fix: server startup on Windows + missing runtime deps in server.txt

[jonathanzhan1975]

Two independent issues that block python scripts/start_web.py after a clean install on Windows. Both block users from reaching the API server, so I'm bundling them into a single PR.

---

Issue 1 — UnicodeDecodeError streaming subprocess output

_spawn() opens the backend / frontend stdout pipes with text=True but no explicit encoding. On Windows, text=True falls back to the system ANSI codepage (e.g. gbk on Chinese Windows). The reader thread crashes the moment a child process emits a UTF-8 byte:

File ""E:\works\study\DeepTutor\scripts\start_web.py"", line 40, in _stream_output
    for line in process.stdout:
UnicodeDecodeError: 'gbk' codec can't decode byte 0xb2 in position 2: illegal multibyte sequence

Fix

Pin encoding=""utf-8"" and errors=""replace"" on the Popen kwargs. Both Node 24 (Next.js dev) and the FastAPI/uvicorn backend write UTF-8 to stdout, so this matches the actual byte stream and is locale-independent.

kwargs: dict[str, object] = {
    ""cwd"": str(cwd),
    ""env"": env,
    ""stdout"": subprocess.PIPE,
    ""stderr"": subprocess.STDOUT,
    ""text"": True,
    ""bufsize"": 1,
+   ""encoding"": ""utf-8"",
+   ""errors"": ""replace"",
}

---

Issue 2 — ModuleNotFoundError: loguru / json_repair after pip install -r requirements/server.txt

deeptutor/services/llm/provider_core/__init__.py unconditionally imports:

• AnthropicProvider → import json_repair
• OpenAICompatProvider → import json_repair
• OpenAICodexProvider → from loguru import logger
• provider_core/base.py → from loguru import logger
• openai_responses/parsing.py → both

Both packages are declared only in requirements/tutorbot.txt. So the documented install path for the web server:

pip install -r requirements/server.txt && pip install -e .
python scripts/start_web.py

crashes the uvicorn worker before it can serve a single request:

File ""...\deeptutor\services\llm\provider_core\anthropic_provider.py"", line 16, in <module>
    import json_repair
ModuleNotFoundError: No module named 'json_repair'

Fix

Add both packages to requirements/server.txt with the same version bounds already used in tutorbot.txt:

loguru>=0.7.3,<1.0.0
json-repair>=0.57.0,<1.0.0

These are core LLM-provider dependencies, not TutorBot-specific, so the right home is server.txt.

---

Reproduction

git clone https://github.com/HKUDS/DeepTutor
cd DeepTutor
python -m venv .venv
.venv\Scripts\activate
pip install -r requirements/server.txt
pip install -e .
python scripts/start_web.py

• Without fix [Question]: I am looking to work on TODO tasks but. #1: backend log streaming crashes the moment uvicorn prints non-ASCII output.
• Without fix avoid false 'Virtual env not detected' warning under conda #2: backend import chain raises ModuleNotFoundError before serving a request.

Verification

Tested on Windows 11 + Python 3.12.1 + a fresh .venv. After applying both fixes, python scripts/start_web.py boots cleanly:

• backend: Uvicorn running on http://0.0.0.0:8001
• frontend: Next.js Ready in <1s

Frontend page loads and successfully reaches the backend /api/... routes.

[pancacake]

Thanks!! This is a good one