Claude Code v2.1.276 Released: Fixes Proxy/Gateway 400 Error Regression

Release Overview

Anthropic has released Claude Code v2.1.276. This is a small but critical patch release focused entirely on fixing a regression bug introduced in the previous version, v2.1.275.

As Anthropic's official command-line coding assistant, Claude Code has accumulated over 146,000 Stars and 23,600 Forks on GitHub, making it one of the most popular AI programming tools available today. The team's rapid release cadence reflects their ongoing commitment to stability and user experience.

The Core Fix

This update contains a single change — but it's a critical one for anyone using a proxy or gateway:

Fixed an issue where every request would fail with 400 … Input tag 'advisor_20260301' when ANTHROPIC_BASE_URL was pointed at a proxy or gateway (a regression introduced in v2.1.275).

In plain terms: in v2.1.275, any user who had configured a custom ANTHROPIC_BASE_URL environment variable to route requests through a third-party proxy or internal enterprise gateway would experience a 100% request failure rate. The advisor_20260301 tag in the error message suggests the root cause was a newly added field in the API request body that was incompatible with the validation logic in intermediate proxy layers, causing the server to return HTTP 400 (Bad Request).

Who Is Affected

• Developers using third-party API proxy services to access Claude
• Teams managing AI API traffic through a centralized enterprise gateway
• Anyone who has configured ANTHROPIC_BASE_URL to point to a non-default endpoint

If you connect directly to Anthropic's official API endpoint, you are not affected by this bug. However, for the large number of enterprise users who deploy proxy layers for compliance, auditing, or access control purposes, this regression was essentially a showstopper — it rendered the entire toolchain completely non-functional.

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ANTHROPIC_BASE_URL is an environment variable provided by Claude Code that allows users to redirect API requests to a custom endpoint instead of Anthropic's default servers. This mechanism is extremely common in enterprise deployments: teams can run a reverse proxy or API gateway on their own infrastructure to centrally manage authentication keys, log requests, enforce access control, or serve as a relay when direct access to external services is restricted. Common implementations include Nginx reverse proxies, cloud provider API gateway products, and AI API aggregation platforms like LiteLLM or OpenRouter. These intermediate layers often perform strict validation on request body formats — so when the official client adds a new field that the proxy doesn't recognize, a 400 error can be triggered. This is precisely the root cause of this regression.

Lessons from a Regression

A "regression" refers to a feature that worked correctly before being broken by a change in a newer version. This is not uncommon in fast-moving software projects, especially one with as high a release frequency as Claude Code.

The tight turnaround from v2.1.275 to v2.1.276 shows that the team responded quickly once the issue was identified. Users relying on proxy-based access should upgrade to v2.1.276 as soon as possible to restore normal functionality. This incident also serves as a reminder to both the team and third-party proxy developers: when the official API client adds new fields to the request body (such as the advisor_20260301 tag here), strict validation policies in intermediate layers can become a source of compatibility issues.

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The advisor_20260301 tag in the error message is worth noting. Its naming format is consistent with the style of feature identifiers commonly seen in the Anthropic API — date suffixes like 20260301 typically represent a version anchor for a specific capability or behavior, similar to tool tags like computer_use_20241022 found in Claude model system prompts. These tags are generally attached to the request body as JSON fields to declare to the backend which feature set or protocol version the client is using. When an intermediate proxy uses a whitelist-based validation strategy rather than a pass-through approach, any unexpected new field will cause the request to be rejected. This highlights a best-practice recommendation for proxy developers: favor "pass through unknown fields" over "filter unknown fields" to achieve better forward compatibility.

Upgrade Recommendations

For current Claude Code users — especially those with proxy or gateway configurations — here's what to do:

• Upgrade from v2.1.275 to v2.1.276 immediately
• After upgrading, verify that requests through your proxy are working normally again
• If you maintain a proxy middleware layer, review whether it passes through or filters new request fields

Community response to this release has been positive, with multiple favorable reactions on the release page. For engineers who have integrated Claude Code into their daily development workflow, staying on top of these stability patches is the best way to avoid unexpected disruptions.