Pre-submission Checklist / 提交前检查

• I have searched the existing issues and this bug has not been reported
• I have read the documentation
• I am using the latest version of AxonHub

FAQ Check / FAQ 检查

• I have checked the FAQ and this bug is not covered there

Bug Description / Bug 描述

When a client sends a request to AxonHub with Content-Encoding: zstd, AxonHub appears to read the compressed request body as raw JSON instead of decoding it first. This causes the OpenAI Responses API request decoder to fail with a JSON parse error.

The observed failure happens on /v1/responses, but the request body is read through the shared HTTP request parsing path, so other JSON endpoints that receive zstd-compressed request bodies may be affected as well.

Steps to Reproduce / 复现步骤

1. Start AxonHub.
2. Send an OpenAI Responses API request to /v1/responses with a valid JSON body compressed using zstd.
3. Include the request header Content-Encoding: zstd.
4. Observe that AxonHub rejects the request while decoding the Responses API payload.

Example request shape:

POST /v1/responses HTTP/1.1
Content-Type: application/json
Content-Encoding: zstd
Authorization: Bearer <redacted>

<zstd-compressed JSON body>

Original uncompressed JSON body is valid JSON, for example:

{"model":"<model>","input":"hello"}

Expected Behavior / 期望行为

AxonHub should detect Content-Encoding: zstd, decode the request body before JSON parsing/transformation, and avoid forwarding an already-decoded body with the stale Content-Encoding: zstd header to upstream providers.

If zstd request body compression is intentionally unsupported, AxonHub should return a clear unsupported content encoding error instead of a misleading JSON parse error.

Error Message / 错误消息

{
  "error": {
    "message": "invalid request: failed to decode responses api request: invalid character '(' looking for beginning of value",
    "type": "invalid_request_error"
  }
}

Operating System / 操作系统

Windows / Windows

AxonHub Version / AxonHub 版本

unstable branch at commit aab5159a (chore: sync model developers data (#1675))

Usage Scenario / 使用场景

API

API Format / API 格式

OpenAI - Responses (/v1/responses)

Channel Type / 渠道类型

Custom/Other

Logs / 日志

No additional server logs captured. The client-visible error is included above.

Additional Context / 其他补充信息

The behavior suggests the compressed body bytes are passed directly into JSON decoding. A likely area to inspect is the shared HTTP request body reading path before inbound transformers parse JSON bodies.