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Build an AI agent by defining a bounded task and its completion conditions, then combine a language model with explicit instructions, a small set of tools, and a controlled run loop. Start with one agent, trace every model and tool call, evaluate failures against repeatable examples, and add routing or additional agents only when measurements show that the simpler design is insufficient.

What makes a system an AI agent?

An agent uses a model to manage workflow execution and decisions on a user’s behalf. It can choose a tool, inspect the result, continue or correct its work, recognize completion, and stop or return control when it cannot proceed. A one-turn chatbot, text generator, or classifier that does not control a workflow is not an agent in this sense.

The smallest useful mental model is:

  • Model: reasons about the current state and proposes the next step.
  • Instructions: define the goal, policies, boundaries, output format, and stopping rules.
  • Tools: retrieve information or perform narrowly defined actions in external systems.

Retrieval, memory, guardrails, approvals, and tracing can augment this core, but they do not replace a clear task definition.

1. Define a bounded job before choosing a model

Write the user goal

State the job in one sentence, such as “Classify an incoming support request, look up the relevant account, draft a response, and ask for approval before sending.” Identify the users, expected inputs, allowed data, permitted actions, and the person or system that receives the result.

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Define completion and stop conditions

A run should end when it produces the required result, has no useful tool call left, encounters an unrecoverable error, reaches a turn limit, or must hand control to a human. Specify what a successful result contains and what the agent must do when information is missing. An explicit stop rule prevents an apparently autonomous loop from running indefinitely.

Decide whether you need an agent

If every case follows a predictable sequence, ordinary code or a fixed language-model workflow is usually easier to test and operate. Agents are most useful when the steps depend on information discovered during the task and cannot be reliably hardcoded in advance.

2. Design the first architecture

Keep the initial toolset small

Expose only task-relevant operations. Each tool needs a clear name, a typed parameter schema, documented side effects, authentication behavior, expected errors, and a concise description that helps the model select it correctly. Separate read tools from write tools so that approval can be required for the latter.

Use structured state

Keep workflow state outside free-form prose where possible. Store identifiers, status, selected records, tool results, and approval decisions in a schema. Structured intermediate data makes validation, redaction, retries, and evaluation more reliable than passing an ever-growing transcript as the sole source of truth.

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Separate instructions from untrusted data

Web pages, emails, uploaded files, and tool results may contain text that attempts to override the agent. Treat those values as data, not as privileged instructions. Do not insert untrusted content into a developer-level instruction field. Mark trust boundaries explicitly and allow the model to quote or summarize content without granting it new authority.

3. Implement a controlled run loop

Every orchestration approach needs a run: a loop that lets the agent operate until an exit condition is reached. A provider-neutral loop looks like this:

state = initialize(user_request)
for turn in range(MAX_TURNS):
    decision = model.decide(instructions, state, tool_schemas)

    if decision.type == "final":
        validate_final(decision.output)
        return decision.output

    if decision.type != "tool_call":
        return handoff("The agent produced no actionable next step")

    tool = approved_tools.get(decision.name)
    if tool is None:
        return handoff("Requested tool is not available")

    validate_arguments(tool.schema, decision.arguments)
    if tool.requires_approval:
        approval = request_human_approval(decision)
        if not approval:
            return handoff("Action declined or expired")

    result = tool.invoke(decision.arguments)
    state = append_observation(state, decision, result)

return handoff("Turn limit reached")

In production, persist a run identifier and state after each step. Record model input and output, selected tool, validated arguments, latency, result status, approval decision, and the reason the run stopped. Redact secrets and unnecessary personal data before storing traces.

Retries and failures

Retry only transient failures, with a bounded count and backoff. Do not blindly repeat a write operation whose first attempt may have succeeded; use idempotency keys or a read-after-write check. Return a clear handoff when a tool is unavailable, its result is malformed, or the model cannot satisfy a required condition.

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4. Make tools safe and predictable

  • Give each credential the minimum scope required for the task.
  • Validate arguments against a schema before invocation; reject unknown fields and unsafe values.
  • Validate important tool responses before adding them to state.
  • Use allowlists for destinations, records, file paths, and resource types where possible.
  • Require human approval for irreversible, financial, externally visible, or high-impact actions.
  • Run experiments in a sandbox with synthetic data and reversible side effects.
  • Apply rate limits, timeouts, payload limits, and per-run budgets.

These controls reduce risk but cannot make an agent error-proof. A prompt alone is not a security boundary.

5. Choose a workflow pattern

Prompt chaining

Use fixed sequential stages when each step has a stable purpose, such as extract facts, validate them, then draft an answer. Intermediate checks can stop a bad result before the next stage.

Routing

Route distinct input classes to specialized processes. For example, billing, account access, and technical incidents can use different instructions and tools after an initial classifier chooses a route.

