The Anthropic provider in BYOK only accepts an API key

The Anthropic provider in BYOK only accepts an API key (sk-ant-) against api.anthropic.com. That means it can only draw on Anthropic Console credits, which are a separate prepaid balance from a Claude Pro/Max subscription, the two don't transfer.

Please add OAuth / subscription-based auth for the Anthropic provider, the same way Claude Code authenticates via `claude login`. Third-party apps that authenticate against a Claude subscription through the Agent SDK currently draw on subscription usage limits, so the mechanism exists.

As it stands, a paying Claude subscriber has no way to use their own plan in Copilot and has to fund a second balance to do the same work (e.g. to use Fable consumption credits).

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| Field | Value |
| --- | --- |
| App version | 1.1.12 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 15 |

首先跟踪 BYOK Anthropic provider，并将其 API key 流程与 Claude Code 通过 `claude login` 进行身份验证的方式进行比较。检查 issue 中描述的 Agent SDK 身份验证行为。当基于 Anthropic 订阅的身份验证能够为 Claude 订阅者工作且无需单独的 API key 余额时，即表示完成。

由索引模型根据 Issue 内容生成。