I found a similar unsolved discussion titled JSON not recognizing a string item from an iteration, which might be related to the issue you're experiencing ^[1].

To address the JSON parsing error when using a string array in the HTTP Request node in Dify version 0.15.1, ensure that the JSON string is correctly formatted before it is parsed. The _init_body method in the Executor class attempts to parse the JSON string using json.loads(json_string, strict=False). If the JSON string is not correctly formatted, a json.JSONDecodeError will be raised.

Here's a snippet of the relevant code:

case "json":
    if len(data) != 1:
        raise RequestBodyError("json body type should have exactly one item")
    json_string = self.variable_pool.convert_template(data[0].value).text
    try:
        json_object = json.loads(json_string, strict=False)
    except json.JSONDecodeError as e:
        raise RequestBodyError(f"Failed to parse JSON: {json_string}") from e
    self.json = json_object

Ensure that the JSON string is valid and properly formatted before it is passed to the HTTP Request node. If the JSON string is generated dynamically, verify that all necessary escaping and formatting are correctly applied. If the error persists, inspect the JSON string being passed to identify any syntax issues ^[2].

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Your suggested case is not using any of string array instance.
Furthermore, we should never pervent segement usage of virables in HTTP node's JSON body, by forcing it pre-generated.

/api/core/workflow/nodes/http_request/executor.py
Change line:
json_object = json.loads(json_string, strict=False)

To:
json_object = json.loads(json_string.replace("'", '"'), strict=False)

This work for me