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feat(mcp): add resource templates and a resources changed hook (#452)
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Anthony Fu (via agent) <reg-github-bot@antfu.me>
Copy file name to clipboardExpand all lines: docs/content/1.guide/18.hub-initiate.md
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@@ -89,7 +89,7 @@ Registrations are validated fail-fast: one module per type (`DF8108`), an existi
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The hub's **single Auth** is one gate at the shared transport for every mounted devframe, built-ins, and the MCP route; one handshake (OTP, magic link, or pre-shared token) unlocks the namespace; `auth: false` disables it for localhost.
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The aggregate MCP route mounts through the `'auto'` default once any mounted devframe (or an agent-flagged hub command) exposes agent tools; `mcp: true` forces it on, `mcp: false` off. It has its own origin gate, independent of this RPC Auth: the mounted route trusts same-machine callers, and `mcp: { authorization }` adds an identity check when the hub is reachable beyond loopback. `mcp: { exposeSharedState }` takes `false` or a `(key) => boolean` filter to choose which shared-state keys agents can read through `devframe_state_read` and `devframe://state/<key>`. `mcp: { instructions }` sets the usage guidance that the route sends in the MCP `initialize` result. The hub exposes one aggregate route over every mounted devframe's tools.
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The aggregate MCP route mounts through the `'auto'` default once any mounted devframe (or an agent-flagged hub command) exposes agent tools; `mcp: true` forces it on, `mcp: false` off. It has its own origin gate, independent of this RPC Auth: the mounted route trusts same-machine callers, and `mcp: { authorization }` adds an identity check when the hub is reachable beyond loopback. `mcp: { exposeSharedState }` takes `false` or a `(key) => boolean` filter to choose which shared-state keys agents can read through `devframe_state_read` and `devframe://state/<key>`. `mcp: { instructions }` sets the usage guidance that the route sends in the MCP `initialize` result. `mcp: { resourceTemplates }` adds resource templates that coding agents list through `resources/templates/list` and read through `resources/read`, and `mcp: { onResourcesChanged }` sends `notifications/resources/list_changed` each time its `notify()` runs. The hub exposes one aggregate route over every mounted devframe's tools.
`createMcpServer` and `createMcpFetchHandler` take the same `instructions` option.
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### Resource templates
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`resourceTemplates` on the route options is a list of resource templates that the MCP server serves through `resources/templates/list`. Each template has a `uriTemplate`, a `name`, an optional `title`, `description` and `mimeType`, a `read(params, uri, ctx)` function, and an optional `list(ctx)` function. Both functions can be sync or async. They get the node-side context (`ctx`), so they can read live state: the route options are resolved before `setup()` runs, so a template cannot keep `ctx` from there.
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```ts
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devframeViteBridge(myDevframe, {
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mcp: {
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resourceTemplates: [{
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uriTemplate: 'my-tool://module/{id}',
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name: 'module',
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mimeType: 'application/json',
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list: ctx=>listModules(ctx).map(id=> ({ uri: `my-tool://module/${encodeURIComponent(id)}`, name: id })),
The `uriTemplate` uses RFC 6570 level 1: plain `{name}` expressions. Each one matches a non-empty value without `/`, and `read` gets its value percent-decoded, so a value with a `/` comes in as `%2F`. When one segment has several values, such as `{from}-{to}`, each value runs up to the first place the next text appears outside a `%XX` escape. Any other template syntax, or two expressions with nothing between them (`{a}{b}`), throws [DF0084](/errors/DF0084) when the route mounts. `read` returns `text`, or `json` that the server sends as JSON text, and an optional `mimeType` (the default is the `mimeType` of the template, then `application/json`).
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`resources/read` sends a `devframe://resource/<id>` or `devframe://state/<key>` URI to the registered resource or the shared state as before. It tries any other URI against the templates in order, and the first template that matches reads it. A URI that nothing matches gets the usual unknown-resource error, and an error thrown by `read` goes to the MCP client like the error of a registered resource. `resources/list` adds the entries that the `list` of each template returns. With an empty or omitted list, the resources stay as they are and `resources/templates/list` answers method not found. `createMcpServer` and `createMcpFetchHandler` take the same `resourceTemplates` option.
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### Telling coding agents the resources changed
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The route sends `notifications/resources/list_changed` when the agent surface changes or a shared-state key is added. For other changes, such as new entries from the `list` of a template, pass `onResourcesChanged`: a function that gets `notify` and the node-side context, and returns an unsubscribe function. Each `notify()` sends `notifications/resources/list_changed`, over HTTP (on the `subscriptions/listen` streams) and over `stdio`. The route calls the unsubscribe function when it closes. `createMcpServer` and `createMcpFetchHandler` take the same `onResourcesChanged` option.
description: 'MCP resource template "{name}" has an invalid `uriTemplate` "{uriTemplate}": {reason}'
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---
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## Message
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> MCP resource template "`{name}`" has an invalid `uriTemplate` "`{uriTemplate}`": `{reason}`
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## Cause
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An entry in the `resourceTemplates` list of the MCP route options (or of `createMcpServer` / `createMcpFetchHandler`) has a `uriTemplate` that goes beyond RFC 6570 level 1. The MCP server supports plain `{name}` expressions only. A name is made of letters, digits, `_` and `.`, and each name appears once. An operator (`{+path}`, `{?q}`, `{/id}`), a modifier (`{path*}`, `{id:3}`), a list of names (`{a,b}`), an empty expression (`{}`), two expressions with nothing between them (`{a}{b}`, where a URI cannot show where one value ends), or a `{` or `}` outside an expression throws this. The check runs once, when the MCP route mounts or `createMcpServer` starts, so no request is served with a broken template.
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## Example
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```ts
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initDevframe(myDevframe, {
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mcp: {
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resourceTemplates: [{
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uriTemplate: 'my-tool://file/{+path}', // `+` is a level 2 operator → DF0084
Use a plain `{name}` expression for each value. A value matches one URI segment, so a value with a `/` (such as a file path) comes in percent-encoded (`my-tool://file/src%2Fmain.ts`) and reaches `read` decoded (`src/main.ts`):
-[`packages/agentic/src/mcp/build-server.ts`](https://github.com/devframes/devframe/blob/main/packages/agentic/src/mcp/build-server.ts): `compileResourceTemplates()` throws this for a `uriTemplate` beyond RFC 6570 level 1. `createMcpFetchHandler()` (which `mountMcpHttp()` and the MCP route use) and `createMcpServer()` call it once, before they serve any request.
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