From b623c23183a1ee25bce1db0ab47715eab05b151e Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Fri, 17 Jul 2026 17:33:57 -0400
Subject: [PATCH] Serialize deep object query parameters (#4523)

---
 .../fastmcp/utilities/openapi/director.py     | 12 +++---
 tests/utilities/openapi/test_director.py      | 39 +++++++++++++++++++
 2 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/fastmcp_slim/fastmcp/utilities/openapi/director.py b/fastmcp_slim/fastmcp/utilities/openapi/director.py
index 461573960..9c8a52695 100644
--- a/fastmcp_slim/fastmcp/utilities/openapi/director.py
+++ b/fastmcp_slim/fastmcp/utilities/openapi/director.py
@@ -313,13 +313,13 @@ class RequestDirector:
                     if not value:
                         continue
                     if explode:
-                        # form,explode=true on objects: each property becomes
-                        # a separate query parameter.
-                        # e.g. {"R": 100, "G": 200} → R=100&G=200
                         for k, v in value.items():
-                            serialized[_query_scalar_to_str(k)] = _query_scalar_to_str(
-                                v
-                            )
+                            # deepObject keeps the parent parameter name;
+                            # form style emits each property as a bare key.
+                            property_name = _query_scalar_to_str(k)
+                            if param_info.style == "deepObject":
+                                property_name = f"{key}[{property_name}]"
+                            serialized[property_name] = _query_scalar_to_str(v)
                     else:
                         style = param_info.style or "form"
                         delimiter = self._STYLE_DELIMITERS.get(style, ",")
diff --git a/tests/utilities/openapi/test_director.py b/tests/utilities/openapi/test_director.py
index 6e8fffb82..651d8c4a9 100644
--- a/tests/utilities/openapi/test_director.py
+++ b/tests/utilities/openapi/test_director.py
@@ -890,6 +890,45 @@ class TestQueryParameterSerialization:
         assert "myAttribute=true" in url
         assert "data=" not in url
 
+    def test_deep_object_explode_true_uses_bracket_notation(self, director):
+        route = HTTPRoute(
+            path="/items",
+            method="GET",
+            operation_id="list_items",
+            parameters=[
+                ParameterInfo(
+                    name="filter",
+                    location="query",
+                    required=True,
+                    schema={
+                        "type": "object",
+                        "properties": {
+                            "eq": {"type": "string"},
+                            "display name": {"type": "string"},
+                        },
+                    },
+                    explode=True,
+                    style="deepObject",
+                )
+            ],
+            parameter_map={
+                "filter": {"location": "query", "openapi_name": "filter"},
+            },
+        )
+
+        request = director.build(
+            route,
+            {"filter": {"eq": "foo/bar", "display name": "active & ready"}},
+            "https://example.com",
+        )
+
+        assert request.url.params["filter[eq]"] == "foo/bar"
+        assert request.url.params["filter[display name]"] == "active & ready"
+        assert "filter%5Beq%5D=foo%2Fbar" in str(request.url)
+        assert "filter%5Bdisplay+name%5D=active+%26+ready" in str(request.url)
+        assert "eq" not in request.url.params
+        assert "display name" not in request.url.params
+
     def test_explode_default_dict_expands_to_separate_params(self, director):
         """Default explode (None → true) on objects expands properties."""
         route = HTTPRoute(