feat(MCP UI) chat widgets

[dimetron]

This pull request adds support for rendering interactive MCP UI widgets (MCP Apps) directly within the chat interface. It introduces backend and frontend changes to detect, handle, and render tools that provide UI resources, ensuring a richer and more interactive user experience. The backend now distinguishes between regular tools and those with UI widgets, compacts tool responses sent to the model to prevent unnecessary repeated calls, and exposes new endpoints for MCP-app interactions. The UI is updated to render these widgets inline and broker interactions between the app and the chat.

Backend: MCP App tool detection and handling

• The CreateToolsets function in registry.go now returns both the toolsets and a set of tool names that support MCP Apps, enabling the agent to treat these tools differently. [1] [2] [3] [4] [5] [6]
• A new callback (MakeMCPAppModelResultCallback in mcp_apps.go) is added to compact the payload sent to the model for MCP App tools, replacing heavy render payloads with a terminal notice to prevent the model from re-invoking rendering tools unnecessarily. [1] [2]
• The agent is updated to wire in the MCP App callback only when MCP App tools are present, ensuring efficient handling.

Testing and validation

• Comprehensive unit tests are added in mcp_apps_test.go to verify correct compaction of responses, error handling, and that only MCP App tools are affected by the new logic.

Documentation and design

• A detailed design document (EP-2046-chat-mcp-ui-widgets.md) describes the motivation, goals, implementation details, test plan, and open questions for this feature.

[Copilot]

Pull request overview

This PR adds end-to-end support for rendering interactive MCP UI widgets (“MCP Apps”) inline in the chat UI, including: backend endpoints to proxy MCP app tool/resource calls, agent-side toolset detection and model-payload compaction for UI-capable tools, and frontend components/context to render and broker interactions with the embedded app.

Changes:

• Backend: detect MCP App-capable tools, filter app-only tools, and compact model-visible tool results to avoid repeated render/poll loops.
• Backend: add /api/mcp-apps/... endpoints to list tools, call tools, and read ui:// resources from RemoteMCPServer.
• Frontend: render MCP apps within tool call UI; add MCP app inspector route; add (adjacent) file attachments + chat minimap UI.

Reviewed changes

Copilot reviewed 32 out of 33 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ui/src/lib/messageHandlers.ts	Preserves raw tool results + surfaces file artifacts and file parts for rendering on reload.
ui/src/lib/fileUpload.ts	Adds client-side file allowlist/size guard and conversion to A2A FilePart.
ui/src/components/ToolDisplay.tsx	Adds MCP App renderer embedding for completed tool calls with UI resources.
ui/src/components/mcp/McpServersView.tsx	Adds “MCP Apps” navigation entry for servers.
ui/src/components/mcp-apps/McpAppsInspector.tsx	Adds a UI to browse/invoke MCP App-capable tools and render their resources.
ui/src/components/mcp-apps/McpAppRenderer.tsx	Implements the sandboxed MCP app renderer and host↔app bridging.
ui/src/components/chat/ToolCallDisplay.tsx	Threads MCP app metadata + raw tool results into the tool call display pipeline.
ui/src/components/chat/FileAttachment.tsx	Renders file parts as thumbnails/download chips.
ui/src/components/chat/ChatMinimap.tsx	Adds a scroll minimap for chat history navigation.
ui/src/components/chat/ChatMessage.tsx	Renders file attachments and forwards MCP app hooks into tool-call rendering.
ui/src/components/chat/ChatMcpAppsContext.tsx	Adds chat-level registry and routing logic for MCP App-capable tools.
ui/src/components/chat/ChatInterface.tsx	Wires MCP app message/tool-call promotion, file upload UI, and minimap integration.
ui/src/components/chat/tests/ChatMcpAppsContext.test.tsx	Unit tests for app/tool registration behavior and app-only filtering.
ui/src/app/servers/[namespace]/[name]/apps/page.tsx	Adds a route for the MCP Apps inspector.
ui/src/app/api/mcp-apps/[namespace]/[name]/tools/[toolName]/call/route.ts	Next.js API proxy for MCP app tool calls.
ui/src/app/api/mcp-apps/[namespace]/[name]/resources/route.ts	Next.js API proxy for MCP app resource reads.
ui/src/app/api/mcp-apps/_utils.ts	Shared proxy helpers + OPTIONS handler for MCP apps API routes.
ui/src/app/actions/mcp-apps.ts	Server actions for listing tools, calling tools, and reading resources.
ui/src/app/actions/tests/mcp-apps.test.ts	Unit tests for MCP apps server actions path encoding and payloads.
ui/public/sandbox_proxy.html	Adds the sandbox proxy document used by the MCP app iframe renderer.
ui/public/mockServiceWorker.js	Updates MSW generated worker version.
ui/package.json	Adds MCP UI / MCP SDK dependencies.
ui/package-lock.json	Locks new MCP UI / MCP SDK dependency tree.
go/core/internal/httpserver/server.go	Adds route wiring for MCP Apps endpoints.
go/core/internal/httpserver/handlers/mcpapps.go	Implements MCP Apps list-tools/call-tool/read-resource handlers.
go/core/internal/httpserver/handlers/handlers.go	Registers the new MCPApps handler.
go/build.err	Adds a build log artifact (should not be committed).
go/adk/pkg/mcp/registry.go	Toolset creation now identifies MCP App tools and filters app-only tools.
go/adk/pkg/mcp/registry_test.go	Adds tests for MCP app detection and app-only visibility logic.
go/adk/pkg/agent/mcp_apps.go	Adds model-result compaction callback for MCP App tool renders.
go/adk/pkg/agent/mcp_apps_test.go	Tests for model-result compaction behavior.
go/adk/pkg/agent/agent.go	Wires MCP App callback only when MCP App-capable tools are present.
design/EP-2046-chat-mcp-ui-widgets.md	Design doc describing goals, approach, and test plan.

Files not reviewed (1)

• ui/package-lock.json: Generated file

Comments suppressed due to low confidence (1)

go/build.err:39

• go/build.err looks like a locally-captured build log and is now stale (it reports CreateToolsets returning 2 values while the PR updates agent.go accordingly). This file shouldn't be committed; it will also confuse CI/reviewers about the current build state. Please remove it from the PR (and rely on CI logs instead).

# github.com/kagent-dev/kagent/go/adk/pkg/agent
adk/pkg/agent/agent.go:58:14: assignment mismatch: 1 variable but mcp.CreateToolsets returns 2 values

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[dimetron]