# Breaking change in Streaming API in the next Height Coordinated Upgrade

**URL:** <https://forum.flow.com/t/breaking-change-in-streaming-api-in-the-next-height-coordinated-upgrade/7072>\
**Category:** Announcements\
**Tags:** access\
**Created:** [February 11, 2025, 1:30am UTC](https://forum.flow.com/t/breaking-change-in-streaming-api-in-the-next-height-coordinated-upgrade/7072 "2025-02-11T01:30:59Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![vishal](https://avatars.discourse-cdn.com/v4/letter/v/49beb7/32.png) [@vishal](https://forum.flow.com/u/vishal)\
**Post date:** [February 11, 2025, 1:30am UTC](https://forum.flow.com/t/breaking-change-in-streaming-api-in-the-next-height-coordinated-upgrade/7072/1 "2025-02-11T01:30:59Z")

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Hello,

Please note - there will be a breaking change to streaming Access API in the next height co-ordinated upgrade (11th Feb on Testnet and **13th Feb on Mainnet** ). However, these streaming APIs are fairly recent, therefore most apps will not be impacted.

Several AccessAPI and ExecutionDataAPI streaming endpoints include a `message_index` field in the response which indicates the order of messages sent by the server so the client can detect if they missed processing any. Previously, some endpoints used 1 as the first index in a stream while others used 0. After the HCU, all endpoints will be consistent regarding message\_index and start from index 0.

APIs that will affected (Protobuf definition) -

1. [SubscribeEvents](https://github.com/onflow/flow/blob/7d925789d0e6d7d54a4108882915a1b6f568d194/protobuf/flow/executiondata/executiondata.proto#L111) - Field [message\_index](https://github.com/onflow/flow/blob/7d925789d0e6d7d54a4108882915a1b6f568d194/protobuf/flow/executiondata/executiondata.proto#L469-L470)
2. [SendAndSubscribeTransactionStatuses](https://github.com/onflow/flow/blob/master/protobuf/flow/access/access.proto#L286) - Field [message\_index](https://github.com/onflow/flow/blob/master/protobuf/flow/access/access.proto#L753-L754)
3. [SubscribeAccountStatuses](https://github.com/onflow/flow/blob/7d925789d0e6d7d54a4108882915a1b6f568d194/protobuf/flow/executiondata/executiondata.proto#L205) - Field [message\_index](https://github.com/onflow/flow/blob/7d925789d0e6d7d54a4108882915a1b6f568d194/protobuf/flow/executiondata/executiondata.proto#L621-L622)

Thank you,  
Flow Team