💡 You are a debugging agent with access to Replay MCP tools and source code. Your job is to investigate a bug, identify the root cause, and suggest a fix.

Bug Report

App: Next.js app using React 19 with server actions. Source at ~/projects/temp/react-udv-bug.

Behavior:

• A textarea input feeds text to a server action (renderAction) which returns Promise<ReactNode>

• The promise is wrapped in useDeferredValue, then consumed via use() inside <Suspense>

• In dev mode (next dev): the deferred output box tracks the input and catches up after typing stops

• In prod mode (next build && next start): the deferred output frequently gets stuck and never catches up to the final input value

Recordings:

• Dev (working): 06447062-f403-49f3-8bd8-476a65b448d2

• Prod (broken): c0b7c6fb-8f2a-461c-9fca-0ba2e9be3083

In the broken prod recording, the user typed "hello world from prod" but the deferred output is stuck showing "hello world from pro" — one character behind, permanently.

What we know:

• No console errors in either recording

• The server action POST requests all return 200 OK with valid data

• The app code is identical between dev and prod — only the build mode differs

• It only manifests in production builds

Goal: Figure out why the deferred value stops updating in production and identify the root cause.

Deliverable

Write a detailed bug report with:

1. Bug Description — What happens

2. Root Cause Analysis — What exactly causes the deferred value to get stuck in prod but not dev

3. Suggested Fix — What code change would fix this

4. Key Evidence Trail — Table of tools used and findings at each step

💡 Bug Report: useDeferredValue Permanently Stalls in React 19 Production with Server Action RSC Payloads

1. Bug Description

When using useDeferredValue with a promise returned from a server action (the useDeferredValue(promise) pattern), the deferred output frequently gets permanently stuck in production builds. The deferred value stops updating and never catches up to the final input.

Repro: A textarea input calls renderAction(value) (a server action returning Promise<ReactNode>), wraps it in useDeferredValue, then consumes via use() inside <Suspense>. In dev mode, the deferred output tracks correctly. In prod, it stalls — e.g., user types "hello world from prod" but deferred output shows "hello world from pro" forever.

Affected version: React 19.3.0-canary (via Next.js 16.1.6 production build)

2. Root Cause Analysis

This is a race condition in React's concurrent renderer where a synchronous ping from an RSC lazy chunk fires during an active render, causing the deferred lane to be permanently lost.

The Bug Mechanism

The failure occurs in pingSuspendedRoot (react-dom-client.production.js L13073-13089) when three conditions converge simultaneously:

1. root.suspendedLanes = 0 — The lane hasn't been marked as suspended yet because markRootSuspended runs after the render completes, but the ping fires during the render

2. executionContext & 2 is truthy — The ping fires inside the render context

3. workInProgressRootExitStatus = 4 (RootSuspended) — Set by renderDidSuspendDelayIfPossible in throwException, before attachPingListener is called

This creates a double failure:

Failure 1 — Lane not pinged (L13076):

root.pingedLanes |= root.suspendedLanes & pingedLanes;  
//                   ^^^^^^^^^^^^^^^^^ = 0, so pingedLanes stays 0  

Failure 2 — No recovery path (L13078-13088):

workInProgressRoot === root                           // TRUE  
  && (workInProgressRootRenderLanes & pingedLanes)    // TRUE  
  && (4 === workInProgressRootExitStatus              // TRUE (RootSuspended)  
      ? 0 === (executionContext & 2)                  // FALSE! In render context.  
        && prepareFreshStack(root, 0)                 // NOT CALLED  
      : (workInProgressRootPingedLanes |= pingedLanes) // NOT REACHED (wrong ternary branch)  
     )  

The ternary evaluates 4 === workInProgressRootExitStatus as true, taking the true-branch where executionContext & 2 blocks prepareFreshStack. The else-branch (workInProgressRootPingedLanes |= pingedLanes) is never reached. The lane is lost with no recovery path.

How the Synchronous Ping Happens

The key trigger is the RSC (React Server Components) client's ReactPromise.prototype.then implementation (react-server-dom-turbopack-client.browser.production.js:539-563):

// L542-549  
if (this.status === "resolved_model") initializeModelChunk(this);  
if (this.status === "fulfilled") {  
  if (typeof resolve === "function") resolve(this.value); // SYNCHRONOUS!  
}  

When attachPingListener calls .then(pingSuspendedRoot) on an RSC chunk that is already fulfilled (or transitions to fulfilled during initializeModelChunk), the ping callback fires synchronously, while still inside throwAndUnwindWorkLoop → throwException, which is inside renderRootConcurrent with executionContext = 2.

