From 2a27e0d13193fd3910600ef2cbda21cabc2da659 Mon Sep 17 00:00:00 2001
From: Stefano Fiorucci <stefanofiorucci@gmail.com>
Date: Wed, 8 Oct 2025 11:30:34 +0200
Subject: [PATCH] fix: fix `OpenAIChatGenerator` `response_format`
 serialization errors (#9858)

* fix: fix OpenAIChatGenerator response_format serialization errors

* relnote fix
---
 haystack/components/generators/chat/openai.py |  2 +-
 ...lization-json-object-925de0a6387ff115.yaml |  5 ++++
 .../components/generators/chat/test_openai.py | 25 +++++++++++++++++++
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/fix-openaichat-serialization-json-object-925de0a6387ff115.yaml

diff --git a/haystack/components/generators/chat/openai.py b/haystack/components/generators/chat/openai.py
index 96ac6f942..676214238 100644
--- a/haystack/components/generators/chat/openai.py
+++ b/haystack/components/generators/chat/openai.py
@@ -220,7 +220,7 @@ class OpenAIChatGenerator:
 
         # If the response format is a Pydantic model, it's converted to openai's json schema format
         # If it's already a json schema, it's left as is
-        if response_format and issubclass(response_format, BaseModel):
+        if response_format and isinstance(response_format, type) and issubclass(response_format, BaseModel):
             json_schema = {
                 "type": "json_schema",
                 "json_schema": {
diff --git a/releasenotes/notes/fix-openaichat-serialization-json-object-925de0a6387ff115.yaml b/releasenotes/notes/fix-openaichat-serialization-json-object-925de0a6387ff115.yaml
new file mode 100644
index 000000000..ee01a51ea
--- /dev/null
+++ b/releasenotes/notes/fix-openaichat-serialization-json-object-925de0a6387ff115.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Ensure that the `OpenAIChatGenerator` is properly serialized when `response_format` in `generation_kwargs` is
+    provided as a dictionary (for example, `{"type": "json_object"}`). Previously, this caused serialization errors.
diff --git a/test/components/generators/chat/test_openai.py b/test/components/generators/chat/test_openai.py
index 190bcbd01..d3d2dc45b 100644
--- a/test/components/generators/chat/test_openai.py
+++ b/test/components/generators/chat/test_openai.py
@@ -341,6 +341,31 @@ class TestOpenAIChatGenerator:
             },
         }
 
+    def test_to_dict_with_response_format_json_object(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "test-api-key")
+        component = OpenAIChatGenerator(
+            api_key=Secret.from_env_var("OPENAI_API_KEY"),
+            model="gpt-4o-mini",
+            generation_kwargs={"response_format": {"type": "json_object"}},
+        )
+        data = component.to_dict()
+        assert data == {
+            "type": "haystack.components.generators.chat.openai.OpenAIChatGenerator",
+            "init_parameters": {
+                "api_key": {"env_vars": ["OPENAI_API_KEY"], "strict": True, "type": "env_var"},
+                "model": "gpt-4o-mini",
+                "api_base_url": None,
+                "organization": None,
+                "streaming_callback": None,
+                "generation_kwargs": {"response_format": {"type": "json_object"}},
+                "tools": None,
+                "tools_strict": False,
+                "max_retries": None,
+                "timeout": None,
+                "http_client_kwargs": None,
+            },
+        }
+
     def test_from_dict(self, monkeypatch):
         monkeypatch.setenv("OPENAI_API_KEY", "fake-api-key")
         data = {