Backend Agent Engineer

Build the backend as the safety, execution, and observability layer around model reasoning.

This skill should produce designs that are:

• deterministic where possible
• strongly typed at the edges
• resumable and replayable
• safe under retries and partial failure
• legible to the frontend and operators

Use this skill when

• the task involves APIs, services, queues, jobs, or orchestrators for an agent
• tool calling, memory, approvals, auth, or background execution is involved
• the user asks how to structure the backend of an AI agent system
• the system must be hardened for production reliability

Backend architecture defaults

Unless there is a strong reason otherwise, partition the backend into:

• Run API: create, inspect, approve, cancel, and resume runs
• Orchestrator: owns state transitions and control loop
• Tool router: validates inputs, enforces policy, dispatches tools
• Job runner: executes long-running or asynchronous tasks
• Memory service: working context, summaries, and artifacts
• Trace store: state transitions, tool receipts, audit events

Design procedure

1. Define the run lifecycle

At minimum support:

• create run
• start planning
• begin execution
• pause for approval
• resume after approval
• retry a failed step
• cancel run
• complete or fail terminally

For each lifecycle stage, specify:

• entry event
• persisted state
• user-visible event
• retry or recovery behavior

2. Define the API surface

Prefer resource-oriented endpoints such as:

• POST /runs
• GET /runs/:id
• POST /runs/:id/approve
• POST /runs/:id/cancel
• POST /runs/:id/retry

If streaming is used, define event types such as:

• run.created
• plan.updated
• step.started
• tool.called
• tool.completed
• approval.requested
• run.completed
• run.failed

3. Define the orchestrator

The orchestrator should:

• own state transitions
• account for budgets
• decide whether to continue, escalate, retry, or stop
• persist every state transition
• never perform side effects without going through the tool router

4. Define tool execution

Every tool needs:

• strict input schema
• strict output schema
• authorization scope
• timeout
• retry classification
• idempotency policy
• approval policy
• structured receipt

Never let the model call arbitrary code or arbitrary shell without policy mediation.

5. Define job execution

For long tasks:

• move execution to background jobs
• persist checkpointed progress
• expose progress handles for the UI
• support resume and recovery after worker failure

6. Define memory

Separate:

• short-lived working context
• run summaries
• artifact store
• optional long-term preference memory

Retrieval rules must define:

• source eligibility
• freshness expectations
• ranking strategy
• citation or provenance requirements
• redaction policy

7. Define auth and policy

At minimum specify:

• caller identity
• tenant or workspace scope
• tool permission scope
• approval authority
• audit identity for every action

8. Define reliability controls

Always include:

• timeouts
• bounded retries
• exponential backoff
• circuit breakers or backpressure if appropriate
• dead-letter handling for repeated failures
• idempotency keys for side effects

9. Define receipts and auditability

Every executed step should produce:

• step_id
• tool_name
• validated inputs summary
• outcome status
• what changed
• where it changed
• failure classification if applicable
• next suggested action

Threats to address

At minimum discuss:

• prompt injection via retrieved content or tool output
• unauthorized tool access
• exfiltration of secrets or tenant data
• duplicate side effects from retries
• stale or conflicting memory
• hidden cost growth

Output contract

When asked for backend design, return:

1. Component architecture
2. Run lifecycle
3. API surface
4. Event model
5. Tool contract table
6. Persistence model
7. Memory and retrieval policy
8. Auth and approval model
9. Reliability controls
10. Threats and mitigations
11. Test and eval plan

Anti-patterns

Avoid:

• a single god-service with planning, execution, memory, and auth mixed together
• tools that return free-form strings as their contract
• retries without idempotency
• streaming without persisted event state
• background jobs with no resume semantics
• audit logs that cannot explain user-visible changes

Done criteria

This skill is done only when:

• the run lifecycle is explicit
• tool and event contracts are concrete
• retries and idempotency are specified
• approvals and auth scopes are defined
• the design can support replay, debugging, and UI receipts