Trigger and scope

Trigger on: XVCL, .xvcl files, VCL transpiler, VCL metaprogramming, #const/#for/#def/#inline in VCL context, writing a VCL script, writing VCL and running it locally, or any Fastly VCL writing task.

Do NOT trigger for: debugging existing .vcl files without XVCL, Fastly API/CLI ops, Fastly Compute, or Terraform — even if they mention VCL.

Writing VCL with XVCL

XVCL is a VCL transpiler that adds metaprogramming to Fastly VCL. Write .xvcl files, compile to .vcl, then test with Falco or deploy to Fastly. All XVCL constructs are resolved at compile time — zero runtime overhead.

Quick Start

# Compile (no install needed with uvx)
uvx xvcl main.xvcl -o main.vcl

# Lint the output
falco lint main.vcl

# Run locally — this is how you "run" VCL on your machine
falco simulate main.vcl
# Listens on localhost:3124 by default
# Then test with: curl http://localhost:3124/

When the user asks to "run locally" or "test locally", always compile and run falco simulate — linting alone doesn't run the VCL.

Minimal Working Example

Backend naming: Fastly VCL requires backends to use F_ prefixed names (e.g., F_origin, F_api). Never use backend default — falco will reject it. Always set req.backend explicitly in vcl_recv.

#const ORIGIN_HOST = "api.example.com"
#const REGIONS = [("us", "us.example.com"), ("eu", "eu.example.com")]

#for name, host in REGIONS
backend F_{{name}} {
  .host = "{{host}}";
  .port = "443";
  .ssl = true;
}
#endfor

sub vcl_recv {
  #FASTLY recv
  set req.backend = F_us;
  return (lookup);
}

sub vcl_deliver {
  #FASTLY deliver
  set resp.http.X-Served-By = "edge";
  return (deliver);
}

XVCL Directives Summary

Read xvcl-directives.md for complete syntax and examples of every directive.

Constants — #const

#const NAME = value              // type auto-inferred
#const NAME TYPE = value         // explicit type
#const TTL INTEGER = 3600
#const ORIGIN = "origin.example.com"
#const ENABLED BOOL = true
#const DOUBLE_TTL = TTL * 2      // expressions supported
#const BACKENDS = ["web1", "web2"] // lists
#const PAIRS = [("api", 8080), ("web", 80)] // tuples

Constants are compile-time only — they do NOT become VCL variables. Always use {{NAME}} to emit their value. A bare constant name in VCL (e.g., error 200 GREETING;) passes through as a literal string, producing invalid VCL. Use error 200 "{{GREETING}}"; instead.

Use in templates: "{{TTL}}", {{ORIGIN}}, backend F_{{name}} { ... }

Template Expressions — {{ }}

{{CONST_NAME}}                   // constant substitution
{{PORT * 2}}                     // arithmetic
{{hex(255)}}                     // → "0xff"
{{format(42, '05d')}}            // → "00042"
{{len(BACKENDS)}}                // list length
{{value if condition else other}} // ternary

Built-in functions: range(), len(), str(), int(), hex(), format(), enumerate(), min(), max(), abs()

For Loops — #for / #endfor

#for i in range(5)               // 0..4
#for i in range(2, 8)            // 2..7
#for item in LIST_CONST          // iterate list
#for name, host in TUPLES        // tuple unpacking
#for idx, item in enumerate(LIST) // index + value

Tables with Loops

Use #for loops to populate VCL table declarations for O(1) lookups, instead of generating inline if-chains.

#const REDIRECTS = [
  ("/blog", "/articles"),
  ("/about-us", "/about"),
  ("/products/old-widget", "/products/widget-v2")
]

// O(1) hash-table lookup — the right pattern for data-driven VCL
table redirects STRING {
#for old_path, new_path in REDIRECTS
  "{{old_path}}": "{{new_path}}",
#endfor
}

sub vcl_recv {
  if (table.contains(redirects, req.url.path)) {
    error 801 table.lookup(redirects, req.url.path);
  }
}

Prefer populating VCL table declarations with #for loops over generating inline if-chains. Tables give O(1) hash lookups and are the idiomatic Fastly pattern for any data-driven routing, redirects, or configuration.

Conditionals — #if / #elif / #else / #endif

#if PRODUCTION
  set req.http.X-Env = "prod";
#elif STAGING
  set req.http.X-Env = "staging";
#else
  set req.http.X-Env = "dev";
#endif

Supports: boolean constants, comparisons (==, !=, <, >), operators (and, or, not).

