# RPC Protocol Reference

RPC mode runs the coding agent as a newline-delimited JSON protocol over stdio.

- **stdin**: commands (`RpcCommand`) and extension UI responses
- **stdout**: command responses (`RpcResponse`), session/agent events, extension UI requests

Primary implementation:

- `src/modes/rpc/rpc-mode.ts`
- `src/modes/rpc/rpc-types.ts`
- `src/session/agent-session.ts`
- `packages/agent/src/agent.ts`
- `packages/agent/src/agent-loop.ts`

## Startup

```bash
omp --mode rpc [regular CLI options]
```

Behavior notes:

- `@file` CLI arguments are rejected in RPC mode.
- RPC mode disables automatic session title generation by default to avoid an extra model call.
- RPC mode resets workflow-altering `todo.*`, `task.*`, and `async.*` settings to their built-in defaults instead of inheriting user overrides.
- The process reads stdin as JSONL (`readJsonl(Bun.stdin.stream())`).
- When stdin closes, the process exits with code `0`.
- Responses/events are written as one JSON object per line.

## Transport and Framing

Each frame is a single JSON object followed by `\n`.

There is no envelope beyond the object shape itself.

### Outbound frame categories (stdout)

1. `RpcResponse` (`{ type: "response", ... }`)
2. `AgentSessionEvent` objects (`agent_start`, `message_update`, etc.)
3. `RpcExtensionUIRequest` (`{ type: "extension_ui_request", ... }`)
4. Extension errors (`{ type: "extension_error", extensionPath, event, error }`)

### Inbound frame categories (stdin)

1. `RpcCommand`
2. `RpcExtensionUIResponse` (`{ type: "extension_ui_response", ... }`)

## Request/Response Correlation

All commands accept optional `id?: string`.

- If provided, normal command responses echo the same `id`.
- `RpcClient` relies on this for pending-request resolution.

Important edge behavior from runtime:

- Unknown command responses are emitted with `id: undefined` (even if the request had an `id`).
- Parse/handler exceptions in the input loop emit `command: "parse"` with `id: undefined`.
- `prompt` and `abort_and_prompt` return immediate success, then may emit a later error response with the **same** id if async prompt scheduling fails.

## Command Schema (canonical)

`RpcCommand` is defined in `src/modes/rpc/rpc-types.ts`:

### Prompting

- `{ id?, type: "prompt", message: string, images?: ImageContent[], streamingBehavior?: "steer" | "followUp" }`
- `{ id?, type: "steer", message: string, images?: ImageContent[] }`
- `{ id?, type: "follow_up", message: string, images?: ImageContent[] }`
- `{ id?, type: "abort" }`
- `{ id?, type: "abort_and_prompt", message: string, images?: ImageContent[] }`
- `{ id?, type: "new_session", parentSession?: string }`

### State

- `{ id?, type: "get_state" }`
- `{ id?, type: "set_todos", phases: TodoPhase[] }`

### Model

- `{ id?, type: "set_model", provider: string, modelId: string }`
- `{ id?, type: "cycle_model" }`
- `{ id?, type: "get_available_models" }`

### Thinking

- `{ id?, type: "set_thinking_level", level: ThinkingLevel }`
- `{ id?, type: "cycle_thinking_level" }`

### Queue modes

- `{ id?, type: "set_steering_mode", mode: "all" | "one-at-a-time" }`
- `{ id?, type: "set_follow_up_mode", mode: "all" | "one-at-a-time" }`
- `{ id?, type: "set_interrupt_mode", mode: "immediate" | "wait" }`

### Compaction

- `{ id?, type: "compact", customInstructions?: string }`
- `{ id?, type: "set_auto_compaction", enabled: boolean }`

### Retry

- `{ id?, type: "set_auto_retry", enabled: boolean }`
- `{ id?, type: "abort_retry" }`

### Bash

- `{ id?, type: "bash", command: string }`
- `{ id?, type: "abort_bash" }`

### Session

- `{ id?, type: "get_session_stats" }`
- `{ id?, type: "export_html", outputPath?: string }`
- `{ id?, type: "switch_session", sessionPath: string }`
- `{ id?, type: "branch", entryId: string }`
- `{ id?, type: "get_branch_messages" }`
- `{ id?, type: "get_last_assistant_text" }`
- `{ id?, type: "set_session_name", name: string }`

### Messages

- `{ id?, type: "get_messages" }`

## Response Schema

All command results use `RpcResponse`:

- Success: `{ id?, type: "response", command: <command>, success: true, data?: ... }`
- Failure: `{ id?, type: "response", command: string, success: false, error: string }`

Data payloads are command-specific and defined in `rpc-types.ts`.

### `get_state` payload

```json
{
  "model": { "provider": "...", "id": "..." },
  "thinkingLevel": "off|minimal|low|medium|high|xhigh",
  "isStreaming": false,
  "isCompacting": false,
  "steeringMode": "all|one-at-a-time",
  "followUpMode": "all|one-at-a-time",
  "interruptMode": "immediate|wait",
  "sessionFile": "...",
  "sessionId": "...",
  "sessionName": "...",
  "autoCompactionEnabled": true,
  "messageCount": 0,
  "queuedMessageCount": 0,
  "todoPhases": [
    {
      "id": "phase-1",
      "name": "Todos",
      "tasks": [
        {
          "id": "task-1",
          "content": "Map the tool surface",
          "status": "in_progress"
        }
      ]
    }
  ]
}
```

### `set_todos` payload

Replaces the in-memory todo state for the current session and returns the normalized phase list:

```json
{
  "id": "req_2",
  "type": "set_todos",
  "phases": [
    {
      "id": "phase-1",
      "name": "Evaluation",
      "tasks": [
        {
          "id": "task-1",
          "content": "Map the read tool surface",
          "status": "in_progress"
        },
        {
          "id": "task-2",
          "content": "Exercise edit operations",
          "status": "pending"
        }
      ]
    }
  ]
}
```

This is useful for hosts that want to pre-seed a plan before the first prompt.

