# 🔄 SubTurn Mechanism

> Back to [README](../../../README.md)

## Overview

The `SubTurn` mechanism is a core feature in Omnipus that allows tools to spawn isolated, nested agent loops to handle complex sub-tasks.

By using a SubTurn, an agent can break down a problem and run a separate LLM invocation in an independent, ephemeral session. This ensures that intermediate reasoning, background tasks, or sub-agent outputs do not pollute the main conversation history.

## Core Capabilities

- **Context Isolation**: Each SubTurn uses an `ephemeralSessionStore`. Its message history does not leak into the parent task and is destroyed upon completion. The ephemeral session holds at most **50 messages**; older messages are automatically truncated when this limit is reached.
- **Depth & Concurrency Limits**: Prevents infinite loops and resource exhaustion.
  - **Maximum Depth**: Up to 3 nested levels.
  - **Maximum Concurrency**: Up to 5 concurrent sub-turns per parent turn (managed via a semaphore with a 30-second timeout).
- **Context Protection**: Supports soft context limits (`MaxContextRunes`). It proactively truncates old messages (while preserving system prompts and recent context) before hitting the provider's hard context window limit.
- **Error Recovery**: Automatically detects and recovers from provider context length exceeded errors and truncation errors by compressing history and retrying.

## Configuration (`SubTurnConfig`)

When spawning a SubTurn, you must provide a `SubTurnConfig`:

| Field | Type | Description |
| :--- | :--- | :--- |
| `Model` | `string` | The LLM model to use for the sub-turn (e.g., `gpt-4o-mini`). **Required.** |
| `Tools` | `[]tools.Tool` | Tools granted to the sub-turn. If empty, it inherits the parent's tools. |
| `SystemPrompt` | `string` | The task description for the sub-turn. Sent as the first user message to the LLM (not as a system prompt override). |
| `ActualSystemPrompt` | `string` | Optional explicit system prompt to replace the agent's default. Leave empty to inherit the parent agent's system prompt. |
| `MaxTokens` | `int` | Maximum tokens for the generated response. |
| `Async` | `bool` | Controls the result delivery mode (Synchronous vs. Asynchronous). |
| `Critical` | `bool` | If `true`, the sub-turn continues running even if the parent finishes gracefully. |
| `Timeout` | `time.Duration` | Maximum execution time (default: 5 minutes). |
| `MaxContextRunes`| `int` | Soft context limit. `0` = auto-calculate (75% of model's context window, recommended), `-1` = no limit (disable soft truncation, rely only on hard context error recovery), `>0` = use specified rune limit. |

> **Note:** The `Async` flag does **not** make the call non-blocking. It only controls whether the result is also delivered to the parent's `pendingResults` channel. Both modes block the caller until the sub-turn completes. For true non-blocking execution, the caller must spawn the sub-turn in a separate goroutine.

## Execution Modes

### Synchronous (`Async: false`)

This is the standard mode where the caller needs the result immediately to proceed.

- The caller blocks until the sub-turn completes.
- The result is **only** returned directly via the function return value.
- It is **not** delivered to the parent's pending results channel.

**Example:**
```go
cfg := agent.SubTurnConfig{
    Model:        "gpt-4o-mini",
    SystemPrompt: "Analyze the provided codebase...",
    Async:        false,
}
result, err := agent.SpawnSubTurn(ctx, cfg)
// Process result immediately
```

### Asynchronous (`Async: true`)

Used for "fire-and-forget" operations or parallel processing where the parent turn collects results later.

- The result is delivered to the parent turn's `pendingResults` channel.
- The result is **also** returned via the function return value (for consistency).
- The parent's Agent Loop will poll this channel in subsequent iterations and automatically inject the results into the ongoing conversation context as `[SubTurn Result]`.

