What is chapplin-mcp-app-builder?

A guide for adding tools with optional user interfaces to projects made with Chapplin. Chapplin is a TypeScript framework for building MCP servers whose tools can include small web interfaces.

Defining tools and their interfaces, passing data between them, supporting React, Preact, Solid, or Hono targets, and checking the result in the preview.

It explains the project conventions and runtime differences that can otherwise cause tool calls and their interfaces to exchange data incorrectly.

chapplin-mcp-app-builder — Skill for Claude Code and Codex by ssssota

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add ssssota/chapplin --skill chapplin-mcp-app-builder

Clone the repo

git clone --depth 1 https://github.com/ssssota/chapplin

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/ssssota/chapplin/chapplin-mcp-app-builder/github.svg)](https://agentmods.dev/skills/ssssota/chapplin/chapplin-mcp-app-builder)

Your own site

<a href="https://agentmods.dev/skills/ssssota/chapplin/chapplin-mcp-app-builder"><img src="https://agentmods.dev/badge/skills/ssssota/chapplin/chapplin-mcp-app-builder/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/ssssota/chapplin/chapplin-mcp-app-builder"><img src="https://agentmods.dev/badge/skills/ssssota/chapplin/chapplin-mcp-app-builder.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 77 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 531 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00077	$0.00531
Opus 5.5	$0.00031	$0.00212
Sonnet 5.5	$0.00015	$0.00106
Haiku 4.5	$0.00008	$0.00053

Measured 1mo ago against content hash a4f19bf0b419, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

chapplin-mcp-app-builder scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

packages/chapplin/skills/chapplin-mcp-app-builder/SKILL.md · 49 lines

How it starts

The opening of the file, as written. The whole thing — 49 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Chapplin MCP App Builder

Overview

Use this skill to extend an existing create-chapplin project with new MCP App-enabled tools while preserving template conventions and minimal diffs.

Quick Workflow

Confirm the project matches the baseline in references/project-baseline.md.
Detect the framework target from vite.config.ts and chapplin({ target }).
Implement defineTool first, then implement defineApp<typeof tool>.
Import useApp from chapplin/<target> only when host actions are needed.
Put machine-facing data in structuredContent and UI-only payload in _meta.
Validate with dev preview and tool-call/UI rendering checks.

Framework/Runtime Selection Rule

Read vite.config.ts.
Resolve target from chapplin({ target: "..." }).
Follow references/framework-deltas.md for plugin, JSX runtime, and useApp import mapping.
Keep existing style patterns in the same target template.

Implementation Guardrails

Do not edit generated artifacts such as dist/, .chapplin/types, or other build outputs.
Do not change HTTP/STDIO transport structure in src/index.ts unless explicitly requested.
Keep diffs focused on the requested tool/app behavior.
Match existing naming, schema style, and JSX conventions in the current project.

Validation Checklist

Run npm run dev and open the chapplin preview UI.
Execute the target tool once and confirm UI renders expected state.
Confirm initial empty state is handled (props.output may be undefined before first result).
Run project checks (typically npm run build and any local test/check scripts).
Re-check framework-specific assumptions against references/framework-deltas.md.

Reference File Map

Baseline project shape and registration flow: references/project-baseline.md
Target-specific runtime differences: references/framework-deltas.md
Reusable implementation patterns: references/implementation-recipes.md
Common failures and fixes: references/troubleshooting.md

Read the full file on GitHub · 49 lines

Files

What ships with it

4 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

1mo ago First seen · 49 lines · 77 tokens per session scan A a4f19bf0b419

Subscribe to this mod's changes

chapplin-mcp-app-builder is a skill published in the GitHub repository ssssota/chapplin (16 stars, last pushed yesterday), licensed MIT. It adds 77 tokens to every session and 531 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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