Use content-type aware deserialization for error responses

[joheredi]

Fix: [Copilot PR] Use content-type aware deserialization for error responses

Problem

When an operation returns an XML error response (e.g., Azure Blob Storage's StorageError), the generated deserialize function always used the JSON deserializer (storageErrorDeserializer(result.body)), ignoring the response's content type. This caused deserialization failures for services that return XML error bodies.

Solution

Updated the error deserialization code generation in operationHelpers.ts to detect whether exception types support XML serialization and generate the appropriate deserializer:

• XML-only exceptions: Directly uses the XML deserializer (e.g., storageErrorXmlDeserializer(result.body))
• Dual-format exceptions (both XML and JSON): Generates a runtime content-type check using isXmlContentType to select the correct deserializer
• JSON-only exceptions (default): Behavior unchanged — uses the JSON deserializer

Example generated output

Before:

const error = createRestError(result);
error.details = storageErrorDeserializer(result.body);
throw error;

After (dual-format):

const error = createRestError(result);
const responseContentType = result.headers?.["content-type"] ?? "";
error.details = isXmlContentType(responseContentType)
  ? storageErrorXmlDeserializer(result.body)
  : storageErrorDeserializer(result.body);
throw error;

Changes

• operationHelpers.ts
  • Added xmlDeserializer and isXmlOnly fields to ExceptionThrowDetail
  • Added defaultXmlDeserializer and defaultIsXmlOnly fields to OperationExceptionDetails
  • Updated getExceptionDetails to check exception contentTypes and model XML serialization metadata, building XML deserializers when applicable
  • Added getExceptionDeserializeExpr helper to generate the correct deserialization expression based on format support
  • Updated getExceptionThrowStatement to emit content-type–aware deserialization for exceptions with XML support

[timovv]

Seems ok in principle. The code seems to be combined with the accept header PR work, could you separate it out?

[timovv]

Ah or I can just review commit by commit

[timovv]

Could you add a test case? would be good to see what each of these cases actually look like

[timovv]

LGTM