[chiliec]

Fixes #25545

Problem

Setting beforeSendSpan — even an identity callback (span) => span — drops the root span's contexts.trace.links from every transaction, so the previous_trace link is lost and the trace view no longer links to the previous trace.

In processBeforeSend, the root span goes through beforeSendSpan(convertTransactionEventToSpanJson(event)) and the result is merged back with merge(event, convertSpanJsonToTransactionEvent(result)). The two converters in packages/core/src/utils/transactionEvent.ts never copied links, and merge (default depth 2) replaces contexts.trace whole, so the links set by spanToTransactionTraceContext are discarded. Thanks to @arobison for the detailed root-cause trace in the issue.

Fix

Carry links through both conversions. TraceContext.links and SpanJSON.links are both SpanLinkJSON[], so this is a direct pass-through.

Tests

Extended the existing transactionEvent.test.ts round-trip cases (convertTransactionEventToSpanJson and convertSpanJsonToTransactionEvent) to assert links (including a previous_trace link) survive.

Verification (local):

npx vitest run test/lib/utils/transactionEvent.test.ts
# with the fix:    Test Files 1 passed (1) | Tests 5 passed (5)
# fix reverted:    Tests 2 failed | 3 passed (5)
#   × convertTransactionEventToSpanJson > should convert a full transaction event to span JSON  (missing "links")
#   × convertSpanJsonToTransactionEvent > should convert a full span JSON to transaction event  (missing "links")

Ran in packages/core with Node 22.12 / vitest 4.1.11.

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[logaretm]

Many thanks! Just a minor change, we need to assert the result. Happy to approve once that's done.

[logaretm]

These assertions cover the converters, but the links actually got lost in the merge(event, ...) step in processBeforeSend.

Could you also assert contexts.trace.links on the sent transaction in client.test.ts?