API 网关对 PDF document 块一律返回无具体原因的 400，且污染会话导致后续所有请求失败

What version of Kimi Code is running?

k3、k3-256k 均复现

Which open platform/subscription were you using?

Claude Code CLI 2.1.220(Windows 11)

Which model were you using?

k3、k3-256k

What platform is your computer?

Windows 11 25H2

What issue are you seeing?

通过 https://api.kimi.com/coding/v1/messages 使用 Claude Code 时，只要请求中包含 Anthropic 标准格式的 PDF document 内容块，网关即返回： {"error":{"type":"invalid_request_error","message":"Invalid request Error"},"type":"error"}

错误信息没有任何可操作的具体原因。更严重的是：Claude Code 每轮请求都会重发完整会话历史，一旦历史中包含 PDF块，之后所有请求（包括纯文本提问）都会持续 400，会话完全不可用，只能清空重来，已消耗的上下文和费用无法恢复

What steps can reproduce the bug?

600 字节的合法单页 PDF 也会失败 —— 与文件大小无关

curl -X POST "https://api.kimi.com/coding/v1/messages"
-H "content-type: application/json"
-H "Authorization: Bearer "
-H "anthropic-version: 2023-06-01"
-H "anthropic-beta: pdfs-2024-09-25"
-d '{"model":"k3","max_tokens":50,"messages":[{"role":"user","content":[
{"type":"document","source":{"type":"base64","media_type":"application/pdf","data":"<600字节PDF的base64>"}},
{"type":"text","text":"概括"}]}]}'

→ 400 Invalid request Error

What is the expected behavior?

返回 API Error: 400 Invalid request Error

Additional information

诉求

1. 在官方文档 (https://www.kimi.com/coding/docs/)中明确标注不支持 PDF document 块（目前只写了"k3-256k 不支持视频输入")，或实现对 PDF输入的支持；
2. 400 错误返回具体原因（如 PDF document blocks are not supported)，而不是笼统的 "Invalid request Error"，方便客户端识别和降级；
3. 如有可能，对因此被污染的会话所产生的费用/额度予以补偿。

Start by reproducing the request against https://api.kimi.com/coding/v1/messages with the supplied curl example, then review the official documentation at https://www.kimi.com/coding/docs/. Confirm the supported behavior for PDF document blocks and define completion as either documented PDF rejection or support, with a specific actionable 400 error that does not leave later requests unusable.