System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse: System.TimeoutException

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1544854
Build error leg or test failing: System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse
Pull request: https://github.com/dotnet/runtime/pull/131671

```
System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse [FAIL]
System.TimeoutException : The operation has timed out.
Stack Trace:
/_/src/libraries/Common/tests/System/Net/Http/Http3LoopbackServer.cs(115,0): at System.Net.Test.Common.Http3LoopbackServerFactory.CreateServerAsync(Func`3 funcAsync, Int32 millisecondsTimeout, GenericLoopbackOptions options)
/_/src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs(1584,0): at System.Net.Http.Functional.Tests.DiagnosticsTest.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse()
--- End of stack trace from previous location ---
```

## Error Message

**DO NOT USE JSON BELOW IF THIS IS A BUILD BREAK** otherwise build analysis will allow pull requests to merge that break the build worse. For a build break, do not use this issue form. Make a regular new issue.

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorMessage": ["SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse [FAIL]", "System.TimeoutException"],
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1544854
**Error message validated:** `[SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse [FAIL] System.TimeoutException`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 8/10/2026 5:18:45 PM UTC

### Report

|Build|Repository|Test|Pull Request|
|---|---|---|---|
|[1595682](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1595682)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1595682&view=ms.vss-test-web.build-test-results-tab&runId=44056366&resultId=237791)|dotnet/runtime#129220|
|[1594905](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594905)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594905&view=ms.vss-test-web.build-test-results-tab&runId=44029780&resultId=208369)|dotnet/runtime#133389|
|[1594658](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594658)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594658&view=ms.vss-test-web.build-test-results-tab&runId=44022680&resultId=194331)|dotnet/runtime#133819|
|[1594502](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594502)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594502&view=ms.vss-test-web.build-test-results-tab&runId=44015696&resultId=237803)|dotnet/runtime#130050|
|[1594338](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594338)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594338&view=ms.vss-test-web.build-test-results-tab&runId=44011088&resultId=228951)|dotnet/runtime#133606|
|[1594314](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594314)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594314&view=ms.vss-test-web.build-test-results-tab&runId=44009978&resultId=227433)||
|[1594266](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594266)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594266&view=ms.vss-test-web.build-test-results-tab&runId=44009076&resultId=228646)|dotnet/runtime#133793|
|[1593618](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1593618)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1593618&view=ms.vss-test-web.build-test-results-tab&runId=43987306&resultId=225827)|dotnet/runtime#133747|
|[1592920](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592920)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592920&view=ms.vss-test-web.build-test-results-tab&runId=43971220&resultId=219468)||
|[1592705](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592705)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592705&view=ms.vss-test-web.build-test-results-tab&runId=43966728&resultId=219384)|dotnet/runtime#133696|
|[1592299](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592299)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592299&view=ms.vss-test-web.build-test-results-tab&runId=43954774&resultId=229502)||
|[1592119](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592119)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592119&view=ms.vss-test-web.build-test-results-tab&runId=43948588&resultId=237471)|dotnet/runtime#133665|
|[1591647](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591647)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591647&view=ms.vss-test-web.build-test-results-tab&runId=43935020&resultId=229233)|dotnet/runtime#133637|
|[1591180](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591180)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591180&view=ms.vss-test-web.build-test-results-tab&runId=43924220&resultId=227914)||
|[1589610](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1589610)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1589610&view=ms.vss-test-web.build-test-results-tab&runId=43876436&resultId=204655)||
|[1588952](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588952)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588952&view=ms.vss-test-web.build-test-results-tab&runId=43858996&resultId=229267)||
|[1588425](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588425)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588425&view=ms.vss-test-web.build-test-results-tab&runId=43840636&resultId=199761)||
|[1587854](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1587854)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1587854&view=ms.vss-test-web.build-test-results-tab&runId=43822204&resultId=209089)||
|[1586665](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586665)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586665&view=ms.vss-test-web.build-test-results-tab&runId=43783220&resultId=225826)|dotnet/runtime#133394|
|[1586039](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586039)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586039&view=ms.vss-test-web.build-test-results-tab&runId=43764418&resultId=237343)|dotnet/runtime#131597|
|[1584429](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584429)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584429&view=ms.vss-test-web.build-test-results-tab&runId=43716086&resultId=215586)|dotnet/runtime#133265|
