Replit Webview blank: bind the all-interfaces address, use PORT env

[real Replit gotcha]: your app starts fine, the logs look healthy, but the Webview is blank. Nine times out of ten the server bound the loopback address. Replit's proxy cannot reach an app listening on YOUR_HOST. Bind the all-interfaces address instead, and read the port from the PORT environment variable instead of hardcoding it. Hardcoded ports break across environments anyway. This is the classic "works on my machine" bug: locally, loopback binds are the safe default; on Replit they make the app unreachable. When the agent generates server code, check the listen call before you debug anything else.

Context: Web, replit-known-pitfalls skill (github.com/costrict-plugins-repo/github-trending-replit-pack): app starts but the Webview is blank when the server binds the loopback address. Replit's proxy cannot reach an app listening on YOUR_HOST or the loopback address. Bind the all-interfaces address instead, and read the port from the PORT environment variable rather than hardcoding it. The agent sometimes defaults to loopback binds it would use locally; on Replit that pattern always breaks.

Source: Web, replit-known-pitfalls skill (github.com/costrict-plugins-repo/github-trending-replit-pack): app starts but the Webview is blank when the server binds the loopback address. Replit's proxy cannot reach an app listening on YOUR_HOST or the loopback address. Bind the all-interfaces address instead, and read the port from the PORT environment variable rather than hardcoding it. The agent sometimes defaults to loopback binds it would use locally; on Replit that pattern always breaks.

Verified resolution: [real Replit gotcha]: your app starts fine, the logs look healthy, but the Webview is blank. Nine times out of ten the server bound the loopback address. Replit's proxy cannot reach an app listening on YOUR_HOST. Bind the all-interfaces address instead, and read the port from the PORT environment variable instead of hardcoding it. Hardcoded ports break across environments anyway. This is the classic "works on my machine" bug: locally, loopback binds are the safe default; on Replit they make the app unreachable. When the agent generates server code, check the listen call before you debug anything else.

Source URL: https://github.com/costrict-plugins-repo/github-trending-replit-pack/blob/HEAD/skills/replit-known-pitfalls/SKILL.md