Fabel 5.1 not working: BYOK Error 400, client version too old for this model

[nikships]

Fabel 5.1 is not usable. Selecting it and sending a request fails immediately with a BYOK error from the upstream API:

BYOK Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.220 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again."}}

The upstream API rejects the request because the client version advertised by droidproxy (Claude Code 2.1.220) is below the minimum version required for this model (2.1.251 or newer).

Expected: Fabel 5.1 requests succeed, or the proxy reports a clear, actionable message instead of an upstream 400.

[nikships]

Investigated. This is an upstream CLIProxyAPI limitation, not a DroidProxy bug, and the fix has to land there first.

Root cause

Anthropic gates claude-fable-5-1 on the client version advertised by the caller. DroidProxy bundles CLIProxyAPI, which emulates the Claude Code wire identity, and the current bundled release (7.2.147) hardcodes:

claude-cli/2.1.220 (external, cli)

in internal/runtime/executor/helps/claude_device_profile.go. Anthropic requires 2.1.251 or newer for this model, so the request is rejected before it ever reaches inference. That is the exact 400 in this report.

Two things worth noting about what is not broken:

• Model routing is fine. The remote catalog (router-for-me/models) already lists claude-fable-5-1, which is why the error is a version 400 rather than unknown provider for model. The DroidProxy model catalog entry is correct.
• Nothing in the DroidProxy proxy layer is involved. The version string is baked into the bundled Go binary.

Upstream status

The CLIProxyAPI project is tracking this, with three competing PRs open and none merged yet:

#	Type	Approach
5402	issue	Add Claude Fable 5.1 OAuth model support
5403	PR	Register the model in the embedded catalog
5405	PR	Register model + bump fingerprint to 2.1.251
5412	PR	Register model + measured 2.1.258 baseline

Why we are not shipping a local workaround

CLIProxyAPI does expose claude-header-defaults in config, so DroidProxy could override the user agent from its bundled config.yaml and clear the version gate with a two-line change. We are deliberately not doing that yet.

Config only overrides the user agent, package version, runtime, OS and arch. It does not override the Anthropic-Beta assembly, which is compiled into the binary and still follows 2.1.220 behavior. Upstream PR 5412 measured a real 2.1.258 client and found the beta set genuinely changed: advanced-tool-use-2025-11-20 is now sent only while tool search is active, afk-mode-2026-01-31 is new, and the Haiku helper transport differs.

A config-only bump would therefore claim to be 2.1.251+ while still sending 2.1.220's betas and SDK version. That mismatched fingerprint is a worse failure mode than a clean 400, since it is exactly what client-gating inspects.

What happens next

Our update-cliproxyapi.yml workflow polls upstream every 12 hours and opens a bump PR automatically, so whichever upstream PR lands will reach a DroidProxy release without manual intervention. There is no ETA while the three PRs are undecided.

Fable 5 (non-5.1), Opus 5 and Sonnet 5 are unaffected and continue to work.