{"slug":"add-capabilities","title":"add-capabilities","summary":"Expose an app operation as a typed pracht capability — one contract projected into direct server calls, an HTTP endpoint, a route-scoped WebMCP page tool, and a remote MCP tool — plus `defineApp({ agents })` trust config, typed clients, `
`, and `pracht eval` scen","platform":"Claude","tags":[],"authorName":"LLM Mart","authorSlug":"llm-mart","score":0,"source":"github","price":null,"verified":false,"createdAt":"2026-10-19T21:17:22.054969Z","repo":{"url":"https://github.com/JoviDeCroock/pracht","stars":99,"forks":5,"license":"MIT","updatedAt":"2026-10-19T16:43:31Z"},"bodyHtml":"
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name: add-capabilities\nversion: 1.3.0\ndescription: |\nExpose an app operation as a typed pracht capability — one contract projected\ninto direct server calls, an HTTP endpoint, a route-scoped WebMCP page tool, and a remote MCP\ntool — plus defineApp({ agents }) trust config, typed clients,\n<Form capability>, and pracht eval scenarios.\nUse for \"add a capability\", \"expose this to agents\", \"add an MCP tool\", \"add\nWebMCP\", \"serve remote MCP\", \"make my app agent-callable\".\nallowed-tools:

\n\n
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Pracht Add Capabilities

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A capability is a protocol-neutral operation. Every\nprojection runs the identical pipeline, so rules never diverge per transport:

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input validation → named middleware chain → run() → output validation\n
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Registration is opt-in and private by default; loaders and API routes are never\ninferred as capabilities, and an app with none ships no dispatch surface.

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Non-Pracht app: use createCapabilityHost() and the signal-owned WebMCP\nregistrar. See https://pracht.resynapse.dev/docs/standalone-capabilities.

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Step 1: Decide the contract before writing code

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Settle these with AskUserQuestion when the request is vague:

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Exposure matrix the runtime, defineCapability(), and pracht verify all\nenforce:

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Effecthttpwebmcpmcp
read / writeyesyes (needs http)yes (needs agents.mcp)
destructiveyes — always confirmation-gatedrejectedyes — needs agents.mcp.destructive and a registered approval store
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Step 2: Install and scaffold

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create-pracht does not add the package, because an app without capabilities\nshould not carry it:

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npm install @pracht/capabilities\npracht generate capability --name notes.search --effect read --expose http,webmcp \\\n  --description \"Find notes whose title or body matches the query.\"\n
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The generator writes and registers src/capabilities/notes-search.ts.\n--description is required with --expose; the MCP generate_capability tool\nhas the same contract.

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If dispatch answers 500 internal_error and pracht inspect capabilities\nprints capabilities as unreadable, the package is missing — that is the\nsymptom.

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Step 3: Write the capability

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// src/capabilities/notes-search.ts\nimport { defineCapability, type CapabilityRunArgs } from \"@pracht/capabilities\";\nimport { searchNotes } from \"../server/notes-store.ts\";\n\ninterface SearchInput {\n  query: string;\n  limit: number;\n}\n\nexport default defineCapability({\n  title: \"Search notes\",\n  description: \"Find notes whose title or body matches the query.\",\n  input: {\n    type: \"object\",\n    properties: {\n      query: { type: \"string\", minLength: 1 },\n      limit: { type: \"integer\", minimum: 1, maximum: 20, default: 10 },\n    },\n    required: [\"query\"],\n    additionalProperties: false,\n  },\n  output: {\n    type: \"object\",\n    properties: { notes: { type: \"array\", items: { type: \"object\" } } },\n    required: [\"notes\"],\n  },\n  effect: \"read\",\n  middleware: [\"auth\"],            // names from the app manifest\n  expose: { http: true, webmcp: true },\n  // webmcp: { untrustedContent: true } advertises untrustedContentHint for\n  // page tools whose results carry user-generated content.\n  // agentPolicy: \"require\",       // verified Web Bot Auth agents only —\n  //                               // never combine with webmcp: page-tool\n  //                               // calls are unsigned and would always 401\n  async run({ input, context, request, signal }: CapabilityRunArgs<SearchInput>) {\n    return { notes: searchNotes(input.query, input.limit) };\n  },\n});\n
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Schema rules that bite:

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Step 4: Register it, and configure agents only if needed

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// src/routes.ts\nimport { defineApp, route } from \"@pracht/core\";\n\nexport const app = defineApp({\n  capabilities: {\n    \"notes.search\": () => import(\"./capabilities/notes-search.ts\"),\n  },\n  agents: {\n    // Verified agent identity (public keys — safe in the manifest).\n    webBotAuth: { policy: \"observe\", directories: [\"https://signature-agent.cloudflare.com\"] },\n    // Destructive prepare/commit tuning.\n    confirmation: { ttlSeconds: 120 },\n    // Remote MCP endpoint; without this, `expose.mcp` serves nothing.\n    mcp: { serverInfo: { name: \"notes\", version: \"1.0.0\" }, instructions: \"…\" },\n  },\n  routes: [\n    route(\"/notes\", \"./routes/notes.tsx\", {\n      // expose.webmcp makes it eligible; this makes it active on this page.\n      capabilities: [\"notes.search\"],\n    }),\n  ],\n});\n
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Pages apps have no manifest: the same capabilities come from\nsrc/capabilities/ and the same agents object is export const agents in\nsrc/pages/_app.config.ts. Activate WebMCP tools on each page that needs them:

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// src/pages/notes.tsx\nexport const CAPABILITIES = [\"notes.search\"];\n
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CAPABILITIES must be an inline array of non-empty registered names and cannot\nappear on _app or 404. In a manifest, group capability lists are additive.

