# Passing custom information from clients to servers

**URL:** <https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075>\
**Category:** Flower Help - Intermediate\
**Created:** [August 8, 2025, 9:38am UTC](https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075 "2025-08-08T09:38:22Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![gxenos](https://dub1.discourse-cdn.com/flex013/user_avatar/discuss.flower.ai/gxenos/32/171_2.png) [@gxenos](https://discuss.flower.ai/u/gxenos)\
**Post date:** [August 8, 2025, 9:38am UTC](https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075/1 "2025-08-08T09:38:22Z")

</div>

Hi,

I am working on a baseline where the clients have to communicate the gradients of the network (the way the gradients are calculated is irrelevant to the question) along with the updated model. Is there a recommended way of doing this? In my mind I have an extra set of NDArrays to return along with the model parameters in the client’s fit method.

I am sure that what I am describing is a common problem but couldn’t find the recommended way.

Thanks

---

<div class="post-metadata">

**Author:** ![williamlm](https://dub1.discourse-cdn.com/flex013/user_avatar/discuss.flower.ai/williamlm/32/162_2.png) [@williamlm](https://discuss.flower.ai/u/williamlm)\
**Post date:** [August 11, 2025, 6:29pm UTC](https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075/2 "2025-08-11T18:29:21Z")

</div>

Hi @gxenos, thanks for your question.

`FitRes.metrics` [is a free-form dict](https://flower.ai/docs/framework/ref-api/flwr.common.FitRes.html) collected on the server. It only supports scalars/strings, so for NDArrays you can serialize (e.g., via `numpy.save` to bytes + base64) and unpack in a **custom Strategy.**

You could use this to communicate the gradients.

Best regards,

William

---

<div class="post-metadata">

**Author:** ![gxenos](https://dub1.discourse-cdn.com/flex013/user_avatar/discuss.flower.ai/gxenos/32/171_2.png) [@gxenos](https://discuss.flower.ai/u/gxenos)\
**Post date:** [August 13, 2025, 12:08pm UTC](https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075/3 "2025-08-13T12:08:36Z")

</div>

Thank you, I assumed that was the way to do it, just wanted to make sure.

---

<div class="post-metadata">

**Author:** ![system](https://europe1.discourse-cdn.com/flex013/uploads/flower/original/1X/bc7fe36843c391cbd0b4e8b6f48275961d356abc.png) [@system](https://discuss.flower.ai/u/system)\
**Post date:** [August 20, 2025, 12:09pm UTC](https://discuss.flower.ai/t/passing-custom-information-from-clients-to-servers/1075/4 "2025-08-20T12:09:27Z")

</div>

This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.