diff --git a/python/packages/gemini/agent_framework_gemini/_chat_client.py b/python/packages/gemini/agent_framework_gemini/_chat_client.py
index 0bc33c56a5..35dc7682fe 100644
--- a/python/packages/gemini/agent_framework_gemini/_chat_client.py
+++ b/python/packages/gemini/agent_framework_gemini/_chat_client.py
@@ -751,6 +751,14 @@ def _prepare_config(
             kwargs["response_mime_type"] = "application/json"
         if schema := options.get("response_schema"):
             kwargs["response_schema"] = schema
+        elif (response_format := options.get("response_format")) is not None:
+            # Forward cross-client ``response_format`` to Gemini's native ``response_schema``
+            # so structured-output requests carry the schema, not just JSON mode.
+            # See https://github.com/microsoft/agent-framework/issues/5888.
+            if isinstance(response_format, type) and issubclass(response_format, BaseModel):
+                kwargs["response_schema"] = response_format
+            elif isinstance(response_format, Mapping):
+                kwargs["response_schema"] = dict(response_format)
         if tools := self._prepare_tools(options):
             kwargs["tools"] = tools
         if tool_config := self._prepare_tool_config(options.get("tool_choice")):
diff --git a/python/packages/gemini/tests/test_gemini_client.py b/python/packages/gemini/tests/test_gemini_client.py
index ab6cde241e..edcb5a5496 100644
--- a/python/packages/gemini/tests/test_gemini_client.py
+++ b/python/packages/gemini/tests/test_gemini_client.py
@@ -931,6 +931,64 @@ async def test_response_schema_added_to_config() -> None:
     assert config.response_schema == schema
 
 
+async def test_response_format_dict_forwarded_as_response_schema() -> None:
+    """Raw JSON schema in response_format should reach Gemini's response_schema, not just JSON mode.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/5888 — the
+    declarative loader hands the chat client ``response_format`` as a plain dict; without
+    this forwarding the model gets ``response_mime_type=application/json`` but no schema
+    and returns arbitrary JSON.
+    """
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="{}")]))
+    schema = {"type": "object", "properties": {"answer": {"type": "string"}}}
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Hi")])],
+        options={"response_format": schema},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    assert config.response_mime_type == "application/json"
+    assert config.response_schema == schema
+
+
+async def test_response_format_pydantic_forwarded_as_response_schema() -> None:
+    """Pydantic ``response_format`` should also surface on Gemini's ``response_schema``."""
+    from pydantic import BaseModel
+
+    class Reply(BaseModel):
+        text: str
+
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="{}")]))
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Hi")])],
+        options={"response_format": Reply},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    assert config.response_mime_type == "application/json"
+    assert config.response_schema is Reply
+
+
+async def test_response_schema_takes_precedence_over_response_format() -> None:
+    """If both are set, the explicit ``response_schema`` wins over ``response_format``."""
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="{}")]))
+    explicit_schema = {"type": "object", "properties": {"score": {"type": "integer"}}}
+    fallback = {"type": "object", "properties": {"answer": {"type": "string"}}}
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Hi")])],
+        options={"response_schema": explicit_schema, "response_format": fallback},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    assert config.response_schema == explicit_schema
+
+
 async def test_streaming_response_format_passed_to_build_response_stream() -> None:
     """Verifies that response_format is forwarded to _build_response_stream when streaming
     so that structured output parsing works correctly on the final assembled response.