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 |

Start by tracing the BYOK Anthropic provider and comparing its API-key flow with how Claude Code authenticates via `claude login`. Review the Agent SDK authentication behavior described in the issue. Done means Anthropic subscription-based authentication works for Claude subscribers without requiring a separate API-key balance.

索引モデルが issue の本文から書いたものです。