detectAgent()

Scans the workspace root for well-known indicator files and returns the Jenkins agent label that matches the project's language. Eliminates the need for every team to hardcode agent { label '...' } in their Jenkinsfile.

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Fallback chain

1. Indicator file scan — checks the repo root for the files below, in order. Returns the label for the first match.
2. JENKINS_DEFAULT_AGENT env var — if no indicator file is found, checks whether this variable is set on the Jenkins controller and uses it. Lets operators define a platform-wide default without touching pipeline code.
3. Hard fail — if neither matched, throws an error listing every supported indicator file so the developer knows exactly what to add.

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Indicator file → agent label

File	Agent label
package.json	nodejs
pom.xml	maven
build.gradle / build.gradle.kts	gradle
go.mod	golang
Cargo.toml	rust
requirements.txt	python
setup.py	python
pyproject.toml	python
Gemfile	ruby
composer.json	php

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Usage

@Library('shared') _

pipeline {
    agent { label detectAgent() }
    stages {
        stage('Build') {
            steps {
                sh 'npm install'
            }
        }
    }
}

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Platform-wide default

Set JENKINS_DEFAULT_AGENT as a global environment variable in casc.yml to provide a fallback for repos that don't match any indicator file:

jenkins:
  globalNodeProperties:
    - envVars:
        env:
          - key: JENKINS_DEFAULT_AGENT
            value: nodejs