> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/ai-sre/ai-sre-for-administrators/get-started/expression-languages.md).

# Create Your First Dynamic Content

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 <a href="#cel-versus-mustache" id="cel-versus-mustache"></a>

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 |

***

### When to use each <a href="#when-to-use-each" id="when-to-use-each"></a>

#### Use CEL when you need: <a href="#use-cel-when-you-need" id="use-cel-when-you-need"></a>

* **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: <a href="#use-mustache-when-you-need" id="use-mustache-when-you-need"></a>

* **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.

***

### Where to use CEL <a href="#where-to-use-cel" id="where-to-use-cel"></a>

CEL is available in these contexts:

#### 1. Alert rule conditions <a href="#id-1-alert-rule-conditions" id="id-1-alert-rule-conditions"></a>

Filter incoming alerts before creating incidents.

**Example:**

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

Go to [Use CEL in Route Alerts](/ai-sre/ai-sre-for-administrators/set-up-alert-management/alert-rules/use-cel-alert-rules.md) to filter alerts.

#### 2. Runbook trigger conditions <a href="#id-2-runbook-trigger-conditions" id="id-2-runbook-trigger-conditions"></a>

Control when runbooks automatically execute.

**Example:**

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

Go to [Use CEL in Runbook Triggers](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/triggers/use-cel-triggers.md) to control runbook execution.

#### 3. Webhook advanced mapping conditions <a href="#id-3-webhook-advanced-mapping-conditions" id="id-3-webhook-advanced-mapping-conditions"></a>

Filter webhook payloads before creating alerts.

**Example:**

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

Go to [Use CEL in Webhooks](/ai-sre/ai-sre-for-administrators/set-up-alert-management/webhooks/use-cel-webhooks.md) to filter webhook payloads.

#### 4. Runbook action fields (inline) <a href="#id-4-runbook-action-fields-inline" id="id-4-runbook-action-fields-inline"></a>

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](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/workflows/use-cel-runbook-actions.md) to embed expressions in action fields.

***

### Where to use Mustache <a href="#where-to-use-mustache" id="where-to-use-mustache"></a>

Mustache is available in these contexts:

#### 1. Runbook action fields <a href="#id-1-runbook-action-fields" id="id-1-runbook-action-fields"></a>

Insert field values into messages, tickets, and notifications.

**Example:**

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

Go to [Use Mustache in Runbook Actions](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/workflows/use-mustache-runbook-actions.md) to insert field values into actions.

#### 2. Map webhook fields <a href="#id-2-map-webhook-fields" id="id-2-map-webhook-fields"></a>

Map webhook payload fields to alert properties.

**Example:**

```
{{webhook.alert.name}}
```

Go to [Use Mustache in Webhooks](/ai-sre/ai-sre-for-administrators/set-up-alert-management/webhooks/use-mustache-webhooks.md) to map payload fields to alerts.

***

### Harness-specific information <a href="#harness-specific-information" id="harness-specific-information"></a>

#### Feature flag <a href="#feature-flag" id="feature-flag"></a>

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

#### Expression limits <a href="#expression-limits" id="expression-limits"></a>

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 <a href="#harness-specific-data" id="harness-specific-data"></a>

**Severity values** are strings, not numbers:

```cel
// ✅ Correct
incident.severity == "0"

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

**Timestamps** are milliseconds since Unix epoch:

```cel
incident.created_at > 1704067200000
```

***

### Common patterns <a href="#common-patterns" id="common-patterns"></a>

#### Severity checks <a href="#severity-checks" id="severity-checks"></a>

**Single severity:**

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

**Multiple severities:**

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

#### String operations <a href="#string-operations" id="string-operations"></a>

**Contains check:**

```cel
incident.title.contains("database")
```

**Regex matching:**

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

**Regex extraction:**

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

**Case conversion:**

```cel
incident.environment.upperAscii()
```

**String trimming:**

```cel
incident.title.trim()
```

**Replace text:**

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

#### Null safety and default values <a href="#null-safety-and-default-values" id="null-safety-and-default-values"></a>

**Always check for null:**

```cel
// ✅ 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():**

```cel
// 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 <a href="#mustache-nested-fields" id="mustache-nested-fields"></a>

**Access nested data:**

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

#### Collection operations <a href="#collection-operations" id="collection-operations"></a>

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

**Get collection size:**

```cel
size(incident.affected_services) > 3
```

**Check if any item matches:**

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

**Extract Harness service IDs from impacted services:**

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

**Get first service ID:**

```cel
Activity.impacted_services[0].id
```

**Filter services by name pattern:**

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

**Count services matching a condition:**

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

**Combine operations:**

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

{% hint style="info" %}
**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.
{% endhint %}

***

### Examples <a href="#examples" id="examples"></a>

#### CEL: dynamic Slack message <a href="#cel-dynamic-slack-message" id="cel-dynamic-slack-message"></a>

```
${{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 <a href="#mustache-simple-jira-ticket" id="mustache-simple-jira-ticket"></a>

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

Link: {{incident.url}}
```

***

### Related concepts <a href="#related-concepts" id="related-concepts"></a>

#### CEL language reference <a href="#cel-language-reference" id="cel-language-reference"></a>

* Go to [cel.dev](https://cel.dev/) to read the official CEL documentation.
* Go to the [CEL language definition](https://github.com/google/cel-spec/blob/master/doc/langdef.md) to review the complete syntax reference.

#### Harness AI SRE guides <a href="#harness-ai-sre-guides" id="harness-ai-sre-guides"></a>

* [Use CEL in Route Alerts](/ai-sre/ai-sre-for-administrators/set-up-alert-management/alert-rules/use-cel-alert-rules.md)
* [Use CEL in Runbook Triggers](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/triggers/use-cel-triggers.md)
* [Use CEL in Webhooks](/ai-sre/ai-sre-for-administrators/set-up-alert-management/webhooks/use-cel-webhooks.md)
* [Use CEL in Runbook Actions](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/workflows/use-cel-runbook-actions.md)
* [Use Mustache in Runbook Actions](/ai-sre/ai-sre-for-administrators/set-up-runbook-management/workflows/use-mustache-runbook-actions.md)
* [Use Mustache in Webhooks](/ai-sre/ai-sre-for-administrators/set-up-alert-management/webhooks/use-mustache-webhooks.md)