[Bug] Claude Messages 转 OpenAI Chat 时单元素 stop_sequences 被转换为 string，导致部分上游 400

Deployment source

Repository release / official image (self-hosted)

Your current newapi version

v1.0.0-rc.30

Submission Checks

• Non-duplicate issue: I have searched existing Issues and confirmed there are no similar issues.
• Read this first: I have fully read the section above, reviewed the docs at https://docs.newapi.ai/ and the project README, and asked AI first, confirming this is not a usage, configuration, or integration question.
• Supported version: I have provided an exact version, commit, or image tag (not latest or unknown) and confirmed that the issue reproduces on an unmodified, supported version from this repository.
• Not a third-party service: I confirm that this issue is not exclusive to a third-party hosting site, relay, API service, or fork that has not been verified against the unmodified repository. Third-party instance issues must be reported to their operator.
• Issue attribution: I have provided evidence that distinguishes the client, new-api, and upstream layers. For relay issues, I compared equivalent redacted requests sent directly upstream and through new-api, confirming that new-api introduces or changes the error rather than merely forwarding the upstream error unchanged.
• Channel and protocol boundary: I confirm that the issue is not caused by a Coding Plan service, reverse-engineered channel, third-party API wrapper, Codex reverse-proxy endpoint, or behavior specific to the Codex API. If first observed through such an interface, I have reproduced it using a standard API protocol supported by this repository.
• Maintainer time: I understand that maintainers have limited time and issues that do not follow this template may be ignored or closed directly.

Issue Description

当通过 /v1/messages 调用 Claude Messages API，并由 NewAPI 转换为 OpenAI Chat Completions 请求时：

如果请求中的 stop_sequences 只有一个元素，例如：

{
  "stop_sequences": ["</block>"]
}

NewAPI 会转换成：

{
  "stop": "</block>"
}

而不是保留数组：

"stop": ["</block>"]

虽然 OpenAI Chat Completions API 本身允许 stop 为 string 或 array，
但部分严格的 OpenAI-compatible provider 只接受 array。

例如 OpenCode Go 的部分上游会因此返回：

status_code=400
JSON parse error:
Cannot construct instance of ArrayList

no String-argument constructor/factory method to deserialize from String value ('</block>')
ChatCompletionRequest["stop"]

Steps to Reproduce

客户端请求：
POST /v1/messages

请求中包含：

{
  "stop_sequences": ["</block>"]
}

NewAPI 在 Claude Messages -> OpenAI Chat Completions 转换后发送：

{
  "stop": "</block>"
}

上游要求：

{
  "stop": ["</block>"]
}

因此返回 HTTP 400。

当前行为
单元素：
[""]

转换为：
""

多元素：
["A", "B"]
则仍然保持数组。

因此该问题表现为间歇性：只有请求包含单个 stop_sequences 时才会触发。

Expected Result

建议 Claude Messages -> OpenAI Chat Completions 转换时始终保持 stop_sequences 的数组结构：

if len(claudeRequest.StopSequences) > 0 {
openAIRequest.Stop = claudeRequest.StopSequences
}

而不是：

if len(claudeRequest.StopSequences) == 1 {
openAIRequest.Stop = claudeRequest.StopSequences[0]
} else if len(claudeRequest.StopSequences) > 1 {
openAIRequest.Stop = claudeRequest.StopSequences
}

这样生成：

{
"stop": [""]
}

仍然是合法的 OpenAI Chat Completions 请求，同时兼容要求 stop 必须为数组的 provider。

Related Screenshots

Start at the Claude Messages to OpenAI Chat conversion handling the /v1/messages path and trace how stop_sequences is mapped. Reproduce the case with a one-element array, then verify that the outgoing request preserves an array while multi-element behavior remains unchanged. Done when strict array-only providers accept the converted request without the reported 400 error.