Detailed Timeline (from prod recording c0b7c6fb)

Time	Event	Key State
5147ms	User types "d" (final keystroke)	Sync render, lanes=2
5150ms	Commit #37 (last ever)	markRootFinished clears suspendedLanes=0, pingedLanes=0
5150ms	1st deferred render starts	use(promise) → pending → SuspenseException
5151ms	Render yields after suspension	markRootSuspended sets suspendedLanes=524800
5158ms	Pings 1 & 2 (fetch response)	executionContext=0, suspendedLanes=524800 → pings work correctly
5159ms	2nd deferred render starts	prepareFreshStack, promise now fulfilled, render proceeds
5163ms	RSC lazy chunk encountered	Pending → SuspenseException → yield
5169ms	Re-entry: chunk not resolved	throwAndUnwindWorkLoop → throwException → renderDidSuspendDelayIfPossible sets exitStatus=4
5169ms	attachPingListener → .then()	RSC chunk .then() fires synchronously (chunk already fulfilled)
5169ms	pingSuspendedRoot (THE BUG)	executionContext=2, suspendedLanes=0, pingedLanes=0 → lane 524288 LOST
5191ms	Render completes	markRootSuspended sets suspendedLanes=524288 but pingedLanes=0 — permanently stuck

Why Dev Mode Works

In development builds, the RSC client likely doesn't trigger synchronous resolution in .then(), or React's dev-mode Suspense handling processes thenables differently. The ping always fires asynchronously (when executionContext = 0), so root.suspendedLanes is properly populated by the time the ping runs.

Evidence: Dev recording shows 125 render commits (every keystroke gets both a SetState and SuspenseResumed). Prod shows only 38 commits, with the last being a SetState with no follow-up SuspenseResumed.

3. Suggested Fix

There are two complementary fixes, either of which would prevent this class of bug:

Fix A: Handle synchronous pings during render in pingSuspendedRoot

The ternary at L13078-13088 has a logic gap. When exitStatus === 4 and executionContext & 2, the code should still update workInProgressRootPingedLanes so the render can retry the pinged lanes when it completes:

// In pingSuspendedRoot, after L13076:  
if (workInProgressRoot === root) {  
  if ((workInProgressRootRenderLanes & pingedLanes) === pingedLanes) {  
    if (4 === workInProgressRootExitStatus || ...) {  
      if (0 === (executionContext & 2)) {  
        prepareFreshStack(root, 0);  
      } else {  
        // NEW: Even if we can't restart now, mark lanes as pinged  
        // so the scheduler picks them up after render completes  
        workInProgressRootPingedLanes |= pingedLanes;  
      }  
    } else {  
      workInProgressRootPingedLanes |= pingedLanes;  
    }  
  }  
}  

Fix B: Prevent synchronous ping resolution from RSC chunks

In attachPingListener or in the RSC client's ReactPromise.prototype.then, ensure the ping callback is always invoked asynchronously (e.g., via microtask), so it never fires inside the render context:

// In attachPingListener, wrap the ping:  
wakeable.then(  
  () => queueMicrotask(() => pingSuspendedRoot(root, pingedLanes)),  
  () => queueMicrotask(() => pingSuspendedRoot(root, pingedLanes))  
);  

Fix C: Also update root.pingedLanes from workInProgressRootPingedLanes after render

In performWorkOnRoot, after renderRootConcurrent returns with exitStatus = 4 (RootSuspended), propagate any in-render pinged lanes to the root before calling markRootSuspended:

// Before markRootSuspended in the exitStatus=4 path:  
root.pingedLanes |= workInProgressRootPingedLanes;  

Recommended approach: Fix A is the most targeted — it addresses the exact ternary logic gap without changing ping timing semantics. Fix B is a defense-in-depth measure. Fix C ensures the information isn't lost even if other paths hit the same issue.

4. Key Evidence Trail

Step	Tool	Finding
1	RecordingOverview (both recordings)	Dev: 125 commits, Prod: 38 commits. Same React 19.3.0-canary.
2	ReactRenders commits (prod)	Last commit #37 is SetState (2 fibers), no SuspenseResumed follows. Recording continues 5+ seconds.
3	NetworkRequest detail (prod)	All 21 POSTs return 200 OK. Final POST body "hello world from prod" returns correct data at 5158ms.
4	ReactRenders trigger-detail	Commit #37 triggered by keypress "d". Previous commit #36 was SuspenseResumed from network.
5	SearchSources / ReadSource	Found updateDeferredValueImpl (L5407-5424): sync renders defer, deferred renders accept.
6	Logpoint on updateDeferredValueImpl	Last 8 hits show hit #6 (5150ms) and #8 (5159ms) accepting deferred value — but no render commits follow.
7	DescribePoint at hit #8 (Point:162)	Promise is fulfilled with correct "hello world from prod" ReactNode.
8	SearchSources for pingSuspendedRoot	Found L13073-13089: lane pinged via `root.pingedLanes
9	Logpoint on pingSuspendedRoot L13076	3 pings after final keystroke. Pings 1-2 at 5158ms work (suspendedLanes=524800). Ping 3 at 5169ms fails (suspendedLanes=0, executionContext=2).
10	Logpoint on markRootSuspended L12149	4 hits. Hit 4 at 5191ms: suspendedLanes goes 0→524288 with pingedLanes=0. Permanent stuck state.
11	ReadSource + DescribePoint on throwException L6205-6218	renderDidSuspendDelayIfPossible() called BEFORE attachPingListener() — sets exitStatus=4 before ping can fire.
12	GetStack at Point:182 (broken ping)	Call stack proves synchronous invocation: pingSuspendedRoot ← RSC .then() ← attachPingListener ← throwException ← throwAndUnwindWorkLoop ← renderRootConcurrent.
13	ReadSource RSC client L539-563	ReactPromise.prototype.then: if status is "fulfilled", calls resolve(this.value) synchronously.
14	ReadSource performWorkOnRoot L11847-11851	exitStatus=0 (yielded): no markRootSuspended called, so root.suspendedLanes stays 0 during re-entry render.
15	Evaluate at Point:182	Confirmed: root.suspendedLanes=0, root.pingedLanes=0, executionContext=2, workInProgressRootExitStatus=4. All conditions for the double failure met.

