One-Shot Agent Management UI with DeepSeek Harness for $0.19

Melvin Vivas · X post · 2026-09-13 · Open on X

Topics: AI Agents, Tool Use & MCP, Prompt & Context Engineering, AI Dev Tools & Productivity · Level: intermediate

Summary

The creator shares the exact prompt he used in DeepSeek Harness with DeepSeek V4.1 Flash (High) to build a 'software factory' UI for managing agents. The run used 231 tool calls and 27.4M tokens and cost only $0.19. The prompt asks for a UI-first approach with React Flow, mock APIs and no database, starting from a list of MVP features.

Key points

• Model: DeepSeek V4.1 Flash (High) in DeepSeek Harness.
• Run stats: 231 tool calls, 27.4M tokens, $0.19 total cost via the DeepSeek API.
• Prompt strategy: build the UI first, mock the APIs, and add no database yet so you can refine the UI quickly.
• Ask the agent to list the features needed for the initial MVP before building.
• Use React Flow for node/graph-based UIs such as agent workflows.
• The app idea: manage agents that connect to sandboxes (a 'software factory').

Resources mentioned

• DeepSeek Harness · repo · github.com · free
  Open-source (MIT) agent harness with plugins, a trajectory/event-stream view and support for any model.
  Also in: DeepSeek Harness desktop app released for macOS and Windows (Melvin Vivas on X · notes), Getting Started with DeepSeek Harness and the DeepSeek API Platform (Melvin Vivas on X · notes), DeepSeek Harness: Open-Source, Browser-Based Agent for DeepSeek V4.1 (Melvin Vivas on X · notes), DeepSeek harness: open-source agent with an inspectable trajectory view (Melvin Vivas on X · notes)
• DeepSeek V4.1 Flash · tool · api-docs.deepseek.com · paid
  A fast DeepSeek language model available through DeepSeek's official API.
  Also in: Using Hugging Face credits: Jobs, Inference Endpoints and Open Models (Melvin Vivas on X · notes), DeepSeek 4.1 Flash speed on the official API: about 325 tokens/s (Melvin Vivas on X · notes), DeepSeek Harness: Open-Source, Browser-Based Agent for DeepSeek V4.1 (Melvin Vivas on X · notes), DeepSeek V4.1 Flash Off-Peak Pricing as a Cheap Fallback Model (Melvin Vivas on X · notes) and 1 more
• React Flow · tool · reactflow.dev · free
  React library for building node-based graph and workflow UIs.

Try this

• Use a UI-first prompt with mock APIs and no database to iterate quickly on an MVP.
• Have the agent list MVP features before it starts building.
• Build a 'software factory' UI with React Flow that lets teams manage coding agents connected to sandboxes, starting with mock APIs.