Dominik Kundel introduces the Codex app server as a protocol around the open-source Codex harness. A live inspector shows messages for threads, configuration and streamed output, illustrating how a custom client can access capabilities used by existing Codex interfaces.
Customisation centres on adding developer instructions and dynamic tools, including deferred tool discovery, rather than replacing the underlying system prompt. Dominik Kundel explains packaging the harness with a client, generating bindings for its version, and choosing standard input/output or WebSocket transport.
A Doom-based interface demonstrates an unconventional client that connects game controls and local shell work to the same agent. Dominik Kundel distinguishes the app server's richer thread control from non-interactive Codex execution, and recommends version pinning, deferred tools and careful instruction extensions.
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