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docs(csp-cors): add a pre-publish checklist for the declared CSP #800

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docs(csp-cors): add a pre-publish checklist for the declared CSP #800

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15 changes: 15 additions & 0 deletions docs/csp-cors.md
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Expand Up @@ -62,3 +62,18 @@ Note that `_meta.ui.csp` and `_meta.ui.domain` are set in the `contents[]` objec

> [!NOTE]
> For full examples that configures CSP, see: [`examples/sheet-music-server/`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/sheet-music-server) (`connectDomains`) and [`examples/map-server/`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/map-server) (`connectDomains` and `resourceDomains`).

## Check the policy before publishing

The host enforces the CSP the server declares, so a mistake in it either breaks the App or widens what script running in it can reach:

- **Every origin the App uses is declared.** Missing origins fail silently in the sandboxed iframe. An App that loads nothing from the network needs no `csp` at all: the host's restrictive default fits it.
- **No `*` or scheme-only entries** such as `https:`. The specification describes origins, optionally with a subdomain wildcard. Hosts may drop other values, and where they apply them, they allow every host, so the App can send what it holds anywhere.
- **Wildcards stay narrow.** `https://*.example.com` allows every subdomain. On a shared service (cloud storage, tunnels, serverless hosting) that includes hosts anyone can create.
- **Development origins stay in development builds.** Drive the `localhost` entry and the runtime URL from the same setting, so the published metadata does not carry it.

[`mcp-app-lint`](https://www.npmjs.com/package/mcp-app-lint) (a third-party MIT tool) checks the declared entries statically over a server's source or build, and can report to GitHub code scanning in CI:

```bash
npx mcp-app-lint --dir . --format text
```
1 change: 1 addition & 0 deletions plugins/mcp-apps/skills/add-app-to-server/SKILL.md
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Expand Up @@ -429,3 +429,4 @@ Configure `SERVERS` with a JSON array of your server URLs (default: `http://loca
3. `ontoolinput` handler fires with tool arguments
4. `ontoolresult` handler fires with tool result
5. Host styling (theme, fonts, colors) applies correctly
6. Optionally, `npx mcp-app-lint --dir . --format text` reports no errors for the declared CSP