
-            (`factory(scope)`) so subgraph mini-muxes get fresh
-            instances. Appended after the built-in `ToolCallTransformer`.
+            the agent defaults. Each factory is invoked as `factory(scope)`
+            so every invocation receives a fresh instance. The final order
+            on the compiled graph is: `ToolCallTransformer`, then any
+            factories declared by middleware via
+            `AgentMiddleware.transformers`, then any factories supplied here.
 
     Returns:
         A compiled `StateGraph` that can be used for chat interactions.
@@ -1662,6 +1665,8 @@ def create_agent(
     if name:
         config["metadata"]["lc_agent_name"] = name
 
+    middleware_transformers = [t for m in middleware for t in getattr(m, "transformers", ())]
+
     return graph.compile(
         checkpointer=checkpointer,
         store=store,
@@ -1670,7 +1675,11 @@ def create_agent(
         debug=debug,
         name=name,
         cache=cache,
-        transformers=[ToolCallTransformer, *(transformers or ())],
+        transformers=[
+            ToolCallTransformer,
+            *middleware_transformers,
+            *(transformers or ()),
+        ],
     ).with_config(config)
 
 

libs/langchain_v1/langchain/agents/middleware/types.py

@@ -40,6 +40,7 @@ if TYPE_CHECKING:
     from langchain_core.language_models.chat_models import BaseChatModel
     from langchain_core.tools import BaseTool
     from langgraph.runtime import Runtime
+    from langgraph.stream._mux import TransformerFactory
     from langgraph.types import Command
 
     from langchain.agents.structured_output import ResponseFormat
@@ -397,6 +398,16 @@ class AgentMiddleware(Generic[StateT, ContextT, ResponseT]):
     tools: Sequence[BaseTool]
     """Additional tools registered by the middleware."""
 
+    transformers: Sequence[TransformerFactory] = ()
+    """Stream transformer factories registered by the middleware.
+
+    Each entry is a scope-aware factory invoked as `factory(scope)` so every
+    invocation receives a fresh instance. Factories are merged with the
+    `transformers` argument of [`create_agent`][langchain.agents.create_agent]
+    at graph compile time, after the `ToolCallTransformer` and before any
+    user-supplied entries.
+    """
+
     @property
     def name(self) -> str:
         """The name of the middleware instance.

libs/langchain_v1/tests/unit_tests/agents/middleware/core/test_transformers.py

@@ -0,0 +1,143 @@
+"""Tests for middleware-registered stream transformers."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+from langchain_core.messages import HumanMessage
+from langgraph.prebuilt import ToolCallTransformer
+from langgraph.stream import StreamChannel, StreamTransformer
+
+from langchain.agents.factory import create_agent
+from langchain.agents.middleware.types import AgentMiddleware
+from tests.unit_tests.agents.model import FakeToolCallingModel
+
+if TYPE_CHECKING:
+    from langgraph.stream._types import ProtocolEvent
+
+
+class _MiddlewareMarker(StreamTransformer):
+    """Marker transformer used to assert registration order."""
+
+    required_stream_modes = ()
+
+    def __init__(self, scope: tuple[str, ...] = ()) -> None:
+        super().__init__(scope)
+        self._log: StreamChannel[int] = StreamChannel()
+
+    def init(self) -> dict[str, Any]:
+        return {"middleware_marker": self._log}
+
+    def process(self, event: ProtocolEvent) -> bool:
+        del event
+        return True
+
+
+class _UserMarker(StreamTransformer):
+    """Second marker to verify user-supplied transformers append last."""
+
+    required_stream_modes = ()
+
+    def __init__(self, scope: tuple[str, ...] = ()) -> None:
+        super().__init__(scope)
+        self._log: StreamChannel[int] = StreamChannel()
+
+    def init(self) -> dict[str, Any]:
+        return {"user_marker": self._log}
+
+    def process(self, event: ProtocolEvent) -> bool:
+        del event
+        return True
+
+
+def test_middleware_transformer_registered_on_compiled_graph() -> None:
+    """A `transformers` factory declared on middleware is wired into the run mux."""
+
+    class _Middleware(AgentMiddleware):
+        transformers = (_MiddlewareMarker,)
+
+    agent = create_agent(model=FakeToolCallingModel(), tools=[], middleware=[_Middleware()])
+
+    run = agent.stream_events({"messages": [HumanMessage("hi")]}, version="v3")
+
+    assert "middleware_marker" in run._mux.extensions  # type: ignore[attr-defined]
+    # Drain to close the run cleanly.
+    list(run.tool_calls)
+
+
+def test_middleware_and_user_transformers_compose_in_order() -> None:
+    """Order is: built-in `ToolCallTransformer` → middleware → user-supplied."""
+
+    class _Middleware(AgentMiddleware):
+        transformers = (_MiddlewareMarker,)
+
+    agent = create_agent(
+        model=FakeToolCallingModel(),
+        tools=[],
+        middleware=[_Middleware()],
+        transformers=[_UserMarker],
+    )
+
+    run = agent.stream_events({"messages": [HumanMessage("hi")]}, version="v3")
+
+    transformers = run._mux._transformers  # type: ignore[attr-defined]
+    tool_call_idx = next(
+        i for i, t in enumerate(transformers) if isinstance(t, ToolCallTransformer)
+    )
+    middleware_idx = next(i for i, t in enumerate(transformers) if isinstance(t, _MiddlewareMarker))
+    user_idx = next(i for i, t in enumerate(transformers) if isinstance(t, _UserMarker))
+
+    assert tool_call_idx < middleware_idx < user_idx, (
+        "transformers must register as: built-in, then middleware, then user-supplied"
+    )
+
+    list(run.tool_calls)
+
+
+def test_transformers_from_multiple_middleware_preserve_middleware_order() -> None:
+    """Transformers across middleware register in middleware-list order."""
+
+    class _MarkerA(_MiddlewareMarker):
+        def init(self) -> dict[str, Any]:
+            return {"marker_a": self._log}
+
+    class _MarkerB(_MiddlewareMarker):
+        def init(self) -> dict[str, Any]:
+            return {"marker_b": self._log}
+
+    class _MwA(AgentMiddleware):
+        transformers = (_MarkerA,)
+
+    class _MwB(AgentMiddleware):
+        transformers = (_MarkerB,)
+
+    agent = create_agent(
+        model=FakeToolCallingModel(),
+        tools=[],
+        middleware=[_MwA(), _MwB()],
+    )
+
+    run = agent.stream_events({"messages": [HumanMessage("hi")]}, version="v3")
+
+    transformers = run._mux._transformers  # type: ignore[attr-defined]
+    idx_a = next(i for i, t in enumerate(transformers) if isinstance(t, _MarkerA))
+    idx_b = next(i for i, t in enumerate(transformers) if isinstance(t, _MarkerB))
+    assert idx_a < idx_b
+
+    list(run.tool_calls)
+
+
+def test_middleware_without_transformers_does_not_affect_registry() -> None:
+    """Middleware that omits `transformers` leaves the default registry intact."""
+
+    class _Middleware(AgentMiddleware):
+        pass
+
+    agent = create_agent(model=FakeToolCallingModel(), tools=[], middleware=[_Middleware()])
+    run = agent.stream_events({"messages": [HumanMessage("hi")]}, version="v3")
+
+    transformers = run._mux._transformers  # type: ignore[attr-defined]
+    assert any(isinstance(t, ToolCallTransformer) for t in transformers)
+    assert not any(isinstance(t, _MiddlewareMarker) for t in transformers)
+
+    list(run.tool_calls)