|[1584171](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584171)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584171&view=ms.vss-test-web.build-test-results-tab&runId=43708508&resultId=233600)||
|[1583519](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583519)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583519&view=ms.vss-test-web.build-test-results-tab&runId=43691426&resultId=231394)|dotnet/runtime#133254|
|[1583332](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583332)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583332&view=ms.vss-test-web.build-test-results-tab&runId=43685952&resultId=218057)|dotnet/runtime#133115|
|[1583196](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583196)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1583196&view=ms.vss-test-web.build-test-results-tab&runId=43681116&resultId=228504)||
|[1582265](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1582265)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1582265&view=ms.vss-test-web.build-test-results-tab&runId=43649144&resultId=218215)|dotnet/runtime#133192|
|[1582067](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1582067)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1582067&view=ms.vss-test-web.build-test-results-tab&runId=43643234&resultId=237702)|dotnet/runtime#133200|
|[1581820](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581820)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581820&view=ms.vss-test-web.build-test-results-tab&runId=43636054&resultId=236571)|dotnet/runtime#132042|
|[1581252](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581252)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581252&view=ms.vss-test-web.build-test-results-tab&runId=43627594&resultId=213202)|dotnet/runtime#132908|
|[1579404](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579404)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579404&view=ms.vss-test-web.build-test-results-tab&runId=43616350&resultId=226898)|dotnet/runtime#132897|
|[1580828](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580828)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580828&view=ms.vss-test-web.build-test-results-tab&runId=43611620&resultId=226505)||
|[1580831](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580831)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580831&view=ms.vss-test-web.build-test-results-tab&runId=43611460&resultId=226885)||
|[1580519](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580519)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580519&view=ms.vss-test-web.build-test-results-tab&runId=43599514&resultId=215334)|dotnet/runtime#133154|
|[1580155](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580155)|dotnet/runtime|[System.Net.Http.Functional.Tests.HttpClientHandlerTest_Http3.SendMoreThanStreamLimitRequestsConcurrently_LastWaits](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580155&view=ms.vss-test-web.build-test-results-tab&runId=43580564&resultId=226642)||
|[1580000](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580000)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1580000&view=ms.vss-test-web.build-test-results-tab&runId=43576952&resultId=237488)|dotnet/runtime#133137|
|[1579975](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579975)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579975&view=ms.vss-test-web.build-test-results-tab&runId=43576312&resultId=215690)|dotnet/runtime#133107|
|[1579420](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579420)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579420&view=ms.vss-test-web.build-test-results-tab&runId=43565342&resultId=226886)||
|[1579488](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579488)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579488&view=ms.vss-test-web.build-test-results-tab&runId=43564518&resultId=226885)||
|[1578769](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1578769)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1578769&view=ms.vss-test-web.build-test-results-tab&runId=43560020&resultId=226505)||
|[1579048](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579048)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579048&view=ms.vss-test-web.build-test-results-tab&runId=43547742&resultId=226222)||
|[1578871](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1578871)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1578871&view=ms.vss-test-web.build-test-results-tab&runId=43543830&resultId=226897)|dotnet/runtime#133091|
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|[1568477](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1568477)|dotnet/runtime|[System.Net.Http.Functional.Tests.WorkItemExecution](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1568477&view=ms.vss-test-web.build-test-results-tab&runId=43467390&resultId=226897)|dotnet/runtime#132782|
|[1575548](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1575548)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1575548&view=ms.vss-test-web.build-test-results-tab&runId=43427918&resultId=224306)|dotnet/runtime#131877|
|[1574314](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1574314)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1574314&view=ms.vss-test-web.build-test-results-tab&runId=43386106&resultId=227635)|dotnet/runtime#132827|
|[1573068](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1573068)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1573068&view=ms.vss-test-web.build-test-results-tab&runId=43335826&resultId=229067)|dotnet/runtime#132904|
|[1572950](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1572950)|dotnet/runtime|[System.Net.Http.Functional.Tests.SocketsHttpHandler_DiagnosticsTest_Http3.SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1572950&view=ms.vss-test-web.build-test-results-tab&runId=43331476&resultId=215083)|dotnet/runtime#132899|
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|1|19|57|

Start with the failing SendAsync_ReuseRequestInHandler_ResetsHeadersForEachReuse test in src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs and the timeout path in src/libraries/Common/tests/System/Net/Http/Http3LoopbackServer.cs. Run the named HTTP/3 test and inspect the linked build failures and repeated reports. Done means the test completes without the System.TimeoutException and continues to verify header reset behavior.

Written by the indexing model from the issue text.