aspire app giving 431 error

Thank you for reaching out.

A 431 — Request Header Fields Too Large means the server refused your request because something in the request headers exceeded what the server will accept. That usually means either one header (for example an oversized Authorization token or a very long Referer) or the combined size of many headers (often a very large cookie string) is too big.
Because your teammates don’t see the error but you do, it strongly suggests the extra/oversized header is coming from your machine or browser (cookies, auth tokens, an enterprise proxy, or a local tool like Fiddler) rather than from the Aspire app itself.

Troubleshooting

1. Incognito / Private Window (Quick Diagnostic)

Open the app in a private window. If it works there, that proves the problem is browser-stored data (cookies/localStorage). This is the fastest way to narrow the cause.

2. Clear All Site Data for the Origin (Not Just Obvious Cookies)

In DevTools → Application (or Storage): use Clear site data for (or your Aspire origin). Remove Cookies, Local Storage, Session Storage, IndexedDB — and specifically delete cookies named like .AspNetCore.Cookies or .AspNetCore.Identity.Application if present. Then fully restart the browser. Many people think “I cleared cookies” but miss origin-scoped storage or session stores.

3. Inspect the Actual Request Headers (Diagnostic, Critical)

Use DevTools Network tab (or Fiddler/Postman) to capture the failing request. Look at the raw Request Headers: the Cookie header, Authorization, Referer, or any custom headers — note their sizes. This tells you the exact offender instead of guessing.

4. Check System-Level / Corporate Tools

If clearing browser data didn’t help, check whether a corporate proxy, VPN, SSO agent, or a system-wide tool like Fiddler is appending headers or injecting tokens. These can affect only your machine and cause 431. Temporarily disable them and retry.

5. Re-authenticate External Identity Tools

Sign out and sign back into Visual Studio, Azure SSO, or any other system that may cache tokens outside the browser. Corrupted or oversized SSO tokens can live outside normal browser cookies.

6. Reset Local Dev Host State (If Needed)

If you suspect local host config corruption (IIS Express / VS debug host), recreate the environment state: close dev tools, remove per-solution caches (they’ll be rebuilt). This is less common but occasionally helpful.

Workarounds

• Temporary Dev Unblock: You can temporarily allow larger headers on your local dev server so you’re not blocked while you investigate. This is a workaround only — it gets you running but doesn’t fix the root cause and should not be used in production.
• Short-Term Fix for Uploads/Referers: If a specific feature sends a very long referer or huge metadata, try sending smaller payloads or split large requests while you diagnose the header issue.

Summary

1. Try Incognito — does it work? If yes → clear site data for the origin and restart browser.
2. If not, capture the failing request in DevTools or Fiddler and identify which header is huge.
3. If the header is coming from a system tool or SSO, disable/re-auth and retry.
4. Only if you remain blocked and need a temporary unblock, use the local dev server workaround (increase header limits) while continuing to find the real cause.

Helpful References

• Here are the Microsoft Learn links related to the HTTP 431 Request Header Fields Too
  • Azure Function App | Header Size
  • 431 RequestHeaderFieldsTooLarge When Calling Azure OpenAI
  • Azure App Services Linux - NetCore 3 with Kestrel
  • StatusCodes.Status431RequestHeaderFieldsTooLarge Field (.NET API)
  • Troubleshoot "Request Header Too Large" - Azure Cosmos DB

Let me know if you need any further help with this. We'll be happy to assist.

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Hello all, I am a colleague of Shivanand's who has been working with him on this issue. It took a few days, but we managed to figure out what happened:

• For OpenIDConnect/JWT tokens, if a user is a member of more than 200 groups the individual groups claims are replaced with a claim telling the system where to find a user's list of groups.
• Last week, our Security team activated Privileged Identity Management (PIM) for a group that grants system engineers the Contributor role across all our subscriptions. Rather than activating/deactivating the role, we activate our membership in that group.
  • When we are in that group, it also grants us membership in various access groups across all subscriptions. During my experimenting, that count was over 2000 groups total.
  • When we are not in that admin group, our total group membership is much lower - in my case, around 175.
• When PIM is off for that group for a user, their total number of "groups" claims may be under the 200-group limit, but still enough that the resulting token is too large. This is exactly what we were running into.

Since we don't need the groups for this application, we resolved this by filtering out the claims in the OnTokenValidated event. If we ever do need the groups, we can manually fetch them.

builder.Services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
	.AddMicrosoftIdentityWebApp(options =>
	{
		// Bind settings from the configuration file
		builder.Configuration.Bind("AzureAd", options);

		// Initialize the Events property if null
		options.Events ??= new OpenIdConnectEvents();

		// Subscribe to events
		options.Events.OnTokenValidated += async context => {
		    // bypass if the principal or identity
		    // don't exist for some reason
		    if (context.Principal is null || 
                context.Principal.Identity is null) 
            {
                return;
            }

		    var claimsIdentity = (ClaimsIdentity)context.Principal.Identity;
		    var issuer = claimsIdentity.FindFirst("iss")?.Value ?? string.Empty;
		    // only do this for Entra tokens
		    if (issuer.StartsWith("https://login.microsoftonline.com/"))
		    {
		        if (claimsIdentity.HasClaim(c => c.Type == "groups"))
		        {
		            // remove all "groups" claims
		            claimsIdentity.FindAll(c => c.Type. == "groups").ToList()
                                .ForEach(c => claimsIdentity.RemoveClaim(c));

		        }
		    }

		    await Task.CompletedTask;
		};
	});