server : return HTTP 400 if prompt exceeds context length (#16486)

In streaming mode when prompt exceeds context length, the server returns
HTTP 200 status code with a JSON error in the body.  This is very
confusing and inconsistent with all other inference engines which return
HTTP 4xx error in this case.

This patch fixes this problem and makes the server return HTTP 400 in
such cases.

tools/server/server.cpp

@@ -3727,7 +3727,7 @@ struct server_context {
                             }
                         } else {
                             if (slot.n_prompt_tokens() >= slot.n_ctx) {
-                                send_error(slot, "the request exceeds the available context size. try increasing the context size or enable context shift", ERROR_TYPE_EXCEED_CONTEXT_SIZE);
+                                send_error(slot, "the request exceeds the available context size, try increasing it", ERROR_TYPE_EXCEED_CONTEXT_SIZE);
                                 slot.release();
                                 continue;
                             }
@@ -4955,9 +4955,17 @@ int main(int argc, char ** argv) {
                 // Everything else, including multimodal completions.
                 inputs = tokenize_input_prompts(ctx_server.vocab, ctx_server.mctx, prompt, true, true);
             }
-
+            const size_t n_ctx_slot = ctx_server.n_ctx / ctx_server.params_base.n_parallel;
             tasks.reserve(inputs.size());
             for (size_t i = 0; i < inputs.size(); i++) {
+                auto n_prompt_tokens = inputs[i].size();
+                if (n_prompt_tokens >= n_ctx_slot) {
+                    json error_data = format_error_response("the request exceeds the available context size, try increasing it", ERROR_TYPE_EXCEED_CONTEXT_SIZE);
+                    error_data["n_prompt_tokens"] = n_prompt_tokens;
+                    error_data["n_ctx"] = n_ctx_slot;
+                    res_error(res, error_data);
+                    return;
+                }
                 server_task task = server_task(type);
 
                 task.id    = ctx_server.queue_tasks.get_new_id();

tools/server/tests/unit/test_chat_completion.py

@@ -408,6 +408,28 @@ def test_context_size_exceeded():
     assert res.body["error"]["n_ctx"] == server.n_ctx // server.n_slots
 
 
+def test_context_size_exceeded_stream():
+    global server
+    server.start()
+    try:
+        for _ in server.make_stream_request("POST", "/chat/completions", data={
+            "messages": [
+                {"role": "system", "content": "Book"},
+                {"role": "user", "content": "What is the best book"},
+            ] * 100, # make the prompt too long
+            "stream": True}):
+                pass
+        assert False, "Should have failed"
+    except ServerError as e:
+        assert e.code == 400
+        assert "error" in e.body
+        assert e.body["error"]["type"] == "exceed_context_size_error"
+        assert e.body["error"]["n_prompt_tokens"] > 0
+        assert server.n_ctx is not None
+        assert server.n_slots is not None
+        assert e.body["error"]["n_ctx"] == server.n_ctx // server.n_slots
+
+
 @pytest.mark.parametrize(
     "n_batch,batch_count,reuse_cache",
     [

tools/server/tests/utils.py

@@ -35,6 +35,12 @@ class ServerResponse:
     body: dict | Any
 
 
+class ServerError(Exception):
+    def __init__(self, code, body):
+        self.code = code
+        self.body = body
+
+
 class ServerProcess:
     # default options
     debug: bool = False
@@ -297,6 +303,8 @@ class ServerProcess:
             response = requests.post(url, headers=headers, json=data, stream=True)
         else:
             raise ValueError(f"Unimplemented method: {method}")
+        if response.status_code != 200:
+            raise ServerError(response.status_code, response.json())
         for line_bytes in response.iter_lines():
             line = line_bytes.decode("utf-8")
             if '[DONE]' in line: