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Overview

FastSkill uses Rust’s conditional compilation feature flags to allow users to customize which functionality is included in the binary. This enables:
  • Smaller binaries - Exclude unused features to reduce binary size
  • Faster compilation - Skip compiling unnecessary dependencies
  • Optional dependencies - Only include dependencies needed for specific use cases
  • Reduced attack surface - Disable features not needed for production
Feature flags are compile-time configuration, not runtime configuration. To change features, you must recompile FastSkill.

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Available Features

FeatureDefaultDescriptionDependenciesWhen to Use
filesystem-storageyesFilesystem-based skill storageNoneDefault, includes base skill storage
registry-publishyesPublishing skills to blob storage with AWS S3aws-sdk-s3, aws-configWhen publishing to registries
hot-reloadnoAutomatic skill reloading on file changesnotifyDevelopment with hot-reload

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filesystem-storage

Default: yes (always enabled) This feature provides the core skill storage implementation using the local filesystem: 
[dependencies]
fastskill = { version = "0.9.16", features = ["filesystem-storage"] }
Included functionality:
  • File-based skill installation and removal
  • Skill metadata caching on disk
  • Directory-based skill organization
  • Standard file operations (read, write, delete, list)
Use cases:
  • Default installations - Most common use case
  • Production deployments - Reliable file-based storage
  • Development - Direct file access for debugging
Location: Skills are stored in .claude/skills/ by default (configurable via skills_directory in .fastskill/config.yaml) Note: This is a required feature and cannot be disabled. It’s included in the default feature set.

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registry-publish

Default: yes (always enabled) This feature enables publishing skills to external blob storage (e.g., AWS S3): 
[dependencies]
fastskill = { version = "0.9.16", features = ["registry-publish"] }
Included functionality:
  • Skill packaging into ZIP artifacts
  • Publishing to S3-compatible blob storage
  • Registry index updates
  • Publish job tracking and status checking
  • Async validation workflows
Dependencies:
  • aws-sdk-s3 - AWS SDK for S3 operations
  • aws-config - AWS configuration loader
Use cases:
  • Running a registry - When you host a public skill registry
  • Team registries - Private skill sharing within teams
  • Organization publishing - Publishing skills to S3 for distribution
  • CI/CD publishing - Automated skill publishing in pipelines
Configuration: Publishing requires blob storage configuration: 
[dependencies]
fastskill = { version = "0.9.16", features = ["registry-publish"] }

# Environment variables
AWS_ACCESS_KEY_ID: your-access-key-id
AWS_SECRET_ACCESS_KEY: your-secret-key
AWS_REGION: us-east-1
AWS_ENDPOINT: https://s3.example.com
Publishing workflow: When to disable:
  • Lightweight deployments without publishing
  • Environments without AWS access
  • Reduced binary size for local-only use

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hot-reload

Default: no (opt-in) This feature enables automatic skill reloading when files change: 
[dependencies]
fastskill = { version = "0.9.16", features = ["hot-reload"] }
Included functionality:
  • File system watching for skill directories
  • Automatic reload on file changes
  • Debouncing to prevent excessive reloads
  • Event-driven skill updates
  • Configurable watch paths
Dependencies:
  • notify - Cross-platform file system notification library
Use cases:
  • Development - Instant feedback when editing skill files
  • Testing - Quick iterations without manual reindexing
  • Skill debugging - See changes reflected immediately
Configuration: Hot-reload behavior is configured: 
# .fastskill/config.yaml
hot_reload:
  enabled: true
  watch_paths:
    - .claude/skills
    - ./custom-skills
  debounce_ms: 500
  auto_reload: true
  max_concurrent_reloads: 5
When to enable:
  • Active development of skills
  • Testing skill changes
  • Debugging skill behavior
When to disable:
  • Production deployments (unnecessary overhead)
  • Resource-constrained environments
  • Reduced attack surface
Note: Hot-reload can be controlled at runtime via configuration even when compiled in.

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Feature Combinations

[dependencies]
fastskill = { version = "0.9.16" }  # All default features
Includes: filesystem-storage, registry-publish
Excludes: hot-reload Best for: General use, production deployments, registry operations

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Lightweight (Minimal Binary)

[dependencies]
fastskill = { version = "0.9.16", default-features = false, features = ["filesystem-storage"] }
Includes: filesystem-storage
Excludes: registry-publish, hot-reload Best for: Local skill management only, no publishing, minimal binary size

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Development (with Hot Reload)

[dependencies]
fastskill = { version = "0.9.16", features = ["filesystem-storage", "hot-reload"] }
Includes: filesystem-storage, hot-reload
Excludes: registry-publish Best for: Active development, local testing with hot-reload

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Full Feature Set

[dependencies]
fastskill = { version = "0.9.16", features = ["filesystem-storage", "registry-publish", "hot-reload"] }
Includes: All features
Excludes: None Best for: Development with hot-reload and registry publishing, complete functionality

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Building with Features

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Using Cargo

# Build with default features
cargo build --bin fastskill

# Build with specific features
cargo build --bin fastskill --features "hot-reload"

# Build with no default features
cargo build --bin fastskill --no-default-features --features "filesystem-storage"

# Build with all features
cargo build --bin fastskill --all-features

# Build with multiple specific features
cargo build --bin fastskill --features "hot-reload,registry-publish"

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Installing from Crates.io (when published)

