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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteWhen a coding agent calls your command-line tool, the error contract determines whether it can recover safely or has to guess. Give failures stable machine-readable codes, define whether retrying the same invocation is safe, keep response fields predictable, and document exactly what the process exit status means.
Why a CLI error is part of the agent API
A human can often infer meaning from a sentence such as “operation failed.” An agent needs to identify the condition reliably and decide what to do next. If it must parse changing prose, infer whether a command made changes, or guess whether a nonzero exit means the wrapper failed or the requested task failed, recovery becomes fragile.
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Design the CLI’s errors as a contract: stable identifiers for branching, messages for explanation, explicit retry and side-effect semantics, a consistent response shape, and documented process-status behavior. This applies whether the caller is a coding agent, an automation script, or another application.
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Give each failure a stable code
Use a specific, stable error code for application logic and reserve the message for people. OpenAI’s Agents API guidance puts it directly: “For structured errors, use error.code in application logic and error.message to explain the failure.” See OpenAI’s Agents API error guidance.
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For example, a code such as permission_denied can remain stable while its message changes to include a path or a more useful explanation. Avoid asking the agent to identify a permission problem by matching phrases in a message that may change across versions or locales.
- Choose codes that distinguish conditions requiring different actions; a single
failedcode is rarely enough. - Document each code’s meaning and the action, if any, a caller may take.
- Require consumers to tolerate unknown codes and missing optional parameters rather than crashing inside their own error handler, as OpenAI’s guidance recommends.
Make retries safe—or explicitly unsafe
A retry decision needs more than a transient/permanent label. The agent must know whether it can repeat the identical invocation unchanged and whether the first attempt may already have caused side effects.
The CLI Agent Spec’s ExitCode schema defines a retryable result as one for which the identical invocation may be retried unchanged and guarantees that no side effects occurred. It treats partial failure as non-retryable. Those are strong, useful semantics: a generic “retryable” flag without a side-effect guarantee could lead an agent to duplicate a payment, write, deployment, or other operation.
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Represent this decision explicitly in the structured result. State whether the operation is safe to retry unchanged, whether any effects occurred, and whether completion was partial. If the command timed out or returned an ambiguous failure, do not imply that nothing happened. OpenAI’s error guidance likewise advises checking completed actions and effects before resubmitting after a failed turn. See the CLI Agent Spec project and its ExitCode schema.
Keep the response shape predictable
Return the same top-level envelope for success and failure wherever practical, with fields consistently present and values that can be absent represented in a defined way. An agent should not have to guess whether an error code moved to a different location or whether a field disappears only for one failure type.
The CLI Agent Spec’s ResponseEnvelope schema describes stable error codes as machine-facing branch values and messages as human-facing explanations. Its invariant envelope is intended to make parsing and recovery less dependent on individual command implementations. Define field types, nullability, and required-versus-optional status, then preserve those choices across commands and versions. See the ResponseEnvelope schema.
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Separate process status from task outcome
Document what the process exit code represents. One valid design is to return nonzero whenever the requested task fails. Another, used by the A2A CLI specification, uses process status to report whether the CLI itself completed its work and reports the remote task outcome in a structured task state. Under that contract, the CLI can exit successfully after correctly conducting and reporting a task that ultimately failed.
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The A2A CLI specification summarizes its choice: “The exit code is the coarse signal for shells and CI, the only result a caller gets without parsing output.” It is an example, not a universal rule. Whichever model you choose, make it consistent and ensure callers can distinguish a CLI execution failure from a task failure. See the A2A CLI specification.
Keep machine output parseable
In machine-readable mode, reserve stdout for the structured payload. Send diagnostics, prompts, progress indicators, and logs to stderr so they cannot corrupt JSON or JSONL consumed by an agent. If the CLI streams output, define the framing and specify how errors appear in that stream as well as in the final status.
The A2A CLI specification documents this stdout/stderr separation. It also distinguishes CLI-local failures from protocol-level failures, a useful reminder that an error contract should locate the layer that failed rather than flattening every problem into one generic result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make the contract discoverable
Agents and the people integrating them benefit when the CLI can describe itself in machine-readable form. The CLI Agent Spec describes a command manifest with commands, flags, types, exit-code maps, and examples. Such a manifest gives callers more than a list of command names: it can expose the expected inputs and the failure vocabulary needed to handle results safely.
Document the same contract for humans, including the meaning of codes, response fields, retry guarantees, side effects, and exit statuses. A manifest is useful only if it stays aligned with the behavior callers actually receive.
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A practical error-contract checklist
- Does every meaningful failure have a stable, specific code?
- Can an agent handle an unknown code or missing optional field without failing its own error handling?
- Does the result say whether the same invocation may be retried unchanged?
- Does it say whether side effects occurred, including partial completion?
- Are response fields and types consistent across success and failure?
- Is the process exit code’s relationship to the structured task outcome explicit?
- Does machine mode keep stdout clean and send diagnostics to stderr?
- Can callers discover commands, arguments, and failure meanings from a manifest or equivalent documentation?
What the available specifications establish
The CLI Agent Spec project reports 75 documented failure modes and 160 requirements in its repository state accessed on October 7, 2026. It also claims that no existing CLI framework covers more than 59% of the failure modes it had mapped at that time. These are the project’s own mutable repository figures, not independently validated industry statistics. The project describes six canonical JSON schemas and a matrix of 12 frameworks over 71 mapped failure modes; those are project-reported scope figures as well. Check the live project repository for current counts.
These materials offer design dimensions, not an independently established head-to-head ranking of CLI frameworks. The useful takeaway is the contract itself: agents need to know what failed, what the result means for side effects, and what action is safe next.
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