diff --git a/codex-rs/tools/src/json_schema.rs b/codex-rs/tools/src/json_schema.rs
index 22a641491e..a31b285920 100644
--- a/codex-rs/tools/src/json_schema.rs
+++ b/codex-rs/tools/src/json_schema.rs
@@ -72,6 +72,13 @@ impl JsonSchema {
         }
     }
 
+    pub fn any(description: Option<String>) -> Self {
+        Self {
+            description,
+            ..Default::default()
+        }
+    }
+
     pub fn boolean(description: Option<String>) -> Self {
         Self::typed(JsonSchemaPrimitiveType::Boolean, description)
     }
@@ -162,6 +169,7 @@ pub fn parse_tool_input_schema(input_schema: &JsonValue) -> Result<JsonSchema, s
 /// Sanitize a JSON Schema (as serde_json::Value) so it can fit our limited
 /// schema representation. This function:
 /// - Ensures every typed schema object has a `"type"` when required.
+/// - Preserves empty schema objects as unconstrained schemas.
 /// - Preserves explicit `anyOf`.
 /// - Collapses `const` into single-value `enum`.
 /// - Fills required child fields for object/array schema types, including
@@ -178,6 +186,10 @@ fn sanitize_json_schema(value: &mut JsonValue) {
             }
         }
         JsonValue::Object(map) => {
+            if map.is_empty() {
+                return;
+            }
+
             if let Some(properties) = map.get_mut("properties")
                 && let Some(properties_map) = properties.as_object_mut()
             {
diff --git a/codex-rs/tools/src/json_schema_tests.rs b/codex-rs/tools/src/json_schema_tests.rs
index 3f13df7638..d3bdd4ce53 100644
--- a/codex-rs/tools/src/json_schema_tests.rs
+++ b/codex-rs/tools/src/json_schema_tests.rs
@@ -250,16 +250,66 @@ fn parse_tool_input_schema_infers_string_from_enum_const_and_format_keywords() {
 }
 
 #[test]
-fn parse_tool_input_schema_defaults_empty_schema_to_string() {
+fn parse_tool_input_schema_preserves_empty_schema_as_any() {
     // Example schema shape:
     // {}
     //
     // Expected normalization behavior:
-    // - With no structural hints at all, the normalizer falls back to a
-    //   permissive string schema.
+    // - Empty schema objects are unconstrained schemas in JSON Schema, so
+    //   preserve them as unconstrained instead of narrowing to a primitive.
     let schema = parse_tool_input_schema(&serde_json::json!({})).expect("parse schema");
 
-    assert_eq!(schema, JsonSchema::string(/*description*/ None));
+    assert_eq!(schema, JsonSchema::any(/*description*/ None));
+}
+
+#[test]
+fn parse_tool_input_schema_preserves_nested_empty_property_schema_as_any() {
+    // Example schema shape:
+    // {
+    //   "type": "object",
+    //   "properties": {
+    //     "body": {
+    //       "type": "object",
+    //       "properties": {
+    //         "parent": {}
+    //       }
+    //     }
+    //   }
+    // }
+    //
+    // Expected normalization behavior:
+    // - Nested `{}` property schemas remain unconstrained so object values are
+    //   not misrepresented to the model as strings.
+    let schema = parse_tool_input_schema(&serde_json::json!({
+        "type": "object",
+        "properties": {
+            "body": {
+                "type": "object",
+                "properties": {
+                    "parent": {}
+                }
+            }
+        }
+    }))
+    .expect("parse schema");
+
+    assert_eq!(
+        schema,
+        JsonSchema::object(
+            BTreeMap::from([(
+                "body".to_string(),
+                JsonSchema::object(
+                    BTreeMap::from([
+                        ("parent".to_string(), JsonSchema::any(/*description*/ None),)
+                    ]),
+                    /*required*/ None,
+                    /*additional_properties*/ None,
+                ),
+            )]),
+            /*required*/ None,
+            /*additional_properties*/ None
+        )
+    );
 }
 
 #[test]