minja: enhance backfill of templates w/o tools description (use example tool call delta!)

common/chat-template.hpp

@@ -41,6 +41,7 @@ class chat_template {
     std::string bos_token_;
     std::string eos_token_;
     std::shared_ptr<minja::TemplateNode> template_root_;
+    std::string tool_call_example_;
 
     std::string try_raw_render(
         const nlohmann::ordered_json & messages,
@@ -176,6 +177,43 @@ class chat_template {
             caps_.supports_tool_responses = contains(out, "Some response!");
             caps_.supports_tool_call_id = contains(out, "call_911_");
         }
+
+        if (!caps_.supports_tools) {
+            const json user_msg {
+                {"role", "user"},
+                {"content", "Hey"},
+            };
+            const json tool_call_msg {
+                {"role", "assistant"},
+                {"content", nullptr},
+                {"tool_calls", json::array({
+                    {
+                        // TODO: detect if requires numerical id or fixed length == 6 like Nemo
+                        {"id", "call_1___"},
+                        {"type", "function"},
+                        {"function", {
+                            {"name", "tool_name"},
+                            {"arguments", (json {
+                                {"arg1", "some_value"},
+                            }).dump()},
+                        }},
+                    },
+                })},
+            };
+            const json tools;
+            auto prefix = apply(json::array({user_msg}), tools, /* add_generation_prompt= */ true);
+            auto full = apply(json::array({user_msg, tool_call_msg}), tools, /* add_generation_prompt= */ false);
+            if (full.find(prefix) != 0) {
+                if (prefix.rfind(eos_token_) == prefix.size() - eos_token_.size()) {
+                    prefix = prefix.substr(0, prefix.size() - eos_token_.size());
+                } else {
+                    throw std::runtime_error("prefix not found at start of full: " + prefix + " vs " + full);
+                }
+            } else {
+
+            }
+            tool_call_example_ = full.substr(prefix.size());
+        }
     }
 
     const std::string & source() const { return source_; }
@@ -229,7 +267,17 @@ class chat_template {
             };
             auto needs_tools_in_system = !tools.is_null() && tools.size() > 0 && !caps_.supports_tools;
 
-            for (const auto & message_ : needs_tools_in_system ? add_system(messages, "Available tools: " + tools.dump(2)) : messages) {
+            json adjusted_messages;
+            if (needs_tools_in_system) {
+                adjusted_messages = add_system(messages,
+                    "\n\n"
+                    "You can call any of the following tools to satisfy the user's requests: " + tools.dump(2) + "\n\n"
+                    "Example tool call syntax:\n\n" + tool_call_example_ + "\n\n");
+            } else {
+                adjusted_messages = messages;
+            }
+
+            for (const auto & message_ : adjusted_messages) {
                 auto message = message_;
                 if (!message.contains("role") || !message.contains("content")) {
                     throw std::runtime_error("message must have 'role' and 'content' fields: " + message.dump());