codex/codex-rs/docs/protocol_v1.md

Overview of Protocol defined in protocol.rs and agent.rs.

The goal of this document is to define terminology used in the system and explain the expected behavior of the system.

NOTE: The code might not completely match this spec. There are a few minor changes that need to be made after this spec has been reviewed, which will not alter the existing TUI's functionality.

Entities

These are entities exit on the codex backend. The intent of this section is to establish vocabulary and construct a shared mental model for the Codex core system.

0. Model
   • In our case, this is the Responses REST API
1. Codex
   • The core engine of codex
   • Runs locally, either in a background thread or separate process
   • Communicated to via a queue pair – SQ (Submission Queue) / EQ (Event Queue)
   • Takes user input, makes requests to the Model, executes commands and applies patches.
2. Session
   • The Codex's current configuration and state
   • Codex starts with no Session, and it is initialized by Op::ConfigureSession, which should be the first message sent by the UI.
   • The current Session can be reconfigured with additional Op::ConfigureSession calls.
   • Any running execution is aborted when the session is reconfigured.
3. Task
   • A Task is Codex executing work in response to user input.
   • Session has at most one Task running at a time.
   • Receiving Op::UserTurn starts a Task (Op::UserInput is legacy)
   • Consists of a series of Turns
   • The Task executes to until:
     • The Model completes the task and there is no output to feed into an additional Turn
     • Additional user-turn input aborts the current task and starts a new one
     • UI interrupts with Op::Interrupt
     • Fatal errors are encountered, eg. Model connection exceeding retry limits
     • Blocked by user approval (executing a command or patch)
4. Turn
   • One cycle of iteration in a Task, consists of:
     • A request to the Model - (initially) prompt + (optional) last_response_id, or (in loop) previous turn output
     • The Model streams responses back in an SSE, which are collected until "completed" message and the SSE terminates
     • Codex then executes command(s), applies patch(es), and outputs message(s) returned by the Model
     • Pauses to request approval when necessary
   • The output of one Turn is the input to the next Turn
   • A Turn yielding no output terminates the Task

The term "UI" is used to refer to the application driving Codex. This may be the CLI / TUI chat-like interface that users operate, or it may be a GUI interface like a VSCode extension. The UI is external to Codex, as Codex is intended to be operated by arbitrary UI implementations.

When a Turn completes, the response_id from the Model's final response.completed message is stored in the Session state to resume the thread given the next user turn. The response_id is also returned in the EventMsg::TurnComplete to the UI, which can be used to fork the thread from an earlier point by providing it in a future user turn.

Since only 1 Task can be run at a time, for parallel tasks it is recommended that a single Codex be run for each thread of work.

Interface

• Codex
  • Communicates with UI via a SQ (Submission Queue) and EQ (Event Queue).
• Submission
  • These are messages sent on the SQ (UI -> Codex)
  • Has an string ID provided by the UI, referred to as sub_id
  • Op refers to the enum of all possible Submission payloads
    • This enum is non_exhaustive; variants can be added at future dates
• Event
  • These are messages sent on the EQ (Codex -> UI)
  • Each Event has a non-unique ID, matching the sub_id from the user-turn op that started the current task.
  • EventMsg refers to the enum of all possible Event payloads
    • This enum is non_exhaustive; variants can be added at future dates
    • It should be expected that new EventMsg variants will be added over time to expose more detailed information about the model's actions.

For complete documentation of the Op and EventMsg variants, refer to protocol.rs. Some example payload types:

