Cline 400 Error With a Custom Anthropic Base URL: Cause and Fix
As of 2026-10-08, Cline 4.1.22 returns a 400 when you call Claude through a custom Anthropic base URL (the official examples are Azure AI Foundry and corporate gateways), because 4.1.22 started sending an option only Anthropic's own API accepts: fallbacks. The fix is to upgrade to 4.1.23, released 2026-10-07, or later (cli-v3.0.69 for the CLI, desktop-v0.0.44 for the desktop app). This page walks through the cause, the affected versions and what to check after upgrading, quoting Cline's official release notes.
Updated 2026-10-08
Versions and the field at a glance
The version that broke
Released 2026-09-30. The 4.1.23 release notes state that 4.1.22 started sending an option only Anthropic's own API accepts.
The fix (VS Code extension)
Released 2026-10-07. Its Fixed section says Claude through a custom Anthropic base URL no longer fails with a 400 error.
The option that gets rejected
Per the SDK v0.0.91 notes, the Anthropic provider now sends the server-side refusal fallbacks option only to api.anthropic.com; custom endpoints such as Azure AI Foundry rejected it with a 400.
The fix for CLI and desktop
Both shipped on 2026-10-07 with the same fix in their release notes: Claude through a custom Anthropic base URL no longer fails with a 400 error.
Where the 400 comes from
As of 2026-10-08, Cline has documented the cause: 4.1.22 started adding an option to Anthropic-provider requests that only Anthropic's own API accepts, so once you tick Use custom base URL and point Cline elsewhere (third-party gateways and relays count too), an endpoint that does not recognize the field answers with a 400. The 4.1.22 release notes describe the change as "Direct Anthropic requests now use Anthropic's server-side refusal fallback."; the SDK v0.0.88 notes from the same day spell out the field as anthropic.fallbacks: "default". 4.1.23 limits the option to api.anthropic.com, so after upgrading a custom base URL no longer receives it. You are affected only when both conditions hold: API Provider is set to Anthropic, and Use custom base URL is ticked.
The official release notes (2026-10-07)
The Fixed section of 4.1.23 reads: "Claude through a custom Anthropic base URL (Azure AI Foundry, corporate gateways) no longer fails with a 400 error. 4.1.22 started sending an option only Anthropic's own API accepts." SDK v0.0.91, from the same day, is more specific: "The Anthropic provider sends the server-side refusal fallbacks option only to api.anthropic.com. Custom endpoints such as Azure AI Foundry rejected it with a 400." The same release fixes other things too: Kimi K3 and other models that only accept certain reasoning levels no longer reject requests, and when a response ends without a recognized finish reason, Cline asks the model to continue once instead of treating it as complete.
From breakage to fix
Cline 4.1.22 ships; its release notes say direct Anthropic requests now use Anthropic's server-side refusal fallback.
A GitHub user reports (issue #14816) that after updating to 4.1.22, requests to a custom Anthropic base URL on Azure AI Foundry fail, with the endpoint returning a 400 "fallbacks: Extra inputs are not permitted".
4.1.23 ships the fix; CLI cli-v3.0.69 and desktop-v0.0.44 ship the same day with the same fix in their release notes.
Confirmed vs not verified
Confirmed (verbatim in the official release notes)
All of the following can be checked word for word in Cline's official release notes and CHANGELOG: 4.1.22 shipped on 2026-09-30 and direct Anthropic requests started using the server-side refusal fallback; 4.1.23 shipped on 2026-10-07, and Claude through a custom Anthropic base URL (Azure AI Foundry, corporate gateways) no longer fails with a 400 error, with the cause given as 4.1.22 sending an option only Anthropic's own API accepts; SDK v0.0.91 states that the fallbacks option now goes only to api.anthropic.com; CLI cli-v3.0.69 and desktop-v0.0.44 carry the same fix. The QCode side comes from the Cline page on docs.qcode.cc: Claude uses the Anthropic provider, with Use custom base URL set to https://api.qcode.cc/api.
Not verified or not stated officially
Four things this page does not conclude: first, the error text "fallbacks: Extra inputs are not permitted" appears only in a GitHub user report (Azure AI Foundry); the official notes say only "400", and other gateways may word it differently. Second, apart from upgrading, Cline gives no official workaround, and this page does not list unofficial ones. Third, this page makes no claim about whether QCode's endpoint accepts the field; from 4.1.23 on, Cline simply stops sending it anywhere other than api.anthropic.com. Fourth, QCode's Cline docs were verified against VS Code extension 4.1.19, and the 4.1.23 interface has not been tested here, so go by what your client shows.
Two comparisons: version and provider
4.1.22 vs 4.1.23
4.1.22 adds the fallbacks option to Anthropic-provider requests, custom base URLs included; from 4.1.23 it goes only to api.anthropic.com, and no other address receives it. So if you use a custom base URL, move to 4.1.23 or later; for the CLI and the desktop app, look for cli-v3.0.69 and desktop-v0.0.44.
