Hammerspoon Development Guide

Overview

Hammerspoon is the macOS automation backbone. It handles hotkeys, window management, notifications, app integrations (Shade, browser, nvim), and system event watchers.

CRITICAL: Before making changes:

1. Verify NO LSP/diagnostic errors in the file
2. Research APIs at https://www.hammerspoon.org/docs/
3. Test in console before committing
4. Performance matters - CPU/memory efficiency is critical

Configuration Structure

config/hammerspoon/
├── init.lua              # Entry point - loads modules in order
├── preflight.lua         # Early setup (IPC, globals, logging)
├── overrides.lua         # Monkey-patches for hs.* modules
├── config.lua            # C.* configuration table (SOURCE OF TRUTH)
├── utils.lua             # U.* utilities (logging, helpers)
├── bindings.lua          # Hotkey definitions
├── hyper.lua             # Hyper key (F19) modal system
├── hypemode.lua          # Hype mode (double-tap) triggers
├── chain.lua             # Window chaining operations
├── clipper.lua           # Clipboard manager
├── contexts/             # Per-app behavior customizations
│   ├── init.lua          # Context loader
│   └── com.*.lua         # App-specific context files
├── watchers/             # Event observers
│   ├── notification.lua  # NC notification capture
│   ├── screen.lua        # Display change watcher
│   └── ...
└── lib/                  # Reusable modules
    ├── state.lua         # S.* centralized state (S.notification.*, etc.)
    ├── canvas.lua        # Canvas drawing utilities
    ├── db.lua            # SQLite database wrapper
    ├── notifications/    # Notification system (N.*)
    │   ├── init.lua      # Main entry (N.send, N.init)
    │   ├── send.lua      # N.send() implementation
    │   ├── notifier.lua  # Canvas notification rendering
    │   └── processor.lua # Rule-based routing
    ├── interop/          # External app integrations
    │   ├── shade.lua     # Shade.app IPC
    │   ├── browser.lua   # Browser JXA bridge
    │   ├── nvim.lua      # Neovim RPC
    │   └── selection.lua # Text selection helpers
    └── meeting/          # Meeting detection

Global References

C

config.lua

U

U.log.i()

U.log.e()

U.log.d()

utils.lua

N

lib/notifications/init.lua

S

S.notification.*

S.watcher.*

lib/state.lua

I

I(obj)

hs.inspect(obj)

init.lua

P

init.lua

TERMINAL

config.lua

BROWSER

config.lua

HYPER

config.lua

Decision Tree: What to Check When Things Break

"Hotkey doesn't work"

1. Is Hammerspoon running?
   └─ pgrep Hammerspoon || open -a Hammerspoon

2. Is hotkey defined?
   └─ rg "the-key" config/hammerspoon/bindings.lua config/hammerspoon/hyper.lua

3. Is it in a modal that's not active?
   └─ Check if it's in hyper.lua (needs F19 held) or hypemode.lua (needs double-tap)

4. Is the handler erroring?
   └─ hs -c "hs.openConsole()" → check for red errors
   └─ Or: log stream --predicate 'subsystem == "org.hammerspoon.Hammerspoon"'

5. Is accessibility permission granted?
   └─ System Settings → Privacy & Security → Accessibility → Hammerspoon ✓

"Window management doesn't work"

1. Is the app excluded from window management?
   └─ rg "bundleID" config/hammerspoon/config.lua (check C.layouts)

2. Does the window have a valid frame?
   └─ hs -c "print(I(hs.window.focusedWindow():frame()))"

3. Is it a special window (floating, panel, etc.)?
   └─ hs -c "print(hs.window.focusedWindow():subrole())"
   └─ Check if subrole is AXFloatingWindow, AXSystemDialog, etc.

4. Is the screen/display configured?
   └─ Check C.displays in config.lua matches actual display names
   └─ hs -c "print(I(hs.screen.allScreens()))"

"Notification not showing"

1. Is notification system initialized?
   └─ hs -c "print(N ~= nil)"

2. Is canvas rendering working?
   └─ hs -c "N.send({title='Test', message='Test', urgency='normal'})"

3. Is the rule suppressing it?
   └─ Check C.notificationRules in config.lua
   └─ Maybe a rule is matching and dismissing

4. Is it a focus mode issue?
   └─ Check C.overrideFocusModes settings

5. Check the notification database:
   └─ sqlite3 ~/.local/share/hammerspoon/hammerspoon.db \
        "SELECT * FROM notifications ORDER BY timestamp DESC LIMIT 5"

"Performance is bad / Hammerspoon laggy"

