sync: minja (google/minja#52)

Author: ochafik

common/chat-template.hpp

@@ -249,16 +249,30 @@ class chat_template {
                     inputs.add_generation_prompt = false;
                     full = apply(inputs);
                 }
-
-                if (full.find(prefix) != 0) {
-                    if (prefix.rfind(eos_token_) == prefix.size() - eos_token_.size()) {
-                        prefix = prefix.substr(0, prefix.size() - eos_token_.size());
+                auto eos_pos_last = full.rfind(eos_token_);
+                if (eos_pos_last == prefix.size() - eos_token_.size() ||
+                      (full[full.size() - 1] == '\n' && (eos_pos_last == full.size() - eos_token_.size() - 1))) {
+                    full = full.substr(0, eos_pos_last);
+                }
+                size_t common_prefix_length = 0;
+                for (size_t i = 0; i < prefix.size() && i < full.size(); ++i) {
+                    if (prefix[i] != full[i]) {
+                        break;
                     }
+                    if (prefix[i] == '<') {
+                        // DeepSeek R1's template (as of 20250209) adds a trailing <think> if add_generation_prompt,
+                        // but it removes thinking tags for past messages.
+                        // The prefix and full strings diverge at <think> vs. <｜tool▁calls▁begin｜>, we avoid consuming the leading <.
+                        continue;
+                    }
+                    common_prefix_length = i + 1;
                 }
-                if (full.find(prefix) != 0) {
+                auto example = full.substr(common_prefix_length);
+                if (example.find("tool_name") == std::string::npos && example.find("some_value") == std::string::npos) {
                     fprintf(stderr, "Failed to infer a tool call example (possible template bug)\n");
+                } else {
+                    tool_call_example_ = example;
                 }
-                tool_call_example_ = full.substr(prefix.size());
             }
         } catch (const std::exception & e) {
             fprintf(stderr, "Failed to generate tool call example: %s\n", e.what());
@@ -363,7 +377,7 @@ class chat_template {
             if (polyfill_tools) {
                 adjusted_messages = add_system(inputs.messages,
                     "You can call any of the following tools to satisfy the user's requests: " + minja::Value(inputs.tools).dump(2, /* to_json= */ true) +
-                    (!polyfill_tool_call_example || tool_call_example_.empty() ? "" : "\n\nExample tool call syntax:\n\n" + tool_call_example_));
+                    (!polyfill_tool_call_example || tool_call_example_.empty() ? "" : "\n\nExample tool call syntax:\n\n" + tool_call_example_ + "\n\n"));
             } else {
                 adjusted_messages = inputs.messages;
             }