Variable interpolation is the mechanism that replaces {{placeholder}} expressions with actual values at runtime. This page covers all interpolation contexts, resolution rules, and advanced patterns.
Interpolation Syntax
All interpolation uses double curly braces:
Where Interpolation Works
Variables can appear in:
Variable Sources
Inline Variables
Defined at the top of a .http file:
Environment Variables
From hitspec.yaml environments section:
System Environment Variables
Using the $env() function:
Built-in Functions
Dynamic values generated at runtime:
Captured Values
Values extracted from prior responses:
Resolution Order
When the same variable name exists in multiple sources, hitspec resolves in this order (later sources win):
- Config environments —
hitspec.yaml environments section for the active --env
- Inline variables —
@variable = value in the file
- Captured values —
{{requestName.captureName}}
Built-in functions ($uuid(), $timestamp(), $env(), …) and system variables ({{$VAR}}) are evaluated wherever they appear.
Capture Chains
Chain captures across dependent requests:
Step 3 references captures from both Step 1 (login.token) and Step 2 (createResource.resourceId).
Nested Interpolation
Variables can reference other variables:
Unresolved Variables
If a variable cannot be resolved, it remains as the literal {{variableName}} string in the request. hitspec does not error on unresolved variables — the request is sent with the raw placeholder, which typically causes a server-side error that surfaces through your assertions.
Use --verbose (or -v) to see resolved variable values in the output, which helps debug interpolation issues.
