Understand and apply how the Codex agent loop works, including request assembly (`instructions`, `tools`, `input`), thread/turn/item mental models, tool iteration, prompt growth, caching, compaction, and where `model_instructions_file` and `AGENTS.md` enter. Keep the local references for the durable conceptual model, but fetch current official docs via `openaiDeveloperDocs` for exact config, prompt-loading, and other evolving Codex behavior.
Understand and apply how the Codex agent loop works, including request assembly (`instructions`, `tools`, `input`), thread/turn/item mental models, tool iteration, prompt growth, caching, compaction, and where `model_instructions_file` and `AGENTS.md` enter. Keep the local references for the durable conceptual model, but fetch current official docs via `openaiDeveloperDocs` for exact config, prompt-loading, and other evolving Codex behavior.
Codex Agent Loop
Use this skill when you need the durable mental model for how Codex actually runs: how the initial request is assembled, how inference and tool calls loop within a turn, how conversation state grows across turns, and where model_instructions_file, AGENTS.md, and local guidance enter.
Read Order
Read references/unrolling-the-codex-agent-loop.md for the end-to-end model of threads, turns, items, and tool iteration.
Read references/building-the-initial-prompt.md for the first request, prompt item ordering, tool definitions, and SSE event flow.
Read references/conversation-growth-and-performance.md for prompt growth, exact-prefix caching, statelessness, previous_response_id, ZDR, and compaction.
Read references/openai-codex-prompt-loading.md only when exact model_instructions_file, AGENTS.md, or project config semantics matter.
Read references/prompt-layering.md only when deciding what belongs in the base prompt versus local guidance or mutable context.
Core Model
A thread is the durable conversation container.
A turn begins with a new user request and may contain many inference and tool-call iterations before it ends.
The Responses API request surface is structured as instructions, tools, and input; the server derives the actual prompt shape from those parts.
Tool calls are part of the normal loop, and their outputs are appended back into later input items.
Stable early prompt content matters because it affects both behavior and prompt caching efficiency.
Use This Skill Well
Start with the conceptual reference before reading the lower-level details.
If the question is narrow, load only the specific reference that answers it.
Keep the distinction between base instructions, later input, and appended tool outputs explicit.
Preserve the blog's mental model: a turn is not a single model call, and the loop does not end until the assistant emits a final message for that turn.
Use the embedded SVGs in the references when you want the original diagrams in context.
Reach for exact docs via MCP when configuration or current product behavior matters.
Reference Policy
Treat references/unrolling-the-codex-agent-loop.md as the main reusable explanation another agent should remember.
Treat references/building-the-initial-prompt.md and references/conversation-growth-and-performance.md as the detailed mechanics that preserve most of the blog's substance.
Fetch official docs via openaiDeveloperDocs before answering questions about exact current prompt loading, AGENTS.md discovery, fallback filenames, byte limits, config keys, or other evolving behavior.
Treat references/openai-codex-prompt-loading.md as a local helper summary of official guidance, not a replacement for checking current docs when precision matters.
Fetch First For
model_instructions_file and config-key semantics
AGENTS.md discovery order, fallback filenames, and byte limits
project-scoped .codex/config.toml behavior
current Codex config reference details
any question phrased as latest, current, official, exact, or up to date
