fix(sdk): export DeepAgentState (#3653)

Follow-up to the `state_schema` parameter added in `create_deep_agent`.
Exposes `DeepAgentState` as a public import so callers can subclass it
for custom schemas, and adds missing test coverage to verify that a
custom schema is actually threaded through declarative subagent
compilation and that the `SubAgentMiddleware` constructor annotations
are runtime-resolvable.

libs/deepagents/CHANGELOG.md

@@ -4,10 +4,9 @@
 
 ## [0.6.6](https://github.com/langchain-ai/deepagents/compare/deepagents==0.6.5...deepagents==0.6.6) (2026-05-28)
 
-
 ### Features
 
-* **sdk:** allow passing `state_schema` in `create_deep_agent` ([#3642](https://github.com/langchain-ai/deepagents/issues/3642)) ([37839bd](https://github.com/langchain-ai/deepagents/commit/37839bd7d67fba8c11ff0ccaaa8ac92b39609450))
+* Allow passing `state_schema` in `create_deep_agent` ([#3642](https://github.com/langchain-ai/deepagents/issues/3642)) ([37839bd](https://github.com/langchain-ai/deepagents/commit/37839bd7d67fba8c11ff0ccaaa8ac92b39609450))
 
 ## [0.6.5](https://github.com/langchain-ai/deepagents/compare/deepagents==0.6.4...deepagents==0.6.5) (2026-05-28)
 

libs/deepagents/deepagents/__init__.py

@@ -6,7 +6,7 @@ from deepagents._subagent_transformer import (
     SubagentTransformer,
 )
 from deepagents._version import __version__
-from deepagents.graph import create_deep_agent
+from deepagents.graph import DeepAgentState, create_deep_agent
 from deepagents.middleware.async_subagents import AsyncSubAgent, AsyncSubAgentMiddleware
 from deepagents.middleware.filesystem import FilesystemMiddleware, FilesystemPermission
 from deepagents.middleware.memory import MemoryMiddleware
@@ -28,6 +28,7 @@ __all__ = [
     "AsyncSubAgentMiddleware",
     "AsyncSubagentRunStream",
     "CompiledSubAgent",
+    "DeepAgentState",
     "FilesystemMiddleware",
     "FilesystemPermission",
     "GeneralPurposeSubagentProfile",

libs/deepagents/tests/unit_tests/middleware/test_subagent_middleware_init.py

@@ -1,5 +1,7 @@
 """Unit tests for SubAgentMiddleware initialization and configuration."""
 
+from typing import get_type_hints
+
 import pytest
 from langchain.agents import create_agent
 from langchain_core.messages import AIMessage
@@ -45,6 +47,12 @@ class TestSubagentMiddlewareInit:
         assert len(middleware.tools) == 1
         assert middleware.tools[0].name == "task"
 
+    def test_public_init_type_hints_are_runtime_resolvable(self) -> None:
+        """Public constructor annotations should support runtime introspection."""
+        hints = get_type_hints(SubAgentMiddleware.__init__)
+
+        assert "state_schema" in hints
+
     def test_subagent_middleware_with_custom_subagent(self) -> None:
         """Test SubAgentMiddleware initialization with a custom subagent."""
         middleware = SubAgentMiddleware(

libs/deepagents/tests/unit_tests/test_graph.py

@@ -16,6 +16,7 @@ from langchain_core.tools import BaseTool, StructuredTool
 from deepagents._api.deprecation import LangChainDeprecationWarning
 from deepagents._tools import _apply_tool_description_overrides, _tool_name
 from deepagents._version import __version__
+from deepagents.backends import StateBackend
 from deepagents.graph import (
     _REQUIRED_MIDDLEWARE_CLASSES,
     _REQUIRED_MIDDLEWARE_NAMES,
@@ -700,7 +701,16 @@ class TestExtraMiddlewareWiring:
 
 
 class TestStateSchema:
-    """End-to-end tests for the `state_schema` parameter on `create_deep_agent`."""
+    """Tests for the `state_schema` parameter on `create_deep_agent`.
+
+    Covers wiring (the schema reaches `create_agent` and `SubAgentMiddleware`) and
+    that a custom schema is applied when declarative subagents compile, so the
+    custom field is exposed as a channel on the subagent graph.
+
+    The round-trip of a custom field through a compiled agent is not retested here:
+    it is `create_agent`'s contract, covered upstream in langchain's
+    `test_state_schema.py`. These tests only assert deepagents' own wiring.
+    """
 
     def test_default_state_schema_uses_deep_agent_state(self) -> None:
         fake_model = GenericFakeChatModel(messages=iter([AIMessage(content="ok")]))
@@ -752,6 +762,30 @@ class TestStateSchema:
         sub_mw = next(m for m in mw_stack if isinstance(m, SubAgentMiddleware))
         assert sub_mw._state_schema is MyState
 
+    def test_declarative_subagent_compiles_with_custom_state_schema(self) -> None:
+        """A declarative subagent's compiled runnable exposes the custom field as a channel."""
+
+        class MyState(DeepAgentState):
+            page_url: str
+
+        middleware = SubAgentMiddleware(
+            backend=StateBackend(),
+            subagents=[
+                {
+                    "name": "researcher",
+                    "description": "Research agent",
+                    "system_prompt": "You are a researcher.",
+                    "model": GenericFakeChatModel(messages=iter([AIMessage(content="ok")])),
+                    "tools": [],
+                }
+            ],
+            state_schema=MyState,
+        )
+
+        runnable = middleware._get_subagents()[0]["runnable"]
+        properties = runnable.get_input_jsonschema()["properties"]
+        assert "page_url" in properties
+
 
 class _OtherStubMW(AgentMiddleware[Any, Any, Any]):
     """Second stub class so exclusion tests can assert coexistence with `_StubMW`."""