1. Check CPU usage:
   └─ top -pid $(pgrep Hammerspoon) -l 1

2. Check memory:
   └─ ps -o rss,vsz -p $(pgrep Hammerspoon)

3. Is a watcher running too often?
   └─ Add logging to watcher callbacks
   └─ U.log.d("watcher fired", ...)

4. Is there an infinite loop?
   └─ Check for circular requires or recursive callbacks

5. Canvas leak?
   └─ hs -c "print(#hs.canvas.list())"
   └─ Should be small (<10 typically)

6. Timer leak?
   └─ Check S.notification.timers or other state for accumulated timers

Reloading Hammerspoon

CRITICAL: Never use

hs -c "hs.reload()"

hs.reload()

hs -c

Safe reload pattern

# Use the hs-reload script (clicks menu, waits for "hammerspork loaded")
hs-reload

How it works: The

hs-reload

Check for errors after reload

hs -c '
  local c = hs.console.getConsole()
  for line in c:gmatch("[^\n]+") do
    if line:match("ERROR") or line:match("attempt to") then print(line) end
  end
'

If Hammerspoon is crashed/not running

open -a Hammerspoon
sleep 3  # Wait for init
hs -c 'print("ok")' && echo "✓ Started"

Quick verification commands

# Test if alive
hs -c 'print("ok")'

# Check IPC is responding
hs -c "return hs.processInfo.processID"

# Check specific module loaded
hs -c 'print(HUD ~= nil and "HUD loaded" or "HUD missing")'

Common API Patterns

Window Management

-- Get focused window
local win = hs.window.focusedWindow()
if not win then return end  -- Always check!

-- Get/set frame
local frame = win:frame()
win:setFrame(frame)

-- Get screen
local screen = win:screen()
local screenFrame = screen:frame()

-- Move to position
win:setTopLeft(hs.geometry.point(100, 100))

-- Resize
win:setSize(hs.geometry.size(800, 600))

-- Move to another screen
win:moveToScreen(hs.screen.find("LG UltraFine"))

-- Center on screen
win:centerOnScreen()

-- Full screen toggle
win:setFullscreen(not win:isFullscreen())

Application Management

-- Find by name or bundle ID
local app = hs.application.find("com.brave.Browser.nightly")
local app = hs.application.get("Brave Browser Nightly")

-- Frontmost app
local frontApp = hs.application.frontmostApplication()

-- Launch or focus
hs.application.launchOrFocusByBundleID("com.brave.Browser.nightly")

-- All windows
local windows = app:allWindows()

-- Main window
local mainWin = app:mainWindow()

-- Activate (bring to front)
app:activate()

-- Hide
app:hide()

Accessibility (AX)

local ax = hs.axuielement

-- Get element for app
local appElement = ax.applicationElement(app)

-- Get attribute
local children = appElement:attributeValue("AXChildren")
local role = appElement:attributeValue("AXRole")
local title = appElement:attributeValue("AXTitle")

-- Common attributes
-- AXRole, AXSubrole, AXTitle, AXDescription, AXValue
-- AXFocused, AXEnabled, AXPosition, AXSize
-- AXChildren, AXParent, AXWindows, AXFocusedWindow

-- Perform action
appElement:performAction("AXPress")
appElement:performAction("AXRaise")

-- Build a tree
local function printAXTree(el, depth)
  depth = depth or 0
  if depth > 3 then return end
  local role = el:attributeValue("AXRole") or "?"
  local title = el:attributeValue("AXTitle") or ""
  print(string.rep("  ", depth) .. role .. ": " .. title)
  for _, child in ipairs(el:attributeValue("AXChildren") or {}) do
    printAXTree(child, depth + 1)
  end
end

Notifications

-- Using the notification system (N.send)
N.send({
  title = "Title",
  message = "Message body",
  urgency = "normal",  -- "low"|"normal"|"high"|"critical"
})

-- With phone notification
N.send({
  title = "Alert",
  message = "Something important",
  urgency = "critical",  -- Auto-sends to phone
  phone = true,          -- Or explicit
})

-- Native hs.notify (goes to NC only)
hs.notify.new({
  title = "Title",
  informativeText = "Body",
}):send()

Distributed Notifications (IPC)

-- Post to external apps (e.g., Shade)
hs.distributednotifications.post("io.shade.toggle", nil, nil)

-- Listen from external apps
local watcher = hs.distributednotifications.new(function(name, object, info)
  U.log.i("Received:", name, object, info)
end, "notification.name")
watcher:start()

Timers

-- One-shot timer (after delay)
hs.timer.doAfter(2, function()
  -- Runs once after 2 seconds
end)

-- Repeating timer
local timer = hs.timer.new(5, function()
  -- Runs every 5 seconds
end)
timer:start()
timer:stop()  -- Don't forget to stop!

