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Freedom251 › Blog › agent safety

I Built an AI Agent That Could Call APIs. Then I Had to Teach It When Not to Call Them

By Freedom251 Team · · 4 min read

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An AI agent’s access to an API does not mean every request should trigger it. A model can propose a tool call; the application decides whether to execute that request. The important engineering lesson is to teach the agent when a tool is appropriate, then enforce consequential boundaries in software before an action can change anything.

This is a design lesson, not a report of a specific build: no verified implementation details are available to describe what an agent did, which call it made, or what changed in its control flow. The practical question is how to design that boundary responsibly.

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What does it mean for an agent to call an API?

In OpenAI’s documented function-calling flow, a model receives descriptions of tools supplied by an application. If it decides that a prompt calls for one, it returns a tool call. The application—not the model—executes that call, provides the result back to the model, and continues the conversation. The model may then respond to the user or request another tool call. See OpenAI’s function-calling documentation.

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That division matters: a tool call is a request to the application, not proof that the action is needed or safe. API access establishes what an agent could ask to do; the application still owns the decision to carry it out.

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Teach the agent when not to use a tool

A tool description should do more than explain its name. OpenAI recommends describing what each function and its parameters do, when to use it and when not to use it, and examples or edge cases for recurring failures. For instance, if an agent has a lookup tool and an action tool, instructions can distinguish requests that need fresh external information from those it can answer without a call. That example is a design pattern, not a claim about a particular implementation.

Instructions help the model choose well, but they are not an execution boundary. OpenAI also recommends making tool interfaces predictable: use clear parameter descriptions, enums and structured objects to limit invalid states, and have application code supply arguments it already knows rather than asking the model to invent them. As the tool surface grows, the function-calling guide suggests limiting initially exposed functions or deferring rarely used ones; its suggestion of fewer than 20 is a soft design recommendation, not a universal cutoff.

Put checks where side effects happen

Not all tools carry the same consequences. OpenAI’s practical guide distinguishes data tools that retrieve context, action tools that can update records or send messages, and orchestration tools that let agents work through other agents. That distinction suggests different review policies: a read-only lookup may need different controls from a tool that changes a record. It is a design inference, not a measured risk ranking.

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OpenAI’s guardrail documentation recommends placing validation close to the tool that can produce a side effect. Tool guardrails apply to the function tools they are attached to, while agent-level input and output guardrails have a more limited workflow scope. A check beside the consequential capability can inspect the proposed action before the application executes it. See OpenAI’s guardrails and human review guide.

Choose a control that matches the decision

  • Instructions and examples explain intended use and non-use to the model. They help resolve ambiguity but do not, by themselves, prevent execution.
  • Schemas and structured data constrain argument shapes and can reduce the risk of passing free-form text as instructions.
  • Application-side validation checks the proposed call against rules enforced by the application before a side effect occurs.
  • Programmatic approval is appropriate when the application can encode the safety decision deterministically.
  • Human approval is useful when a person must review a sensitive action before execution.
  • Trace evaluation helps examine tool decisions and find workflow mistakes that need correction.

These controls solve different problems. Instructions influence model behavior; schemas constrain data; validation and approval determine whether a proposed action proceeds.

Use approval when a call needs review

For a sensitive tool call, an application can pause for review rather than execute immediately. OpenAI documents a review flow that returns an interruption and resumable state; after the review, the same run can resume. For MCP tools in the Agents SDK, approval can be configured for all calls or selected tools. An approval callback can decide programmatically, while the human-in-the-loop flow is available when a person needs to make the decision. The current SDK details are in OpenAI Agents SDK’s MCP guide.

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Approval should be scoped to the action that warrants it. Requiring human review for every harmless lookup may add friction without addressing the key boundary; allowing a consequential write to proceed solely because it was suggested by a model leaves the application without that review point.

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Treat outside text as untrusted input

Prompt injection is one reason not to let arbitrary text decide what tools an agent uses. OpenAI defines it as untrusted text or data that attempts to override instructions. Its safety guide warns that downstream effects can include private-data exposure and unintended actions. As the guide puts it, “Risk rises when agents process arbitrary text that influences tool calls.”

OpenAI recommends keeping untrusted variables out of developer messages, constraining data passed between workflow steps with structured outputs, using clear policy examples, adding input guardrails and approvals, and grading traces. These are layers of mitigation, not a guarantee that an agent will always behave correctly or cannot be tricked. See OpenAI’s safety guide for building agents.

What the build lesson really is

Granting API access creates a possible action, not an obligation to act. Clear positive and negative instructions can make tool selection more deliberate, while structured interfaces, application-side checks and approvals govern what actually executes. Keep those layers distinct, put checks beside consequential capabilities, and evaluate traces for mistakes. None makes an agent infallible; together, they make the boundary between a model’s request and an application’s action explicit.

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