feat(claude): add Claude Fable 5.1 OAuth model support

[nejim123]

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 is another issue.

Describe the bug
Anthropic released Claude Fable 5.1 on September 1, 2026 with the API model ID claude-fable-5-1. CLIProxyAPI v7.2.147 does not register the model, does not show it in /v1/models or the CPAMC supported-model list, and rejects requests before they can be routed to an enabled Claude OAuth credential.

Official model documentation: https://platform.claude.com/docs/en/models/fable-5-1/overview

The current registry contains claude-fable-5 but not claude-fable-5-1:
https://github.com/router-for-me/CLIProxyAPI/blob/17a65ee5470fbaf0e22fc219381e6a4ae9e07624/internal/registry/models/models.json

CLI Type
Claude Code OAuth

Model Name
claude-fable-5-1

LLM Client
T3 Code using the Codex/OpenAI-compatible harness

Request Information
Sanitized equivalent request:

curl http://127.0.0.1:8317/v1/responses \
  -H "Authorization: Bearer <proxy-api-key>" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-fable-5-1","input":"What model are you?"}'

Response:

{"error":{"type":"invalid_request_error","code":"model_not_found","message":"unknown provider for model claude-fable-5-1","param":"model"}}

The Claude OAuth credential was refreshed successfully before testing. CPAMC still lists only claude-fable-5.

Expected behavior
CLIProxyAPI should recognize claude-fable-5-1 as a Claude provider model, expose it through the model catalog and CPAMC, and route it through eligible Claude OAuth credentials. Fable 5.1 has protocol differences documented by Anthropic, so support may require more than adding registry metadata.

Screenshots
Not attached because the error response above contains the complete failure and avoids exposing account information.

OS Type

• OS: Ubuntu
• Version: 26.04 Resolute Raccoon

Additional context

• CLIProxyAPI: v7.2.147
• Commit: 17a65ee5470fbaf0e22fc219381e6a4ae9e07624
• Release: https://github.com/router-for-me/CLIProxyAPI/releases/tag/v7.2.147
• Four Claude OAuth credentials are enabled, but the behavior reproduces independently of credential refresh.

[matheustimbo]

Confirming the report and adding a finding from a local build.

Adding the missing claude-fable-5-1 entry to the model catalog (PR #5403 for the embedded copy, router-for-me/models#49 for the live catalog) removes the local rejection: /v1/models lists the model and requests are routed to an enabled Claude OAuth credential.

The request is then rejected upstream by Anthropic:

invalid_request_error: Claude Code 2.1.220 does not support this model;
version 2.1.251 or newer is required. Run 'claude update' ...

Anthropic gates Fable 5.1 on the observed Claude Code version, and the cloaking layer emulates 2.1.220, so a real 2.1.257 client behind the proxy is still refused. Fully supporting the model therefore needs the emulated Claude Code baseline re-measured at >= 2.1.251 in addition to the catalog entry.

[davidfarah2003]

I measured this against native Claude Code rather than only changing the advertised version.

Reproduction and successful path

• Adding claude-fable-5-1 to the Claude model catalog is necessary, but not sufficient.
• With the catalog entry present, a real Claude Code 2.1.257 request passed through a local CLIProxyAPI build and completed successfully against claude-fable-5-1.
• Merely changing the emulated version from 2.1.220 to 2.1.257 is not a complete fix for non-Claude-Code clients, because the native request shape changed too.

Native captures

Controlled captures of Claude Code 2.1.251 and 2.1.257 showed:

• 2.1.251 user agent: claude-cli/2.1.251 (external, sdk-cli)
• 2.1.257 user agent: claude-cli/2.1.257 (external, sdk-cli)
• SDK version: 0.112.1
• runtime version: v26.3.0

The native 2.1.251 request used these top-level fields:

model, messages, system, tools, metadata, max_tokens, thinking,
context_management, output_config, diagnostics, stream

The native 2.1.257 Fable 5.1 request additionally included fallbacks, extra system content, and these beta capabilities:

server-side-fallback-2026-06-01
fallback-credit-2026-06-01
cache-diagnosis-2026-04-07

cache-diagnosis-2026-04-07 was also present in the captured 2.1.251 request.

CCH/signing result

CLIProxyAPI's existing Anthropic Messages body signer reproduced the signatures from the captured native 2.1.251 and native 2.1.257 Fable requests byte-for-byte. The signing algorithm therefore appears compatible; the incomplete part is the hard-coded 2.1.220 fingerprint/request template.

Suggested scope for the fix

1. Add the model to both the embedded and remotely refreshed Claude catalogs, with its 1M context/output/thinking metadata.
2. Re-measure and update the full Claude Code baseline, not just the version string.
3. Preserve evolving headers, beta capabilities, system-array content, and body fields instead of treating them as a closed list. Anthropic's gateway protocol explicitly recommends forwarding these unchanged: https://code.claude.com/docs/en/llm-gateway-protocol
4. Add regression captures for at least native 2.1.251 and 2.1.257 requests.

Scope note: the exact comparison above covered controlled sdk-cli requests without tools. It does not yet prove equivalence for interactive sessions, count-tokens, tool-heavy requests, subagents, or compaction paths.

[owen800q]

I get this issue
● API Error: 400 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.

[carljborg]

Reproduced on macOS with CLIProxyAPI 7.2.145 and native Claude Code 2.1.258. The request reaches CPA with User-Agent: claude-cli/2.1.258 (external, cli) and X-Stainless-Package-Version: 0.112.1, but CPA sends upstream claude-cli/2.1.220 (external, cli) and package version 0.94.0, producing the version-gate 400. The model is already recognized and routed, so the remaining blocker is the fingerprint/protocol rewrite. Happy to test an updated build against native interactive, tool-call, and subagent paths.

[er77]

ubuntu 22.04
cpa - v7.2.137

{"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."}}

[KJ7LNW]

Looks like (at least) these PRs are needed to fix the issue above:

• fix(claude): recognize model-free native thinking signatures and warn on unknown generations #5422
• fix(claude): align beta assembly and Haiku helper detection with the 2.1.258 baseline #5412

The authors of each pull request appear to have put a lot of thought into them.

[yuya-iwabuchi]

Here is a quick workaround to unblock until #5412 is merged:

Add the User-Agent override in your config (Homebrew: /opt/homebrew/etc/cliproxyapi.conf) to match your installed version:

claude-header-defaults:
  user-agent: "claude-cli/2.1.259 (external, cli)"

Restart the service, and claude-fable-5-1 should now work. Tested on Homebrew 7.2.145 with Claude Code 2.1.259.
Hope this helps!

[owen800q]

@yuya-iwabuchi it works, thanks

[ikun-404]

@yuya-iwabuchi good work