Support continue final message (#2733)

* feat: support continue_final_message param in chat request

* feat: add test for continue final message

* fix: bump openapi docs

* fix: remove continue_final_message chat request param

* fix: remove unneeded launcher args in continue test

* fix: bump test output

* fix: remove accidentally included guideline from rebase

* fix: remove guideline tests

* fix: adjust continuation tests expected text

* fix: replace expected output for continue test

integration-tests/models/__snapshots__/test_continue_final_message/test_llama_completion_single_prompt.json

+{
+  "choices": [
+    {
+      "finish_reason": "length",
+      "index": 0,
+      "logprobs": null,
+      "message": {
+        "content": "Both an elephant and a mouse are mammals. However, the differences between elephants and mice are:\n\n1",
+        "role": "assistant"
+      }
+    }
+  ],
+  "created": 1732541189,
+  "id": "",
+  "model": "TinyLlama/TinyLlama-1.1B-Chat-v1.0",
+  "object": "chat.completion",
+  "system_fingerprint": "2.4.1-dev0-native",
+  "usage": {
+    "completion_tokens": 30,
+    "prompt_tokens": 49,
+    "total_tokens": 79
+  }
+}

integration-tests/models/__snapshots__/test_continue_final_message/test_llama_completion_single_prompt_continue.json

+{
+  "choices": [
+    {
+      "finish_reason": "length",
+      "index": 0,
+      "logprobs": null,
+      "message": {
+        "content": " the royal mouse? It is a little more slender and only weighs around 1.5 pounds for males and 1.3 pounds",
+        "role": "assistant"
+      }
+    }
+  ],
+  "created": 1732541190,
+  "id": "",
+  "model": "TinyLlama/TinyLlama-1.1B-Chat-v1.0",
+  "object": "chat.completion",
+  "system_fingerprint": "2.4.1-dev0-native",
+  "usage": {
+    "completion_tokens": 30,
+    "prompt_tokens": 73,
+    "total_tokens": 103
+  }
+}

integration-tests/models/test_continue_final_message.py

+import pytest
+import requests
+
+
+@pytest.fixture(scope="module")
+def llama_continue_final_message_handle(launcher):
+    with launcher("TinyLlama/TinyLlama-1.1B-Chat-v1.0") as handle:
+        yield handle
+
+
+@pytest.fixture(scope="module")
+async def llama_continue_final_message(llama_continue_final_message_handle):
+    await llama_continue_final_message_handle.health(300)
+    return llama_continue_final_message_handle.client
+
+
+def test_llama_completion_single_prompt(
+    llama_continue_final_message, response_snapshot
+):
+    response = requests.post(
+        f"{llama_continue_final_message.base_url}/v1/chat/completions",
+        json={
+            "model": "tgi",
+            "messages": [
+                {"role": "system", "content": "system message"},
+                {"role": "user", "content": "Which is bigger an elephant or a mouse?"},
+            ],
+            "max_tokens": 30,
+            "stream": False,
+            "seed": 1337,
+        },
+        headers=llama_continue_final_message.headers,
+        stream=False,
+    )
+    response = response.json()
+    print(response)
+    assert len(response["choices"]) == 1
+    content = response["choices"][0]["message"]["content"]
+    assert (
+        content
+        == "Both an elephant and a mouse are mammals. However, the differences between elephants and mice are:\n\n1"
+    )
+    assert response == response_snapshot
+
+
+def test_llama_completion_single_prompt_continue(
+    llama_continue_final_message, response_snapshot
+):
+    response = requests.post(
+        f"{llama_continue_final_message.base_url}/v1/chat/completions",
+        json={
+            "model": "tgi",
+            "messages": [
+                {"role": "system", "content": "system message"},
+                {"role": "user", "content": "Which is bigger an elephant or a mouse?"},
+                {
+                    "role": "assistant",
+                    "content": "the elephant, but have you heard about",
+                },
+            ],
+            "max_tokens": 30,
+            "stream": False,
+            "seed": 1337,
+        },
+        headers=llama_continue_final_message.headers,
+        stream=False,
+    )
+    response = response.json()
+    print(response)
+    assert len(response["choices"]) == 1
+    content = response["choices"][0]["message"]["content"]
+    assert (
+        content
+        == " the royal mouse? It is a little more slender and only weighs around 1.5 pounds for males and 1.3 pounds"
+    )
+    assert response == response_snapshot

router/src/infer/chat_template.rs

@@ -75,8 +75,9 @@ impl ChatTemplate {
         };
 
         let messages: Vec<TextMessage> = messages.into_iter().map(|c| c.into()).collect();
-
-        self.template
+        let final_message = messages.last().cloned();
+        let mut rendered_template = self
+            .template
             .render(ChatTemplateInputs {
                 messages,
                 bos_token: self.bos_token.as_deref(),
@@ -84,7 +85,24 @@ impl ChatTemplate {
                 add_generation_prompt: true,
                 tools,
             })
-            .map_err(InferError::TemplateError)
+            .map_err(InferError::TemplateError)?;
+
+        // if the last message is from the assistant, continue the generation prompt
+        rendered_template = match final_message {
+            Some(msg) if msg.role == "assistant" => {
+                match rendered_template.rfind(msg.content.as_str()) {
+                    // implementation based on feature in transformers pipeline
+                    // https://github.com/huggingface/transformers/blob/1cf17077bf2d4affed31387c0943251a4ba8fab7/src/transformers/pipelines/text_generation.py#L418
+                    Some(index) => rendered_template[..index + msg.content.len()]
+                        .trim_end()
+                        .to_string(),
+                    None => rendered_template,
+                }
+            }
+            _ => rendered_template,
+        };
+
+        Ok(rendered_template)
     }
 }