-- Delayed timer (common pattern)
hs.timer.delayed.new(0.5, function()
  -- Runs 0.5s after last trigger
end):start()

Canvas (Drawing)

local canvas = hs.canvas.new({ x = 100, y = 100, w = 200, h = 100 })
canvas:appendElements({
  {
    type = "rectangle",
    fillColor = { red = 0, green = 0, blue = 0, alpha = 0.8 },
    roundedRectRadii = { xRadius = 10, yRadius = 10 },
  },
  {
    type = "text",
    text = "Hello",
    textColor = { white = 1 },
    textAlignment = "center",
    frame = { x = 0, y = 35, w = 200, h = 30 },
  },
})
canvas:show()

-- IMPORTANT: Always clean up canvases
canvas:delete()  -- or canvas:hide() then delete later

Debugging Commands

# Open Hammerspoon console
hs -c "hs.openConsole()"

# Check if running
pgrep Hammerspoon

# View logs
log stream --predicate 'subsystem == "org.hammerspoon.Hammerspoon"' --level debug

# Test a module
hs -c "print(I(require('lib.interop.shade')))"

# Check state
hs -c "print(I(S.notification))"

# Check config
hs -c "print(I(C.displays))"

# List loaded modules
hs -c "for k,v in pairs(package.loaded) do print(k) end"

# Memory usage (canvas count is a good indicator)
hs -c "print('Canvases:', #hs.canvas.list())"

# Check notification rules
hs -c "print(I(C.notificationRules))"

# Query notification database
sqlite3 ~/.local/share/hammerspoon/hammerspoon.db \
  "SELECT datetime(timestamp,'unixepoch','localtime') as time, sender, title, message FROM notifications ORDER BY timestamp DESC LIMIT 10"

Performance Monitoring

-- Memory tracking
local function logMemory(label)
  collectgarbage("collect")
  local mem = collectgarbage("count")
  U.log.i(label .. " memory:", string.format("%.2f KB", mem))
end

-- Timer audit
local function auditTimers()
  -- Check S.* for timer accumulation
  local count = 0
  for k, v in pairs(S.notification.timers or {}) do
    count = count + 1
  end
  U.log.i("Active notification timers:", count)
end

-- Canvas audit
local function auditCanvases()
  local canvases = hs.canvas.list()
  U.log.i("Active canvases:", #canvases)
  for i, c in ipairs(canvases) do
    U.log.d("  Canvas", i, c:frame())
  end
end

Common Issues and Fixes

"Error: attempt to index nil value"

Cause: Trying to access property on nil object (window closed, app quit, etc.) Fix: Always check for nil before accessing:

local win = hs.window.focusedWindow()
if not win then return end

"Hammerspoon uses too much CPU"

Cause: Watcher firing too often, infinite loop, or timer leak Fix:

1. Add debouncing to watchers
2. Check for recursive callbacks
3. Ensure timers are stopped when not needed

"Canvas not appearing"

Cause: Canvas behind other windows, wrong coordinates, or not shown Fix:

canvas:level(hs.canvas.windowLevels.overlay)  -- Above everything
canvas:behavior(hs.canvas.windowBehaviors.canJoinAllSpaces)
canvas:show()

"Accessibility not working"

Cause: Permission not granted or app not accessibility-enabled Fix:

1. System Settings → Privacy & Security → Accessibility → Hammerspoon ✓
2. Some apps (like browsers) need extra permissions
3. Try

   hs.accessibilityState()

   to check

"Module not found after reload"

Cause: Cached require not cleared Fix:

package.loaded["module.name"] = nil
require("module.name")

Files to Check First

bindings.lua

hyper.lua

config.lua

chain.lua

lib/notifications/*.lua

config.lua

contexts/com.bundleid.lua

lib/interop/shade.lua

lib/interop/browser.lua

lib/state.lua

init.lua

Discovering Hammerspoon Capabilities

List All Available Modules

-- In Hammerspoon console: list all hs.* modules
for k, v in pairs(hs) do
  if type(v) == "table" then
    print("hs." .. k)
  end
end

-- Check what a module provides
print(I(hs.window))  -- See all methods/properties

-- Get help for a function
help("hs.window.focusedWindow")