Summary

This is a React 19 scheduler bug where a synchronous ping from an RSC lazy chunk fires during an active concurrent render, hitting a logic gap in pingSuspendedRoot's ternary condition. The deferred lane is neither pinged on the root nor recorded in workInProgressRootPingedLanes, causing it to be permanently suspended with no recovery mechanism. The bug is prod-only because the RSC client in production resolves chunks synchronously in .then(), while the dev bundle resolves them asynchronously.

💡 You are a debugging agent with access to Replay MCP tools and source code. Your job is to investigate a bug, identify the root cause, and suggest a fix.

Bug Report

App: Next.js app using React 19 with server actions. Source at ~/projects/temp/react-udv-bug.

Behavior:

• A textarea input feeds text to a server action (renderAction) which returns Promise<ReactNode>

• The promise is wrapped in useDeferredValue, then consumed via use() inside <Suspense>

• In dev mode (next dev): the deferred output box tracks the input and catches up after typing stops

• In prod mode (next build && next start): the deferred output frequently gets stuck and never catches up to the final input value

Recordings:

• Dev (working): 06447062-f403-49f3-8bd8-476a65b448d2

• Prod (broken): c0b7c6fb-8f2a-461c-9fca-0ba2e9be3083

In the broken prod recording, the user typed "hello world from prod" but the deferred output is stuck showing "hello world from pro" — one character behind, permanently.

What we know:

• No console errors in either recording

• The server action POST requests all return 200 OK with valid data

• The app code is identical between dev and prod — only the build mode differs

• It only manifests in production builds

Goal: Figure out why the deferred value stops updating in production and identify the root cause.

Investigation Methodology

Follow this structured process:

Step 1: Establish the baseline

Get an overview of both recordings. Compare high-level stats: render commit counts, network request counts, timing. The quantitative difference between dev and prod is your first clue.

Step 2: Confirm the visual bug

Take screenshots at the end of both recordings to verify what each shows. Confirm the prod recording is stuck one character behind.

Step 3: Compare render commit patterns

Look at the sequence of React render commits in both recordings. Focus on the tail end — the last few commits. In a working recording, you'd expect the render pattern to complete cleanly. In the broken one, something is missing or different. What's the last commit in each? What triggered it? What components re-rendered?

Step 4: Map the data flow end-to-end

Trace the full lifecycle: keystroke → server action POST → response → promise resolution → deferred value update → component re-render. Verify each step completes in both recordings. If all steps complete but the render doesn't happen, the bug is in React's scheduling, not the data flow.

Step 5: Find the divergence point

The app code is identical. The data flow completes in both. So what's different at the React runtime level? Read the relevant React source code in the recorded bundles. Focus on understanding the algorithm before looking for bugs — understand what SHOULD happen first, then look for where reality diverges.

Step 6: Trace state through the critical path

Once you've identified the relevant scheduling/reconciler functions, trace mutable state values through the critical time window. Focus on the last update that fails in prod — what state does React see? Compare the same state in the dev recording for the equivalent update. The divergence in state values IS the root cause.

Step 7: Explain why dev vs prod

Dev and prod React builds differ: dev is slower (extra validation, warnings, double-renders in StrictMode), prod is faster (optimized, no dev checks). This timing difference can change when asynchronous callbacks (microtasks, promise .then()) fire relative to React's render cycle. If a callback fires at a different point in the render lifecycle between dev and prod, that's likely the mechanism.

Step 8: Red-team your conclusion

Before finalizing: Does your explanation account for WHY it's prod-only? Does your fix address the root cause or just a symptom? Could there be other scenarios that trigger the same bug?