Anthropic provider vs OpenAI Compatible
The fix targets the Anthropic provider, and switching to OpenAI Compatible is not a way around it: QCode's docs state that OpenAI-compatible mode cannot use Claude, and a claude-* id returns model_not_available_on_endpoint. Claude goes only through the Anthropic provider (https://api.qcode.cc/api); OpenAI Compatible (https://api.qcode.cc/openai/v1) is for GPT and Chinese models.
How to fix it
① Check your version: open the VS Code Extensions panel (Ctrl+Shift+X / Cmd+Shift+X), search for Cline and look at the installed version; 4.1.22 is the one the release notes name. ② Upgrade to 4.1.23 or later; on the CLI, to cli-v3.0.69 or later; on the desktop app, to desktop-v0.0.44 or later. ③ Leave your settings as they are: API Provider stays Anthropic, and Use custom base URL stays ticked with your address; for QCode that is https://api.qcode.cc/api, not /api/v1/messages, and without a trailing slash. ④ Send a message to confirm. Cline gives no official workaround other than upgrading.
On QCode
To call Claude from Cline on QCode, the docs give this setup: API Provider Anthropic, API Key your QCode key (starts with cr_), tick Use custom base URL and enter https://api.qcode.cc/api, Model claude-sonnet-5 (everyday) or claude-opus-5 (harder tasks). As tested on 2026-10-08, with the Base URL set to https://api.qcode.cc/api, requests to both /api/messages and /api/v1/messages return 200, so the address works whether the client appends /messages or /v1/messages. In North America and Europe you can use https://us.qcode.cc/api (Los Angeles) instead. This page covers only the 400; for the full setup, including GPT and Chinese models through OpenAI Compatible, see the Cline OpenAI-compatible guide. Billed per token; see /models for each model's price.
Frequently asked questions
Why does Cline 4.1.22 return a 400 with a custom base URL?
Because 4.1.22 started sending an option in Anthropic requests that only Anthropic's own API accepts. The 4.1.23 release notes say verbatim "4.1.22 started sending an option only Anthropic's own API accepts.", and SDK v0.0.91 names it the server-side refusal fallbacks option, which custom endpoints such as Azure AI Foundry rejected with a 400.
How do I fix it, and which version do I need?
Upgrade to 4.1.23 or later (released 2026-10-07). From 4.1.23 the option goes only to api.anthropic.com, so a custom base URL no longer receives it. The Cline CLI fix is cli-v3.0.69 and the desktop fix is desktop-v0.0.44, both released 2026-10-07.
What is the exact error message?
The official release notes say only "a 400 error" and give no message text. A GitHub user report (issue #14816, endpoint on Azure AI Foundry) shows the message "fallbacks: Extra inputs are not permitted"; that is a user report, and your gateway may word it differently.
Is there a workaround without upgrading?
Not an official one. The 4.1.23 and SDK v0.0.91 release notes describe only the fix itself and list no setting to turn the option off; this page does not cover unofficial workarounds, so upgrading is the way to go.
Can I get around it by switching to OpenAI Compatible?
Not for Claude on QCode. QCode's docs state that OpenAI-compatible mode cannot use Claude and that a claude-* id returns model_not_available_on_endpoint; Claude needs the Anthropic provider with Base URL https://api.qcode.cc/api. GPT and Chinese models can use OpenAI Compatible (https://api.qcode.cc/openai/v1).
I upgraded and still get errors with QCode. What should I check first?
Check three things against QCode's docs: API Provider is Anthropic; the Base URL stops at /api, with no trailing slash and not /api/v1/messages; the key starts with cr_ and was copied in full. The docs note that a 401 usually means the path is correct and only auth failed. If the main domain is unstable, switch to a backup node such as https://us.qcode.cc/api.
Sources
Cause, affected versions and fix: the Releases page and CHANGELOG.md of Cline's GitHub repository (release notes for v4.1.22, v4.1.23, SDK v0.0.88 and v0.0.91, cli-v3.0.69 and desktop-v0.0.44), crawled 2026-10-08. User report: Cline GitHub issue #14816 and fix PR #14834, crawled the same day and used only to describe the symptom. Cline's field names: the Anthropic provider page on docs.cline.bot. QCode setup: the Cline page on docs.qcode.cc (last verified 2026-09-18, against VS Code extension 4.1.19) and the Endpoints & API Paths page (updated 2026-09-25), both crawled 2026-10-08.
Upgrade Cline and keep using Claude
In Cline pick the Anthropic provider, set Use custom base URL to https://api.qcode.cc/api and Model to claude-sonnet-5; billed per token, with each model's price on /models.
Related reading
Cline with OpenAI-compatible APIs: 2026 guide
The full setup: GPT and Chinese models via OpenAI Compatible, Claude via the Anthropic provider.
Claude Code gateway errors 2026: 400 and beta header fixes
400 and beta-header errors in Claude Code behind a gateway, matched to the versions that fix them.
BYOK AI coding tools: 16 with custom API support
Protocol, config location and Claude support for each tool.
Versions and quotes on this page were checked on 2026-10-08 against Cline's official release notes and QCode's docs; upstream changes may happen without notice. The error text from the GitHub issue is a user report and does not represent every gateway's response. Model availability is whatever /models shows.