[BUG]: GraphMockResponsePlugin Mocked JSON date format depends on local machine culture settings

[bluepurplee]

Description

When using GraphMockResponsePlugin in Dev Proxy, the dates contained in the mocked JSON responses are formatted according to the operating system's regional culture settings. This causes inconsistent output across different environments. Since Dev Proxy intends to mock Microsoft Graph responses, date formats should be predictable and aligned with Microsoft Graph’s ISO-8601 conventions.

Expected behaviour

All mocked Microsoft Graph date/time fields should be serialized using a consistent culture-invariant ISO-8601 format (e.g. 2025-11-01T14:30:00Z), independent of the host machine’s locale/region settings. Like the sample response present in the documentation https://learn.microsoft.com/en-us/graph/throttling

Actual behaviour

Date/time values in mocked JSON responses vary depending on the OS regional settings. For example:

01/11/2025 14:30:00 on a French machine

11/01/2025 2:30 PM on a US machine

This leads to non-deterministic behavior.

Steps to reproduce

Configure Dev Proxy with GraphMockResponsePlugin enabled.

Run Dev Proxy on a machine with French regional settings.

Observe the different date fields in mocked responses

Run the same proxy on a machine with US regional settings.

Compare the JSON responses and note that date formats differ between environments.

Dev Proxy Version

1.3

Operating system (environment)

Windows

Shell

PowerShell

Configuration file

{
"$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v1.3.0/rc.schema.json",
"plugins": [
{
"name": "GraphSelectGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll"
},
{
"name": "GraphBetaSupportGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
"urlsToWatch": [
"https://graph.microsoft.com/beta/",
"https://graph.microsoft.us/beta/",
"https://dod-graph.microsoft.us/beta/",
"https://microsoftgraph.chinacloudapi.cn/beta/"
]
},
{
"name": "GraphConnectorGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
"urlsToWatch": [
"https://graph.microsoft.com//external/connections//schema",
"https://graph.microsoft.us//external/connections//schema",
"https://dod-graph.microsoft.us//external/connections//schema",
"https://microsoftgraph.chinacloudapi.cn//external/connections//schema"
]
},
{
"name": "GraphSdkGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll"
},
{
"name": "ODataPagingGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll"
},
{
"name": "GraphClientRequestIdGuidancePlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll"
},
{
"name": "GraphRandomErrorPlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
"configSection": "graphRandomErrorsPlugin"
},
{
"name": "ExecutionSummaryPlugin",
"enabled": true,
"pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
"configSection": "executionSummaryPlugin"
}
],
"urlsToWatch": [
"https://graph.microsoft.com/v1.0/",
"https://graph.microsoft.com/beta/",
"https://graph.microsoft.us/v1.0/",
"https://graph.microsoft.us/beta/",
"https://dod-graph.microsoft.us/v1.0/",
"https://dod-graph.microsoft.us/beta/",
"https://microsoftgraph.chinacloudapi.cn/v1.0/",
"https://microsoftgraph.chinacloudapi.cn/beta/"
],
"graphRandomErrorsPlugin": {
"$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v1.3.0/graphrandomerrorplugin.schema.json",
"allowedErrors": [
429,
500,
502,
503,
504,
507
],
"rate": 50
},
"executionSummaryPlugin": {
"$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v1.3.0/executionsummaryplugin.schema.json",
"groupBy": "url"
},
"labelMode": "text",
"logLevel": "information"
}

Additional Info

No response

[waldekmastykarz]

Good catch and a clear bug on our end. We'll get to fix it asap. Sorry for the trouble and thanks for pointing it out.

[waldekmastykarz]

Actually there's a bit nuance to it: in response data, Graph uses indeed ISO date, BUT, we're not sending mocked data in our code, because mocked responses are content provided by users. In the code we set the Date response header which uses the RFC 2822 format. So while we still need to fix our code, we need to use the RFC format instead of ISO.

[waldekmastykarz]

Looking more closely at the example you shared, I wonder if the date in the innerError property is an exception that we need to handle separately.