Deliverable

Write a detailed bug report with:

1. Bug Description — What happens

2. Root Cause Analysis — What exactly causes the deferred value to get stuck in prod but not dev

3. Suggested Fix — What code change would fix this

4. Key Evidence Trail — Table of tools used and findings at each step

💡 Bug Report: useDeferredValue Gets Permanently Stuck in Production with RSC Server Actions

1. Bug Description

When using useDeferredValue with a promise returned by a React Server Action inside <Suspense>, the deferred output permanently stops updating in production builds. The last update is lost — the output gets stuck one (or more) characters behind the input and never catches up, even though the server action completes successfully and returns correct data.

Reproduction: Type rapidly into a textarea that triggers a server action on each keystroke. The server action returns Promise<ReactNode>, which is passed through useDeferredValue and consumed via use() inside <Suspense>. In dev mode, output catches up after typing stops. In prod mode, output freezes permanently behind.

2. Root Cause Analysis

This is a race condition between React's Suspense ping mechanism and RSC chunk resolution, exposed only in production due to faster React execution.

The Two Interacting Bugs

Bug A: isThenableResolved doesn't recognize RSC intermediate states

React's isThenableResolved function (react-dom-client.production.js:3394) only checks for status === "fulfilled" or status === "rejected". But RSC flight client chunks use an intermediate "resolved_model" status, meaning HTTP data has arrived but hasn't been fully deserialized yet. When React's renderRootConcurrent resumes a suspended render (case 7: SuspendedOnData) and checks isThenableResolved(thrownValue), it returns false for "resolved_model" chunks and falls through to throwAndUnwindWorkLoop — treating already-available data as still-pending.

Bug B: Synchronous ping fires before lane is marked suspended, creating a dead lane

After throwAndUnwindWorkLoop, React calls attachPingListener which attaches .then(pingSuspendedRoot) to the RSC chunk. But RSC's ReactPromise.prototype.then (line 539) detects the "resolved_model" status, synchronously calls initializeModelChunk() (transitioning to "fulfilled"), and synchronously invokes pingSuspendedRoot — all within the same call stack as attachPingListener.

At this moment, the render is still in progress. root.suspendedLanes is 0 because markRootSuspended hasn't run yet. So pingSuspendedRoot computes:

root.pingedLanes |= root.suspendedLanes & pingedLanes  
//                    ^^^^^^^^^^^^^^^^  
//                    = 0 (not yet set!)  
// Result: pingedLanes stays 0. Ping is wasted.  

Later (~26ms), the render completes with exit status 4 (RootSuspended). markRootSuspended finally sets root.suspendedLanes |= 524288. But the ping already fired and won't fire again. The lane is now:

• pendingLanes: has bit 19 ✓

• suspendedLanes: has bit 19 ✓

• pingedLanes: 0 ✗

getNextLanes computes nonIdlePendingLanes & ~suspendedLanes = 0. The lane is permanently stuck — pending but unreachable.

The Fatal Timeline (from prod recording)

Time (ms)	Event	State
5147	Final keystroke "d"	setPromise(renderAction("hello world from prod"))
5149	POST request sent	Server action in flight
5158	POST response arrives	RSC chunk status → "resolved_model"
5165	renderRootConcurrent case 7	isThenableResolved("resolved_model") → false
5165	throwAndUnwindWorkLoop	Suspense boundary catches, render continues unwinding
5168	attachPingListener	.then(pingSuspendedRoot) attached to chunk
5169	RSC .then() fires synchronously	initializeModelChunk → "fulfilled" → pingSuspendedRoot
5169	Wasted ping	suspendedLanes=0, so pingedLanes stays 0
5191	Render completes	Exit status 4 (RootSuspended)
5191	markRootSuspended	suspendedLanes = 524288 (too late for ping)
5192	scheduleTaskForRootDuringMicrotask	getNextLanes → 0. Dead lane. No more renders.

Why Dev Mode Doesn't Hit This

Dev React has extra overhead: StrictMode double-renders, validation checks, debug IO tracking in trackUsedThenable (lines 3838-3854). This extra time creates a window where the RSC chunk transitions from "resolved_model" → "fulfilled" before React's case 7 check runs. So isThenableResolved returns true, replaySuspendedUnitOfWork is called instead of throwAndUnwindWorkLoop, and the value is consumed directly — no ping needed.

3. Suggested Fix

There are two complementary fixes, either of which would prevent the dead lane:

Fix A (React reconciler): Recognize RSC intermediate resolved states

In isThenableResolved, also check for "resolved_model" (and potentially other RSC intermediate resolved states like "resolved_module"):

// react-reconciler: ReactFiberThenable.js  
function isThenableResolved(thenable) {  
  var status = thenable.status;  
  return status === "fulfilled" || status === "rejected"  
    || status === "resolved_model" || status === "resolved_module";  
}  

This way, when case 7 checks the thenable, it recognizes the data has arrived and calls replaySuspendedUnitOfWork instead of throwAndUnwindWorkLoop. The render succeeds without needing a ping at all.

