docs: Add plugin launch flow design document (#13084)

Co-authored-by: openhands <openhands@all-hands.dev> Co-authored-by: Engel Nyst <engel.nyst@gmail.com>
2026-03-22 05:37:20 +08:00 · 2026-02-26 14:03:40 -05:00
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+# Plugin Launch Flow
+
+This document describes how plugins are launched in OpenHands Saas / Enterprise, from the plugin directory through to agent execution.
+
+## Architecture Overview
+
+```
+Plugin Directory ──▶ Frontend /launch ──▶ App Server ──▶ Agent Server ──▶ SDK
+    (external)           (modal)           (API)        (in sandbox)    (plugin loading)
+```
+
+| Component | Responsibility |
+|-----------|---------------|
+| **Plugin Directory** | Index plugins, present to user, construct launch URLs |
+| **Frontend** | Display confirmation modal, collect parameters, call API |
+| **App Server** | Validate request, pass plugin specs to agent server |
+| **Agent Server** | Run inside sandbox, delegate plugin loading to SDK |
+| **SDK** | Fetch plugins, load contents, merge skills/hooks/MCP into agent |
+
+## User Experience
+
+### Plugin Directory
+
+The plugin directory presents users with a catalog of available plugins. For each plugin, users see:
+- Plugin name and description (from `plugin.json`)
+- Author and version information
+- A "Launch" button
+
+When a user clicks "Launch", the plugin directory:
+1. Reads the plugin's `entry_command` to know which slash command to invoke
+2. Determines what parameters the plugin accepts (if any)
+3. Redirects to OpenHands with this information encoded in the URL
+
+### Parameter Collection
+
+If a plugin requires user input (API keys, configuration values, etc.), the frontend displays a form modal before starting the conversation. Parameters are passed in the launch URL and rendered as form fields based on their type:
+
+- **String values** → Text input
+- **Number values** → Number input
+- **Boolean values** → Checkbox
+
+Only primitive types are supported. Complex types (arrays, objects) are not currently supported for parameter input.
+
+The user fills in required values, then clicks "Start Conversation" to proceed.
+
+## Launch Flow
+
+1. **Plugin Directory** (external) constructs a launch URL to the OpenHands app server when user clicks "Launch":
+   ```
+   /launch?plugins=BASE64_JSON&message=/city-weather:now%20Tokyo
+   ```
+
+   The `plugins` parameter includes any parameter definitions with default values:
+   ```json
+   [{
+     "source": "github:owner/repo",
+     "repo_path": "plugins/my-plugin",
+     "parameters": {"api_key": "", "timeout": 30, "debug": false}
+   }]
+   ```
+
+2. **OpenHands Frontend** (`/launch` route, [PR #12699](https://github.com/OpenHands/OpenHands/pull/12699)) displays modal with parameter form, collects user input
+
+3. **OpenHands App Server** ([PR #12338](https://github.com/OpenHands/OpenHands/pull/12338)) receives the API call:
+   ```
+   POST /api/v1/app-conversations
+   {
+     "plugins": [{"source": "github:owner/repo", "repo_path": "plugins/city-weather"}],
+     "initial_message": {"content": [{"type": "text", "text": "/city-weather:now Tokyo"}]}
+   }
+   ```
+
+   Call stack:
+   - `AppConversationRouter` receives request with `PluginSpec` list
+   - `LiveStatusAppConversationService._finalize_conversation_request()` converts `PluginSpec` → `PluginSource`
+   - Creates `StartConversationRequest(plugins=sdk_plugins, ...)` and sends to agent server
+
+4. **Agent Server** (inside sandbox, [SDK PR #1651](https://github.com/OpenHands/software-agent-sdk/pull/1651)) stores specs, defers loading:
+
+   Call stack:
+   - `ConversationService.start_conversation()` receives `StartConversationRequest`
+   - Creates `StoredConversation` with plugin specs
+   - Creates `LocalConversation(plugins=request.plugins, ...)`
+   - Plugin loading deferred until first `run()` or `send_message()`
+
+5. **SDK** fetches and loads plugins on first use:
+
+   Call stack:
+   - `LocalConversation._ensure_plugins_loaded()` triggered by first message
+   - For each plugin spec:
+     - `Plugin.fetch(source, ref, repo_path)` → clones/caches git repo
+     - `Plugin.load(path)` → parses `plugin.json`, loads commands/skills/hooks
+     - `plugin.add_skills_to(context)` → merges skills into agent
+     - `plugin.add_mcp_config_to(config)` → merges MCP servers
+
+6. **Agent** receives message, `/city-weather:now` triggers the skill
+
+## Key Design Decisions
+
+### Plugin Loading in Sandbox
+
+Plugins load **inside the sandbox** because:
+- Plugin hooks and scripts need isolated execution
+- MCP servers run inside the sandbox
+- Skills may reference sandbox filesystem
+
+### Entry Command Handling
+
+The `entry_command` field in `plugin.json` allows plugin authors to declare a default command:
+
+```json
+{
+  "name": "city-weather",
+  "entry_command": "now"
+}
+```
+
+This flows through the system:
+1. Plugin author declares `entry_command` in plugin.json
+2. Plugin directory reads it when indexing
+3. Plugin directory includes `/city-weather:now` in the launch URL's `message` parameter
+4. Message passes through to agent as `initial_message`
+
+The SDK exposes this field but does not auto-invoke it—callers control the initial message.
+
+## Related
+
+- [OpenHands PR #12338](https://github.com/OpenHands/OpenHands/pull/12338) - App server plugin support
+- [OpenHands PR #12699](https://github.com/OpenHands/OpenHands/pull/12699) - Frontend `/launch` route
+- [SDK PR #1651](https://github.com/OpenHands/software-agent-sdk/pull/1651) - Agent server plugin loading
+- [SDK PR #1647](https://github.com/OpenHands/software-agent-sdk/pull/1647) - Plugin.fetch() for remote plugin fetching