Variable Shorthand — #let

#let cache_key STRING = req.url.path;
// expands to:
// declare local var.cache_key STRING;
// set var.cache_key = req.url.path;

Functions — #def / #enddef

// Single return value
#def normalize_path(path STRING) -> STRING
  declare local var.result STRING;
  set var.result = std.tolower(path);
  return var.result;
#enddef

// Tuple return (multiple values)
#def parse_pair(s STRING) -> (STRING, STRING)
  declare local var.key STRING;
  declare local var.value STRING;
  set var.key = regsub(s, ":.*", "");
  set var.value = regsub(s, "^[^:]*:", "");
  return var.key, var.value;
#enddef

// Call sites
set var.clean = normalize_path(req.url.path);
set var.k, var.v = parse_pair("host:example.com");

Functions compile to VCL subroutines with parameters passed via req.http.X-Func-* headers.

Inline Macros — #inline / #endinline

#inline cache_key(url, host)
digest.hash_md5(url + "|" + host)
#endinline

// Zero-overhead text substitution. Auto-parenthesizes arguments
// containing operators to prevent precedence bugs.
set req.hash += cache_key(req.url, req.http.Host);

Includes — #include

#include "includes/backends.xvcl"  // relative path
#include <stdlib/security.xvcl>    // include path (-I)

Include-once semantics. Circular includes are detected and reported.

Compilation

# Basic
uvx xvcl input.xvcl -o output.vcl

# With include paths
uvx xvcl main.xvcl -o main.vcl -I ./includes -I ./shared

# Debug mode (shows expansion traces)
uvx xvcl main.xvcl -o main.vcl --debug

# Source maps (adds BEGIN/END INCLUDE markers)
uvx xvcl main.xvcl -o main.vcl --source-maps

Option	Description
-o, --output	Output file (default: replace .xvcl with .vcl)
-I, --include	Add include search path (repeatable)
--debug / -v	Show expansion traces
--source-maps	Add source location comments
--error-format	Error output format: text (default) or json

Common Mistakes

• Bare constant names in VCL: error 200 GREETING; passes through as a literal string. Use error 200 "{{GREETING}}"; with template syntax.
• Generating if-chains instead of tables: When you have data-driven routing or redirects, always populate a VCL table with #for — not an inline if-chain. If-chains are O(n); tables are O(1).
• Forgetting #FASTLY macros: Every VCL subroutine (vcl_recv, vcl_fetch, vcl_deliver, vcl_error, vcl_hit, vcl_miss, vcl_pass) needs #FASTLY recv (or the appropriate name) at the top.
• Using backend default: Fastly VCL requires F_ prefixed backend names. Use backend F_origin { ... } and set req.backend = F_origin;.

VCL Gotchas

VCL runtime pitfalls that are easy to get wrong:

• No modulo operator: VCL has no % operator. For traffic splitting, use substr() on a hash digest: if (substr(digest.hash_sha256(client.ip), 0, 1) ~ "^[0-7]$") gives ~50%. Or use randomint(0, 99) < 50.
• Vary MUST be set in vcl_fetch: The Vary header controls the cache key. Setting it only in vcl_deliver is too late — the object is already cached without Vary dimensions. Always append Vary in vcl_fetch (and optionally mirror in vcl_deliver for client-visible headers). Never overwrite existing Vary: check and append.
• req.url.path is read-only in falco tests: In test subroutines, use set req.url = "/path" instead of set req.url.path = "/path". The .path property is computed from req.url and cannot be set directly.
• req.request is deprecated: Use req.method instead. Falco accepts both but req.method is the modern form.
• Cookie parsing: Use subfield(req.http.Cookie, "name", ";") instead of regex. Regex like Cookie ~ "name=(\w+)" false-matches cookies with similar prefixes (e.g., name_v2=X).

References

For VCL basics (request lifecycle, return actions, variable types), see the VCL syntax and subroutines references below.

Read the relevant reference file completely before implementing specific features.

Topic	File	Use when...
XVCL Directives	xvcl-directives.md	Writing any XVCL code — complete syntax for all directives
VCL Syntax	vcl-syntax.md	Working with data types, operators, control flow
Subroutines	subroutines.md	Understanding request lifecycle, custom subs
Headers	headers.md	Manipulating HTTP headers
Backends	backends.md	Configuring origins, directors, health checks
Caching	caching.md	Setting TTL, grace periods, cache keys
Strings	strings.md	String manipulation functions
Crypto	crypto.md	Hashing, HMAC, base64 encoding
Tables/ACLs	tables-acls.md	Lookup tables, access control lists
Testing VCL	testing-vcl.md	Writing unit tests, assertions, test helpers

Project Structure

vcl/
├── main.xvcl
├── config.xvcl          # shared constants
└── includes/
    ├── backends.xvcl
    ├── security.xvcl
    ├── routing.xvcl
    └── caching.xvcl