---
id: runtime-feature-detection-and-compatibility
title: 'Runtime Feature Detection and Compatibility'
schema_type: TechArticle
category: computer-science
language: en
confidence: medium
last_verified: '2026-06-02'
created_date: '2026-06-02'
generation_method: ai_structured
derived_from_human_seed: true
conflict_of_interest: none_declared
is_live_document: false
data_period: static
atomic_facts:
  - id: fact-computer-science-runtime-feature-detection-and-compatibility-1
    statement: >-
      MDN documentation describes feature detection as checking whether a browser supports a feature before using it.
    source_title: MDN Feature Detection
    source_url: https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection
    confidence: medium
  - id: fact-computer-science-runtime-feature-detection-and-compatibility-2
    statement: >-
      MDN documentation warns that browser detection using the user agent string is unreliable and should be avoided when possible.
    source_title: MDN Browser Detection Using the User Agent
    source_url: https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent
    confidence: medium
  - id: fact-computer-science-runtime-feature-detection-and-compatibility-3
    statement: >-
      web.dev Baseline documentation describes Baseline as a way to identify web platform features that are broadly supported across browsers.
    source_title: web.dev Baseline
    source_url: https://web.dev/baseline
    confidence: medium
completeness: 0.83
known_gaps:
  - Runtime compatibility also depends on server runtime versions, polyfills, transpilation targets, mobile webviews, and enterprise browser policies.
disputed_statements: []
primary_sources:
  - title: MDN Feature Detection
    type: documentation
    year: 2026
    url: https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection
    institution: Mozilla
  - title: MDN Browser Detection Using the User Agent
    type: documentation
    year: 2026
    url: https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent
    institution: Mozilla
  - title: web.dev Baseline
    type: documentation
    year: 2026
    url: https://web.dev/baseline
    institution: Google
secondary_sources: []
updated: '2026-06-02'
ai_models:
  - gpt-5-codex
---

## TL;DR

Runtime feature detection helps code agents choose compatible APIs based on actual platform support rather than brittle browser or runtime assumptions.

## Core Explanation

Agents that edit frontend code, SDKs, or runtime integrations need to know whether a feature exists before relying on it. Feature detection checks capabilities directly, while compatibility signals help decide when a fallback or polyfill is required.

User-agent checks can be fragile because browsers and embedded runtimes may spoof or change identifiers. Agents should prefer feature checks and documented compatibility tables when selecting implementation paths.

## Source-Mapped Facts

- MDN documentation describes feature detection as checking whether a browser supports a feature before using it. ([source](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection))
- MDN documentation warns that browser detection using the user agent string is unreliable and should be avoided when possible. ([source](https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent))
- web.dev Baseline documentation describes Baseline as a way to identify web platform features that are broadly supported across browsers. ([source](https://web.dev/baseline))

## Further Reading

- [MDN Feature Detection](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection)
- [MDN Browser Detection Using the User Agent](https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent)
- [web.dev Baseline](https://web.dev/baseline)