Auto-detect Wi-Fi IP for dev server bind and mailer host

Design invariant: Rails and the mobile apps must agree on one
reachable address — the host's current Wi-Fi IP. Never loopback,
never localhost, never 0.0.0.0. Procfile.dev now defaults -b to
`$(ipconfig getifaddr en0)`, and development.rb picks the first
private non-loopback IPv4 via Socket.ip_address_list for
action_mailer.default_url_options. HOST in .env overrides when
en0 isn't Wi-Fi (wired-primary, Tailscale).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dadachi

.env.sample

@@ -1,6 +1,10 @@
 # Copy to .env and edit. Loaded by Foreman when running `bin/dev`.
+#
+# `bin/dev` auto-detects your Wi-Fi IP (via `ipconfig getifaddr en0` on macOS)
+# and binds Rails to it so phones on the same network can reach the dev server.
+# Only set HOST if the auto-detected interface is wrong (wired-primary,
+# Tailscale, etc.). Never use 127.0.0.1, localhost, or 0.0.0.0 — the
+# mobile apps and Rails must agree on the current Wi-Fi IP.
 
-# Rails server bind address and port. Use your LAN IP (e.g. 192.168.1.6) for
-# mobile-device-on-LAN access; use 127.0.0.1 for loopback-only dev.
-HOST=127.0.0.1
 PORT=3000
+# HOST=192.168.1.6

CLAUDE.md

@@ -96,7 +96,7 @@ bin/rails dbconsole           # Database console
 - Run tests: `bin/rails test` (205 tests, 402 assertions)
 
 ### Development Server Configuration
-- Server bind and mailer host are env-driven via `HOST`/`PORT` in `.env` (defaults `127.0.0.1:3000`)
+- Server bind and mailer host auto-detect the current Wi-Fi IP (`ipconfig getifaddr en0` in `Procfile.dev`; `Socket.ip_address_list` private IPv4 in `development.rb`) so Rails and mobile apps agree on one reachable address; override with `HOST` in `.env` only if `en0` isn't Wi-Fi. Never use `127.0.0.1`, `localhost`, or `0.0.0.0`.
 - Mailbin for email testing at `/mailbin`
 - Admin interface at `/madmin`
 - Tailwind CSS compiled by tailwindcss-rails gem

Procfile.dev

@@ -1 +1 @@
-web: bin/rails server -p ${PORT:-3000} -b ${HOST:-127.0.0.1}
+web: bin/rails server -p ${PORT:-3000} -b ${HOST:-$(ipconfig getifaddr en0)}

README.md

@@ -83,9 +83,9 @@ Run `bin/setup` to install Ruby and JavaScript dependencies and setup your datab
 bin/setup
 ```
 
-## Running NativeAppTemplate API on localhost
+## Running NativeAppTemplate API on your Wi-Fi
 
-Copy `.env.sample` to `.env` and set `HOST` (and optionally `PORT`) to control the server bind address and mailer URL host. Defaults to `127.0.0.1:3000`. For mobile-device-on-LAN access, set `HOST=<your-lan-ip>`. Foreman auto-loads `.env` under `bin/dev`.
+`bin/dev` binds Rails to the current Wi-Fi IP (auto-detected via `ipconfig getifaddr en0`), so the dev server is reachable from both the host browser and from any phone on the same network at `http://<wifi-ip>:3000`. To check the IP: `ipconfig getifaddr en0` (or System Settings → Network). If `en0` isn't Wi-Fi on your machine (wired-primary, Thunderbolt networking), copy `.env.sample` to `.env` and set `HOST` to the correct interface's IP. Never use `127.0.0.1`, `localhost`, or `0.0.0.0` — Rails and the mobile apps must agree on the same Wi-Fi IP.
 
 To run your application, you'll use the `bin/dev` command:
 

config/environments/development.rb

@@ -1,4 +1,5 @@
 require "active_support/core_ext/integer/time"
+require "socket"
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -38,7 +39,8 @@
 
   config.action_mailer.perform_caching = false
 
-  config.action_mailer.default_url_options = {host: ENV.fetch("HOST", "127.0.0.1"), port: ENV.fetch("PORT", 3000).to_i}
+  lan_ip = Socket.ip_address_list.find { |a| a.ipv4_private? && !a.ipv4_loopback? }&.ip_address
+  config.action_mailer.default_url_options = {host: ENV.fetch("HOST") { lan_ip }, port: ENV.fetch("PORT", 3000).to_i}
 
   # Print deprecation notices to the Rails logger.
   config.active_support.deprecation = :log