steve sparks
24 of 42
ContentsPart III · Chapter 11

Chapter 11How it all fits together

Build your own

Model, prompt, harness and tools are separate, swappable parts, and what you experience is the combination.

Build your own AI system

Illustration
Good for
Not good for
Where your words go
Who approves actions

Illustration. The results are general rules of thumb written for this page, not a product review, and the model each preset picks is illustrative: each product lets you choose among models. Presets: Claude app,1 Claude Code,2 ChatGPT,3 Codex,4 Hermes Agent.5

Model thinks. Prompt informs. Harness manages. Tools act.

Go deeper: the parts in two ecosystems
Examples only, checked October 26, 2026. Both ecosystems support MCP for connecting tools and offer skills for packaging know-how.678
PartClaude ecosystemChatGPT ecosystem
Brain (models)Haiku, Sonnet, Opus and Fable; Mythos for vetted users only.910GPT-6 Luna, GPT-6.1 Sol, GPT-6 Astra.11
Briefing (prompts and context)Published system prompts; Projects; memory.121314The Model Spec's layers of instructions.15
Body (harnesses)The Claude app, Claude in Chrome, Claude Code.1162ChatGPT and Codex.34
Hands (tools)Web search, connectors (MCP), skills, code execution.1718719Web search, remote MCP servers, skills, shell, computer use.8
Notes (19)
  1. 1Anthropic, “Claude Cowork and chat are now one Claude,” Claude blog, September 16, 2026, https://claude.com/blog/cowork-is-now-claude (accessed October 26, 2026).
  2. 2Anthropic, “Claude Code overview,” Claude Code Docs, https://code.claude.com/docs/en/overview (accessed October 26, 2026).
  3. 3OpenAI, “ChatGPT: Optimizing language models for dialogue (now 'Introducing ChatGPT'),” November 30, 2022, https://openai.com/index/chatgpt/ (accessed October 26, 2026).
  4. 4OpenAI, “openai/codex,” GitHub, https://github.com/openai/codex (accessed October 26, 2026).
  5. 5Nous Research, “hermes-agent,” Nous Research (GitHub), https://github.com/NousResearch/hermes-agent (accessed October 26, 2026).
  6. 6Anthropic, “Donating the Model Context Protocol and establishing the Agentic AI Foundation,” December 9, 2025, https://www.anthropic.com/news/donating-the-model-context-protocol-and-establishing-of-the-agentic-ai-foundation (accessed October 26, 2026).
  7. 7Anthropic, “Equipping agents for the real world with Agent Skills,” Anthropic Engineering, October 16, 2025, https://www.anthropic.com/engineering/equipping-agents-for-the-real-world-with-agent-skills (accessed October 26, 2026).
  8. 8OpenAI, “Using tools,” OpenAI API docs, https://developers.openai.com/api/docs/guides/tools (accessed October 26, 2026).
  9. 9Anthropic, “Models overview,” Claude Platform Docs, https://platform.claude.com/docs/en/models/overview (accessed October 26, 2026).
  10. 10Anthropic, “Introducing Claude Fable 5.1 and Claude Mythos 5.1,” September 2026, https://www.anthropic.com/claude-fable-and-mythos-5-1 (accessed October 26, 2026).
  11. 11OpenAI, “GPT-6 Family Model Guide,” October 2, 2026, https://openai.com/index/practical-guide-building-gpt-6/ (accessed October 26, 2026).
  12. 12Anthropic, “System prompts (release notes),” Claude Platform Docs, https://platform.claude.com/docs/en/release-notes/system-prompts/overview (accessed October 26, 2026).
  13. 13Anthropic, “What are projects?” Claude Help Center, https://support.claude.com/en/articles/9517075-what-are-projects (accessed October 26, 2026).
  14. 14Anthropic, “Use Claude's chat search and memory to build on previous context,” Claude Help Center, https://support.claude.com/en/articles/11817273-use-claude-s-chat-search-and-memory-to-build-on-previous-context (accessed October 26, 2026).
  15. 15OpenAI, “OpenAI Model Spec (2025-10-27 version),” October 27, 2025, https://model-spec.openai.com/2025-10-27 (accessed October 26, 2026).
  16. 16Anthropic, “Get started with Claude in Chrome,” Claude Help Center, https://support.claude.com/en/articles/12012173-get-started-with-claude-in-chrome (accessed October 26, 2026).
  17. 18Anthropic, “Get started with custom connectors using remote MCP,” Claude Help Center, https://support.claude.com/en/articles/11175166-get-started-with-custom-connectors-using-remote-mcp (accessed October 26, 2026).
  18. 19Anthropic, “How tool use works,” Claude Platform Docs, https://platform.claude.com/docs/en/agents-and-tools/tool-use/how-tool-use-works (accessed October 26, 2026).

Chapter 11 · page 2 of 2

Turn pages with the ← and → keys.

Cite this page

https://aifromthepartsup.com/how-it-all-fits-together/2/

Using this book

© 2026 Steve Sparks. All rights reserved. You're welcome to quote short passages and cite any page, with a link back. To reuse diagrams or larger parts, ask through GitHub.

Quoted material and figures from other sources remain their owners'.