⚗️ Add 128-bit trace IDs for Browser RUM

[BeltranBulbarellaDD]

Draft: waiting for approval on the RFC.

Motivation

Browser RUM currently generates 64-bit trace IDs and stores them as decimal on resource events. APM, mobile SDKs, and LLM Observability use 128-bit hexadecimal IDs, so Browser-started traces cannot be joined through one shared representation without extracting the low 64 bits and converting it to hexadecimal.

This PR introduces 128-bit trace IDs behind the trace_id_128_bit experimental feature. The existing 64-bit behavior remains the default while we dogfood and validate end-to-end correlation and CORS behavior.

Changes

• Generate trace IDs using <32-bit Unix timestamp><32 zero bits><64 random bits>.
• Add explicit low-decimal, full-hex, and high-hex identifier representations while preserving the existing toString() behavior.
• When trace_id_128_bit is enabled:
  • keep x-datadog-trace-id as the low 64 bits in decimal;
  • add x-datadog-tags: _dd.p.tid=<high 64 bits in hex>;
  • send the full 128-bit hexadecimal ID in W3C traceparent, B3, and B3 multi headers;
  • store _dd.trace_id on RUM resource events as a 32-character lowercase hexadecimal string.
• Keep span ID generation and session-consistent trace sampling unchanged; sampling still uses the low 64 bits.
• Add restricted-CORS E2E coverage for the new x-datadog-tags header and the tracecontext-only fallback.

Enable it with:

DD_RUM.init({
  // ...
  enableExperimentalFeatures: ['trace_id_128_bit'],
})

Example from manual testing:

Representation	Value
Full trace ID, 128-bit hex	6a6ca9f1000000000d5e267e2ee09948
High 64 bits, hex (_dd.p.tid)	6a6ca9f100000000
Low 64 bits, decimal (x-datadog-trace-id)	963249693698070856
Span ID, decimal	6901709716850828458

The full trace ID is the concatenation of the high 64-bit hex value and the low 64 bits converted to 16-character hexadecimal.

Playground

Screenshots:

link of the example trace in DD

Test instructions

1. Build and serve the local Browser SDK, then override the Playground SDK with the developer extension.

2. Initialize RUM with:

   {
     "applicationId": "<application-id>",
     "clientToken": "<client-token>",
     "site": "datadoghq.com",
     "service": "browser-sdk-128bit-local",
     "allowedTracingUrls": ["https://datadoghq.dev"],
     "enableExperimentalFeatures": ["trace_id_128_bit"]
   }

3. Trigger a traced fetch or XHR request.

4. Verify that:

   • traceparent contains a non-zero 32-character trace ID;
   • x-datadog-tags contains _dd.p.tid matching the first 16 characters of traceparent;
   • x-datadog-trace-id remains a decimal representation of the low 64 bits;
   • the RUM resource event stores the same full trace ID as 32-character hexadecimal.

5. For cross-origin Datadog propagation, allow x-datadog-tags in Access-Control-Allow-Headers; otherwise the browser preflight is expected to fail.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[BeltranBulbarellaDD]

@codex pls review

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	180.54 KiB	181.07 KiB	+546 B	+0.30%	✅
Rum Profiler	8.43 KiB	8.43 KiB	0 B	0.00%	✅
Rum Recorder	21.12 KiB	21.12 KiB	0 B	0.00%	✅
Logs	57.05 KiB	57.09 KiB	+38 B	+0.07%	✅
Rum Salesforce	N/A	138.93 KiB	N/A	N/A	N/A
Rum Slim	138.41 KiB	138.92 KiB	+524 B	+0.37%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	200.64 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.72 KiB	N/A	N/A	N/A

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: 26b264f2d4

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[datadog-official]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 62.50%
• Overall Coverage: 77.13% (-0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 26b264f | Docs | Datadog PR Page | Give us feedback!}