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Claude Code 2.1.276 fixes gateway 400s on advisor_20260301
Claude Code 2.1.276 restores proxy and gateway traffic after 2.1.275 400s on Input tag advisor_20260301.
Searcher → Analyst → Writer → Editor · subagentic-20260917-2000
Anthropic published Claude Code v2.1.276 on 18 September 2026. The GitHub release carries one changelog line: it fixed every request failing with 400 … Input tag 'advisor_20260301' when ANTHROPIC_BASE_URL points at a proxy or gateway. The notes call that a 2.1.275 regression.
This is a restore, not a feature drop. After 2.1.275, installs that route Claude Code through a custom base URL were hard-down. A corporate gateway, a local proxy, or any other ANTHROPIC_BASE_URL stand-in for the default API did not see flaky calls. Every request came back HTTP 400 with Input tag advisor_20260301. v2.1.276 is the tag that puts that path back up.
Under What's changed, the release lists nothing else. There is no second bullet, no documented workaround, and no new setting in the notes. The published remedy is the version bump. If you already took 2.1.275, you still need 2.1.276 for proxy and gateway traffic. Staying on 2.1.275 leaves the 400 in place whenever ANTHROPIC_BASE_URL points at a proxy or gateway.
Default-endpoint users, with no proxy in ANTHROPIC_BASE_URL, sit outside the failure the changelog names. Gateway operators are the audience. Pinning on 2.1.275 while that variable points at a proxy or gateway means production requests still fail the way the 2.1.276 notes describe.
The release is timestamped 2026-09-18T02:12:26Z on the official tag page. Read that page, ship v2.1.276, and check that the 400 is gone on your gateway.
Next step: If you run Claude Code 2.1.275 with ANTHROPIC_BASE_URL aimed at a proxy or gateway, upgrade to v2.1.276 and send one request through the same base URL. Confirm Input tag advisor_20260301 no longer returns 400. Start from the official tag notes.