**Example:**
```go
cfg := agent.SubTurnConfig{
    Model:        "gpt-4o-mini",
    SystemPrompt: "Run a background security scan...",
    Async:        true,
}
result, err := agent.SpawnSubTurn(ctx, cfg)
// The result will also be injected into the parent loop later via channel
```

## Error Recovery and Retries

SubTurns implement automatic retry mechanisms for transient errors:

| Error Type | Max Retries | Recovery Action |
|:-----------|:------------|:----------------|
| Context Length Exceeded | 2 | Force compress history and retry |
| Response Truncated (`finish_reason="truncated"`) | 2 | Inject recovery prompt and retry |

### Truncation Recovery
When the LLM response is truncated (`finish_reason="truncated"`), SubTurn automatically:
1. Detects the truncation from `turnState.lastFinishReason`
2. Injects a recovery prompt: "Your previous response was truncated due to length. Please provide a shorter, complete response..."
3. Retries up to 2 times

### Context Error Recovery
When the provider returns a context length error (e.g., `context_length_exceeded`):
1. Force compresses the message history (drops oldest 50% of conversation)
2. Retries with the compressed context
3. Up to 2 retries before failing

## Lifecycle and Cancellation

SubTurns operate within an independent context but maintain a structural link to their parent `turnState`.

### Graceful Parent Finish
When the parent task finishes naturally (`Finish(false)`):
- **Non-critical** sub-turns receive a signal to exit gracefully without throwing an error.
- **Critical** (`Critical: true`) sub-turns continue running in the background. Once finished, their results are emitted as **Orphan Results** so the data is not lost.

### Hard Abort
When the parent task is forcefully aborted (e.g., user interrupts with `/stop`):
- A cascading cancellation is triggered, instantly terminating all child and grandchild sub-turns.
- The root turn's session history rolls back to the snapshot taken at turn start (`initialHistoryLength`), preventing dirty context. SubTurns are not affected by this rollback as they use ephemeral sessions that are discarded anyway.

## Agent Loop Integration

### Bus Draining During Processing

When a message enters the `Run()` loop, the agent starts a `drainBusToSteering` goroutine before calling `processMessage`. This goroutine runs concurrently with the entire processing lifecycle and continuously consumes any new inbound messages from the bus, redirecting them into the **steering queue** instead of dropping them.

This ensures that if a user sends a follow-up message while the agent is processing (including during SubTurn execution), the message is not lost — it will be picked up between tool call iterations via `dequeueSteeringMessages`.

The drain goroutine stops automatically when `processMessage` returns (via a cancellable context).

### Pending Result Polling

The agent loop polls for async SubTurn results at two points per iteration:
1. **Before the LLM call**: injects any arrived results as `[SubTurn Result]` messages into the conversation context.
2. **After all tool executions**: polls again during the tool loop to catch results that arrived during tool execution.
3. **After the final iteration**: one last poll before the turn ends to avoid losing late-arriving results.

### Turn State Tracking

All active root turns are registered in `AgentLoop.activeTurnStates` (`sync.Map`, keyed by session key). This allows `HardAbort` and `/subagents` observability commands to find and operate on active turns.

## Event Bus Integration

SubTurns emit specific events to the Omnipus `EventBus` for observability and debugging:

| Event Kind | When Emitted | Payload |
|:------|:-------------|:--------|
| `subturn_spawn` | Sub-turn successfully initialized | `SubTurnSpawnPayload{AgentID, Label, ParentTurnID}` |
| `subturn_end` | Sub-turn finishes (success or error) | `SubTurnEndPayload{AgentID, Status}` |
| `subturn_result_delivered` | Async result successfully delivered to parent | `SubTurnResultDeliveredPayload{TargetChannel, TargetChatID, ContentLen}` |
| `subturn_orphan` | Result cannot be delivered (parent finished or channel full) | `SubTurnOrphanPayload{ParentTurnID, ChildTurnID, Reason}` |

## API Reference

### SpawnSubTurn (Public Entry Point)

```go
func SpawnSubTurn(ctx context.Context, cfg SubTurnConfig) (*tools.ToolResult, error)
```

This is the exported package-level entry point for agent-internal code (e.g., tests, direct invocations). It retrieves `AgentLoop` and `turnState` from context and delegates to the internal `spawnSubTurn`.

**Requirements:**
- `AgentLoop` must be injected into context via `WithAgentLoop()`
- Parent `turnState` must exist in context (automatically set when called from tools)

**Returns:**
- `*tools.ToolResult`: Contains `ForLLM` field with the sub-turn's output
- `error`: One of the defined error types or context errors

### AgentLoopSpawner (Interface Implementation)

```go
type AgentLoopSpawner struct { al *AgentLoop }

func (s *AgentLoopSpawner) SpawnSubTurn(ctx context.Context, cfg tools.SubTurnConfig) (*tools.ToolResult, error)
```

This implements the `tools.SubTurnSpawner` interface for use by tools that need to spawn sub-turns without a direct import of the `agent` package (avoiding circular dependencies). It converts `tools.SubTurnConfig` → `agent.SubTurnConfig` before delegating to the internal `spawnSubTurn`.

