[BUG]: MockResponsePlugin tries to parse non-JSON bodies as JSON and emits noisy warning

[waldekmastykarz]

Description

MockResponsePlugin attempts to parse request bodies as JSON even when the request uses the application/x-www-form-urlencoded content-type. This results in a warning:

MockResponsePlugin:  Failed to parse request body as JSON.  Placeholders in the mock response won't be replaced.

The plugin should only attempt to parse bodies as JSON when the content-type is a known JSON type or is absent (for backward compatibility).

Root Cause

The issue is in the [ReplacePlaceholders method in MockResponsePlugin.cs](https://github.com/dotnet/dev-proxy/blob/33a7bc4a7807f4a56ee847e3b12e4756dac6414a/DevProxy. Plugins/Mocking/MockResponsePlugin.cs#L528-L550). It unconditionally attempts to deserialize the request body as JSON without checking the Content-Type header first:

try
{
    var requestBody = JsonSerializer. Deserialize<JsonElement>(request.BodyString, ProxyUtils.JsonSerializerOptions);
    response.Body = ReplacePlaceholdersInObject(response. Body, requestBody, logger);
}
catch (Exception ex)
{
    logger.LogDebug(ex, "Failed to parse request body as JSON");
    logger.LogWarning("Failed to parse request body as JSON.  Placeholders in the mock response won't be replaced.");
}

Suggested Fix

Before attempting JSON parsing, check the request's Content-Type header. Only parse JSON if:

• The content-type is a JSON type (e.g., application/json, application/vnd.api+json, etc.)
• No content-type is set (for backwards compatibility)

Expected behaviour

The plugin should only parse the request body as JSON if the content-type indicates a JSON type or if there is no content-type header. There should be no parsing attempts (and thus no warnings) for other content-types such as application/x-www-form-urlencoded.

Actual behaviour

When the content-type is set to application/x-www-form-urlencoded, MockResponsePlugin still attempts to parse the body as JSON and emits a warning that it failed to parse. This is both noisy and misleading for users sending form-encoded requests.

Steps to reproduce

1. Start Dev Proxy.

2. Send a POST request with content-type set to application/x-www-form-urlencoded:

   curl -ikx http://127.0.0.1:8000 -X POST https://api.twilio.com/2010-04-01/Accounts/AC00000000000000000000000000000000/Messages. json \
       -d 'To=+15559876543' \
       -d 'From=+15551234567' \
       -d 'Body=Hello from Dev Proxy! '

3. Observe the warning in the proxy logs:

MockResponsePlugin: Failed to parse request body as JSON.  Placeholders in the mock response won't be replaced.

Dev Proxy Version

latest from main

Operating system (environment)

macOS

Shell

zsh

Configuration file

n/a

Additional Info

The warning should only appear when the content-type is JSON or unset, not for non-JSON content-types. This change will prevent unnecessary log noise and clarify actual parsing failures.