AWS JSON errors discard modeled response members

[ikolomiets]

Disclaimer: The Issue and PR were created using Codex gpt-5.6-sol xhigh.

Problem

AWS JSON 1.0/1.1 service errors can contain modeled response members in addition to a message. The SDK currently identifies the correct modeled error variant but does not expose those additional members to callers.

A concrete example is DynamoDB conditional writes. DynamoDB supports ReturnValuesOnConditionCheckFailure=ALL_OLD, which allows the item responsible for a failed condition to be returned in ConditionalCheckFailedException. This is useful for optimistic-concurrency and conflict-resolution flows that need to inspect a stored value or hash.

The generated SDK already accepts the request option:

.return_values_on_condition_check_failure = .all_old

The pinned DynamoDB model also defines ConditionalCheckFailedException.Item as an AttributeMap.

Reproduction

1. Insert an item containing a partition key, sort key, and another value such as a stored hash.
2. Issue a conditional PutItem that fails its condition.
3. Set .return_values_on_condition_check_failure = .all_old.
4. Pass a ServiceError diagnostic and inspect the .conditional_check_failed_exception variant.

Expected behavior

The modeled diagnostic should expose the returned old item, allowing callers to read its keys and stored values:

switch (diagnostic.kind) {
    .conditional_check_failed_exception => |failure| {
        const old_item = failure.item;
    },
    else => {},
}

Actual behavior

The request returns error.ServiceError and the diagnostic contains the correct .conditional_check_failed_exception variant, but the generated exception type exposes only normalized message and synthetic request_id fields.

The service response body contains the old item, but it is discarded before the diagnostic is returned. Consequently, callers cannot access the item or its stored hash.

ReturnValues=ALL_OLD is not a substitute because it applies to successful writes. Conditional failures require ReturnValuesOnConditionCheckFailure.

Impact

This is not limited to DynamoDB. Modeled exception members beyond message and request_id are unavailable across AWS JSON 1.0/1.1 services. That includes other structured diagnostic data such as cancellation and throttling details, even when those fields are present in the pinned Smithy models and returned by the service.