Building on the Codex Harness — Dominik Kundel, OpenAI

Instead of building a harness, you can embed the Codex App Server and control instructions, tools, models and sandboxing per thread. Users can sign in with their ChatGPT subscription.

• The Codex App Server is an open-source JSON-RPC protocol wrapping the Codex harness. It powers the Codex app, the IDE extensions and third-party clients in Xcode and JetBrains, and is not tied to one model provider.
• Kundel says clients get more than 120 messages to work with, plus developer instructions layered on top of the system prompt and dynamic and deferred tools.
• To integrate it you ship the binary and generate bindings or use the Python SDK; the talk also covers when the simpler codex exec is the better fit.

• agent harness — The scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.
• sandbox — An isolated environment where AI-generated code or agent actions run without being able to touch anything real.
• system prompt — The standing instructions a model receives before any user input — defining its role, rules, tools, and tone for the whole conversation.

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• videoCodex, Behind the Harness — Dominik Kundel, OpenAIAI Engineer
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• videoGetting started with CodexOpenAI

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