# docs/reference/encyclopedia.md · stienswout/t3code

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```
# Encyclopedia

This is a living glossary for T3 Code. It explains what common terms mean in this codebase.

## Table of contents

- [Project and workspace](#project-and-workspace)
- [Thread timeline](#thread-timeline)
- [Orchestration](#orchestration)
- [Provider runtime](#provider-runtime)
- [Checkpointing](#checkpointing)

## Concepts

### Project and workspace

#### Project

The top-level workspace record in the app. In [the orchestration contracts][1], a project has a `workspaceRoot`, a title, and one or more threads. See [workspace-layout.md][2].

#### Workspace root

The root filesystem path for a project. In [the orchestration model][1], it is the base directory for branches and optional worktrees. See [workspace-layout.md][2].

#### Worktree

A Git worktree used as an isolated workspace for a thread. If a thread has a `worktreePath` in [the contracts][1], it runs there instead of in the main working tree. Git operations live in [GitCore.ts][3].

### Thread timeline

#### Thread

The main durable unit of conversation and workspace history. In [the orchestration contracts][1], a thread holds messages, activities, checkpoints, and session-related state. See [projector.ts][4].

#### Turn

A single user-to-assistant work cycle inside a thread. It starts with user input and ends when follow-up work like checkpointing settles. See [the contracts][1], [ProviderRuntimeIngestion.ts][5], and [CheckpointReactor.ts][6].

#### Activity

A user-visible log item attached to a thread. In [the contracts][1], activities cover important non-message events like approvals, tool actions, and failures. They are projected into thread state in [projector.ts][4].

### Orchestration

Orchestration is the server-side domain layer that turns runtime activity into stable app state. The main entry point is [OrchestrationEngine.ts][7], with core logic in [decider.ts][8] and [projector.ts][4].

#### Aggregate

The domain object a command or event belongs to. In [the contracts][1], that is usually `project` or `thread`. See [decider.ts][8].

#### Command

A typed request to change domain state. In [the contracts][1], commands are validated in [commandInvariants.ts][9] and turned into events by [decider.ts][8].
Examples include `thread.create`, `thread.turn.start`, and `thread.checkpoint.revert`.

#### Domain Event

A persisted fact that something already happened. In [the contracts][1], events are the source of truth, and [projector.ts][4] shows how they are applied.
Examples include `thread.created`, `thread.message-sent`, and `thread.turn-diff-completed`.

#### Decider

The pure orchestration logic that turns commands plus current state into events. The core implementation is in [decider.ts][8], with preconditions in [commandInvariants.ts][9].

#### Projection

A read-optimized view derived from events. See [projector.ts][4], [ProjectionPipeline.ts][11], and [ProjectionSnapshotQuery.ts][10].

#### Projector

The logic that applies domain events to the read model or projection tables. See [projector.ts][4] and [ProjectionPipeline.ts][11].

#### Read model

The current materialized view of orchestration state. In [the contracts][1], it holds projects, threads, messages, activities, checkpoints, and session state. See [ProjectionSnapshotQuery.ts][10] and [OrchestrationEngine.ts][7].

#### Reactor

A side-effecting service that handles follow-up work after events or runtime signals. Examples include [CheckpointReactor.ts][6], [ProviderCommandReactor.ts][12], and [ProviderRuntimeIngestion.ts][5].

#### Receipt

A lightweight typed runtime signal emitted when an async milestone completes. See [RuntimeReceiptBus.ts][13].
Examples include `checkpoint.baseline.captured`, `checkpoint.diff.finalized`, and `turn.processing.quiesced`, which are emitted by flows such as [CheckpointReactor.ts][6].

#### Quiesced

"Quiesced" means a turn has gone quiet and stable. In [the receipt schema][13], it means the follow-up work has settled, including work in [CheckpointReactor.ts][6].

### Provider runtime

The live backend agent implementation and its event stream. The main service is [ProviderService.ts][14], the adapter contract is [ProviderAdapter.ts][15], and the overview is in [provider-architecture.md][16].

#### Provider

The backend agent runtime that actually performs work. See [ProviderService.ts][14], [ProviderAdapter.ts][15], and [CodexAdapter.ts][17].

#### Session

The live provider-backed runtime attached to a thread. Session shape is in [the orchestration contracts][1], and lifecycle is managed in [ProviderService.ts][14].