• Op
  • Op::UserTurn – Any input from the user to kick off a Turn
  • Op::UserInput – Legacy form of user input
  • Op::Interrupt – Interrupts a running turn
  • Op::ExecApproval – Approve or deny code execution
  • Op::UserInputAnswer – Provide answers for a request_user_input tool call
  • Op::ListSkills – Request skills for one or more cwd values (optionally force_reload)
  • Op::UserTurn and Op::OverrideTurnContext accept an optional personality override that updates the model’s communication style
• EventMsg
  • EventMsg::AgentMessage – Messages from the Model
  • EventMsg::AgentMessageContentDelta – Streaming assistant text
  • EventMsg::PlanDelta – Streaming proposed plan text when the model emits a <proposed_plan> block in plan mode
  • EventMsg::ExecApprovalRequest – Request approval from user to execute a command
  • EventMsg::RequestUserInput – Request user input for a tool call (questions can include options plus isOther to add a free-form choice)
  • EventMsg::TurnStarted – Turn start metadata including model_context_window and collaboration_mode_kind
  • EventMsg::TurnComplete – A turn completed successfully
  • EventMsg::Error – A turn stopped with an error
  • EventMsg::Warning – A non-fatal warning that the client should surface to the user
  • EventMsg::TurnComplete – Contains a response_id bookmark for last response_id executed by the turn. This can be used to continue the turn at a later point in time, perhaps with additional user input.
  • EventMsg::ListSkillsResponse – Response payload with per-cwd skill entries (cwd, skills, errors)

UserInput items

Op::UserTurn content items can include:

• text – Plain text plus optional UI text elements.
• image / local_image – Image inputs.
• skill – Explicit skill selection (name, path to SKILL.md).
• mention – Explicit app/connector selection (name, path in app://{connector_id} form).

Note: For v1 wire compatibility, EventMsg::TurnStarted and EventMsg::TurnComplete serialize as task_started / task_complete. The deserializer accepts both task_* and turn_* tags.

The response_id returned from each turn matches the OpenAI response_id stored in the API's /responses endpoint. It can be stored and used in future Sessions to resume threads of work.

Transport

Can operate over any transport that supports bi-directional streaming. - cross-thread channels - IPC channels - stdin/stdout - TCP - HTTP2 - gRPC

Non-framed transports, such as stdin/stdout and TCP, should use newline-delimited JSON in sending messages.

Example Flows

Sequence diagram examples of common interactions. In each diagram, some unimportant events may be eliminated for simplicity.

Basic UI Flow

A single user input, followed by a 2-turn task

sequenceDiagram
    box UI
    participant user as User
    end
    box Daemon
    participant codex as Codex
    participant session as Session
    participant task as Task
    end
    box Rest API
    participant agent as Model
    end
    user->>codex: Op::ConfigureSession
    codex-->>session: create session
    codex->>user: Event::SessionConfigured
    user->>session: Op::UserTurn
    session-->>+task: start task
    task->>user: Event::TurnStarted
    task->>agent: prompt
    agent->>task: response (exec)
    task->>-user: Event::ExecApprovalRequest
    user->>+task: Op::ExecApproval::Allow
    task->>user: Event::ExecStart
    task->>task: exec
    task->>user: Event::ExecStop
    task->>user: Event::TurnComplete
    task->>agent: stdout
    agent->>task: response (patch)
    task->>task: apply patch (auto-approved)
    task->>agent: success
    agent->>task: response<br/>(msg + completed)
    task->>user: Event::AgentMessage
    task->>user: Event::TurnComplete
    task->>-user: Event::TurnComplete

Task Interrupt

Interrupting a task and continuing with additional user input.

sequenceDiagram
    box UI
    participant user as User
    end
    box Daemon
    participant session as Session
    participant task1 as Task1
    participant task2 as Task2
    end
    box Rest API
    participant agent as Model
    end
    user->>session: Op::UserTurn
    session-->>+task1: start task
    task1->>user: Event::TurnStarted
    task1->>agent: prompt
    agent->>task1: response (exec)
    task1->>task1: exec (auto-approved)
    task1->>user: Event::TurnComplete
    task1->>agent: stdout
    task1->>agent: response (exec)
    task1->>task1: exec (auto-approved)
    user->>task1: Op::Interrupt
    task1->>-user: Event::Error("interrupted")
    user->>session: Op::UserTurn w/ response bookmark
    session-->>+task2: start task
    task2->>user: Event::TurnStarted
    task2->>agent: prompt + Task1 last_response_id
    agent->>task2: response (exec)
    task2->>task2: exec (auto-approve)
    task2->>user: Event::TurnComplete
    task2->>agent: stdout
    agent->>task2: msg + completed
    task2->>user: Event::AgentMessage
    task2->>user: Event::TurnComplete
    task2->>-user: Event::TurnComplete