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Chapter 3.3 — Agent runtime and agent harness

🎯 Objective

Distinguish what executes the agent (runtime) from what makes the agent operable (harness).

🧠 Definitions

  • Agent runtime — receives input, assembles context, calls the model, interprets tool calls, executes tools, updates state, decides when to stop.
  • Agent harness — the engineering around the model: orchestration, retries, timeouts, fallbacks, policies, observability, evals, sandbox, HITL, auditing.

In practical terms:

Model:    predicts, classifies, generates, decides or proposes actions.
Harness:  makes the capability safe, testable, observable and operable.
Product:  harness + model + integrations + governance.

🏗️ Mandatory harness components

Component Responsibility
Orchestrator Controls flow and loop
State manager Maintains resumable state
Tool executor Executes external actions with a timeout
Policy engine Applies rules (policy-as-code)
AuthZ/AuthN Controls access
Eval harness Measures quality
Observability Makes execution debuggable
Cost controller Enforces budgets
Sandbox Isolates dangerous execution
Human approval Mitigates critical actions

🔁 Basic loop

sequenceDiagram
    participant U as User/API
    participant H as Harness
    participant P as Policy/Auth
    participant C as Context Engine
    participant M as Model
    participant T as Tool Executor
    participant O as Observability

    U->>H: Request
    H->>P: Check scope and permission
    H->>C: Assemble context
    C->>H: Structured context
    H->>M: Call the model
    M->>H: Response or tool call
    H->>P: Authorize tool call
    H->>T: Execute tool with timeout
    T->>H: Structured result
    H->>O: Record spans, cost and tokens
    H->>M: Send the tool result
    M->>H: Final response
    H->>U: Return with evidence

📚 References