marcus / feature-flags

Feature Flags

Feature flags gate experimental functionality behind user-configurable settings, enabling safe rollout (default off), user opt-in, and easy rollback.

Checking Feature State

import "github.com/marcus/sidecar/internal/features"

if features.IsEnabled("tmux_interactive_input") {
    // Feature-gated code
}

Adding a New Feature Flag

1. Define the feature in internal/features/features.go:

var MyNewFeature = Feature{
    Name:        "my_new_feature",
    Default:     false,
    Description: "Description of what this enables",
}

1. Add to the allFeatures slice:

var allFeatures = []Feature{
    TmuxInteractiveInput,
    MyNewFeature, // Add here
}

1. Use the feature check in your code:

if features.IsEnabled("my_new_feature") {
    // New functionality
}

That is all a flag needs to be reachable. Configuration → System → **Feature
Flags** derives its list from features.ListAll(), so registering a feature
puts a working switch on the page with the registry's own Name as the label
and Description as the help text. There is no second list to remember.

Giving a flag better copy (optional)

previewCopy in internal/configui/page_flags.go overrides the registry's
wording per flag. Only fill in what you want to change:

features.MyNewFeature.Name: {
    label:   "Human-readable name",
    help:    "One sentence on what turning this on does.",
    restart: true, // only if a consumer reads it once at startup
    note:    "An honest scope line for a flag that applies live but not retroactively.",
},

restart must be checked against what actually consumes the flag, never added
as blanket caution — a flag read at the point of use applies immediately and
must not claim otherwise. Both restart and note render when both are set.

A flag that another page already owns as a first-class setting sets owner
and ownerControl instead. It is then listed read-only with a jump to the
control that owns it, because two switches over one value is how surfaces start
disagreeing. If the owning page's control means more than the raw flag — Panels'
Conversations switch is the flag and the plugin's own enabled key — also set
reads so the row reports the owner's answer rather than the flag's.

Keep the page shorter than a small terminal. The Configuration detail pane
truncates rather than scrolling, and the row cursor still walks onto rows that
were cut. Rows are one line each with the explanation shown only under the
focused row for that reason; TestFlagsPageFitsAnOrdinaryTerminal guards it.

User Configuration

Config file (~/.config/sidecar/config.json)

{
  "features": {
    "flags": {
      "tmux_interactive_input": true
    }
  }
}

CLI override (takes precedence over config)

sidecar --enable-feature=tmux_interactive_input
sidecar --disable-feature=tmux_interactive_input
sidecar --enable-feature=feature1,feature2   # Multiple features

Unknown feature names in CLI flags produce a warning but do not prevent startup.

Priority Order

Feature state resolves in this order (first match wins):

1. CLI override (--enable-feature, --disable-feature)
2. Config file (features.flags in config.json)
3. Default value (defined in code)

Available Features

API Reference

features.IsEnabled(name string) bool           // Check if enabled
features.List() map[string]bool                 // All features with current state
features.ListAll() []Feature                    // All features with metadata
features.SetEnabled(name string, enabled bool) error  // Persist to config
features.SetOverride(name string, enabled bool) // Runtime override (not persisted)
features.IsKnownFeature(name string) bool       // Check if registered

Best Practices

• Use snake_case for feature names (e.g., my_new_feature)
• New experimental features should default to false
• Provide clear descriptions for each feature
• Document features in docs/guides/deprecated/feature-flags.md when adding them
• Remove feature flags once features are stable