Stable User-Agent Identity in Anthropic’s TypeScript SDK

Engineering objective

System challenge

The SDK generated its User-Agent from constructor.name. Subclassing or JavaScript minification can rename that constructor, producing unstable client identity in network traffic and weakening observability across builds.

Constraints

Working within real limits

• ✕Preserve the established Anthropic/JS <version> header format
• ✕Remain stable under subclassing and minification
• ✕Keep the change focused on SDK identity rather than request behavior
• ✕Classify the outcome accurately because PR #1135 was not merged

Approach

How it was built

1. 01

   Demonstrated the failure with a subclass whose constructor name differs from the canonical SDK name.

2. 02

   Specified a stable Anthropic/JS versioned token independent of runtime class names.

3. 03

   Added a regression that rejects leaked subclass names while preserving the public header format.

4. 04

   Validated the focused Jest suite, formatting, linting, and TypeScript compilation.

Workflow

Implementation flow

Technical validation

Implementation evidence

Observable engineering evidence. Discipline: UPSTREAM SHIPPED

Public evidence

Review the upstream contribution

Outcome

What was delivered

• Anthropic confirmed that the same fix shipped in SDK v0.116.0

• The released implementation included regression coverage for the subclass/minified-name scenario

• The focused Jest run passed all 51 tests alongside lint, format, and TypeScript checks

• PR #1135 was closed without merge because upstream had already shipped the correction

Technologies

Stack and tools

Next engineering system