Request variables

A test often has to send something different each time: a fresh idempotency key, the current time, a unique order id. Write a variable in double braces and each attempt fills it in. Variables work in an HTTP test’s URL, header values, and body, and in an MCP test’s URL and tool arguments.

The variables

Variable	Value
{{uuid}}	a random UUID
{{now}}	the current time, ISO 8601 in UTC
{{date}}	today’s date in UTC, YYYY-MM-DD
{{timestamp}}	Unix time in seconds
{{timestampMs}}	Unix time in milliseconds
{{timestampNs}}	Unix time in nanoseconds
{{randomInt}}	a random integer from 0 to 1000, inclusive
{{traceId}}	the run’s trace id (see below)

When a variable is filled in

A variable has one value per attempt, so {{uuid}} in a header and in the body are the same UUID, and a retry gets fresh values. The exception is {{traceId}}, which stays the same across a run’s retries: it is the run’s W3C trace id, the same one Fixter sends in its traceparent header (see the run detail). For example, this body creates a distinct order on every attempt:

{ "orderId": "probe-{{uuid}}", "placedAt": "{{now}}" }

Names are case-sensitive, and spaces inside the braces are allowed: {{ now }} is the same as {{now}}. A name that is not in the table, or a {{ that is never closed, is sent exactly as written. To send the literal text {{now}}, write \{{now}} in the field.

Secrets in the body or in a custom header are recorded as sent

The run detail keeps the request that was sent. It redacts the headers Authorization, Proxy-Authorization, Cookie, X-Api-Key and Api-Key, and everything a stored credential adds. It does not redact the request body or a custom header you typed yourself, such as X-My-Token. Put secrets in a credential, not in the body or in a header of your own naming.

In an MCP test

On a Tool call test, the Tool arguments (JSON) field (toolArgumentsJson) takes the same variables as the URL. These arguments send a new query id on every attempt:

{ "query": "{{uuid}}", "since": "{{date}}" }

Fields that do not take variables

These fields are sent or compared exactly as written; a {{ in them is not replaced:

• The HTTP method.
• HTTP header names. Header values do take variables.
• The JSON sample you paste for JSON_CONTAINS, JSON_SHAPE or EXACT_MATCH. The response is compared to it after the request; it is never sent, so a {{uuid}} in it stays the literal text. Under EXACT_MATCH that text never equals a real id; under the other two tiers values are ignored anyway.
• The MCP tool name.
• The MCP expected tools.