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Unknown names, capabilities without expose.webmcp, and activation on\nhydration: \"none\" routes are rejected. Initial hydration registers the matched\nroute's set; every committed client navigation replaces it, so never assume a\ntool exposed on one page persists globally.

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In dev, the tab's pracht_page_tools WebMCP tool lists the tools active on\nthe route and why a declared capability is not one.

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Each agents sub-option is independent — add only what the app uses. Web Bot\nAuth policy: \"require\" gates capability HTTP endpoints (not pages or API\nroutes) with 401 agent_required; agentPolicy: \"require\" on a capability\nfails closed even when webBotAuth is unconfigured.

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Authenticating the MCP endpoint (agents.mcp.auth)

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agents: { mcp: {} } alone serves an open endpoint — anyone who can reach\nthe URL can call every expose.mcp tool, and authentication is whatever the\ncapability's named middleware does with the forwarded Authorization header.\nThat is fine for a public read surface and wrong for anything scoped to a user.

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Add auth and /mcp becomes an OAuth 2.0 protected resource: pracht publishes\nRFC 9728 metadata at /.well-known/oauth-protected-resource, answers\nunauthenticated calls with the WWW-Authenticate challenge MCP hosts follow,\nand calls your verify module. This is what makes a real host (Claude, a\nChatGPT connector) able to connect at all.

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mcp: {\n  serverInfo: { name: \"notes\", version: \"1.0.0\" },\n  auth: {\n    resource: \"https://app.example.com/mcp\",       // absolute; token audience\n    authorizationServers: [\"https://auth.example.com\"],\n    scopesSupported: [\"notes.read\", \"notes.write\"],\n    requiredScopes: [\"notes.read\"],                // optional per-request gate\n    verify: () => import(\"./server/mcp-token.ts\"), // server-only module\n  },\n},\n
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Rules to hold the user to:

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See https://pracht.resynapse.dev/docs/capabilities for the metadata document and the verify recipe.

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Step 5: Destructive capabilities

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destructive (delete, publish, pay, send, change access) may be exposed over\nhttp and mcp, never webmcp, and every dispatch is gated:

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    \n
  1. Set PRACHT_CONFIRMATION_SECRET in the server environment (or call\nsetCapabilityConfirmationSecret() from @pracht/core/server). Without it,\ncalls fail closed with 403 confirmation_unavailable and pracht verify\nfails — verify reads the environment, so the variable must be set even when\nthe app registers the secret programmatically.
    2. \n
  2. A call without a token answers 409 confirmation_required with a token\nbound to principal + capability + canonical input + expiry.
    3. \n
  3. The commit repeats the call with byte-identical input plus the confirmation\nheader.
    4. \n
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Be honest about what this buys, and say so to the user\n(https://pracht.resynapse.dev/docs/agent-trust): stateless HMAC cannot prevent replay inside the TTL,\nthe calling agent can hand the token back to itself, and without Web Bot Auth\nor setCapabilityApprovalPrincipalResolver() both phases run as \"anonymous\".\nRegister a CapabilityApprovalStore for exactly-once commits, and\nconfirmation: { mode: \"human\" } for a real human decision — that mode fails\nclosed without both a store and an end-user identity (a principal resolver, or\nagents.mcp.auth on remote MCP); a Web Bot Auth key alone is refused.

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createSqlApprovalStore({ execute }) from @pracht/core/server is the\nfirst-party durable store — one implementation for Postgres, Cloudflare D1, and\nSQLite/Turso. Pass a parameterized-query function and run the migration from\nthe Agent Trust page; use dialect: \"postgres\" for $1 placeholders.\ncreateMemoryApprovalStore() is for tests only. A non-SQL backend needs atomic\nconditional writes (Durable Objects, Redis — not Cloudflare KV).

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Destructive over remote MCP

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Off by default. To serve one:

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    \n
  1. agents: { mcp: { destructive: true } } in defineApp().
    2. \n
  2. Register an approval store from a server entry or a capability module, so it\nexists before the graph is served — a token handed to the committing agent\nmust be consumable exactly once. The endpoint refuses to serve at all when\nthe store, PRACHT_CONFIRMATION_SECRET, or (in human mode) any resolvable\nprincipal is missing; pracht verify warns when it cannot find the\nregistration in the configured source directories.
    3. \n
  3. The flow is unchanged; only the channel differs. Prepare answers\nisError: true with the token in _meta[\"io.pracht/error\"], and the commit\nrepeats tools/call with identical arguments plus\n_meta[\"io.pracht/confirmation\"].
    4. \n
\n

Nested invokeCapability() under an MCP tool still refuses destructive callees\nunless the served tool is itself a destructive capability that already cleared\nprepare/commit.

