Agent Runtime Hooks

Agent Runtime Hooks

Lifecycle hooks for observing and intercepting agent execution. Hooks are registered per-operation via execAgent({ hooks }) and dispatched by HookDispatcher.

Hook Types

16 hook types across 5 categories:

execAgent({ hooks })
  │
  ├─ beforeStep ──────────── Before each step executes
  │     │
  │     ├─ [call_llm]        LLM inference
  │     │
  │     ├─ [call_tool]
  │     │     ├─ beforeToolCall ── Before tool executes (supports mocking)
  │     │     ├─ (tool execution)
  │     │     ├─ afterToolCall ─── After tool completes (observation only)
  │     │     └─ onToolCallError ─ Tool threw an exception
  │     │
  │     ├─ [request_human_approve]
  │     │     ├─ beforeHumanIntervention ── Before agent pauses
  │     │     ├─ afterHumanIntervention ─── After approve/reject + resume
  │     │     └─ onStopByHumanIntervention ── User rejected, agent halted
  │     │
  │     ├─ [compress_context]
  │     │     ├─ beforeCompact ──── Before compression starts
  │     │     ├─ afterCompact ───── After compression completes
  │     │     └─ onCompactError ─── Compression failed
  │     │
  │     ├─ [callAgent] (via execSubAgentTask)
  │     │     ├─ beforeCallAgent ── Before sub-agent starts
  │     │     ├─ afterCallAgent ─── After sub-agent completes
  │     │     └─ onCallAgentError ── Sub-agent failed
  │     │
  │     └─ afterStep ──────────── After step completes
  │
  ├─ (next step...)
  │
  ├─ onComplete ───────────── Operation reaches terminal state
  └─ onError ──────────────── Error during execution

Key Files

Registration Flow

const hooks: AgentHook[] = [
  { id: 'my-hook', type: 'afterStep', handler: async (event) => { ... } },
];
await aiAgentService.execAgent({ agentId, prompt, hooks });
// Internally: hookDispatcher.register(operationId, hooks)
// Cleanup:    hookDispatcher.unregister(operationId)

Hook Reference

Step Level

beforeStep — Before each step. event: AgentHookEvent afterStep — After each step. event: AgentHookEvent (content, toolsCalling, totalCost, etc.) onComplete — Terminal state. event: AgentHookEvent (reason: done/error/interrupted/max_steps/cost_limit) onError — Error occurred. event: AgentHookEvent (errorMessage, errorDetail)

Tool Call Level

beforeToolCall — Before tool executes. Supports mocking via event.mock().

// event: ToolCallHookEvent
{
  (identifier, apiName, args, callIndex, stepIndex, operationId, mock);
}
// Mock example:
event.mock({ content: '{"error":"rate limited"}' });

Dispatch method: hookDispatcher.dispatchBeforeToolCall() (returns mock result or null).

afterToolCall — After tool completes. Observation only.

// event: AfterToolCallHookEvent
{
  (identifier, apiName, args, callIndex, content, success, mocked, executionTimeMs, stepIndex);
}

onToolCallError — Tool threw an exception (catch block, not just success=false).

// event: ToolCallErrorHookEvent
{
  (identifier, apiName, args, callIndex, error, stepIndex);
}

Human Intervention

beforeHumanIntervention — Before agent pauses for approval.

// event: BeforeHumanInterventionHookEvent
{ operationId, stepIndex, pendingTools: [{ identifier, apiName }] }

afterHumanIntervention — After approve/reject, agent resumes.

// event: AfterHumanInterventionHookEvent
{ operationId, action: 'approve' | 'reject' | 'rejectAndContinue', toolCallId?, rejectionReason? }

onStopByHumanIntervention — User rejected, agent halted.

// event: StopByHumanInterventionHookEvent
{ operationId, toolCallId?, rejectionReason? }

Context Compression

beforeCompact — Before compression starts.

// event: BeforeCompactHookEvent
{
  (operationId, stepIndex, messageCount, tokenCount);
}

afterCompact — After compression completes.

// event: AfterCompactHookEvent
{
  (operationId, stepIndex, groupId, messagesBefore, messagesAfter, summary);
}

onCompactError — Compression failed.

// event: CompactErrorHookEvent
{
  (operationId, stepIndex, tokenCount, error);
}

Sub-Agent (CallAgent)

beforeCallAgent — Before calling sub-agent. Dispatched on parent operation.

// event: BeforeCallAgentHookEvent
{
  (operationId, agentId, instruction);
}

afterCallAgent — Sub-agent completed. Dispatched on parent operation.

// event: AfterCallAgentHookEvent
{
  (operationId, agentId, subOperationId, threadId, success);
}

onCallAgentError — Sub-agent failed. Dispatched on parent operation.

// event: CallAgentErrorHookEvent
{
  (operationId, agentId, error);
}

Note: CallAgent hooks require parentOperationId in ExecSubAgentTaskParams.

Design Notes

• Fire-and-forget: All handlers return Promise<void>. Errors are non-fatal.
• Exception: beforeToolCall supports mock via event.mock() — uses dispatchBeforeToolCall() which returns the mock result.
• Sequential: Same-type hooks run in registration order.
• Local only: beforeToolCall mock only works in local mode (in-memory hooks). Webhook mode does not support mocking.
• Scoped per operation: Auto-cleaned via hookDispatcher.unregister() on completion.
• Sandbox/MCP: No separate hooks — they go through executeTool, so beforeToolCall/afterToolCall cover them. Use event.identifier to filter.

Real-World Example: agent-evals

See devtools/agent-evals/helpers/runner.ts — createEvalHooks() uses afterStep, onComplete, afterToolCall, and beforeToolCall (for mock).