# Install with default features
cargo install fastskill

# Install with specific features
cargo install fastskill --features "hot-reload"

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Installing from Source

# Clone repository
git clone https://github.com/gofastskill/fastskill.git
cd fastskill

# Install with features
cargo install --path . --bin fastskill --features "hot-reload"

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Feature Interaction Matrix

Feature CombinationBinary SizeSkills StoragePublishingHot Reload
DefaultMedium✅ Filesystem✅ AWS S3❌
LightweightSmall✅ Filesystem❌❌
DevelopmentMedium✅ Filesystem❌✅
FullLarge✅ Filesystem✅ AWS S3✅

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Runtime Detection

FastSkill can detect which features were compiled in: 
use fastskill::service::FastSkillService;

let service = FastSkillService::new(config).await?;

// Check features at runtime
#[cfg(feature = "hot-reload")]
{
    println!("Hot reload enabled");
}

#[cfg(not(feature = "hot-reload"))]
{
    println!("Hot reload not available");
}

#[cfg(feature = "registry-publish")]
{
    println!("Registry publishing available");
}

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Performance Impact

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Binary Size Comparison

Feature SetBinary Size (x86_64-linux)Size Reduction
Full (all features)~12 MB0% (baseline)
Default~10 MB17% smaller
Lightweight~6 MB50% smaller

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Compilation Time Comparison

Feature SetClean Build TimeIncremental Build Time
Full~5 minutes~30 seconds
Default~4 minutes~20 seconds
Lightweight~3 minutes~15 seconds

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Runtime Impact

FeatureRuntime Overhead
filesystem-storageNegligible (required)
registry-publishOnly when publishing (AWS SDK initialization)
hot-reloadFile system watcher thread (~1-2 MB memory, minimal CPU)

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Use Case Examples

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Production Deployment

# Production: Use default features
[dependencies]
fastskill = { version = "0.9.16" }

# Build
cargo build --release --bin fastskill

# Deploy
./target/release/fastskill serve --host 0.0.0.0 --port 8080

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Development with Hot Reload

# Development: Enable hot-reload
[dependencies]
fastskill = { version = "0.9.16", features = ["hot-reload"] }

# Build
cargo build --bin fastskill --features "hot-reload"

# Run with hot-reload
./target/debug/fastskill serve --enable-hot-reload

# Edit skill files and see them reload automatically

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Local-Only Deployment

# Local: No publishing, smaller binary
[dependencies]
fastskill = { version = "0.9.16", default-features = false, features = ["filesystem-storage"] }

# Build
cargo build --release --bin fastskill --no-default-features --features "filesystem-storage"

# Deploy (smaller binary, no publishing)
./target/release/fastskill install

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Registry Publisher

# Registry: All features for registry server
[dependencies]
fastskill = { version = "0.9.16", features = ["filesystem-storage", "registry-publish"] }

# Build
cargo build --release --bin fastskill --features "registry-publish"

# Run registry with publishing
export AWS_ACCESS_KEY_ID=...
export AWS_SECRET_ACCESS_KEY=...
./target/release/fastskill serve --enable-registry

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Troubleshooting

Feature not compiled: You’re trying to use functionality that wasn’t included in the build.
Resolution: Rebuild with the required feature.
cargo build --bin fastskill --features "hot-reload"
Dependency error: A required dependency for a feature is not available.
Resolution: Ensure all required dependencies are installed.
cargo install --features "registry-publish"
# This will install aws-sdk-s3 and aws-config
Unwanted features: Binary includes features you don’t need.
Resolution: Build with --no-default-features and only include what you need.
# In your project or Cargo.toml
fastskill = { version = "0.9.16", default-features = false, features = ["filesystem-storage"] }

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Best Practices

1

Use default features for production

Default features provide complete functionality and are well-tested. Only customize features for specific needs.
2

Enable hot-reload only for development

Hot-reload adds runtime overhead and is unnecessary in production. Disable it for deployments.
3

Test feature combinations before committing

Verify that your chosen feature set includes all necessary functionality for your use case.
4

Document feature requirements in deployment guides

When deploying FastSkill, document which features were used so others know what to expect.
5

Consider binary size for edge deployments

For edge devices or containers, use --no-default-features to build smaller binaries.
6

Use feature flags for optimization

Don’t include features you don’t need. This reduces compilation time, binary size, and runtime overhead.

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Feature Flag Reference

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Complete Feature List

[features]
default = ["filesystem-storage", "registry-publish"]

# Storage backends
filesystem-storage = []

# Optional features
hot-reload = ["notify"]

# Registry publishing (requires AWS SDK)
registry-publish = ["aws-sdk-s3", "aws-config"]

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Enabling/Disabling Features

# Enable specific feature
cargo build --features "hot-reload"

# Disable all default features
cargo build --no-default-features

# Enable multiple features
cargo build --features "hot-reload,registry-publish"

# Use with cargo install
cargo install --features "hot-reload"

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See Also