[Kielek]

This fix addresses a ANT-2026-YXEDG1G8 finding from Anthropic's OSS Scanner.
Project enrollment: anthropics/oss-scanner#117.

Fixes #7449, it is 3rd attempt to solve this. This time, with AppContext opt-out switch.
Is is similar to was done directly in .NET for https://learn.microsoft.com/en-us/dotnet/core/compatibility/serialization/10/xmlserializer-obsolete-properties

Changes

Ambient baggage writes now publish a replacement holder in the current execution
context, so they no longer change baggage observed by previously forked contexts.
The correction covers Baggage.Current and every static baggage mutator, with
both explicit and omitted baggage arguments. It addresses the behavior tracked
in #7449.

This is a breaking behavior change: enrichment performed in an awaited
asynchronous helper no longer flows back to its caller. Enable the
OpenTelemetry.Baggage.AllowUnsafeLegacyFlow AppContext switch at startup to
temporarily restore the original shared holder and locking behavior. An unset
or false switch keeps isolation. The selected mode is cached when Baggage
initializes; later configuration changes do not update it.
The API documentation explains the unsafe sharing risk and shows how to migrate
by returning updated baggage to the caller. No public signatures or dependencies
change.

The compatibility option is a migration aid, not a claim of backward compatibility
or an approved 1.x release exception. The history in #5208, #5227, and #7191.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, successful build/tests, etc.)
• Tests added/updated
• CHANGELOG.md file(s) updated for non-trivial changes
• Changes in public API reviewed (if applicable)
• AI usage disclosure:
  • No AI used
  • AI-assisted
  • Bulk AI-generated
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR¹

Note

This PR description was generated with OpenAI Codex.

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR without using AI. ↩

[opentelemetry-pr-dashboard]

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Waiting on the author · refreshed 2026-11-10 13:04 UTC

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[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.01%. Comparing base (f7eaaab) to head (c15cf9c).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry.Api/Baggage.cs	53.33%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8042      +/-   ##
==========================================
- Coverage   93.03%   93.01%   -0.03%     
==========================================
  Files         398      400       +2     
  Lines       23473    23734     +261     
  Branches     4389     4439      +50     
==========================================
+ Hits        21838    22075     +237     
- Misses       1115     1125      +10     
- Partials      520      534      +14     

Flag	Coverage Δ
unittests-Project-Experimental	92.98% <53.33%> (-0.15%)	⬇️
unittests-Project-Stable	92.97% <53.33%> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Api/Baggage.cs	92.20% <53.33%> (-4.25%)	⬇️

... and 9 files with indirect coverage changes

[martincostello]

Waiting on anything else to merge this?

[Kielek]

I would like to have opinion from @alanwest or @rajkumar-rangaraj here. I hope they will find some time to review.

[rajkumar-rangaraj]

LGTM on the direction and the implementation. Isolating writes matches the spec's immutable-context model, and the opt-out switch gives users a migration path. A few things before merging:

Breaking default: this is essentially #5208, which was reverted in #5227 after user feedback that baggage set in awaited helpers no longer flowed back to the caller. Since this changes stable OpenTelemetry.Api 1.x behavior, can we confirm it at the SIG and call it out prominently in the release notes?

Timing: 1.21.0-rc.2 shipped today. I'd prefer this not go straight into 1.21.0 stable without an RC. Either cut another RC or merge after 1.21.0.

Contrib impact: ASP.NET TelemetryHttpModule saves the baggage holder object at request start and puts it back when the async context is lost (ActivityHelper.cs). With this change that restore rolls baggage back to its request-start value. If there was no incoming baggage, nothing is restored. I'll open a contrib follow-up so it saves the current Baggage value instead.

Nit: the legacy-switch path no longer has tests. That's fine since the code is unchanged, but worth noting.

[Kielek]

Contrib impact: ASP.NET TelemetryHttpModule saves the baggage holder object at request start and puts it back when the async context is lost (ActivityHelper.cs). With this change that restore rolls baggage back to its request-start value. If there was no incoming baggage, nothing is restored. I'll open a contrib follow-up so it saves the current Baggage value instead.

PR open-telemetry/opentelemetry-dotnet-contrib#5948 is ready. It can be merged even now, to avoid problems if anybody update only API and forgot to update instrumentation package.

Nit: the legacy-switch path no longer has tests. That's fine since the code is unchanged, but worth noting.

Intentionally omited.

[Kielek]

Timing: 1.21.0-rc.2 shipped today. I'd prefer this not go straight into 1.21.0 stable without an RC. Either cut another RC or merge after 1.21.0.

I hope it will work for you:

1. Stable release with .NET11
2. Merge this PR.
3. RC.1 release with the fix couple days later. Hopefully next week, then we can include it into stable in the next+1 version.