Fix B (React reconciler): Guard against synchronous pings during render

In pingSuspendedRoot, if the lane isn't yet in suspendedLanes (because render is still in progress), defer the ping or ensure the lane gets re-pinged after markRootSuspended:

// react-reconciler: ReactFiberWorkLoop.js  
function markRootSuspended(root, suspendedLanes, ...) {  
  root.suspendedLanes |= suspendedLanes;  
  root.pingedLanes &= ~suspendedLanes;  
  // NEW: Check if any of these lanes were pinged while render was in progress  
  // If so, immediately re-ping them  
  if (root._pendingPingsDuringRender & suspendedLanes) {  
    root.pingedLanes |= root._pendingPingsDuringRender & suspendedLanes;  
    root._pendingPingsDuringRender = 0;  
  }  
}  

Or, alternatively in pingSuspendedRoot, detect that a render is in progress and record the ping for later application.

Fix A is the cleaner solution — it avoids the ping mechanism entirely for already-resolved data and handles the root cause (misidentification of RSC chunk state). Fix B is a defense-in-depth measure for any other scenario where synchronous pings could race with in-progress renders.

4. Key Evidence Trail

Step	Tool	Finding
Baseline comparison	RecordingOverview (both recordings)	Dev: 125 commits, 318ms render. Prod: 38 commits, 107ms render. 3x render difference.
Visual confirmation	Screenshot (both, final timestamp)	Dev output matches input. Prod stuck at "hello world from pro" — one char behind.
Last render commits	ReactRenders commits mode (prod)	Last commit #37 at 5.15s is a setState for the typing event, not a Suspense resolution. No commit for final value.
Network verification	NetworkRequest detail (prod, last 2 POSTs)	Both return 200 OK. Final POST contains "hello world from prod" — correct data.
Trigger chains	ReactRenders trigger-detail (prod, commits 35-37)	Commit 36 triggered by network response for "hello world from pro". No commit triggered by final response.
Ping analysis	Logpoint on pingSuspendedRoot	3 pings after final keystroke. Ping #3 (Point:114, 5169ms): suspendedLanes=0, ping is a no-op.
Re-suspension	Logpoint on markRootSuspended	Point:118 (5191ms): Lane 524288 re-suspended. pingedLanes already 0.
Scheduling dead-end	Logpoint on scheduleTaskForRootDuringMicrotask	After re-suspension: getNextLanes returns 0. Lane stuck forever.
isThenableResolved	Logpoint on line 3394 + DescribePoint	Returns false for status === "resolved_model". Only checks "fulfilled"/"rejected".
Case 7 path	DescribePoint on renderRootConcurrent case 7	isThenableResolved false → throwAndUnwindWorkLoop instead of replaySuspendedUnitOfWork.
RSC chunk .then()	ReadSource on RSC client code	ReactPromise.prototype.then synchronously resolves "resolved_model" → "fulfilled" and invokes callback inline.
Dev vs prod timing	DescribePoint comparison of trackUsedThenable	Dev has extra debug IO code (lines 3838-3854) creating enough delay for chunk to reach "fulfilled" before case 7 check.

5. Impact Assessment

• Severity: High — causes permanent UI staleness with no recovery

• Scope: Any Next.js app using useDeferredValue + use() + RSC server actions with rapid updates

• Workarounds: None clean. Adding artificial delays would mask but not fix. Avoiding useDeferredValue with server actions works but defeats the purpose.

• React version affected: 19.3.0-canary-f93b9fd4-20251217 (and likely all React 19 builds with RSC support)

💡 You are a debugging agent with access to Replay MCP tools and source code. Your job is to investigate a bug, identify the root cause, and suggest a fix.

Bug Report

App: Next.js app using React 19 with server actions. Source at ~/projects/temp/react-udv-bug.

Behavior:

• A textarea input feeds text to a server action (renderAction) which returns Promise<ReactNode>

• The promise is wrapped in useDeferredValue, then consumed via use() inside <Suspense>

• In dev mode (next dev): the deferred output box tracks the input and catches up after typing stops

• In prod mode (next build && next start): the deferred output frequently gets stuck and never catches up to the final input value

Recordings:

• Dev (working): 06447062-f403-49f3-8bd8-476a65b448d2

• Prod (broken): c0b7c6fb-8f2a-461c-9fca-0ba2e9be3083

In the broken prod recording, the user typed "hello world from prod" but the deferred output is stuck showing "hello world from pro" — one character behind, permanently.

What we know:

• No console errors in either recording

• The server action POST requests all return 200 OK with valid data

• The app code is identical between dev and prod — only the build mode differs

• It only manifests in production builds

Goal: Figure out why the deferred value stops updating in production and identify the root cause.

Investigation Methodology

Follow this structured process:

Step 1: Establish the baseline

Get an overview of both recordings. Compare high-level stats: render commit counts, network request counts, timing. The quantitative difference between dev and prod is your first clue.

Step 2: Confirm the visual bug

Take screenshots at the end of both recordings to verify what each shows. Confirm the prod recording is stuck one character behind.

