Always attach some URL/target data point to fetch/xhr/request errors

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The Sentry Chief Product Officer has brought to our attention that Fetch errors sometimes suck. (example that will expire in 90 days)

We technically have a stack trace here but sourcemaps are broken so the user has basically no idea what fetch request failed. It's very hard to gauge from breadcrumbs what exact request failed.

Side note, the fetch call that failed looked like fetch(""). We render this as in the product, you guessed it "". Which is crap because it actually means the request went to the current route. We should at least coerce that string to URL(fetchTarget, window.location.href).toString() in the SDK.

Hii, i was working on this issue & i think there are two separate things going on;

Message: enhanceFetchErrorMessages already appends the host, but it calls new URL(url) without a base, so relative URLs (including "") throw and are silently skipped.

sentry-javascript/packages/core/src/instrument/fetch.ts

Lines 138 to 153 in 3d47e12

    
             try { 
        
               const url = new URL(handlerData.fetchData.url); 
        
               const hostname = url.host; 
        
               if (enhanceOption === 'always') { 
        
                 // Modify the error message directly 
        
                 error.message = `${error.message} (${hostname})`; 
        
               } else { 
        
                 // Store hostname as non-enumerable property for Sentry-only enhancement 
        
                 // This preserves the original error message for third-party packages 
        
                 addNonEnumerableProperty(error, '__sentry_fetch_url_host__', hostname); 
        
               } 
        
             } catch { 
        
               // ignore it if errors happen here 
        
             } 
        
           }

I added test in core/test/lib/instrument/fetch.test.ts , it failed.
Before:

For this part I've made a PR - #25555 with 6 unit tests, (relative URL, empty URL, no location, file:// origin, opaque null origin, absolute URL on another origin) all passed.

Breadcrumbs: The empty URL in breadcrumbs is a separate spot - "" is stored as-is in breadcrumbs.ts. Coercing it with new URL(target, location.href) would work, but since new URL('', base).href keeps the base’s query string (not the hash, that’s dropped), it would leak query params like ?token=... into the breadcrumb.
want the full resolved URL anyway, or should the query be stripped before storing it?

big

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Always attach some URL/target data point to fetch/xhr/request errors #13168

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	try {
	const url = new URL(handlerData.fetchData.url);
	const hostname = url.host;

	if (enhanceOption === 'always') {
	// Modify the error message directly
	error.message = `${error.message} (${hostname})`;
	} else {
	// Store hostname as non-enumerable property for Sentry-only enhancement
	// This preserves the original error message for third-party packages
	addNonEnumerableProperty(error, '__sentry_fetch_url_host__', hostname);
	}
	} catch {
	// ignore it if errors happen here
	}
	}

Always attach some URL/target data point to fetch/xhr/request errors #13168

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kunal649 commented on Oct 20, 2026

lforst commented on Oct 20, 2026

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