Refactor to use Content for tool calls

include/json_types.hpp

@@ -400,9 +400,11 @@ struct ContentPart {
 DRH_DEFINE_TYPE_OPTIONAL(ContentPart, type, text, image_url)
 
 
+using Content = std::variant<std::string, std::vector<ContentPart>>;
+
 struct Message {
     std::string role = "";
-    std::variant<std::string, std::vector<ContentPart>> content = "";
+    Content content = "";
     std::string reasoning = "";
     std::string reasoning_content = "";
     std::string tool_call_id = "";

include/tool.hpp

@@ -7,9 +7,9 @@
 
 // Template function that wraps a typed lambda for tool execution
 // Handles JSON deserialization internally, eliminating std::any
-template<typename ParamsType>
-std::function<std::string(const nlohmann::json&)> create_tool_execute(
-    std::function<std::string(const ParamsType&)> fn
+template<typename ParamsType, typename F>
+std::function<Content(const nlohmann::json&)> create_tool_execute(
+    F const& fn
 ) {
     return [fn = std::move(fn)](const nlohmann::json& json) -> std::string {
         auto result = json_to<ParamsType>(json);
@@ -24,7 +24,7 @@ struct Tool {
     std::string name;
     std::string description;
     nlohmann::json parameters_schema;
-    std::function<std::string(const nlohmann::json&)> execute;
+    std::function<Content(const nlohmann::json&)> execute;
 
     nlohmann::json to_api_json() const {
         return {

include/tool_registry.hpp

@@ -11,7 +11,7 @@ public:
     std::expected<void, std::string> register_tool(Tool tool);
     std::optional<const Tool*> find(const std::string& name) const;
     std::vector<Tool> tools() const;
-    std::string invoke(const std::string& name, const nlohmann::json& args) const;
+    Content invoke(const std::string& name, const nlohmann::json& args) const;
     std::expected<ToolRegistry, std::string> create_subregistry(std::vector<std::string> const& names) const;
     std::vector<std::string> const& tool_names() const;
 

src/harness.cpp

@@ -114,7 +114,7 @@ std::expected<std::string, std::string> Harness::chat(const Message& user_messag
                 if (config_.verbose)
                     std::cerr << prefix << "[tool call] " << tool_name << "(" << args_str << ")\n";
 
-                std::string result;
+                Content result = "";
                 auto args_json = parse_json_safely(args_str);
                 if (args_json.is_discarded()) {
                     result = "Error: invalid JSON in tool arguments";
@@ -123,9 +123,16 @@ std::expected<std::string, std::string> Harness::chat(const Message& user_messag
                 }
 
                 if (config_.verbose)
-                    std::cerr << prefix << "[tool result] "
-                              << result.substr(0, 300)
-                              << (result.size() > 300 ? "..." : "") << "\n";
+                {
+                    std::cerr << prefix << "[tool result] ";
+                    if (std::holds_alternative<std::string>(result))
+                    {
+                        std::string str = std::get<std::string>(result);
+                        std::cerr
+                                  << str.substr(0, 300)
+                                  << (str.size() > 300 ? "..." : "") << "\n";
+                    }
+                }
 
                 Message tool_result{};
                 tool_result.tool_call_id = tc.id;

src/tool_registry.cpp

@@ -24,8 +24,8 @@ std::vector<Tool> ToolRegistry::tools() const {
     return ret;
 }
 
-std::string ToolRegistry::invoke(const std::string& name,
-                                 const nlohmann::json& args) const {
+Content ToolRegistry::invoke(const std::string& name,
+                             const nlohmann::json& args) const {
     auto it = tools_.find(name);
     if (it == tools_.end())
         return "Error: unknown tool: " + name;