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Tags are key-value pairs attached to spans. They let you filter, group, and search traces in the Datadog UI. This guide covers how to add tags at the span level, in bulk, and globally across all spans.

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Adding tags to a span

Use span.setTag(key, value) to add a single tag to a span: 
const tracer = require('dd-trace').init()

tracer.trace('web.request', (span) => {
  span.setTag('customer.id', '42')
  span.setTag('request.region', 'us-east-1')
  span.setTag('feature.flag', 'new-checkout')

  // ... your code
})
Tag values must be strings or numbers. Nested objects are not supported — serialize complex values manually.

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Adding multiple tags at once

Use span.addTags(object) to set multiple tags in a single call: 
const tracer = require('dd-trace').init()

tracer.trace('order.process', (span) => {
  span.addTags({
     'order.id': order.id,
     'order.total': order.total.toString(),
     'order.currency': order.currency,
     'customer.tier': customer.tier
  })

  // ... your code
})

​
Tagging spans from integrations

For spans created automatically by integrations, use span hooks to add custom tags without modifying the integration: 
const tracer = require('dd-trace').init()

tracer.use('express', {
  hooks: {
    request: (span, req, res) => {
      // Add tags to every Express request span
      span.setTag('customer.id', req.query.customerId)
      span.setTag('request.version', req.headers['x-api-version'])
    }
  }
})
Span hooks are currently supported for web framework integrations.

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Tagging the active span

When you need to tag a span created by an integration (e.g., inside a route handler), access it from the current scope: 
const tracer = require('dd-trace').init()
const express = require('express')
const app = express()

app.get('/users/:id', (req, res) => {
  const span = tracer.scope().active()

  if (span) {
    span.setTag('user.id', req.params.id)
    span.setTag('user.role', req.user?.role)
  }

  // ... handle request
})

​
Global tags

Global tags are applied to every span produced by the tracer. Use them for environment-level metadata like service, env, and version.
The most common approach for Datadog unified service tagging:
DD_SERVICE=my-api \
DD_ENV=production \
DD_VERSION=1.4.2 \
node app.js
Add arbitrary tags with DD_TAGS (comma-separated key:value pairs):
DD_TAGS=region:us-east-1,team:platform node app.js

​
Service, env, and version tags

These three tags are the foundation of Datadog unified service tagging. Set them consistently across your services to enable filtering in APM, logs, and infrastructure views.
TagEnvironment variableInit optionDescription
serviceDD_SERVICEserviceName of your service. Inferred from package.json if not set.
envDD_ENVenvDeployment environment (e.g., production, staging).
versionDD_VERSIONversionApplication version. Inferred from package.json if not set.

​
Resource naming

The resource name identifies the specific operation within a span. For web requests this is usually the route pattern. Set it using tracer.trace options or by tagging the span directly: 
tracer.trace(
  'db.query',
  {
    resource: 'SELECT users WHERE id = ?',
    service: 'users-db',
    type: 'sql'
  },
  (span) => {
    // execute query
  }
)

​
Tag naming best practices

  • Use dot-separated namespaces to group related tags: user.id, user.role, order.id, order.status.
  • Use lowercase letters, digits, underscores, and dots. Avoid spaces and special characters.
  • Keep tag values concise and low-cardinality when possible (e.g., avoid using raw SQL or full URLs as tag values).
  • Reserve high-cardinality identifiers (UUIDs, user IDs, order IDs) for tags that you specifically need to filter by — high cardinality can increase costs.
  • Follow the Datadog tagging documentation for reserved tag names and naming conventions.
Nested objects as tag values are not supported. When adding an object as a tag value, only its direct (own) properties are included. If you need nested properties, add them individually:
// Correct
span.setTag('user.id', obj.user.id)
span.setTag('user.role', obj.user.role)

// Incorrect — nested properties are not traversed
span.setTag('user', obj.user)

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