### NewSubTurnSpawner

```go
func NewSubTurnSpawner(al *AgentLoop) *AgentLoopSpawner
```

Creates a new spawner instance for the given AgentLoop. Pass the returned value to `SpawnTool.SetSpawner()` or `SubagentTool.SetSpawner()` during tool registration.

### Continue

```go
func (al *AgentLoop) Continue(ctx context.Context, sessionKey string) error
```

Resumes an idle agent turn by injecting any queued steering messages as a new LLM iteration. Used when the agent is waiting and a deferred steering message needs to be processed without a new inbound message arriving.

## Context Propagation

SubTurn relies on context values for proper operation:

| Context Key | Purpose |
|:------------|:--------|
| `agentLoopKey` | Stores `*AgentLoop` for tool access and SubTurn spawning |
| `turnStateKey` | Stores `*turnState` for hierarchy tracking and result delivery |

### Injecting Dependencies

```go
// Before calling tools that may spawn SubTurns
ctx = WithAgentLoop(ctx, agentLoop)
ctx = withTurnState(ctx, turnState)
```

### Independent Child Context

**Important**: The child SubTurn uses an **independent context** derived from `context.Background()`, not from the parent context. This design choice:

- Allows critical SubTurns to continue after parent cancellation
- Prevents parent timeout from affecting child execution
- Child has its own timeout for self-protection (`Timeout` config or 5 minutes default)

## Error Types

| Error | Condition |
|:------|:----------|
| `ErrDepthLimitExceeded` | SubTurn depth exceeds 3 levels |
| `ErrInvalidSubTurnConfig` | Required field `Model` is empty |
| `ErrConcurrencyTimeout` | All 5 concurrency slots occupied for 30+ seconds |
| Context errors | Parent context cancelled during semaphore acquisition |

## Thread Safety

SubTurns are designed for concurrent execution:

- **Parent-child relationships**: Managed under mutex (`parentTS.mu.Lock()`)
- **Active turn tracking**: Uses `sync.Map` for concurrent access to `activeTurnStates`
- **ID generation**: Uses `atomic.Int64` for unique SubTurn IDs (format: `subturn-N`, globally monotonic per `AgentLoop` instance)
- **Result delivery**: Reads parent state under lock, releases before channel send (small race window acceptable)

## Orphan Results

An orphan result occurs when:
1. Parent turn finishes before the SubTurn completes
2. The `pendingResults` channel is full (buffer size: 16)

When a result becomes orphan:
- `SubTurnOrphanResultEvent` is emitted to EventBus
- The result is **NOT** delivered to the LLM context
- External systems can listen to this event for custom handling

### Preventing Orphan Results
- Use `Critical: true` for important SubTurns that must complete
- Monitor `SubTurnOrphanResultEvent` for observability
- Consider the 16-buffer limit when spawning many async SubTurns

## Tool Inheritance

### When `cfg.Tools` is empty:
- SubTurn inherits **all** tools from the parent agent
- Tools are registered in a new `ToolRegistry` instance
- Tool TTL is managed independently from parent

### When `cfg.Tools` is specified:
- Only the specified tools are available to the SubTurn
- Parent tools are **NOT** merged
- Use this to restrict SubTurn capabilities for security or focus

**Example - Restricted SubTurn:**
```go
cfg := agent.SubTurnConfig{
    Model: "gpt-4o-mini",
    Tools: []tools.Tool{readOnlyTool}, // Only read-only access
    SystemPrompt: "Analyze the file structure...",
}
```

## Reference

| Constant | Value |
|:---------|:------|
| `maxSubTurnDepth` | 3 |
| `maxConcurrentSubTurns` | 5 |
| `concurrencyTimeout` | 30s |
| `defaultSubTurnTimeout` | 5m |
| `maxEphemeralHistorySize` | 50 messages |
| `pendingResults` buffer | 16 |
| `MaxContextRunes` default | 75% of model context window |
