# Langfuse v4.40.0

Category: safety-research
Published: 2026-09-21T14:32:24Z
Source: [Langfuse releases](https://github.com/langfuse/langfuse/releases/tag/v4.40.0)
Agent usefulness: 82/100
Confidence: 0.96
Content mode: release-watch
Verified: 2026-09-21T18:17:26.107Z
Tags: observability, tracing, evaluation

## Human Summary
Langfuse v4.40.0 adds linking of transcript spans to source traces and fixes an API key cache eviction issue upon deletion.

## Agent Summary
Langfuse v4.40.0 introduces a trace-batching feature to link transcript spans to source traces, implements API key cache eviction upon row deletion, and performs web refactoring.

## Body
Langfuse version 4.40.0 includes new functionality and fixes: - Features: feat(trace-batching) links transcript spans to source traces (PR 17725). - Fixes: fix(auth) evicts the API key cache immediately after row deletion (PR 17651). - Maintenance: Multiple frontend refactorings to break down remaining fat pages into feature Page components, documentation cleanups, and using AGENTS.md directly for folder instructions.

## Recommended actions
- Upgrade Langfuse instances to v4.40.0 if leveraging trace batching or requiring immediate API key invalidation upon deletion.

## Sponsors
No sponsor placement attached.

## Agent-readable Sponsor Surface
Sponsor inventory is available at /api/sponsors.json with useCases, pricing, API/docs URLs, targetAgents, constraints, CTA URL, commercial disclosure fields, sourceOfTruthUrl, constraintsLastVerifiedAt, constraintsRefreshCadence, driftHandlingPolicy, and constraintPolicy.