falco
Trigger and scope
Trigger on: VCL files, .vcl extensions, XVCL files, .xvcl extensions, falco CLI, VCL unit tests, VCL linting/simulation/formatting, VCL REPL, beresp/bereq/req.http variables, subroutine scopes, backend/ACL/director/table declarations, edge dictionaries, validating VCL in Terraform plans, or running/testing XVCL scripts locally.
Do NOT use for: generic non-Fastly VCL, Fastly Compute/WASM, Fastly API/dashboard ops, CDN comparison, cache purging, or authoring Terraform resources.
Falco — VCL Development Tool & Reference
Falco is a Fastly VCL development tool for linting, testing, simulating, and formatting VCL code. Equally important, the falco source code is the most complete machine-readable specification of Fastly's VCL dialect — its parser, interpreter, and type system document every variable, function, type, and scope rule in VCL.
Official VCL documentation: https://www.fastly.com/documentation/guides/full-site-delivery/fastly-vcl/about-fastly-vcl/
Falco documentation: https://github.com/ysugimoto/falco
Using Falco Source as VCL Reference
If you need to understand how VCL works — what variables exist, which scopes they're available in, what functions are built-in, how types coerce — the falco source code is your best reference. It's a complete Go implementation of Fastly's VCL 2.x and is more precise than prose documentation.
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