Create Your First Dynamic Content

Build dynamic content with CEL and Mustache expression languages.

Harness AI SRE supports two expression languages for creating dynamic content: CEL (Common Expression Language) for logic and conditions, and Mustache for simple variable substitution.

CEL versus Mustache

Choose the right expression language based on your needs:

Feature	CEL (${{expression}})	Mustache ({{variable}})
Purpose	Logic and computation	Simple variable substitution
Use cases	Conditions, transformations, calculations	Display values, basic templating
Capabilities	Boolean logic, regex, math, functions	Variable interpolation only
Syntax	${{incident.severity == "0"}}	{{incident.severity}}
Return types	Any type (string, number, boolean, array, object)	String only
Mixing	Cannot mix with Mustache in same field	Cannot mix with CEL in same field

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When to use each

Use CEL when you need:

• Conditional logic: Filter or route based on conditions.

• Regex matching: Pattern matching on service names or messages.

• Calculations: Math operations, thresholds, percentages.

• Transformations: String manipulation, datetime formatting.

• Complex logic: Multi-field boolean expressions.

Use Mustache when you need:

• Simple display: Show field values in messages.

• Basic templating: Insert variables into text.

• Field mapping: Map source fields to destination fields.

• Static substitution: Replace placeholders with values.

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Where to use CEL

CEL is available in these contexts:

1. Alert rule conditions

Filter incoming alerts before creating incidents.

Example:

alert.severity == "critical" && alert.source.matches("prod-.*")

Go to Use CEL in Route Alerts to filter alerts.

2. Runbook trigger conditions

Control when runbooks automatically execute.

Example:

incident.severity == "0" && incident.environment == "production"

Go to Use CEL in Runbook Triggers to control runbook execution.

3. Webhook advanced mapping conditions

Filter webhook payloads before creating alerts.

Example:

webhook.priority == "P1" && webhook.region.matches("us-.*")

Go to Use CEL in Webhooks to filter webhook payloads.

4. Runbook action fields (inline)

Embed CEL expressions in text fields for dynamic content.

Example:

Incident ${{incident.title}} has severity ${{incident.severity == "0" ? "CRITICAL" : "Normal"}}

Go to Use CEL in Runbook Actions to embed expressions in action fields.

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Where to use Mustache

Mustache is available in these contexts:

1. Runbook action fields

Insert field values into messages, tickets, and notifications.

Example:

Incident {{incident.title}} detected in {{incident.environment}}

Go to Use Mustache in Runbook Actions to insert field values into actions.

2. Map webhook fields

Map webhook payload fields to alert properties.

Example:

{{webhook.alert.name}}

Go to Use Mustache in Webhooks to map payload fields to alerts.

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Harness-specific information

Feature flag

CEL expressions require the feature flag IR_CEL_CONDITIONS. Contact your Harness account team to enable this feature.

Expression limits

CEL and Mustache expressions have the following limits:

• Max CEL expression length: 4,096 characters

• No mixing: Cannot use both CEL and Mustache in the same field

• No preview: Expressions cannot be tested before execution

Harness-specific data

Severity values are strings, not numbers:

// ✅ Correct
incident.severity == "0"

// ❌ Wrong - comparing string to number
incident.severity == 0

Timestamps are milliseconds since Unix epoch:

incident.created_at > 1704067200000

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Common patterns

Severity checks

Single severity:

incident.severity == "0"    // SEV0 (Critical)

Multiple severities:

incident.severity in ["0", "1", "2"]

String operations

Contains check:

incident.title.contains("database")

Regex matching:

incident.service.matches("^payment-.*")

Regex extraction:

regex.extract(Webhook.parsed_body, r"\"AlarmName\"\:\"(.*)\"")

Case conversion:

incident.environment.upperAscii()

String trimming:

incident.title.trim()

Replace text:

regex.replace(incident.description, r"\n", ", ")

Null safety and default values

Always check for null:

// ✅ Safe
incident.owner != null && incident.owner.contains("@example.com")

// ❌ May fail if owner is null
incident.owner.contains("@example.com")

Provide default values with orValue():

// Returns "Unknown" if field is null or empty
regex.extract(Webhook.parsed_body, r"\"AlarmName\"\:\"(.*)\"").orValue("")

// Chain multiple operations with safe defaults
incident.owner.orValue("Unassigned").trim()

Mustache nested fields

Access nested data:

{{webhook.metadata.environment}}
{{alert.resource.name}}

Collection operations

CEL provides powerful collection operations for working with arrays and lists.

Get collection size:

size(incident.affected_services) > 3

Check if any item matches:

incident.tags.exists(t, t == "customer-impact")

Extract Harness service IDs from impacted services:

Activity.impacted_services.map(s, s.id)
// Returns: ["9280f15c-8c59-4c32-834b-3b36c06d269b", "a1b2c3d4-..."]

Get first service ID:

Activity.impacted_services[0].id

Filter services by name pattern:

Activity.impacted_services.filter(s, s.name.contains("api"))

Count services matching a condition:

size(Activity.impacted_services.filter(s, s.name.contains("api")))

Combine operations:

// Get IDs of all API services
Activity.impacted_services
  .filter(s, s.name.contains("api"))
  .map(s, s.id)

ACTIVITY NAMESPACE

Activity.* provides access to incident and activity data in runbook action fields. Use Activity.impacted_services to get the list of Harness services affected by an incident, not just service names.

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Examples

CEL: dynamic Slack message

${{incident.severity == "0" ? "[CRITICAL]" : "[ALERT]"}}
**Service**: ${{incident.service}}
**Environment**: ${{incident.environment}}
**Status**: ${{incident.status}}

${{incident.severity in ["0", "1"] ?
  "**IMMEDIATE ACTION REQUIRED**" :
  "Monitor and triage as needed"}}

View: ${{incident.url}}

Mustache: simple Jira ticket

**Incident**: {{incident.short_id}}
**Title**: {{incident.title}}
**Service**: {{incident.service}}
**Environment**: {{incident.environment}}
**Severity**: SEV{{incident.severity}}

Link: {{incident.url}}

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Related concepts

CEL language reference

• Go to cel.dev to read the official CEL documentation.

• Go to the CEL language definition to review the complete syntax reference.

Harness AI SRE guides

• Use CEL in Route Alerts

• Use CEL in Runbook Triggers

• Use CEL in Webhooks

• Use CEL in Runbook Actions

• Use Mustache in Runbook Actions

• Use Mustache in Webhooks