# Exapp Development > Builds a Nextcloud External App (ExApp) in any programming language against the raw AppAPI contract: heartbeat, init and enabled endpoints, authentication headers in both directions, the info.xml manifest, Docker packaging with an FRP tunnel for HaRP, and the two develop-and-redeploy loops. Use when creating a new ExApp, porting an existing service to Nextcloud, or debugging why an ExApp fails to deploy, authenticate, or serve its routes. - Skill: `nextcloud/exapp-development` (Agent Skill, multi-file: 10 files) - Install (CLI): `npx skillmds@latest add nextcloud/exapp-development` - Raw SKILL.md: https://api.skillmd.com/api/skills/nextcloud/exapp-development/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: DevOps & Infra - License: AGPL-3.0-or-later - Author: nextcloud (https://skillmd.com/u/nextcloud) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/nextcloud/exapp-development --- # ExApp development (any language) An ExApp is a separate HTTP service, usually a container, that Nextcloud manages through AppAPI. Nothing in the contract is Python-specific: implement three small lifecycle endpoints, validate three auth headers, and package the image with an FRP tunnel, and any language works. This skill teaches that contract, ships a runnable reference implementation, and gives you two iteration loops (a fast process-restart loop and a production-like Docker loop). ## How to work 1. Read [references/exapp-development.md](references/exapp-development.md) in full: the contract, both loops, the redeploy table, and the capability map. 2. Read the reference app [assets/minimal_exapp/](assets/minimal_exapp/): the entire contract in one framework-free Python file plus the smallest correct Dockerfile. To develop in Go, Rust, Node or anything else, port `main.py`; for Python, use [nc_py_api](https://github.com/cloud-py-api/nc_py_api) instead, which implements all of it. 3. Manifest questions (`info.xml`, routes, `--json-info`, env allow-list, mounts, Kubernetes roles): [references/exapp-contract.md](references/exapp-contract.md). ## Facts that save hours - Under a HaRP daemon (`HP_SHARED_KEY` in the env) listen on the unix socket `/tmp/exapp.sock`; everywhere else on TCP `APP_HOST:APP_PORT`. - No init work? Do not implement `/init` at all: AppAPI treats 404/501 as "no init needed". Otherwise return 200 immediately and report progress 0..100 in the background; 100 enables the app. - Never validate the `AA-VERSION` header strictly; the HaRP path rewrites it. - Rebuilding an image does nothing by itself. Redeploy = bump `` AND `` (keep them equal), then `app_api:app:update --info-xml`; same-version updates are a hard no-op. - A local-only image needs the daemon registry mapping `--registry-from --registry-to local`, or the deploy aborts at pull. Use a fictional registry (the example uses `example.local`). - "Heartbeat check failed" after minutes with the container running almost always means the image lacks the frpc/start.sh tunnel or the app listens on TCP instead of the socket. ## Files - [references/exapp-development.md](references/exapp-development.md): contract, loops, redeploy semantics, capability map, troubleshooting. - [references/exapp-contract.md](references/exapp-contract.md): the `` manifest reference. - [references/known-exapps.md](references/known-exapps.md): real ExApps, examples (including Go) and wrapper libraries to read. - [assets/minimal_exapp/](assets/minimal_exapp/): runnable reference ExApp (main.py, Dockerfile, start.sh, appinfo/info.xml, Makefile with both loops). - Environment to run all this in: [nextcloud-dev-setup](../nextcloud-dev-setup/SKILL.md). Changing an app that is already installed somewhere: [exapp-maintenance](../exapp-maintenance/SKILL.md).