Parallelization

Run independent lookups together to reduce waiting, or ask independent reviewers to assess the same result. Merge outputs with an explicit conflict policy rather than allowing whichever response arrives last to win.

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Orchestrator and workers

An orchestrator can inspect a request and dynamically assign subtasks to specialists. This is useful when the required subtasks depend on the input, but it adds coordination, state, and failure-handling overhead.

Evaluator and optimizer

When quality criteria are explicit, generate an output, evaluate it against those criteria, and revise only when the evaluator identifies a fixable problem. Set a maximum number of revisions and preserve the original output for comparison.

Open-ended agent loop

For genuinely unpredictable work, let the agent plan, act, observe environmental results, and continue or stop. Expect higher latency, cost, and compounding-error risk; sandbox extensively before granting broad permissions.

6. Start with one agent, then justify more

Put the initial tools in one agent while their descriptions and selection logic remain understandable. Consider specialist agents when instructions have become difficult to maintain, tools overlap and confuse selection, or the task separates into clear domains. A manager can call specialists, or peer agents can hand work to one another. Define the handoff payload, ownership of state, timeout, and failure behavior. More agents are not automatically more capable; coordination itself creates new failure modes and overhead.

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7. Evaluate behavior, not just final text

Build a representative dataset

Include ordinary successful requests, ambiguous inputs, missing or malformed tool results, permission-boundary cases, prompt-injection attempts, transient errors, and tasks that should stop or ask a human. Record the expected completion state and acceptable alternatives.

Use traces and graders

A trace should show model calls, tool calls, guardrails, handoffs, and the final stop reason. Trace graders can check whether the right tool was selected, arguments were valid, a handoff occurred at the right time, and a safety policy was followed. Promote stable examples into a repeatable dataset.

Change one variable at a time

Run a capable model first to establish a baseline. Then compare less costly or faster models on the same cases and acceptance criteria. When a run fails, make the failure reproducible before changing the prompt, tool schema, model, or routing. Compare the revised trace with the baseline rather than relying on an anecdotal improvement.

8. Select an SDK or runtime deliberately

An SDK runtime is useful when you want it to manage turns, function-tool schema validation, handoffs, guardrails, sessions, human involvement, MCP integrations, and tracing. Owning the loop through a direct model API can be preferable when the workflow is short-lived or your application must control dispatch, state, and persistence itself. This is a division of responsibilities, not a universal framework ranking.

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Decision area Questions to answer
State ownership Where are sessions, checkpoints, retries, and run limits stored?
Orchestration Are steps fixed, dynamically routed, or a mixture?
Tools Does the runtime validate schemas, handle MCP, and isolate credentials?
Approvals Can sensitive calls pause for a human and resume safely?
Operations Are traces, redaction, replay, and repeatable evaluations available?
Deployment Does it fit your language, latency budget, hosting, and compliance constraints?

Measure model cost, latency, and reliability on your actual task. The documented guidance does not establish a universal benchmark winner.

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9. Troubleshoot common failures

The agent calls the wrong tool

Reduce overlapping tools, rename them around user intent, tighten descriptions and schemas, and add a trace grader for tool choice. If the route is deterministic, move routing into ordinary code.

The loop never finishes

Check that the model can emit a final result, define a maximum turn count, and require a stop reason in state. Add a handoff path for repeated equivalent calls or unchanged observations.

Tool results are unreliable

Validate response schemas, distinguish empty data from an error, set timeouts, and return machine-readable error types. Retry only known transient conditions.

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Private data leaks into output

Minimize retrieved fields, redact before tracing, enforce output validation, and require approval before external transmission. Test with deliberately sensitive fixtures.

Prompt injection changes behavior

Keep untrusted text out of privileged instructions, label its origin, restrict tools by policy, and test injection attempts in a sandbox. A successful-looking answer is not evidence that the boundary held.

10. Operate the agent after launch

Monitor stop reasons, tool error rates, approval declines, latency, token usage, and evaluation scores. Version instructions, tool schemas, model settings, and routing rules together so a trace can be reproduced. Review a sample of successful runs as well as failures: silent policy violations can look like good answers. Expand permissions gradually, and keep a manual fallback for cases outside the agent’s contract.

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FAQ

Can an agent be deterministic?

Its model decisions are probabilistic, but deterministic tools, schemas, limits, approvals, and fixed stages can constrain the overall workflow and make outcomes testable.

Should memory be added immediately?

No. Add persistent memory only when a demonstrated task requires information across runs. Otherwise, scoped run state is easier to inspect and delete.

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When should a run ask a person?

Use a handoff when authority is unclear, required data is missing, a policy boundary is reached, or the next action is irreversible or high impact.

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