iot describe_thing returns 500 InternalError after update_thing sets thingTypeName

[yaxi-lei-simplisafe]

Calling update_thing with a thingTypeName (either the same value that create_thing set, or a new value) leaves the internal Thing record in a state where any subsequent describe_thing, list_things, or search_index call fails to serialize its response and returns 500 InternalError.

Minimal reproducer (AWS CLI against localstack-pro)

  aws --endpoint-url http://localhost:4566 --region us-east-1 iot create-thing-type --thing-type-name thingType
  aws --endpoint-url http://localhost:4566 --region us-east-1 iot create-thing --thing-name t1 --thing-type-name thingType
  aws --endpoint-url http://localhost:4566 --region us-east-1 iot update-thing --thing-name t1 --thing-type-name thingType --attribute-payload 'attributes={test1=A,test2=B},merge=false'
  aws --endpoint-url http://localhost:4566 --region us-east-1 iot describe-thing --thing-name t1
  # InternalError (500)

Expected: describe-thing returns {"thingName":"t1","thingTypeName":"thingType","attributes":{...}} — matches real AWS behavior (verified against a real AWS account, same sequence works).

Actual: describe-thing returns HTTP 500 with:

An error occurred (InternalError) when calling the DescribeThing operation:
exception while calling iot.DescribeThing: An unknown error occurred when trying to serialize the response.

Server-side traceback:

File ".../localstack/aws/protocol/serializer.py", line 1290, in _serialize_body_params
return json.dumps(body)
TypeError: Object of type FakeThingType is not JSON serializable

Not affected

• create_thing(name, thingTypeName=...) alone → subsequent describe_thing returns 200.
• update_thing without thingTypeName on a typed Thing → subsequent describe_thing returns 200.
• Things with no type → describe_thing returns 200.

Only the combination "update_thing passes thingTypeName" corrupts subsequent reads.

Environment tested

  ┌──────────────────────────────────┬──────────────┬───────────────────────────────┐
  │          localstack-pro          │ bundled moto │ describe_thing after sequence │
  ├──────────────────────────────────┼──────────────┼───────────────────────────────┤
  │ 3.0.2                            │ 4.2.10.dev   │ 500 InternalError             │
  ├──────────────────────────────────┼──────────────┼───────────────────────────────┤
  │ 3.8.1                            │ 4.2.10.dev   │ 500 InternalError             │
  ├──────────────────────────────────┼──────────────┼───────────────────────────────┤
  │ 4.4.0                            │ (4.x)        │ 500 InternalError             │
  ├──────────────────────────────────┼──────────────┼───────────────────────────────┤
  │ 4.7.0                            │ (4.x)        │ 500 InternalError             │
  ├──────────────────────────────────┼──────────────┼───────────────────────────────┤
  │ latest nightly (2026.5.0.dev170) │ 5.1.21.dev   │ 500 InternalError             │
  └──────────────────────────────────┴──────────────┴───────────────────────────────┘

Isolation: pure moto is NOT the culprit

Running the exact same sequence under pure moto (via @mock_aws, no localstack involved) succeeds:

  from moto import mock_aws
  import boto3

  @mock_aws
  def go():
      c = boto3.client('iot', region_name='us-east-1')
      c.create_thing_type(thingTypeName='thingType')
      c.create_thing(thingName='t1', thingTypeName='thingType')
      c.update_thing(thingName='t1', thingTypeName='thingType',
                     attributePayload={'attributes': {'test1': 'A', 'test2': 'B'}, 'merge': False},
                     removeThingType=False)
      print(c.describe_thing(thingName='t1'))
      # {'thingName': 't1', 'thingTypeName': 'thingType', 'attributes': {'test1': 'A', 'test2': 'B'}, ...}
  go()

moto/iot/models.py:FakeThing.to_dict() correctly maps self.thing_type.thing_type_name to the string "thingType". moto.iot.responses.IoTResponse.describe_thing does json.dumps(thing.to_dict(...)) and returns the correct payload. moto is fine.

Root cause (revised): LocalStack Pro's own IoT provider

Provider registration in localstack/pro/core/providers.py:

  @pro_aws_provider(api='iot')
  def iot():
      from localstack.pro.core.services.iot.provider import IotProvider as A
      return Service.for_provider(A(), dispatch_table_factory=MotoFallbackDispatcher)

MotoFallbackDispatcher routes each operation to IotProvider if implemented, else falls back to moto. Since pure moto works but the Pro image doesn't, IotProvider (in localstack/pro/core/services/iot/provider.py — shipped as an encrypted .enc file in the image) is overriding describe_thing / update_thing and bypassing moto's to_dict(). The override appears to hand the raw moto.iot.models.FakeThing (or a proxy of it) to the AWS-protocol serializer, which picks up the thing_type attribute — a FakeThingType object — into the response body dict before json.dumps. Either the Pro provider should call to_dict() on the FakeThing before returning, or it should project thing_type → thing_type.thing_type_name (string) itself.

Impact

Any integration test that exercises the real AWS pattern "create a thing, then UpdateThing to associate/change its type, then read it back" is forced into a workaround (update_thing(removeThingType=True) before reads) because LocalStack Pro's read APIs 500. This is a common pattern for healing or migrating thing types in production code.