Step 3: Compare render commit patterns

Look at the sequence of React render commits in both recordings. Focus on the tail end — the last few commits. In a working recording, you'd expect the render pattern to complete cleanly. In the broken one, something is missing or different. What's the last commit in each? What triggered it? What components re-rendered?

Step 4: Map the data flow end-to-end

Trace the full lifecycle: keystroke → server action POST → response → promise resolution → deferred value update → component re-render. Verify each step completes in both recordings. If all steps complete but the render doesn't happen, the bug is in React's scheduling, not the data flow.

Step 5: Find the divergence point

The app code is identical. The data flow completes in both. So what's different at the React runtime level? Read the relevant React source code in the recorded bundles. Focus on understanding the algorithm before looking for bugs — understand what SHOULD happen first, then look for where reality diverges.

Step 6: Trace state through the critical path

Once you've identified the relevant scheduling/reconciler functions, trace mutable state values through the critical time window. Focus on the last update that fails in prod — what state does React see? Compare the same state in the dev recording for the equivalent update. The divergence in state values IS the root cause.

Step 7: Explain why dev vs prod

Dev and prod React builds differ: dev is slower (extra validation, warnings, double-renders in StrictMode), prod is faster (optimized, no dev checks). This timing difference can change when asynchronous callbacks (microtasks, promise .then()) fire relative to React's render cycle. If a callback fires at a different point in the render lifecycle between dev and prod, that's likely the mechanism.

Step 8: Red-team your conclusion

Before finalizing: Does your explanation account for WHY it's prod-only? Does your fix address the root cause or just a symptom? Could there be other scenarios that trigger the same bug?

Conceptual Context: React's Scheduling System

These concepts will help you navigate the React internals you'll encounter:

• Lanes: React uses a bitmask-based priority system called "lanes." Each update is assigned a lane (e.g., SyncLane, TransitionLane, DeferredLane). useDeferredValue schedules work on deferred lanes, which are lower priority than user-initiated updates.

• Suspension and pinging: When a component throws a Promise (Suspense), React marks the update's lanes as "suspended." When the promise resolves, a "ping" fires that marks those lanes as "pinged," telling the scheduler to retry. The lifecycle is: pending → rendering → suspended → pinged → retry rendering → committed.

• workInProgress state: During rendering, React tracks the current render's state in module-scoped variables prefixed with workInProgress — things like what root is being rendered, what lanes, what the exit status is, which lanes have been pinged during this render. These are mutable and change throughout the render cycle.

• markRootSuspended****: When a render suspends, this function marks the lanes as suspended on the root. It uses workInProgressRootPingedLanes to exclude lanes that were pinged during the current render (so they don't get stuck as suspended). It also adds lanes to warmLanes which affects future scheduling.

• Concurrent rendering and yielding: In prod, React uses time-slicing — renders can yield back to the browser mid-work and resume later. This means microtasks (like promise .then() callbacks) can fire between render chunks. In dev mode, renders tend to run longer without yielding due to extra dev checks, so microtasks fire after the render completes.

• The key timing question: When a suspended promise resolves, the .then() ping callback might fire during an active render (prod, due to yielding) or after the render completes (dev, due to slower execution). If the ping handling code doesn't correctly handle the "ping arrives mid-render" case, the lane can get stuck.

Deliverable

Write a detailed bug report with:

1. Bug Description — What happens

2. Root Cause Analysis — What exactly causes the deferred value to get stuck in prod but not dev

3. Suggested Fix — What code change would fix this

4. Key Evidence Trail — Table of tools used and findings at each step

💡 Bug Report: useDeferredValue Permanently Stalls in Production React Builds

1. Bug Description

When using useDeferredValue with a server action that returns Promise<ReactNode>, wrapped in <Suspense> and consumed via use(), the deferred output frequently stops updating in production builds. The user types "hello world from prod" but the deferred display is stuck showing "hello world from pro" — one character behind, permanently. The same app code works correctly in dev mode.

2. Root Cause Analysis

The bug is in React's pingSuspendedRoot function — specifically, a code path that silently drops a Suspense ping when it arrives during an active concurrent render with RootSuspendedWithDelay exit status.

The mechanism step by step:

1. User types the final 'd' in "prod" (5149ms). React:

• Schedules a new server action POST for "hello world from prod"

• Begins a concurrent render on lane 524288 (DeferredLane)

2. The server action responds quickly (5155ms, ~6ms round-trip). The promise's .then() callback fires pingSuspendedRoot at 5169ms — while React is still mid-render of the same lane due to concurrent time-slicing.

