Runtime Context, Compaction, Replay

Understand how Codex keeps a long conversation coherent.

A source-pinned technical book about context as runtime state: turn envelopes, history normalization, typed injections, optional-context budgets, compaction, rollout reconstruction, rollback, fork, realtime surfaces, and token telemetry.

Start reading Source atlas

Codex context management as a pipeline from ledger, turn envelope, and fragments to projection, clients, compaction, and replay — Codex keeps context coherent by separating the durable ledger, turn envelope, typed fragments, model projection, client views, compaction, and replay.

Boundary

Start with context as runtime state and the turn envelope that freezes one unit of work.

Projection

Then read history normalization, typed fragments, and optional-context budgets as prompt-shaping owners.

Survival

Study compaction, replacement history, rollout reconstruction, rollback, and fork as recovery mechanics.

Exposure

Finish with TUI, realtime, app-server, token usage, and trace as downstream views.

Runtime-first

Context is treated as governed runtime state, not as a growing prompt string.

Checkpointed

Compaction installs replacement history with enough evidence to resume and audit later.

Source-pinned

All source links point to 569ff6a1c400bd514ff79f5f1050a684dc3afde3, so the book can be audited against a fixed tree.

Part 1

Define the Context Contract

Context is not text around a model. It is the runtime boundary every turn must obey.

1 Context Is a Runtime Boundary The thesis: Codex governs context as runtime state, not as a text buffer. 2 TurnContext: The Envelope Around a Turn How model identity, policy, tools, hooks, and workspace facts become one turn envelope.

Part 2

Shape Prompt State

A prompt is assembled from ledgers, fragments, policies, and optional evidence.

3 ContextManager: History as Prompt-Ready State How the history ledger stores, normalizes, trims, and prepares model-visible items. 4 Typed Fragments and Settings Diffs How Codex injects changing runtime facts without repeating the entire world. 5 Budgeting Optional Context Skills, plugins, memory, tool output, and images as budgeted context planes.

Part 3

Survive Long Threads

A long-lived agent survives only if forgetting is explicit, auditable, and reversible enough.

6 Compaction as a Checkpoint Protocol Local and remote compaction as governed replacement history, not lossy summarization. 7 Resume, Rollback, Fork, and Replay How rollout evidence reconstructs prompt state after time, rollback, and branching.

Part 4

Expose Context Without Losing Control

Clients may render context, but the runtime must remain the source of truth.

8 Client-Facing Context How TUI, realtime, app-server, token usage, and trace surfaces expose context safely.

E Epilogue: The Context Discipline Transferable lessons for any agent runtime that must stay coherent over long work.

Reference

Source Atlas Pinned source anchors for the context-management book.