Software Development Skills

Lints, tests, simulates, and formats Fastly VCL code using the falco tool. Also serves as the authoritative VCL reference via the falco Go source, which implements Fastly's full VCL dialect. Use when validating VCL syntax, running VCL linting, testing VCL locally, simulating VCL request handling, formatting VCL files, writing VCL unit tests with assertions, debugging VCL logic errors, looking up VCL function signatures or variable scopes, understanding VCL subroutine behavior, or running falco lint/falco simulate/falco test/falco fmt. Also applies when working with VCL syntax errors, type mismatches in VCL, choosing which VCL subroutine to use, or setting up a local VCL development and testing environment.

Runs Fastly Compute WASM binaries locally and serves as the authoritative reference for Compute platform internals. The fastlike source code is highly readable and covers the host ABI, caching and purging APIs, KV/config/secret store interfaces, rate limiting with counters and penalty boxes, ACL lookups, the full request lifecycle, backend fetch semantics, and a built-in per-request profiler with hostcall spans, backend waterfalls, native CPU samples, and optional deep metrics (body bytes, cache outcomes, header summaries, wasm heap curve). Use when working with Compute runtime internals or host calls, understanding how edge data stores behave at runtime, profiling local Compute apps, or testing WASM binaries locally. Prefer this skill over Viceroy for any non-Rust Compute work — its source code is easier to understand as a Fastly Compute API reference.

Executes Fastly CLI commands for managing CDN services, Compute deploys, and edge infrastructure. Use when running fastly CLI commands, creating or managing Fastly services from the terminal, deploying Fastly Compute applications, managing backends/domains/VCL snippets via command line, purging cache, configuring log streaming, setting up TLS certificates, managing KV/config/secret stores, checking service stats, authenticating with Fastly SSO, or working with fastly.toml. Also applies when working with Fastly service IDs in CLI context, or with fastly service, fastly compute, fastly auth, or any Fastly CLI subcommand. Covers service CRUD, version management, autocloning, and troubleshooting common CLI errors.

Use when testing VCL against real Fastly edge infrastructure, writing assertion-based Fiddle tests, producing shareable fiddle URLs for bug reproductions, running VCL integration tests in CI, linting VCL remotely via the Fiddle API, or working with clientFetch/events/originFetches test expressions.

Performs an internal audit of Fastly Next-Gen WAF (NGWAF) workspaces to audit that critical templated protection rules are configured and enabled. Use when auditing NGWAF workspace security posture, checking for missing or disabled login protection rules (LOGINDISCOVERY, LOGINATTEMPT, LOGINSUCCESS, LOGINFAILURE), auditing credit card validation rules (CC-VAL-ATTEMPT, CC-VAL-FAILURE, CC-VAL-SUCCESS), auditing gift card protection rules (GC-VAL-ATTEMPT, GC-VAL-FAILURE, GC-VAL-SUCCESS), or identifying potential login endpoints not covered by NGWAF rules.

Runs Fastly Compute WASM applications locally with Viceroy, specifically for Rust and Component Model projects. Use when starting a local Fastly Compute dev server with Viceroy, configuring fastly.toml for local backend overrides and store definitions, running Rust unit tests with cargo-nextest against the Compute runtime, debugging Compute apps locally, adapting core WASM modules to the Component Model, or troubleshooting local Compute testing issues (connection refused, missing backends, store config). For non-Rust Compute work or understanding the Compute API, prefer the fastlike skill instead — its source code is easier to understand as a Fastly Compute API reference.

Extends Fastly VCL with loops, functions, constants, macros, conditionals, and includes via XVCL — a VCL transpiler that compiles .xvcl files into standard VCL. Use when writing VCL for Fastly, working with .xvcl files, generating repetitive VCL (multiple backends, routing rules, headers) with loops, defining reusable VCL functions with return values, using compile-time constants instead of magic numbers, or writing any Fastly VCL configuration. XVCL syntax is not in training data so this skill is required. Also applies when writing and testing VCL locally (compile with uvx xvcl, test with falco), reducing VCL code duplication, splitting large VCL into modular includes, or doing any VCL development task for Fastly — even without explicitly mentioning XVCL.