From 74da3d1c8abf71ce77bff1df4e4e9d1348dcbe3d Mon Sep 17 00:00:00 2001
From: tstadel <60758086+tstadel@users.noreply.github.com>
Date: Fri, 31 Oct 2025 08:12:27 +0100
Subject: [PATCH] fix: Agent deserialization when `state_schema=None` (#9984)

* fix: agent deserialization when state_schema None

* add reno
---
 haystack/components/agents/agent.py           |  2 +-
 ...none-deserialization-23e967b51a5a56ee.yaml |  4 ++
 test/components/agents/test_agent.py          | 70 +++++++++++++++++++
 3 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/fix-state-schema-none-deserialization-23e967b51a5a56ee.yaml

diff --git a/haystack/components/agents/agent.py b/haystack/components/agents/agent.py
index b8b7739ee..9b870cec5 100644
--- a/haystack/components/agents/agent.py
+++ b/haystack/components/agents/agent.py
@@ -239,7 +239,7 @@ class Agent:
 
         deserialize_chatgenerator_inplace(init_params, key="chat_generator")
 
-        if "state_schema" in init_params:
+        if init_params.get("state_schema") is not None:
             init_params["state_schema"] = _schema_from_dict(init_params["state_schema"])
 
         if init_params.get("streaming_callback") is not None:
diff --git a/releasenotes/notes/fix-state-schema-none-deserialization-23e967b51a5a56ee.yaml b/releasenotes/notes/fix-state-schema-none-deserialization-23e967b51a5a56ee.yaml
new file mode 100644
index 000000000..64821c556
--- /dev/null
+++ b/releasenotes/notes/fix-state-schema-none-deserialization-23e967b51a5a56ee.yaml
@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix deserialization of state schema when it is None in Agent.from_dict.
diff --git a/test/components/agents/test_agent.py b/test/components/agents/test_agent.py
index 24b977304..1cc0e94e0 100644
--- a/test/components/agents/test_agent.py
+++ b/test/components/agents/test_agent.py
@@ -464,6 +464,76 @@ class TestAgent:
         assert agent.tools[0].function is weather_function
         assert agent.exit_conditions == ["text"]
 
+    def test_from_dict_state_schema_none(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "fake-key")
+        data = {
+            "type": "haystack.components.agents.agent.Agent",
+            "init_parameters": {
+                "chat_generator": {
+                    "type": "haystack.components.generators.chat.openai.OpenAIChatGenerator",
+                    "init_parameters": {
+                        "model": "gpt-4o-mini",
+                        "streaming_callback": None,
+                        "api_base_url": None,
+                        "organization": None,
+                        "generation_kwargs": {},
+                        "api_key": {"type": "env_var", "env_vars": ["OPENAI_API_KEY"], "strict": True},
+                        "timeout": None,
+                        "max_retries": None,
+                        "tools": None,
+                        "tools_strict": False,
+                        "http_client_kwargs": None,
+                    },
+                },
+                "tools": [
+                    {
+                        "type": "haystack.tools.tool.Tool",
+                        "data": {
+                            "name": "weather_tool",
+                            "description": "Provides weather information for a given location.",
+                            "parameters": {
+                                "type": "object",
+                                "properties": {"location": {"type": "string"}},
+                                "required": ["location"],
+                            },
+                            "function": "test_agent.weather_function",
+                            "outputs_to_string": None,
+                            "inputs_from_state": None,
+                            "outputs_to_state": None,
+                        },
+                    },
+                    {
+                        "type": "haystack.tools.component_tool.ComponentTool",
+                        "data": {
+                            "component": {
+                                "type": "haystack.components.builders.prompt_builder.PromptBuilder",
+                                "init_parameters": {
+                                    "template": "{{parrot}}",
+                                    "variables": None,
+                                    "required_variables": None,
+                                },
+                            },
+                            "name": "parrot",
+                            "description": "This is a parrot.",
+                            "parameters": None,
+                            "outputs_to_string": None,
+                            "inputs_from_state": None,
+                            "outputs_to_state": None,
+                        },
+                    },
+                ],
+                "system_prompt": None,
+                "exit_conditions": ["text", "weather_tool"],
+                "state_schema": None,
+                "max_agent_steps": 100,
+                "raise_on_tool_invocation_failure": False,
+                "streaming_callback": None,
+                "tool_invoker_kwargs": {"max_workers": 5, "enable_streaming_callback_passthrough": True},
+            },
+        }
+        agent = Agent.from_dict(data)
+        assert agent.state_schema == {"messages": {"type": list[ChatMessage], "handler": merge_lists}}
+
     def test_serde(self, weather_tool, component_tool, monkeypatch):
         monkeypatch.setenv("FAKE_OPENAI_KEY", "fake-key")
         generator = OpenAIChatGenerator(api_key=Secret.from_env_var("FAKE_OPENAI_KEY"))