#### Runtime mode

The safety/access mode for a thread or session. In [the contracts][1], the main values are `approval-required` and `full-access`. See [runtime-modes.md][18].

#### Interaction mode

The agent interaction style for a thread. In [the contracts][1], the main values are `default` and `plan`. See [runtime-modes.md][18].

#### Assistant delivery mode

Controls how assistant text reaches the thread timeline. In [the contracts][1], `streaming` updates incrementally and `buffered` delivers a completed result. See [ProviderService.ts][14].

#### Snapshot

A point-in-time view of state. The word is used in multiple layers, including orchestration, provider, and checkpointing. See [ProjectionSnapshotQuery.ts][10], [ProviderAdapter.ts][15], and [CheckpointStore.ts][19].

### Checkpointing

Checkpointing captures workspace state over time so the app can diff turns and restore earlier points. The main pieces are [CheckpointStore.ts][19], [CheckpointDiffQuery.ts][20], and [CheckpointReactor.ts][6].

#### Checkpoint

A saved snapshot of a thread workspace at a particular turn. In practice it is a hidden Git ref in [CheckpointStore.ts][19] plus a projected summary from [ProjectionCheckpoints.ts][21]. Capture and lifecycle work happen in [CheckpointReactor.ts][6].

#### Checkpoint ref

The durable identifier for a filesystem checkpoint, stored as a Git ref. It is typed in [the contracts][1], constructed in [Utils.ts][22], and used by [CheckpointStore.ts][19].

#### Checkpoint baseline

The starting checkpoint for diffing a thread timeline. This flow is surfaced through [RuntimeReceiptBus.ts][13], coordinated in [CheckpointReactor.ts][6], and supported by [Utils.ts][22].

#### Checkpoint diff

The patch difference between two checkpoints. Query logic lives in [CheckpointDiffQuery.ts][20], diff parsing lives in [Diffs.ts][23], and finalization is coordinated by [CheckpointReactor.ts][6].

#### Turn diff

The file patch and changed-file summary for one turn. It is usually computed in [CheckpointDiffQuery.ts][20], represented in [the contracts][1], and recorded into thread state by [projector.ts][4].

## Practical Shortcuts

- If you see `requested`, think "intent recorded".
- If you see `completed`, think "result applied".
- If you see `receipt`, think "async milestone signal".
- If you see `checkpoint`, think "workspace snapshot for diff/restore".
- If you see `quiesced`, think "all relevant follow-up work has gone idle".

## Related Docs

- [architecture.md][24]
- [provider-architecture.md][16]
- [runtime-modes.md][18]
- [workspace-layout.md][2]

[1]: ../packages/contracts/src/orchestration.ts
[2]: ./workspace-layout.md
[3]: ../apps/server/src/git/Layers/GitCore.ts
[4]: ../apps/server/src/orchestration/projector.ts
[5]: ../apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts
[6]: ../apps/server/src/orchestration/Layers/CheckpointReactor.ts
[7]: ../apps/server/src/orchestration/Layers/OrchestrationEngine.ts
[8]: ../apps/server/src/orchestration/decider.ts
[9]: ../apps/server/src/orchestration/commandInvariants.ts
[10]: ../apps/server/src/orchestration/Layers/ProjectionSnapshotQuery.ts
[11]: ../apps/server/src/orchestration/Layers/ProjectionPipeline.ts
[12]: ../apps/server/src/orchestration/Layers/ProviderCommandReactor.ts
[13]: ../apps/server/src/orchestration/Services/RuntimeReceiptBus.ts
[14]: ../apps/server/src/provider/Layers/ProviderService.ts
[15]: ../apps/server/src/provider/Services/ProviderAdapter.ts
[16]: ./provider-architecture.md
[17]: ../apps/server/src/provider/Layers/CodexAdapter.ts
[18]: ./runtime-modes.md
[19]: ../apps/server/src/checkpointing/CheckpointStore.ts
[20]: ../apps/server/src/checkpointing/CheckpointDiffQuery.ts
[21]: ../apps/server/src/persistence/Services/ProjectionCheckpoints.ts
[22]: ../apps/server/src/checkpointing/Utils.ts
[23]: ../apps/server/src/checkpointing/Diffs.ts
[24]: ./architecture.md

```