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When an AI agent appears to succeed, the failure is usually one of three things: a wrong step that the final message hides, a tool call that changed real state incorrectly, or an evaluation that scored the wrong thing. Debug from the execution trace and the resulting state, not from how fluent the last answer sounds.
This guide covers the failure patterns that recur in agent builds rather than a log of one particular system. Because the agent’s task is not specified here, the examples are general or labeled as hypothetical. Where a claim comes from a vendor or published report, the source and date are named.
Judge the outcome in the environment, not the final message
An agent’s final text is a claim about what happened. The environment is where you check that claim. For a support agent, that means the ticket status and the refund record. For a coding agent, it means the file on disk and the test result. For a booking agent, it means the reservation in the provider’s system. Write the success condition as a check you can run against that state.
Consider a hypothetical refund agent that reports “Your refund has been issued.” If the payment call failed with a schema error and the agent wrote a plausible confirmation anyway, a grader that reads only the text will pass the run. A check of the payment record will fail it.
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Anthropic’s January 9, 2026 engineering article on agent evaluations defines the record you need to keep: “A transcript (also called a trace or trajectory) is the complete record of a trial, including outputs, tool calls, reasoning, intermediate results, and any other interactions.” OpenAI’s evaluation documentation describes the same idea at the level of one run: “A trace captures the end-to-end record of model calls, tool calls, guardrails, and handoffs for one run.”
Log what you need before you change anything
Most debugging time is lost when the run that failed was not recorded in enough detail to reconstruct. Capture the following for every run, and keep the tool definitions as the model saw them at that moment, since descriptions change over time.
| Trace element | What to record | What it helps you find |
|---|---|---|
| Input and instructions | The exact user input and system instructions sent, with a version identifier | Whether a prompt change or the input itself caused the behavior |
| Model call | Model name, parameters, and the output or chosen action | Which decision was wrong |
| Tool definition | Tool name, description, and input schema at the time of the call | Vague or overlapping tools |
| Tool arguments and response | Arguments sent, and the full response including errors | Schema mismatches and swallowed errors |
| Handoffs (multi-agent builds) | Which agent received control and what context came with it | Context dropped or duplicated between agents |
| Guardrail results | Which checks ran and what they flagged or blocked | Whether a safety control fired or was skipped |
| Stop reason | Completed, turn limit reached, tool error, guardrail exception, or timeout | Whether the run ended cleanly or only looked finished |
| Persisted state | What was saved, replayed, or resumed between turns | Duplicate or stale context |
| Final environment state | The external record checked after the run | Whether the task actually succeeded |
Redact credentials and personal data before you share any trace.
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The failure classes that show up most often
Wrong or confusing tool use
A Partnership on AI report notes that agents can misuse tools, or pick one that does not match the user’s intent, when interfaces are vague or tool descriptions overlap. Suppose a hypothetical agent has two tools, search_orders and lookup_customer, that both accept an email address. The agent may call the wrong one, receive a valid response, and build a confident answer on it. The tool call looks successful, but the choice was wrong.
To diagnose this, reconstruct the decision: which tool was chosen, its description and schema at that moment, the arguments, the response, and what the agent did next. Ask which tool should have been chosen and what in the descriptions should have made that clear. Fixes usually involve narrowing or merging overlapping descriptions, tightening argument schemas, or rejecting ambiguous calls. Keep the root cause open until the trace supports one. A poor description and weak model reasoning can produce the same final output.
Loops, turn limits, and partial results presented as complete
OpenAI’s runtime documentation treats max-turn limits, guardrail exceptions, and tool errors as distinct failure classes. The agent runner keeps cycling through model and tool calls until it reaches a stopping point, so a run can end because it ran out of turns rather than because the task was done. The dangerous case is a run that stops at a limit and still produces fluent output.
Record the stop reason for every run. When a run hits a limit or an error, check whether the system marked a partial result as partial, reported a timeout, or presented the output as complete. A “turn limit” stop paired with a confident completion message is a defect in its own right, even if the model’s reasoning was otherwise sound.
State carried between turns
OpenAI documents several ways to carry conversation state between turns, and advises choosing one strategy per conversation in most applications. Mixing approaches causes trouble. If your code replays the full history locally while the platform also keeps server-side state, the same context can appear twice, and the model may act on repeated or stale instructions.
- Check whether each turn sends history that the platform already holds.
- Check whether a resumed run reloads state that a previous attempt had already changed.
- Check whether a retry after a tool error repeats side effects such as a payment or an outgoing email.
Unsafe instructions and unintended actions
OpenAI’s safety guidance describes prompt injection as malicious content in untrusted text or data that tries to override the agent’s instructions. The same guidance identifies two related risks: disclosure of private data, and unintended actions that result from hallucination, misunderstanding, or ambiguous input. An agent that reads a web page, an email, or a document is reading content you do not control.
The controls OpenAI documents include clear policy prompts with examples, structured outputs, approval steps for tool calls, input guardrails, and trace graders or evals that check behavior. These reduce risk. None of them is a guarantee. In a postmortem, record whether the unsafe action was blocked, approved by a human, or executed, and whether an eval now covers that input.
Evaluations that score the wrong thing
An evaluation can fail an agent for the wrong reason, or pass it for the wrong reason. Anthropic’s January 9, 2026 article describes both. In one example, Opus 4.5 solved a flight-booking task by using a policy loophole. As written, the evaluation failed that run, even though the agent had found a better outcome for the user. A grader has to test the intended outcome and the policy, not one narrow output shape.
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When a score surprises you, check the grader before the agent. Does it accept equivalent answers within a sensible tolerance? Does the task state exactly what success means? For tasks whose outputs vary, run repeated trials and report the spread rather than a single pass or fail.
Multi-agent coordination
Anthropic’s June 13, 2025 engineering article on multi-agent systems states: “Systems with multiple agents introduce new challenges in agent coordination, evaluation, and reliability.” Every added agent adds a handoff, and every handoff is a place where context can be dropped or duplicated. Before you split a task across agents, make sure each handoff appears in the trace and check what the receiving agent was given. Do not assume that more agents will make the system more reliable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A postmortem procedure you can run
- Write the task and a testable success definition that names the environment check, such as a record, file, or message that must exist afterward.
- Pull one representative failing trace together with the matching state snapshot. Redact credentials and personal data.
- Classify the break as task specification, tool selection or execution, state handling, runtime limits, safety, or evaluator error. Choose one primary class.
- Separate evidence from hypothesis. State the root cause only as far as the trace supports it, and list the alternatives you have not ruled out.
- Make the smallest change that addresses the cause, such as one tool description, one schema, or one stop rule.
- Rerun the same failing case, then run nearby cases to catch regressions, checking the environment state each time. Report the result as a change in the task outcome, not a change in wording.
What published evidence can and cannot tell you
Public data on agents is thin, which limits how far you can compare your results with others. The MIT AI Agent Index (2025) reports that 135 of 240 safety, evaluation, and social-impact fields had no information available. Of the 30 agents it indexed, 25 disclosed no internal safety results and 23 had no third-party testing information. These figures describe what the index’s sample published. They are not measurements showing that those agents are unsafe, and they do not predict how your own build will behave.
Platform timelines to check
OpenAI’s safety documentation, checked on October 7, 2026, states that Agent Builder is scheduled to shut down on November 30, 2026, and that ChatKit remains available. If your build depends on Agent Builder, plan the migration now and confirm these dates against OpenAI’s current documentation, since schedules change.
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