3. Inside pingSuspendedRoot (line 13073), the ping enters the critical branch:

function pingSuspendedRoot(root, wakeable, pingedLanes) {  
  // Line 13076: Try to mark root.pingedLanes  
  root.pingedLanes |= root.suspendedLanes & pingedLanes;  
  //                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
  //  BUG: root.suspendedLanes is STILL 0 here!  
  //  The lane hasn't been marked suspended yet (render is still in progress).  
  //  So this is: 0 |= (0 & 524288) = 0. NO EFFECT.  
  
  root.warmLanes &= ~pingedLanes;  // warmLanes also 0 at this point  
  
  // Line 13078: Are we currently rendering this root+lanes?  
  workInProgressRoot === root &&           // TRUE (same root)  
  (workInProgressRootRenderLanes & pingedLanes) === pingedLanes &&  // TRUE (same lane)  
  
  // Line 13080: Ternary — exitStatus check  
  (4 === workInProgressRootExitStatus ||   // TRUE (RootSuspendedWithDelay = 4)  
   (...))  
    // FIRST BRANCH taken:  
    ? 0 === (executionContext & 2) && prepareFreshStack(root, 0)  
    //       ^^^^^^^^^^^^^^^^^^^^^^  
    //  BUG: executionContext IS 2 (RenderContext)! We're mid-render!  
    //  So 0 === (2 & 2) is FALSE. prepareFreshStack is NOT called.  
    //  This entire branch is a no-op.  
  
    // ELSE BRANCH (never reached because ternary was truthy):  
    : (workInProgressRootPingedLanes |= pingedLanes)  
    //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
    //  This would have saved us! But it's unreachable.  
  
  ensureRootIsScheduled(root);  // Called, but getNextLanes returns 0  
}  

4. The render completes and calls markRootSuspended (line 12141):

function markRootSuspended(root, suspendedLanes, spawnedLane, didAttemptEntireTree) {  
  suspendedLanes &= ~workInProgressRootPingedLanes;  // 524288 &= ~0 = 524288 (no exclusion)  
  root.suspendedLanes |= suspendedLanes;              // Lane 524288 marked SUSPENDED  
  root.pingedLanes &= ~suspendedLanes;                // Clears any pings (already 0)  
  didAttemptEntireTree && (root.warmLanes |= suspendedLanes);  // Lane marked WARM  
}  

5. The lane is now permanently stuck:

• root.pendingLanes = 524288 ✓ (work pending)

• root.suspendedLanes = 524288 ✗ (excluded from scheduling)

• root.pingedLanes = 0 ✗ (no ping recorded to rescue it)

• root.warmLanes = 524288 ✗ (excluded from cold-retry path)

6. getNextLanes (line 706) returns 0 — all three rescue paths fail:

1. pendingLanes & ~suspendedLanes = 0 (suspended)

2. pingedLanes & nonIdlePendingLanes = 0 (not pinged)

3. nonIdlePendingLanes & ~warmLanes = 0 (warm)

The scheduler permanently sees no work. The UI is stuck.

Why dev mode works

In development mode, React runs significantly slower due to:

• Extra validation checks, warning infrastructure, DevTools hooks

• StrictMode double-rendering

This means renders take longer and never yield to the browser mid-render. Promise .then() microtask callbacks therefore always fire after the render completes. In every single ping across 126 hits in the dev recording, workInProgressRoot !== root and executionContext === 0 — the ping always takes the fast path of just setting root.pingedLanes (line 13076, where root.suspendedLanes is already set) and calling ensureRootIsScheduled. The buggy ternary at line 13078 is never entered.

In production, concurrent time-slicing allows yields mid-render, so microtasks from resolved promises can fire during the render. This creates the timing window where the ping arrives before markRootSuspended runs.

3. Suggested Fix

The fix is in pingSuspendedRoot. When the ping fires mid-render (executionContext has RenderContext bit set) and the exit status prevents prepareFreshStack, the function must still record the ping in workInProgressRootPingedLanes so that markRootSuspended can exclude the lane from suspension.

Option A (minimal fix): When the prepareFreshStack guard fails, fall through to set workInProgressRootPingedLanes:

// In pingSuspendedRoot, replace the ternary at lines 13080-13086:  
  
// BEFORE (buggy):  
(4 === workInProgressRootExitStatus || (...))  
  ? 0 === (executionContext & 2) && prepareFreshStack(root, 0)  
  : (workInProgressRootPingedLanes |= pingedLanes)  
  
// AFTER (fixed):  
if (4 === workInProgressRootExitStatus || (...)) {  
  if (0 === (executionContext & 2)) {  
    prepareFreshStack(root, 0);  
  } else {  
    // Ping arrived mid-render. Can't restart now, but record the ping  
    // so markRootSuspended won't mark this lane as suspended.  
    workInProgressRootPingedLanes |= pingedLanes;  
  }  
} else {  
  workInProgressRootPingedLanes |= pingedLanes;  
}  

Option B (defensive): Additionally ensure root.pingedLanes is set even when the lane isn't yet suspended, so the ping survives regardless of ordering:

// At the top of pingSuspendedRoot, unconditionally record the ping on the root  
// for lanes that are pending (not just suspended):  
root.pingedLanes |= root.suspendedLanes & pingedLanes;  
root.pingedLanes |= root.pendingLanes & pingedLanes;  // NEW: catch pre-suspension pings  

Option A is the more surgical fix addressing the exact logic error. Option B is belt-and-suspenders.

