[Bug] Claude Code 2.1.37 random cch in x-anthropic-billing-header causes severe prompt-cache miss on third-party upstreams

[IIXINGCHEN]

Is it a request payload issue?
[ x ] Yes, this is a request payload issue. I am using a client/cURL to send a request payload, but I received an unexpected error.
[ ] No, it's another issue.

If it's a request payload issue, you MUST know
Attached below are reproducible payload/cURL examples.
I verified that the payload is identical except for cch=... in x-anthropic-billing-header.

Describe the bug
After upgrading Claude Code to 2.1.37 (observed since 2026-02-08), prompt cache hit rate dropped sharply when routing to third-party Anthropic-compatible upstreams.
Before upgrade, cache hit rate was usually above 90%.
After upgrade, cache hit rate dropped to around 30%-40%, and in some cases near 0%.

The only unstable part we found is in system[].text:
x-anthropic-billing-header:cc_version=2.1.37.3a3; cc_entrypoint=claude-vscode; cch=<random>;

cch changes per request, which causes cache key instability on cache-sensitive upstreams.

CLI Type
claude code

Model Name
claude-3-5-sonnet-20240620
(also observed on claude-sonnet-4 variants)

LLM Client
claude code (cc_entrypoint=claude-vscode observed in payload)

Request Information
Two requests with same model, same messages, same system content except cch.

Request A:

curl -sS -X POST "http://127.0.0.1:8080/api/provider/anthropic/v1/messages" \
  -H "Authorization: Bearer <LOCAL_API_KEY>" \
  -H "Content-Type: application/json" \
  --data-raw '{
    "model":"claude-3-5-sonnet-20240620",
    "system":[{"type":"text","text":"x-anthropic-billing-header:cc_version=2.1.37.3a3; cc_entrypoint=claude-vscode; cch=AAA111;"}],
    "messages":[{"role":"user","content":[{"type":"text","text":"Please say hello."}]}],
    "max_tokens":64
  }'

Request B:

curl -sS -X POST "http://127.0.0.1:8080/api/provider/anthropic/v1/messages" \
  -H "Authorization: Bearer <LOCAL_API_KEY>" \
  -H "Content-Type: application/json" \
  --data-raw '{
    "model":"claude-3-5-sonnet-20240620",
    "system":[{"type":"text","text":"x-anthropic-billing-header:cc_version=2.1.37.3a3; cc_entrypoint=claude-vscode; cch=BBB222;"}],
    "messages":[{"role":"user","content":[{"type":"text","text":"Please say hello."}]}],
    "max_tokens":64
  }'

In our environment, this difference alone significantly reduces prompt-cache hit rate.

Expected behavior
For otherwise identical requests, prompt caching should remain stable.
Volatile cch should not cause cache key churn.
A practical mitigation in proxy layer is to normalize/strip volatile cch before forwarding to upstream cache-sensitive backends.

Screenshots
Cache hit-rate before/after upgrade and request-log snippets can be provided (sanitized).

OS Type

- OS: Windows
- Version: Microsoft Windows 10.0.26200.7840

Additional context

- Repository: github.com/router-for-me/CLIProxyAPI/v6
- Branch: main
- Commit tested: ae1e8a5
- This looks different from #1562 and #1366:
    - #1562 is primarily about cache usage reporting.
    - #1366 is primarily about prompt cache_control handling.
    - This issue is payload volatility caused by random cch.