# Composable Mini Apps > Build many focused workspace apps that compose through agent discovery and A2A. Use when designing headless mini-apps or cross-app workflows. - Skill: `builderio/composable-mini-apps` (Agent Skill) - Install (CLI): `npx skillmds@latest add builderio/composable-mini-apps` - Raw SKILL.md: https://api.skillmd.com/api/skills/builderio/composable-mini-apps/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Product & Planning - Author: Builder.io (https://skillmd.com/u/builderio) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/builderio/composable-mini-apps --- # Composable Mini-Apps ## Rule Prefer many one-job apps in a workspace over one oversized app. A headless app can own a provider, dataset, workflow, or specialist action surface without a full UI; the main agent composes those apps through discovery and A2A. ## Shape - Give each mini-app one clear job, a concise `package.json` description, and action names that describe the job it owns. - Keep provider credentials and upstream API details in the app that owns that provider or workflow. Other apps should delegate to it instead of copying its integration code. - Use a tiny status/config screen only when users need to inspect state. A pure headless app is fine when its job is invoked by agents, automations, or sibling apps. - If two workflows only share a helper, put the helper in `packages/shared`; keep the workflow actions in separate apps. ## Discovery And Invocation The main agent should discover available siblings before assuming capability: - Runtime agents receive an `` block built from `discoverAgents()`. Workspace siblings are layered in by `discoverWorkspaceAgents()`. It carries one line per app — enough to know a sibling exists, not enough to know what it can do. - Use the built-in `describe-workspace-apps` tool for actual capability. It reads each peer's live `/.well-known/agent-card.json` and returns its purpose plus any optional stable machine contracts; pass `app: ""` for one peer's full description. Call it before building something a sibling may already own, before telling a user what is or is not possible across apps, and whenever someone asks which app to use for a job. - Never hand-maintain a markdown or code list of what each workspace app does. A stale catalog is worse than none: it reads as authoritative while pointing at capabilities that moved or vanished. An app's purpose belongs in its own `package.json` `description` (which flows into the workspace manifest and the `` block), and its capabilities belong in its exposed actions — both of which `describe-workspace-apps` reads live. - UI shells, headless surfaces, and scripts can read the same registry through `GET /_agent-native/agents?selfAppId=`. - Code or CLI callers should use the first-class message-based A2A invocation path (`invokeAgent()` / `agent-native invoke`) when they need to ask an app by id, name, or URL. - In the agent loop, use `call-agent` with the sibling app id when another app owns the work or data. Never call the current app through `call-agent`; use local actions instead. - Send a natural-language objective by default so the sibling can apply its own instructions, skills, schemas, data dictionary, credentials, and tools. `invokeAgentAction()` or `call-agent` with `action` + `input` is only for an explicit stable semantic read contract whose complete input is already known. Never expose or call an implementation action as a workaround for slow or failed message delegation. Send narrow prompts to siblings: name the exact question, relevant ids, date ranges, and expected output shape. Preserve returned ids and URLs verbatim. ## Artifact Handoff Mini-apps should hand off compact artifacts, not giant pasted transcripts or provider dumps. When a mini-app creates something another app may use, return or store an artifact with: - `artifactType` - what kind of output this is, such as `deal-set`, `call-evidence`, `brief`, `dashboard`, or `report`. - `artifactId` - the stable app-owned id, file path, or resource id. - `createdAt` - an ISO timestamp. - `source` - provider/app/source ids used to create it. - `summary` - a short human-readable explanation. - `items` or `records` - the bounded structured data downstream apps need. - `links` - fully qualified URLs for user-visible artifacts. Downstream apps should receive artifact ids, URLs, and narrow follow-up questions. If a downstream app needs more detail, it should call back to the artifact-owning app instead of asking the orchestrator to paste the whole corpus into a prompt. Example: `hubspot-pipeline` returns `{ artifactType: "deal-set", artifactId: "hubspot-pipeline:deal-set:2026-06-18" }`. `deal-brief` passes that id to `gong-evidence`, which returns a `call-evidence` artifact id and URLs. `deal-brief` then synthesizes the final brief from the artifact ids and bounded summaries. ## Provider APIs Provider-specific actions are shortcuts, not limits. When the upstream API can answer the question better than a first-class shortcut, call `provider-api-catalog` and `provider-api-docs` as needed, then `provider-api-request` against the real provider endpoint. For broad joins, searches, or absence claims, stage the bounded corpus with `stageAs` and reduce it with `query-staged-dataset` or code. When composing apps, make the provider-owning mini-app do those `provider-api-request` calls. The orchestrator should delegate a bounded job; it should not reimplement every provider endpoint locally. ## Example For a sales-intelligence workspace, split the job into small apps: | App | Owns | Calls | | --- | --- | --- | | `hubspot-pipeline` | CRM deals, contacts, companies, associations | `provider-api-request` with provider `hubspot` | | `gong-evidence` | Calls, transcripts, snippets, speaker evidence | `provider-api-request` with provider `gong` | | `knowledge-base` | Internal docs, pricing rules, playbooks | local search/read actions | | `deal-brief` | Orchestration and final brief | message-based `invokeAgent()` or `call-agent` to the three apps | Flow: `deal-brief` asks `hubspot-pipeline` for the target account and open deals, asks `gong-evidence` for recent transcript evidence about those deals, asks `knowledge-base` for relevant playbook guidance, then synthesizes the answer. That is a HubSpot→Gong→knowledge-base chain made of focused apps, not a single app that clones every provider integration. ## Don't - Do not clone Mail, Calendar, Analytics, Brain, Assets, or another first-party app just to reuse its data. Delegate or link to the existing app. - Do not hide a multi-provider workflow inside a giant "misc tools" app. - Do not add one-off provider endpoints when `provider-api-request` can express the upstream API safely. - Do not create wrapper routes that only re-export another app's action or A2A result. ## Related Skills - **a2a-protocol** - How apps expose and call A2A endpoints. - **actions** - How each mini-app exposes its own operation surface. - **external-agents** - How external MCP hosts route through workspace apps. - **storing-data** - How app-owned data stays SQL-backed and portable.