apr serve: chat completions with array-form message content fail with HTTP 422 (Qwen Code can't use apr serve) #5919

apr serve run rejects a chat request whose message content is an array of content parts, with HTTP 422. The OpenAI Chat Completions API allows content to be either a string or an array of parts ([{"type":"text","text":"..."}]), and some clients always send the array form. Qwen Code (@qwen-code/qwen-code 0.25.3) is one of them, so it can't use apr serve as its backend at all.

Versions

• apr 0.71.4 (5027da6), from cargo install aprender
• Model: Qwen3.5-4B-Q4_K_M.gguf
• @qwen-code/qwen-code 0.25.3
• Linux, RTX 4090

Reproduce

apr serve run -v Qwen3.5-4B-Q4_K_M.gguf --gpu -p 8199

String content works:

curl -s -w '\nHTTP %{http_code}\n' http://127.0.0.1:8199/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"x","max_tokens":40,"messages":[{"role":"user","content":"What is 2+2? Answer with one number."}]}'

... {"role":"assistant","content":"4"},"finish_reason":"stop"} ...
HTTP 200

The same message with array content is rejected:

curl -s -w '\nHTTP %{http_code}\n' http://127.0.0.1:8199/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"x","max_tokens":40,"messages":[{"role":"user","content":[{"type":"text","text":"What is 2+2? Answer with one number."}]}]}'

{"error":"Invalid request body. Check that the JSON structure matches the expected schema."}
HTTP 422

A system message with array content plus a tools list gives the same 422.

Through Qwen Code

OPENAI_BASE_URL=http://127.0.0.1:8199/v1 OPENAI_API_KEY=local OPENAI_MODEL=qwen3.5-4b \
  qwen "What is 2+2? Answer with one number." -o json

"subtype":"error_during_execution", ... "error":{"message":"[API Error: 422 \"Invalid request body. Check that the JSON structure matches the expected schema.\"]"}

Expected

Accept content as either a string or an array of content parts, at least for "type": "text" parts (concatenated in order), as other OpenAI-compatible servers do. llama.cpp's llama-server, for example, accepts both forms. A part type the server can't handle should get a clear 400 that names the unsupported part type, not a generic 422.