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falco is VCL parser and linter optimized for Fastly.



This is a VCL parser, but dedicated to Fastly’s VCL (version 2.x), so we don’t care about the latest Varnish (7.x or later) syntax. The Varnish may have additional syntax, builtin function, predefined variables, but this tool may not parse correctly.

Additionally, Fastly provides its special builtin function, predefined variables. It’s not compatible with Varnish. But this tool is optimized for them, we could parse and lint their declarations.


Fastly is a really fantastic CDN, but sometimes we have problems with deployment operations. On deploy custom VCL to the Fastly, VCLs are validated when activating a new service version. Typically our deployment flow using custom VCLs is following:

  1. Clone active service and create new version
  2. Delete existing custom VCLs
  3. Upload new VCL files to the Fastly
  4. Activate new device version <= Validate VCLs on the Fastly cloud

Above flows take a time, and then if we have some mistakes on VCL e.g. missing semicolon X(, the deployment will fail. Additionally, unnecessary service versions will be created by our trivial issue.

To solve them, we made a Fastly dedicated VCL parser and linter tool to notice syntax errors and unexpected mistakes before starting the deployment flow.


Download binary from releases page according to your platform and place it under the $PATH.


Command help displays following:

falco -h
  falco: Fastly VCL parser / linter
    falco [main vcl file]

    -I, --include_path : Add include path
    -t, --transformer  : Specify transformer
    -h, --help         : Show this help
    -V, --version      : Display build version
    -v,                : Verbose warning lint result
    -vv,               : Verbose all lint result

    falco -I . -vv /path/to/vcl/main.vcl


Your VCL will have dependent modules loaded via include [module]. falco accept include path from -I, --include_path flag and search and load destination module from include path.

User defined subroutine

On linting, falco could not recognize when the user-defined subroutine is called, so you should apply the subroutine scope by adding annotation or its subroutine name. falco understands call scope by following rules:

Subroutine name

If the subroutine name has a suffix of _[scope], falco lint within that scope.

sub custom_recv { // name has `_recv` suffix, lint with RECV scope

sub custom_fetch { // name has `_fetch` suffix, lint with FETCH scope

Following table describes subroutine name and recognizing scope:

suffix scope example
_recv RECV sub custom_recv {}
_miss MISS sub custom_miss {}
_hash HASH sub custom_hash {}
_pass PASS sub custom_pass {}
_fetch FETCH sub custom_fetch {}
_error ERROR sub custom_error {}
_deliver DELIVER sub custom_deliver {}
_log LOG sub custom_log {}


For some reasons, the subroutine name could not be changed. Then, if you apply a hint of scope on annotation, falco also understands scope:

// @recv
sub custom_process { // subroutine has `recv` annotation, lint with RECV scope

// @fetch
sub custom_request { // subroutine has `fetch` annotation, lint with FETCH scope

Following table describes annotation name and recognizing scope:

annotation scope example
@recv RECV // @recv
sub custom {}
@miss MISS // @miss
sub custom {}
@hash HASH // @hash
sub custom {}
@pass PASS // @pass
sub custom {}
@fetch FETCH // @fetch
sub custom {}
@error ERROR // @error
sub custom {}
@deliver DELIVER // @deliver
sub custom {}
@log LOG // @log
sub custom {}

Fastly related features

Currently, we don’t support snippets which are managed in Fastly:

  • Edge Dictionary
  • VCL Snippets
  • Log definitions
  • Etc

Above snippets will be injected to your VCL top or extracting FASTLY XXX macro, but this tool aims to run locally, not communicating with Fastly service. However, we’re planning to solve them using the Fastly API.

Lint error

falco has built in lint rules. see rules in detail. falco may report lots of errors and warnings because falco lints with strict type checks, disallows implicit type conversions even VCL is fuzzy typed language.

Overriding Severity

To avoid them, you can override severity levels by putting a configuration file named .falcorc on working directory. the configuration file contents format is following:

## /path/to/working/directory/.falcorc
regex/matched-value-override: IGNORE

Format is simply a yaml key-value object. The key is rule name, see rules.md and value should be one of IGNORE, INFO, WARNING and ERROR, case insensitive.

In the above case, the rule of regex/matched-value-override reports INFO as default, but overrides IGNORE which does not report it.

Error Levels

falco reports three of severity on linting:


VCL may cause errors on Fastly, or may cause unexpected behavior for actual works.


VCL could work, but may have potential bug and cause unexpected behavior for actual works.

falco does not output warnings as default. To see them, run with -v option.


VCL is fine, but we suggest you improve your VCL considering Fastly recommendation.

falco does not output information as default. To see them, run with -vv option.


falco is planning to transpile Fastly VCL to the other programming language e.g Go (HTTP service), node.js ([email protected]) to use temporal CDN instead of Fastly.


  • Fork this repository
  • Customize / Fix problem
  • Send PR 🙂
  • Or feel free to create issues for us. We’ll look into it


MIT License