Key hs.* Modules Reference

hs.window

hs.application

hs.screen

hs.hotkey

hs.eventtap

hs.canvas

hs.notify

hs.distributednotifications

hs.axuielement

hs.osascript

hs.timer

hs.task

hs.socket

hs.http

hs.json

hs.fs

hs.audiodevice

hs.battery

hs.caffeinate

hs.pasteboard

hs.chooser

hs.alert

hs.menubar

hs.pathwatcher

hs.usb

hs.wifi

hs.location

hs.speech

Reading Hammerspoon Source

The Hammerspoon codebase reveals capabilities not always in docs:

# Clone Hammerspoon source for deep reference
git clone https://github.com/Hammerspoon/hammerspoon.git /tmp/hs-source

# Search for specific functionality
rg "AX" /tmp/hs-source/extensions --type objc

# Check how a module is implemented
cat /tmp/hs-source/extensions/window/window.lua

# Find undocumented features
rg "@objc func" /tmp/hs-source/Hammerspoon --type swift

Checking Documentation Gaps

# Hammerspoon docs source
git clone https://github.com/Hammerspoon/hammerspoon.github.io.git /tmp/hs-docs

# Compare extension implementations vs docs
ls /tmp/hs-source/extensions/
# vs
ls /tmp/hs-docs/docs/

Known Limitations and Issues

Platform Limitations (macOS)

Common GitHub Issues

Check these resources for known bugs:

# Current open issues
open "https://github.com/Hammerspoon/hammerspoon/issues"

# Search for specific problem
open "https://github.com/Hammerspoon/hammerspoon/issues?q=is%3Aissue+canvas+leak"

# Check if issue is fixed in newer version
open "https://github.com/Hammerspoon/hammerspoon/releases"

Notable recurring issues:

• Canvas memory leaks (always delete canvases)
• hs.reload() hanging (use timeout pattern)
• Accessibility permission revoked after macOS updates
• Window filters not catching all events
• Some apps not responding to AX actions

Version Compatibility

-- Check Hammerspoon version
print(hs.processInfo.version)

-- Check if feature exists (defensive coding)
if hs.canvas then
  -- Use canvas
else
  hs.alert("Canvas not available in this version")
end

-- Check macOS version for compatibility
local osVersion = hs.host.operatingSystemVersion()
print(osVersion.major, osVersion.minor, osVersion.patch)

Undocumented But Useful

-- hs.inspect (aliased as I in this config)
print(hs.inspect(someTable, { depth = 2 }))

-- hs.printf (like printf)
hs.printf("Value: %s", someValue)

-- hs.fnutils (functional programming)
local mapped = hs.fnutils.map(list, function(x) return x * 2 end)
local filtered = hs.fnutils.filter(list, function(x) return x > 5 end)

-- hs.geometry helpers
local rect = hs.geometry.rect(0, 0, 100, 100)
rect:move(10, 20)
rect:scale(1.5)

-- hs.settings (persistent storage)
hs.settings.set("myKey", "myValue")
local value = hs.settings.get("myKey")

-- hs.ipc (command line communication)
-- This is how `hs -c "..."` works

Extensions Not in Default Build

Some extensions require manual compilation or are experimental:

-- Check if extension exists
if hs.razer then print("Razer support available") end
if hs.streamdeck then print("Stream Deck support available") end
if hs.tangent then print("Tangent panel support available") end

Self-Discovery Pattern

When you don't know if Hammerspoon can do something:

1. Check if module exists:
   └─ hs -c "print(hs.modulename ~= nil)"

2. List module contents:
   └─ hs -c "for k,v in pairs(hs.modulename) do print(k, type(v)) end"

3. Check docs:
   └─ open "https://www.hammerspoon.org/docs/hs.modulename.html"

4. Search GitHub issues:
   └─ open "https://github.com/Hammerspoon/hammerspoon/issues?q=modulename"

5. Search source code:
   └─ rg "modulename" /path/to/hammerspoon/source

6. Ask in community:
   └─ https://github.com/Hammerspoon/hammerspoon/discussions
   └─ IRC: #hammerspoon on Libera.Chat

Related Resources

• Hammerspoon Expert Agent: Spawn for deep debugging/exploration tasks
• Shade Skill: For Shade.app integration
• Smart-ntfy Skill: For notification system details
• Hammerspoon Docs: https://www.hammerspoon.org/docs/
• Hammerspoon GitHub: https://github.com/Hammerspoon/hammerspoon
• Hammerspoon Wiki: https://github.com/Hammerspoon/hammerspoon/wiki
• Notification DB:

  ~/.local/share/hammerspoon/hammerspoon.db

• Spoons (plugins): https://www.hammerspoon.org/Spoons/