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Step 6: Call it

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// Server: loaders, API routes, middleware — works for private capabilities too.\nimport { invokeCapability } from \"@pracht/core/server\";\nconst result = await invokeCapability(\"notes.search\", { query: \"roadmap\" }, { request, context, signal });\n
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// Browser: generated, typed, http-exposed names only.\nimport { capabilities, useCapability } from \"virtual:pracht/capabilities\";\nconst result = await capabilities.notes.search({ query: \"roadmap\" });\n
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Unresolvable in TS? Add \"@pracht/vite-plugin/virtual\" to tsconfig types.

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// One contract for the human form and the agent tool.\n<Form capability=\"notes.create\" onCapabilityResult={(result) => { /* … */ }}>\n  <input name=\"title\" />\n  <button type=\"submit\">Create</button>\n</Form>\n
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Step 7: Types, inspection, and proof

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pracht typegen                 # emits src/pracht-capabilities.d.ts\npracht inspect capabilities --json\npracht verify --json           # contract, exposure, and projection checks\npracht eval --start \"pracht preview\"\n
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Once the declaration exists the compiler rejects unknown names, bad input,\nbrowser calls to private capabilities, destructive calls without\nprepare/confirm, and computed names (assert as HttpCapabilityName).\nRe-run pracht typegen --check in CI.

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pracht eval runs JSON scenarios against the live app and exits 1 on a failed\nexpectation. Steps can reference earlier results\n($steps[0].error.confirmationToken); a scenario-level signAs block signs\nevery step as a verified agent.

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A scenario targets HTTP by default; scenario-level \"transport\": \"mcp\" runs\nthe same steps over the remote MCP endpoint (initialize, then one\ntools/call per step, names mapped notes.search → notes_search). Write one\nof each for any expose.mcp capability — passing over HTTP does not prove an\nMCP host can reach it.\nIf agents.mcp.auth protects the endpoint, add scenario-level\n\"mcpHeaders\": { \"authorization\": \"Bearer …\" }; it applies to initialize and\nevery later request, and step-level headers.authorization overrides it for\none call. Inject test tokens in CI. Expectations are portable: expect.status\nis the capability dispatch status on both transports.

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Three MCP limits fail loudly: a step for a capability without expose.mcp, a\nstep header other than authorization (the projection forwards nothing else),\nand a destructive step whose app has not enabled agents.mcp.destructive with\nan approval store. For an exposed destructive MCP tool, confirm completes the\nsame prepare/commit round trip as HTTP, with the token in the call's\n_meta[\"io.pracht/confirmation\"].

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createCapabilityTestHost() from @pracht/core covers the same pipeline in\nunit tests, without a server.

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WebMCP: names must fit the draft's 1–128 ASCII [a-zA-Z0-9_.-] grammar;\nChrome advises 30 chars/name, 500/tool description, 150/parameter description,\nand 1.5K/result. pracht verify checks static limits; bound outputs yourself.\nStable Chrome needs an origin-trial token. No mainstream production agent\nconsumes these tools yet, so retain HTTP or remote MCP. See the capabilities\nsite page for current compatibility. An effective agentPolicy: \"require\"\nalways 401s unsigned page-tool calls and is warned.

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To audit what the whole agent surface exposes, run /audit-agent-surface.

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Rules

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    \n
  1. Never expose a destructive capability over webmcp; expose it over mcp\nonly with agents.mcp.destructive and a durable approval store, and say so\nto the user. Never reclassify a destructive operation as write to escape\nthe confirmation gate.
    2. \n
  2. Never widen a schema (drop required, open additionalProperties, raise a\nmaximum) without saying so — pracht plan reports it as a widening of the\nagent-reachable surface for a reason.
    3. \n
  3. Keep expose and effect inline; reuse a Standard JSON Schema validator for\ninput/output when the app already has one.
    4. \n
  4. Treat expose.webmcp as eligibility, not activation. Add the capability to\nonly the manifest routes/groups or Pages CAPABILITIES exports where an\nin-page agent should see it.
    5. \n
  5. Put authentication, authorization, and rate limiting in named middleware —\nthe framework ships no rate limiting, no write-idempotency helper, and no\nresult-size budget. Bound outputs with a limit input and a schema\nmaximum.
    6. \n
  6. Design write inputs to be safely repeatable; agents retry, and only\ndestructive calls are token-gated.
    7. \n
  7. Never register an app-wide approval endpoint or UI without your own\nauthorization — who may approve is an application decision.
    8. \n
  8. Re-run pracht typegen after changing a schema, name, exposure, or route activation, and\npracht verify before committing.
    9. \n
\n

$ARGUMENTS

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