Default args and locals params in debugger snapshot hooks

[watson]

Motivation

The Live Debugger build plugin (in DataDog/build-plugins) generates instrumented code that wraps every monitored function with calls to four global hooks: $dd_probes, $dd_entry, $dd_return, and $dd_throw. The entry/return/throw hooks currently require the build plugin to always pass an args helper, and $dd_return also requires a locals helper, even when:

• the function has no parameters (nothing to capture as args), or
• the probe is not configured to capture locals (so locals would always be empty).

Today the plugin works around this by emitting () => ({}) helpers in those cases, which adds dead code to the hot path and inflates the instrumented bundle. Making these parameters optional on the runtime side unlocks a follow-up build-plugin change that simply omits the argument at the call site.

Changes

packages/debugger/src/domain/api.ts:

• onEntry(probes, self, args) → onEntry(probes, self, args = {})
• onReturn(probes, value, self, args, locals) → onReturn(probes, value, self, args = {}, locals = {})
• onThrow(probes, error, self, args) → onThrow(probes, error, self, args = {})

No behavioral change for any existing call site: callers that pass a Record<string, any> (including the build plugin's current $dd_e() / $dd_l() helpers) behave identically. The defaults only kick in when an argument is omitted — which is what the build plugin will start doing for parameter-less functions and probes without local capture in a follow-up.

Notes:

• args is spread into the context used for condition / message evaluation and into the captured snapshot (captureFields(args, …)). Spreading {} is a no-op, so behavior is preserved.
• locals in onReturn is treated the same way (...locals + captureFields(locals, …)).
• This is a pure typing/runtime relaxation; no public API surface changes.

Test instructions

End-to-end behavior with the build plugin will be exercised once the corresponding build-plugins change lands (omitting the trailing args/locals helpers in generated code).

Checklist

• Tested locally
• Tested on staging — not applicable; pure runtime relaxation with no protocol or transport change.
• Added unit tests for this change — covered by the existing api.spec.ts suite; the change is a type-level relaxation (defaulting optional parameters to {}). Happy to add an explicit "called with no args/locals" case if reviewers prefer.
• Added e2e/integration tests for this change — will be covered end-to-end by the follow-up build-plugins PR that exercises the new call shape.
• Updated documentation and/or relevant AGENTS.md file — no documented contract changes.

[watson]

• Default debugger version and embed build identity in snapshots #4511
• Default args and locals params in debugger snapshot hooks #4609 👈 (View in Graphite)
• Switch debugger delivery to public ClientTokenAuth endpoint #4480
• Add Live Debugger package #4449 : 4 other dependent PRs (#4503 , #4508 , #4509 and 1 other)
• Fix TanStack Router lint resolution in test app #4513
• main

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.51 KiB	169.51 KiB	0 B	0.00%	✅
Rum Profiler	5.97 KiB	5.97 KiB	0 B	0.00%	✅
Rum Recorder	21.23 KiB	21.23 KiB	0 B	0.00%	✅
Logs	54.70 KiB	54.70 KiB	0 B	0.00%	✅
Rum Slim	127.85 KiB	127.85 KiB	0 B	0.00%	✅
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0031	0.0019	-38.71%
RUM - add action	0.0164	0.0095	-42.07%
RUM - add error	0.0137	0.0101	-26.28%
RUM - add timing	0.0007	0.0005	-28.57%
RUM - start view	0.0151	0.012	-20.53%
RUM - start/stop session replay recording	0.0013	0.0007	-46.15%
Logs - log message	0.0234	0.0194	-17.09%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	38.75 KiB	38.61 KiB	-146 B
RUM - add action	64.74 KiB	64.97 KiB	+241 B
RUM - add timing	36.82 KiB	37.33 KiB	+517 B
RUM - add error	70.81 KiB	69.54 KiB	-1.27 KiB
RUM - start/stop session replay recording	41.37 KiB	42.58 KiB	+1.22 KiB
RUM - start view	486.34 KiB	483.21 KiB	-3.13 KiB
Logs - log message	55.35 KiB	54.48 KiB	-884 B

🔗 RealWorld

[datadog-datadog-prod-us1]

✨ Fix all issues with BitsAI or with Cursor

⚠️ Warnings

🧪 1 Test failed

cookieAccess document.cookie fallback observable should notify when cookie is changed externally from Chrome Headless 148.0.0.0 (Linux 0.0.0)     (Fix with Cursor)

Error: Expected spy observer to have been called once. It was called 0 times.
    at <Jasmine>
    at UserContext.<anonymous> (/go/src/github.com/DataDog/browser-sdk/packages/core/src/browser/cookieAccess.spec.ts:179:23 <- /tmp/_karma_webpack_870888/commons.js:359849:33)

ℹ️ Info

No other issues found (see more)

❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 76.96% (+0.00%)

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