CONFIRMED: Claude Code 2.1.276 fixes a regression breaking gateways and proxies

Anthropic released Claude Code 2.1.276 on September 17 in Brasília time, with a small changelog entry and a major consequence for teams operating the product through a proxy or gateway. The previous version, 2.1.275, introduced a regression that made every request fail with a 400 error when ANTHROPIC_BASE_URL pointed to an intermediary layer. The new package adds no model and no flashy feature. It restores the ability to work.

The problem was in the compatibility layer

The official message identifies the visible cause: the client sent an entry with the advisor_20260301 tag, which the proxy or gateway rejected. The practical result was a total failure, not a partial degradation. Instead of losing one feature, teams could lose responses across every turn. The fix was released as v2.1.276 and is directly linked to the v2.1.275 regression.

Why this matters for teams running their own infrastructure

Gateways are common in companies that need centralized authentication, policy enforcement, usage observability, or routing across environments. They also appear in operations that use endpoints compatible with the Anthropic API without accepting exactly the same set of fields as the official service. A schema change that goes unnoticed can break development, automations, and agents at the same time. This case shows that the client version is part of the infrastructure, not merely a local tool.

The impact on a Brazilian small or midsize business

For a small or midsize company, the recommendation is direct: anyone who upgraded to 2.1.275 and uses ANTHROPIC_BASE_URL should prioritize 2.1.276 before investigating prompts, credentials, or application code. The first test should reproduce a simple request through the same gateway used in production, checking the response, authentication, streaming, and tools. It is also worth recording the client version alongside gateway logs, because that greatly shortens diagnosis when an update changes the input contract.

The fix matters more than the novelty

The editorial reading is that this is not a release to chase out of excitement, but one to install if the environment was affected. Anthropic quickly fixed a broad break caused by the previous version, which is what one expects from a mature vendor, but the incident is a reminder that automatic updates of agentic tools need staging. The counterpoint is the cost of holding every patch for too long: in this case, staying on 2.1.275 leaves a known regression inside the operation.

What to do now

Environments without a proxy or gateway have no relevant functional change to explore according to the published note. Teams using an intermediary should update, repeat the failing test, and watch whether traffic reaches the destination without the 400 error. The previous article can then remain the historical record for the series, now with the new fact that the regression was fixed. The classification remains CONFIRMED because the information comes from Anthropic's official release, with a verifiable public record and independent tracking of the release series.

Sources

Anthropic's official GitHub release: https://github.com/anthropics/claude-code/releases/tag/v2.1.276 | Structured release record: https://api.github.com/repos/anthropics/claude-code/releases/tags/v2.1.276 | Independent release-series tracking: https://updatify.io/releases/claude-code