[Bug]: Anthropic Messages adapter drops sessionId when metadata is absent

[JakobM7]

Product or interface

CLI - interactive TUI

Version

MiniMax Code 0.5.2

Platform

Windows

OS version and architecture

Windows 11 x64

Issue area

MCP / OAuth / integration

Desktop log upload ID (optional)

No response

Steps to reproduce

1. Configure MiniMax Code to use an Anthropic-compatible endpoint backed by OpenCodex.
2. Start a long conversation with mostly unchanged context.
3. Run several consecutive turns, including a subagent or branch request.
4. Inspect the proxy usage data for observed cache reads.
5. Compare the behavior with a local diagnostic change that forwards
   sessionId when metadata.user_id is absent.

Expected and actual behavior

Expected:

If the internal request contains a sessionId, the Anthropic request should
preserve the session identity, or the supported field/header for doing so
should be documented.

Actual:

The generated Anthropic request builder appears to forward metadata.user_id
only when a metadata object exists:

if (r?.metadata) {
  const userId = r.metadata.user_id;
  typeof userId === "string" &&
    (a.metadata = { user_id: userId });
}

When the request contains sessionId but no separate metadata object, no
metadata.user_id is emitted.
In a MiniMax Code -> OpenCodex setup, this was associated with
inconsistent observed prompt-cache reads, especially for subagent and branch
requests. A local diagnostic change using sessionId as a fallback resulted
in high cache reads in subsequent requests.

Redacted error summary

No runtime exception occurs. The issue is behavioral: repeated requests
sometimes report zero or very low observed cache reads despite mostly
unchanged context.

Screenshots

No response

Before submitting

• I have searched existing issues.
• I have included my version and removed sensitive information.

[hetaoBackend]

This was generated by AI during triage.

Thanks for the careful analysis — your reading of the request builder is correct, but the behavior is intentional, and there is a supported mechanism for what you're after.

Why metadata.user_id is not auto-filled from sessionId: in the Anthropic Messages API, metadata.user_id is an abuse-detection/attribution field, not a cache-affinity key. The builder (third_party/pi-mono/packages/ai/src/providers/anthropic.ts:1070-1074) only forwards it when the caller explicitly sets metadata, so the client never silently reuses an internal session id as a user identifier.

The supported session-affinity path: the Anthropic-compatible provider layer can send an x-session-affinity: <sessionId> header instead. See anthropic.ts:953-954 — when the model's compat has sendSessionAffinityHeaders: true, every request carries the header (this exists precisely for proxies/providers that route prompt-cache hits by session, e.g. Fireworks). It defaults to false, and for BYOK custom providers it is configurable: sendSessionAffinityHeaders is an accepted boolean compat key in the custom-provider config (packages/local-runtime-v2/src/service/model-system/resolution/model-resolver-byok.ts:194).

Suggested next step: enable sendSessionAffinityHeaders: true in the compat section of your OpenCodex provider entry in config.yaml, and have the proxy key its cache routing on x-session-affinity (or map that header to whatever it currently derives from metadata.user_id). Could you try that and report whether observed cache reads recover for subagent/branch requests?

Moving to needs-info pending that verification. If the header works for you, the remaining action here would be documenting this compat key.