mongodb output fails with error when using MongoDB Extended JSON ($date)

[mladBlum]

I want to insert a JSON document into MongoDB that contains a date field expressed in MongoDB Extended JSON format (see https://www.mongodb.com/docs/manual/reference/mongodb-extended-json/#mongodb-extended-json--v2-). This is valid JSON — an object with a "$date" field containing an ISO8601 string.

Example input:

{
  "thing_id": "t1",
  "timestamp": {
    "$date": "2025-10-13T09:13:52Z"
  }
}

Expected behavior:

• Connect should forward this JSON unchanged to MongoDB. MongoDB should interpret { "$date": "2025-10-13T09:13:52Z" } as a Date and store ISODate("2025-10-13T09:13:52Z").

Actual behavior:

Instead of inserting the document, Connect crashes with the following error mesage:

rpk connect run config.yaml
INFO[2025-11-17T17:38:01+01:00] Running main config from specified file       @service=redpanda-connect benthos_version=4.70.0 path=config.yaml
INFO[2025-11-17T17:38:01+01:00] Successfully loaded Redpanda license          @service=redpanda-connect expires_at="2035-11-15T17:38:01+01:00" license_org="" license_type="open source"
INFO[2025-11-17T17:38:01+01:00] Listening for HTTP requests at: http://0.0.0.0:4195  @service=redpanda-connect
INFO[2025-11-17T17:38:01+01:00] Input type file is now active                 @service=redpanda-connect label="" path=root.input
INFO[2025-11-17T17:38:01+01:00] Launching a Redpanda Connect instance, use CTRL+C to close  @service=redpanda-connect
INFO[2025-11-17T17:38:01+01:00] Output type mongodb is now active             @service=redpanda-connect label="" path=root.output
ERRO[2025-11-17T17:38:01+01:00] Failed to send message to mongodb: failed to execute document_map: invalid JSON input; expected {  @service=redpanda-connect label="" path=root.output
INFO[2025-11-17T17:38:01+01:00] Pipeline has terminated. Shutting down the service  @service=redpanda-connect

How to reproduce:

1. Start MongoDB:

docker run -d -p 27017:27017 --name=mongo-example1 mongo:latest

2. Create a config.yaml file:

input:
  file:
    paths: [ ./*.json ]
    scanner:
      json_documents: {}

output:
  mongodb:
    url: mongodb://localhost
    database: admin
    collection: message
    operation: insert-one
    write_concern:
      w: majority
      j: false
      w_timeout: ""
    document_map:
      root = this

3. Create a input.json file:

{
  "thing_id": "t1",
  "timestamp": {
    "$date": "2025-10-13T09:13:52Z"
  }
}

4. Run

rpk connect run config.yaml

Strangely enough, the following JSON file works fine without an error. So expect that something happen if Connect tries to process $date.

{
  "$device_id": "fs1",
  "timestamp": {
    "$someThing": "2025-10-13T09:13:52Z"
  }
}

I hope this fits as an issue; I don't think it's a discussion.