Building Effective MCP Apps: Data-First Design

The Data-First Architecture

Building effective Model Context Protocol (MCP) applications requires prioritizing data availability over UI presentation. When a UI widget is tightly coupled to a search tool, the model often stops exploring after a single call because it perceives the results as already displayed. To maintain the model's reasoning capabilities, developers must separate data processing from UI rendering. This involves creating a standard, text-based search tool that the model can invoke multiple times to filter and analyze data, followed by a separate, dedicated render tool that accepts specific IDs or data payloads to display the final selection to the user.

Maintaining Model Context

To avoid the "black box" problem where the model remains unaware of what the user is seeing or interacting with, developers must treat the UI as a data-driven extension of the model's context.

• Sync Data and Display: Every piece of information rendered in the UI must be simultaneously provided to the model as structured data. If the UI displays a job listing, the model needs the underlying metadata to answer follow-up questions about that specific job.
• Explicit Tool Descriptions: Update tool descriptions to explicitly inform the model that a UI component exists. This prevents the model from redundantly narrating results that are already visible in the widget.
• Feedback Loops: Use updateModelContext to push user interactions back into the model's state. If a user clicks a "View Details" button, the model must receive that event to understand which specific item is currently being examined.
• Composable Tooling: Design small, composable tools rather than monolithic ones. By separating search logic from rendering logic, the model gains the flexibility to perform complex multi-step filtering and then choose exactly which subset of results to pass to the render tool, potentially even injecting its own reasoning for why a specific result is a good fit.