Function app freezes when receiving POST request larger than about 2 million characters (locally) on python 3.13

[BenKGIT]

Setup

When I run my Azure function app locally via func start in PowerShell I try to send a POST request to an endpoint I created. The POST request simply consists of one key and its value with more than about 2 million random alphanumeric characters.

function_app.py

import azure.functions as func
import json
import logging

app = func.FunctionApp()

@app.route("test",methods=["POST"])
def test(req: func.HttpRequest) -> func.HttpResponse:
    logging.info("Processing request at /test endpoint.")

    try:
        return func.HttpResponse(
            json.dumps("Received document successfully."),
            status_code=200,
            mimetype="application/json"
        )
    
    except ValueError:
        return func.HttpResponse(
            "Invalid JSON payload.",
            status_code=400
        )
    except Exception as e:
        logging.error(f"Unexpected error: {e}")
        return func.HttpResponse(
            "Internal server error.",
            status_code=500
        )

test.py

import requests
import random
import string

rnd = ''.join(random.choices(string.ascii_letters + string.digits, k=2_500_000))
print(len(rnd))

data = {"base64_document": rnd}

response = requests.post("http://localhost:7071/api/test", json=data)
print(response.status_code)

Problem

Now when I run func start and after that python test.py I get the output in the log

'local.settings.json' found in root directory (REDACTED).
Resolving worker runtime to 'python'.
Found Python version 3.13.9 (python3).
Azure Functions Core Tools
Core Tools Version:       4.5.0+e74aae22c9630777c9f58354f290a6e214218546 (64-bit)
Function Runtime Version: 4.1044.400.25520
[2025-11-30T06:02:59.501Z] Worker process started and initialized.
Functions:
        test: [POST] http://localhost:7071/api/test
For detailed output, run func with --verbose flag.
[2025-11-30T06:03:17.837Z] Worker process started and initialized.
[2025-11-30T06:03:23.816Z] Executing 'Functions.test' (Reason='This function was programmatically called via the host APIs.', Id=ca3951a9-cb7a-41ea-9267-edb2367ba261)
[2025-11-30T06:13:23.834Z] Timeout value of 00:10:00 exceeded by function 'Functions.test' (Id: 'ca3951a9-cb7a-41ea-9267-edb2367ba261'). Initiating cancellation.
[2025-11-30T06:13:23.843Z] Executed 'Functions.test' (Failed, Id=ca3951a9-cb7a-41ea-9267-edb2367ba261, Duration=600076ms)
[2025-11-30T06:13:23.844Z] Microsoft.Azure.WebJobs.Host: Timeout value of 00:10:00 was exceeded by function: Functions.test.

but nothing after that. It runs for a 10 minutes (the timeout duration I set) and eventually gives me a 500 or 504.

I have tried all configurations for my host.json and local.settings.json I could find and change them to the maximum values but that did not fix my problem.

This problem should be easily reproducible though because the problem also occurs with the default settings and the provided code when running on python version 3.13. Does anyone have an idea how to fix this without changing the request data type (like multipart) or chunking?

I tested out different python versions and it seems to be an issue exclusive to python 3.13 (I tested 3.13.9, 3.13.8, 3,12.12, 3.10.11). 3.10 and 3.12 worked.

[hallvictoria]

Hi @BenKGIT, thanks for reporting.

This has been fixed in #1805. It'll be part of host version 4.1046, and ETA for the release to complete is Jan - Feb. The recommendation for this scenario currently is to stay on python 3.12.

Closing this as the fix has been merged & release is in-progress, but please feel free to re-open if you have any further questions or if your app picks up the new host update and the issue persists.