diff --git a/pkg/model/provider/openai/schema.go b/pkg/model/provider/openai/schema.go
index 28149ab3b..f7db79e3a 100644
--- a/pkg/model/provider/openai/schema.go
+++ b/pkg/model/provider/openai/schema.go
@@ -55,12 +55,42 @@ func walkSchema(schema map[string]any, fn func(map[string]any)) {
 // makeAllRequired makes all object properties "required" throughout the schema,
 // because that's what the OpenAI Response API demands.
 // Properties that were not originally required are made nullable.
+// Also ensures all object-type schemas have additionalProperties: false.
 func makeAllRequired(schema shared.FunctionParameters) shared.FunctionParameters {
 	if schema == nil {
 		schema = map[string]any{"type": "object", "properties": map[string]any{}}
 	}
 
 	walkSchema(schema, func(node map[string]any) {
+		// Check if this node is an object type (either "object" or ["object", ...])
+		isObject := false
+		if typeVal, ok := node["type"]; ok {
+			switch t := typeVal.(type) {
+			case string:
+				isObject = t == "object"
+			case []any:
+				for _, v := range t {
+					if s, ok := v.(string); ok && s == "object" {
+						isObject = true
+						break
+					}
+				}
+			case []string:
+				isObject = slices.Contains(t, "object")
+			}
+		}
+
+		// All object types must have additionalProperties: false for OpenAI Responses API strict mode
+		// But only set it if additionalProperties is not already defined as an object schema
+		if isObject {
+			if addProps, exists := node["additionalProperties"]; !exists || addProps == nil || addProps == true {
+				node["additionalProperties"] = false
+			}
+			// If additionalProperties is already set to false or is an object schema (map[string]any),
+			// leave it as is - the object schema case will be walked separately
+		}
+
+		// If the node has explicit properties, make them all required
 		properties, ok := node["properties"].(map[string]any)
 		if !ok {
 			return
@@ -88,7 +118,6 @@ func makeAllRequired(schema shared.FunctionParameters) shared.FunctionParameters
 		}
 
 		node["required"] = newRequired
-		node["additionalProperties"] = false
 	})
 
 	return schema
diff --git a/pkg/model/provider/openai/schema_test.go b/pkg/model/provider/openai/schema_test.go
index 65562b774..d9331aac7 100644
--- a/pkg/model/provider/openai/schema_test.go
+++ b/pkg/model/provider/openai/schema_test.go
@@ -333,6 +333,45 @@ func TestRemoveFormatFields_NoProperties(t *testing.T) {
 	assert.Equal(t, schema, updated)
 }
 
+func TestMakeAllRequired_TypeArrayWithObject(t *testing.T) {
+	// Reproduces the user_prompt tool schema where a property has
+	// type: ["object", "null"] with nested properties. OpenAI requires
+	// these nested properties to also have additionalProperties: false.
+	schema := shared.FunctionParameters{
+		"type": "object",
+		"properties": map[string]any{
+			"schema": map[string]any{
+				"type": []string{"object", "null"},
+				"properties": map[string]any{
+					"name": map[string]any{"type": "string"},
+					"age":  map[string]any{"type": "number"},
+				},
+				"required": []any{"name"},
+			},
+		},
+		"required": []any{"schema"},
+	}
+
+	updated := makeAllRequired(schema)
+
+	// Top-level should have additionalProperties: false
+	assert.Equal(t, false, updated["additionalProperties"])
+
+	// The schema property should also have additionalProperties: false
+	schemaProps := updated["properties"].(map[string]any)["schema"].(map[string]any)
+	assert.Equal(t, false, schemaProps["additionalProperties"])
+
+	// All properties in schema should be required
+	schemaRequired := schemaProps["required"].([]any)
+	assert.Len(t, schemaRequired, 2)
+	assert.Contains(t, schemaRequired, "name")
+	assert.Contains(t, schemaRequired, "age")
+
+	// age was not originally required, so its type should be nullable
+	age := schemaProps["properties"].(map[string]any)["age"].(map[string]any)
+	assert.Equal(t, []string{"number", "null"}, age["type"])
+}
+
 func TestFixSchemaArrayItems(t *testing.T) {
 	schema := `{
   "properties": {