## Event Stream Schema

RPC mode forwards `AgentSessionEvent` objects from `AgentSession.subscribe(...)`.

Common event types:

- `agent_start`, `agent_end`
- `turn_start`, `turn_end`
- `message_start`, `message_update`, `message_end`
- `tool_execution_start`, `tool_execution_update`, `tool_execution_end`
- `auto_compaction_start`, `auto_compaction_end`
- `auto_retry_start`, `auto_retry_end`
- `ttsr_triggered`
- `todo_reminder`
- `todo_auto_clear`

Extension runner errors are emitted separately as:

```json
{ "type": "extension_error", "extensionPath": "...", "event": "...", "error": "..." }
```

`message_update` includes streaming deltas in `assistantMessageEvent` (text/thinking/toolcall deltas).

## Prompt/Queue Concurrency and Ordering

This is the most important operational behavior.

### Immediate ack vs completion

`prompt` and `abort_and_prompt` are **acknowledged immediately**:

```json
{ "id": "req_1", "type": "response", "command": "prompt", "success": true }
```

That means:

- command acceptance != run completion
- final completion is observed via `agent_end`

### While streaming

`AgentSession.prompt()` requires `streamingBehavior` during active streaming:

- `"steer"` => queued steering message (interrupt path)
- `"followUp"` => queued follow-up message (post-turn path)

If omitted during streaming, prompt fails.

### Queue defaults

From `packages/agent/src/agent.ts` defaults:

- `steeringMode`: `"one-at-a-time"`
- `followUpMode`: `"one-at-a-time"`
- `interruptMode`: `"immediate"`

### Mode semantics

- `set_steering_mode` / `set_follow_up_mode`
  - `"one-at-a-time"`: dequeue one queued message per turn
  - `"all"`: dequeue entire queue at once
- `set_interrupt_mode`
  - `"immediate"`: tool execution checks steering between tool calls; pending steering can abort remaining tool calls in the turn
  - `"wait"`: defer steering until turn completion

## Extension UI Sub-Protocol

Extensions in RPC mode use request/response UI frames.

### Outbound request

`RpcExtensionUIRequest` (`type: "extension_ui_request"`) methods:

- `select`, `confirm`, `input`, `editor`
- `notify`, `setStatus`, `setWidget`, `setTitle`, `set_editor_text`

Runtime note:

- Automatic session title generation is disabled in RPC mode, and `setTitle` UI
  requests are also suppressed by default because most hosts do not have a
  meaningful terminal-title surface. Set `PI_RPC_EMIT_TITLE=1` to opt back in to
  the UI event only.

Example:

```json
{ "type": "extension_ui_request", "id": "123", "method": "confirm", "title": "Confirm", "message": "Continue?", "timeout": 30000 }
```

### Inbound response

`RpcExtensionUIResponse` (`type: "extension_ui_response"`):

- `{ type: "extension_ui_response", id: string, value: string }`
- `{ type: "extension_ui_response", id: string, confirmed: boolean }`
- `{ type: "extension_ui_response", id: string, cancelled: true }`

If a dialog has a timeout, RPC mode resolves to a default value when timeout/abort fires.

## Error Model and Recoverability

### Command-level failures

Failures are `success: false` with string `error`.

```json
{ "id": "req_2", "type": "response", "command": "set_model", "success": false, "error": "Model not found: provider/model" }
```

### Recoverability expectations

- Most command failures are recoverable; process remains alive.
- Malformed JSONL / parse-loop exceptions emit a `parse` error response and continue reading subsequent lines.
- Empty `set_session_name` is rejected (`Session name cannot be empty`).
- Extension UI responses with unknown `id` are ignored.
- Process termination conditions are stdin close or explicit extension-triggered shutdown.

## Compact Command Flows

### 1) Prompt and stream

stdin:

```json
{ "id": "req_1", "type": "prompt", "message": "Summarize this repo" }
```

stdout sequence (typical):

```json
{ "id": "req_1", "type": "response", "command": "prompt", "success": true }
{ "type": "agent_start" }
{ "type": "message_update", "assistantMessageEvent": { "type": "text_delta", "delta": "..." }, "message": { "role": "assistant", "content": [] } }
{ "type": "agent_end", "messages": [] }
```

### 2) Prompt during streaming with explicit queue policy

stdin:

```json
{ "id": "req_2", "type": "prompt", "message": "Also include risks", "streamingBehavior": "followUp" }
```

### 3) Inspect and tune queue behavior

stdin:

```json
{ "id": "q1", "type": "get_state" }
{ "id": "q2", "type": "set_steering_mode", "mode": "all" }
{ "id": "q3", "type": "set_interrupt_mode", "mode": "wait" }
```

### 4) Extension UI round trip

stdout:

```json
{ "type": "extension_ui_request", "id": "ui_7", "method": "input", "title": "Branch name", "placeholder": "feature/..." }
```

stdin:

```json
{ "type": "extension_ui_response", "id": "ui_7", "value": "feature/rpc-host" }
```

## Notes on `RpcClient` helper

`src/modes/rpc/rpc-client.ts` is a convenience wrapper, not the protocol definition.

Current helper characteristics:

- Spawns `bun <cliPath> --mode rpc`
- Correlates responses by generated `req_<n>` ids
- Dispatches only recognized `AgentEvent` types to listeners
- Does **not** expose helper methods for every protocol command (for example, `set_interrupt_mode` and `set_session_name` are in protocol types but not wrapped as dedicated methods)

Use raw protocol frames if you need complete surface coverage.