diff --git a/nanochat/engine.py b/nanochat/engine.py
index aa2e6a98..f133d891 100644
--- a/nanochat/engine.py
+++ b/nanochat/engine.py
@@ -17,7 +17,7 @@ import signal
 import warnings
 from contextlib import contextmanager
 from collections import deque
-from nanochat.common import compute_init, autodetect_device_type
+from nanochat.common import compute_init, autodetect_device_type, COMPUTE_DTYPE
 from nanochat.checkpoint_manager import load_model
 
 # -----------------------------------------------------------------------------
@@ -172,18 +172,16 @@ class Engine:
         self.model = model
         self.tokenizer = tokenizer # needed for tool use
 
+    def _get_kv_cache_dtype(self):
+        """Use the repo-wide compute dtype for inference cache allocation."""
+        return COMPUTE_DTYPE
+
     @torch.inference_mode()
     def generate(self, tokens, num_samples=1, max_tokens=None, temperature=1.0, top_k=None, seed=42):
         """Same as generate, but does single prefill and then clones the KV cache."""
         assert isinstance(tokens, list) and isinstance(tokens[0], int), "expecting list of ints"
         device = self.model.get_device()
-        # NOTE: setting the dtype here and in this way is an ugly hack.
-        # Currently the repo assumes that cuda -> bfloat16 and everything else -> float32.
-        # We need to know the dtype here to call __init__ on KVCache and pre-allocate its tensors.
-        # As a quick hack, we're making generate() function inherit and know about this repo-wise assumption.
-        # I think there has to be a bigger refactor to deal with device/dtype tracking across the codebase.
-        # In particular, the KVCache should allocate its tensors lazily
-        dtype = torch.bfloat16 if device.type == "cuda" else torch.float32
+        dtype = self._get_kv_cache_dtype()
         rng = torch.Generator(device=device)
         rng.manual_seed(seed)
 
diff --git a/scripts/chat_web.py b/scripts/chat_web.py
index ffaf7dab..9cce083e 100644
--- a/scripts/chat_web.py
+++ b/scripts/chat_web.py
@@ -33,6 +33,7 @@ Abuse Prevention:
 import argparse
 import json
 import os
+import sys
 import torch
 import asyncio
 import logging
@@ -59,18 +60,29 @@ MAX_TOP_K = 200
 MIN_MAX_TOKENS = 1
 MAX_MAX_TOKENS = 4096
 
-parser = argparse.ArgumentParser(description='NanoChat Web Server')
-parser.add_argument('-n', '--num-gpus', type=int, default=1, help='Number of GPUs to use (default: 1)')
-parser.add_argument('-i', '--source', type=str, default="sft", help="Source of the model: sft|rl")
-parser.add_argument('-t', '--temperature', type=float, default=0.8, help='Default temperature for generation')
-parser.add_argument('-k', '--top-k', type=int, default=50, help='Default top-k sampling parameter')
-parser.add_argument('-m', '--max-tokens', type=int, default=512, help='Default max tokens for generation')
-parser.add_argument('-g', '--model-tag', type=str, default=None, help='Model tag to load')
-parser.add_argument('-s', '--step', type=int, default=None, help='Step to load')
-parser.add_argument('-p', '--port', type=int, default=8000, help='Port to run the server on')
-parser.add_argument('--device-type', type=str, default='', choices=['cuda', 'cpu', 'mps'], help='Device type for evaluation: cuda|cpu|mps. empty => autodetect')
-parser.add_argument('--host', type=str, default='0.0.0.0', help='Host to bind the server to')
-args = parser.parse_args()
+def build_parser():
+    parser = argparse.ArgumentParser(description='NanoChat Web Server')
+    parser.add_argument('-n', '--num-gpus', type=int, default=1, help='Number of GPUs to use (default: 1)')
+    parser.add_argument('-i', '--source', type=str, default="sft", help="Source of the model: sft|rl")
+    parser.add_argument('-t', '--temperature', type=float, default=0.8, help='Default temperature for generation')
+    parser.add_argument('-k', '--top-k', type=int, default=50, help='Default top-k sampling parameter')
+    parser.add_argument('-m', '--max-tokens', type=int, default=512, help='Default max tokens for generation')
+    parser.add_argument('-g', '--model-tag', type=str, default=None, help='Model tag to load')
+    parser.add_argument('-s', '--step', type=int, default=None, help='Step to load')
+    parser.add_argument('-p', '--port', type=int, default=8000, help='Port to run the server on')
+    parser.add_argument('--device-type', type=str, default='', choices=['cuda', 'cpu', 'mps'], help='Device type for evaluation: cuda|cpu|mps. empty => autodetect')
+    parser.add_argument('--host', type=str, default='0.0.0.0', help='Host to bind the server to')
+    return parser
+
+
+def parse_args(argv=None):
+    parser = build_parser()
+    if argv is None:
+        argv = sys.argv[1:]
+    return parser.parse_args(argv)
+
+
+args = parse_args()
 
 # Configure logging for conversation traffic
 logging.basicConfig(
@@ -186,7 +198,7 @@ def validate_chat_request(request: ChatRequest):
         if message.role not in ["user", "assistant"]:
             raise HTTPException(
                 status_code=400,
-                detail=f"Message {i} has invalid role. Must be 'user', 'assistant', or 'system'"
+                detail=f"Message {i} has invalid role. Must be 'user' or 'assistant'"
             )
 
     # Validate temperature
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
new file mode 100644
index 00000000..ed332d1f
--- /dev/null
+++ b/tests/test_regressions.py
@@ -0,0 +1,36 @@
+import importlib
+import sys
+
+import pytest
+import torch
+from fastapi import HTTPException
+
+import nanochat.engine as engine_module
+from nanochat.engine import Engine
+
+
+class DummyModel:
+    def get_device(self):
+        return torch.device("cpu")
+
+
+def test_engine_kv_cache_uses_compute_dtype(monkeypatch):
+    monkeypatch.setattr(engine_module, "COMPUTE_DTYPE", torch.float16)
+    engine = Engine(DummyModel(), tokenizer=None)
+    assert engine._get_kv_cache_dtype() == torch.float16
+
+
+def test_chat_web_rejects_system_role_with_consistent_error(monkeypatch):
+    monkeypatch.setattr(sys, "argv", ["chat_web_test"])
+    sys.modules.pop("scripts.chat_web", None)
+    chat_web = importlib.import_module("scripts.chat_web")
+
+    request = chat_web.ChatRequest(
+        messages=[chat_web.ChatMessage(role="system", content="You are helpful.")]
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        chat_web.validate_chat_request(request)
+
+    assert exc_info.value.status_code == 400
+    assert exc_info.value.detail == "Message 0 has invalid role. Must be 'user' or 'assistant'"