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Tool error text can contain adversarial instructions. This is not a substitute for “Return an explicit not-found outcome with its semantics rather than silently fabricating a record”; that control is necessary in the stated circumstances. A validly formed lookup that matches no record is a business-level not-found outcome, not permission to fabricate a customer.

Uncertain writes must not be repeated blindly. That addresses a different part of the system, not the requirement to make absence of a requested entity explicit.

Retry-after metadata is appropriate when a provider has throttled a request. A valid customer lookup that returns 'not found' is a different outcome: no matching entity exists. Report an explicit not-found status without implying the result will appear after a timing delay.

Fabricating a record hides the not-found condition and can cause unauthorized follow-on actions. A nonexistent record must remain not found, with no fabricated balance, customer or entitlement. Returning a made-up successful object could trigger billing decisions against someone who never passed the lookup.

An upstream timeout is not proof that no account records matched. Reporting an empty successful list conceals a dependency failure and may cause the model to tell the customer no account exists. Propagate the provider timeout with a correlation ID so the client can apply a bounded recovery policy.

Per-record statuses are necessary when some items in a bulk update succeed, but this CRM lookup did not return an authoritative account list at all. Its upstream call timed out. Propagate that failure with a safe provider/correlation reference rather than describing any number of records as successful.

A correlation token lets support teams investigate without dumping stack traces or private data into model context. This capability does not directly preserve upstream failure semantics in the tool layer.

That makes the agent misinterpret an outage as an empty query. An empty successful result means the operation ran and found no records. A provider exception is different: callers need a failure state and a safe retry or escalation path, not evidence falsely indicating no matching data.

Exposing raw secrets creates an additional compromise path. Request credentials are secrets, not debugging context for a language model. Redact authorization headers and sensitive tokens, log protected diagnostics privately, and return only an error category and safe correlation identifier.

A rate-limit category explains why a request should be delayed and retried. This bulk operation completed with eight successes and two failures, not one global 429. Report each record's identity and actual status so clients can reconcile and selectively handle the two unsuccessful items.

Repeated identical requests cannot fix an access policy. That addresses a different part of the system, not the requirement to represent partial successes explicitly for safe recovery.

Diagnostics are useful but must not leak secrets into model-visible messages or logs. This alternative—Redact credentials while preserving an actionable failure code and safe context—does not implement the specific operation that the question asks about. The required control is Return per-record outcome statuses and identifiers instead of one blanket success flag. A bulk tool must report which records succeeded, which failed and which have an uncertain outcome, using stable identifiers for each item.

An authentication-required error is one useful category, but the tool currently reports every failure as UnknownError, including invalid input and rate limits. A single auth classification would mislabel other causes. Provide a taxonomy that distinguishes validation, authorization, missing records, throttling and transient service errors.

Optional tool failures need not invalidate a task if scope can be reduced honestly. The decisive requirement here is to classify failures by how callers can recover.

Tool error text can contain adversarial instructions. This alternative does not establish how to classify failures by how callers can recover in the stated workflow.

A correlation token lets support teams investigate without dumping stack traces or private data into model context. The scenario instead requires a way to distinguish protocol errors from tool-domain errors.

An MCP tool_result with isError is appropriate after a valid JSON-RPC call reaches a tool and its execution fails. A server that cannot parse the JSON-RPC envelope has not reached the tool-dispatch stage. Return a protocol-level error for that malformed request, reserving isError for failures within executed tools.

Optional tool failures need not invalidate a task if scope can be reduced honestly. Although useful elsewhere, it is not the direct mechanism to distinguish protocol errors from tool-domain errors.

A correlation token lets support teams investigate without dumping stack traces or private data into model context. This capability does not directly support safe reconciliation of uncertain write outcomes.

Tool error text can contain adversarial instructions. That addresses a different part of the system, not the requirement to support safe reconciliation of uncertain write outcomes.

Refreshing credentials addresses a confirmed authentication failure. A network timeout after a charge request leaves uncertainty about whether the financial side effect committed. Do not classify that as expired authentication; return the transaction's idempotency reference and require outcome reconciliation before retrying.

Optional tool failures need not invalidate a task if scope can be reduced honestly. Although useful elsewhere, it is not the direct mechanism to separate safe model-visible errors from detailed internal logs.

A throttled call may succeed later, unlike permanent authentication failure. This alternative does not establish how to separate safe model-visible errors from detailed internal logs in the stated workflow.

A 403 denial should not be retried without new permission, but the problem here is connecting a safe user-facing message to technical diagnostics. Preserve a nonsecret correlation ID that lets authorized engineers find restricted provider logs without exposing sensitive exception content to the model.

A correlation token lets support teams investigate without dumping stack traces or private data into model context. The scenario instead requires a way to degrade gracefully while disclosing unavailable optional data.

A detailed error taxonomy improves retry handling, but it does not decide whether optional enrichment may be omitted after a verified primary result. Mark the secondary source unavailable and restrict the final answer to confirmed primary data; do not let an auxiliary failure invalidate unrelated verified evidence.

Tool error text can contain adversarial instructions. This is not a substitute for “Mark enrichment unavailable and continue only with explicitly supported primary results”; that control is necessary in the stated circumstances. Label the supplemental source unavailable and avoid adding facts the failed service never confirmed.

Ignoring hostile error text is necessary for prompt-injection safety, but this provider returned an expired access token. The problem is correct error classification, not instruction following. Report authentication required so the authorized credential manager can refresh or reauthorize before another call.

Uncertain writes must not be repeated blindly. This capability does not directly route authentication problems to credential remediation. Read-after-write reconciliation is appropriate after an uncertain side effect, not an expired authentication token. This 401-like condition calls for credential refresh or approved reauthorization.

A permission denial means the caller lacks authorization for a resource. An expired token is more specifically an authentication-state problem, which may be resolved by refreshing credentials. Report that distinction instead of treating every expired-session response as permanent access denial.

A rate-limit response asks the caller to back off and retry after provider guidance. The current tool failed authentication, which will not be fixed by waiting for quota. Return an MCP error result with a safe authentication cause; do not turn it into either an empty successful list or a rate-limit diagnosis.

Actionable argument errors support a safe corrected retry without inventing data. Here, an MCP tool fails authentication and returns an empty successful list. Return an isError-marked response with a specific authentication failure. Empty success is semantically different from inability to access records, and the caller must know the operation failed.

A timeout is not proof of nonexecution, so blind retries can duplicate the side effect. The direct remedy is “Return an isError-marked response with a specific authentication failure”, which the alternative does not itself accomplish. Empty success is semantically different from inability to access records, and the caller must know the operation failed.