From 19d34aacecc2f89155dc233177290017b236ccbf Mon Sep 17 00:00:00 2001
From: Luan Vu <luanvuseo@gmail.com>
Date: Sat, 21 Mar 2026 07:35:24 +0700
Subject: [PATCH] fix(mcp): add empty properties to object schemas for OpenAI
 compatibility (#321)

MCP tools that accept no parameters return {type: "object"} without a
properties field, which is valid per JSON Schema spec. However, OpenAI
API strictly requires "properties" on object schemas and rejects
requests with HTTP 400: "object schema missing properties".

Add default empty properties map when type is "object" and the MCP
server provides none.

Co-authored-by: Luvu182 <208665161+Luvu182@users.noreply.github.com>
---
 internal/mcp/bridge_tool.go      |  3 +++
 internal/mcp/bridge_tool_test.go | 13 +++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/internal/mcp/bridge_tool.go b/internal/mcp/bridge_tool.go
index d7ac8582..bb5e5472 100644
--- a/internal/mcp/bridge_tool.go
+++ b/internal/mcp/bridge_tool.go
@@ -125,6 +125,9 @@ func inputSchemaToMap(schema mcpgo.ToolInputSchema) map[string]any {
 	}
 	if len(schema.Properties) > 0 {
 		m["properties"] = schema.Properties
+	} else if m["type"] == "object" {
+		// OpenAI requires "properties" even when empty for object schemas.
+		m["properties"] = map[string]any{}
 	}
 	if len(schema.Required) > 0 {
 		m["required"] = schema.Required
diff --git a/internal/mcp/bridge_tool_test.go b/internal/mcp/bridge_tool_test.go
index 01e19d6d..783048e9 100644
--- a/internal/mcp/bridge_tool_test.go
+++ b/internal/mcp/bridge_tool_test.go
@@ -47,6 +47,19 @@ func TestInputSchemaToMap_EmptyType(t *testing.T) {
 	}
 }
 
+func TestInputSchemaToMap_ObjectNoProperties(t *testing.T) {
+	schema := mcpgo.ToolInputSchema{Type: "object"}
+	m := inputSchemaToMap(schema)
+
+	props, ok := m["properties"].(map[string]any)
+	if !ok || props == nil {
+		t.Fatal("expected empty properties map for object schema, got nil — OpenAI rejects object schemas without properties")
+	}
+	if len(props) != 0 {
+		t.Errorf("expected empty properties, got %v", props)
+	}
+}
+
 func TestExtractTextContent(t *testing.T) {
 	result := &mcpgo.CallToolResult{
 		Content: []mcpgo.Content{