4. Key Evidence Trail

Step	Tool	Finding
Recording overview	RecordingOverview (both)	Dev: 125 commits, 63 POSTs. Prod: 38 commits, 21 POSTs. Massive gap.
Visual confirmation	Screenshot (both at final timestamps)	Dev shows "hello world from prod" in output. Prod shows "hello world from pro" — stuck.
Render commit tails	ReactRenders commit mode (both)	Dev ends with SuspenseResumed at 8.47s. Prod ends with SetState at 5.15s — no final SuspenseResumed.
Network completion	NetworkRequest detail (prod #36)	Final POST "hello world from prod" returns 200 OK at 5155ms with valid RSC payload. Data arrived.
App source code	grepika_get on app/client.tsx	useDeferredValue(promise) → use() in <Suspense>. Standard pattern.
pingSuspendedRoot analysis	ReadSource + Logpoint on line 13073	46 total hits. Hit #5 (Point:114, 5169ms) fires mid-render: executionContext=2, root.suspendedLanes=0, workInProgressRootExitStatus=4. Ping silently dropped.
markRootSuspended aftermath	Logpoint on line 12147 + post-state eval	After line 12161: root.suspendedLanes=524288, root.pingedLanes=0, root.warmLanes=524288. Lane permanently stuck.
getNextLanes deadlock	ReadSource on line 706	Three rescue paths all return 0 with stuck state. No work scheduled.
Dev comparison	Logpoint on line 13073 (dev recording)	All 126 pings: executionContext=0, workInProgressRoot !== root. Buggy branch never entered.
markRootSuspended exclusion logic	ReadSource on line 12147	suspendedLanes &= ~workInProgressRootPingedLanes — would exclude pinged lanes, but workInProgressRootPingedLanes=0 because the ping was dropped.

5. Summary

This is a race condition in React's concurrent rendering scheduler. When a Suspense promise resolves during an active concurrent render (via microtask firing during a yield), pingSuspendedRoot enters a code path where:

1. root.pingedLanes can't be set (lane not yet suspended on root)

2. prepareFreshStack can't be called (currently in render context)

3. workInProgressRootPingedLanes isn't set (unreachable else-branch of ternary)

The ping is completely lost. The subsequent markRootSuspended marks the lane as suspended with no ping, and getNextLanes can never find work for it again. The fix is to ensure workInProgressRootPingedLanes is set in the mid-render case, so markRootSuspended excludes the lane from suspension.

💡 You are a debugging agent with access to Replay MCP tools and source code. Your job is to investigate a bug, identify the root cause, and suggest a fix.

Bug Report

App: Next.js app using React 19 with server actions. Source at ~/projects/temp/react-udv-bug.

Behavior:

• A textarea input feeds text to a server action (renderAction) which returns Promise<ReactNode>

• The promise is wrapped in useDeferredValue, then consumed via use() inside <Suspense>

• In dev mode (next dev): the deferred output box tracks the input and catches up after typing stops

• In prod mode (next build && next start): the deferred output frequently gets stuck and never catches up to the final input value

Recordings:

• Dev (working): 06447062-f403-49f3-8bd8-476a65b448d2

• Prod (broken): c0b7c6fb-8f2a-461c-9fca-0ba2e9be3083

In the broken prod recording, the user typed "hello world from prod" but the deferred output is stuck showing "hello world from pro" — one character behind, permanently.

What we know:

• No console errors in either recording

• The server action POST requests all return 200 OK with valid data

• The app code is identical between dev and prod — only the build mode differs

• It only manifests in production builds

Goal: Figure out why the deferred value stops updating in production and identify the root cause.

Investigation Methodology

Follow this structured process:

Step 1: Establish the baseline

Get an overview of both recordings. Compare high-level stats: render commit counts, network request counts, timing. The quantitative difference between dev and prod is your first clue.

Step 2: Confirm the visual bug

Take screenshots at the end of both recordings to verify what each shows. Confirm the prod recording is stuck one character behind.

Step 3: Compare render commit patterns

Look at the sequence of React render commits in both recordings. Focus on the tail end — the last few commits. In a working recording, you'd expect the render pattern to complete cleanly. In the broken one, something is missing or different. What's the last commit in each? What triggered it? What components re-rendered?

Step 4: Map the data flow end-to-end

Trace the full lifecycle: keystroke → server action POST → response → promise resolution → deferred value update → component re-render. Verify each step completes in both recordings. If all steps complete but the render doesn't happen, the bug is in React's scheduling, not the data flow.

Step 5: Find the divergence point

The app code is identical. The data flow completes in both. So what's different at the React runtime level? Read the relevant React source code in the recorded bundles. Focus on understanding the algorithm before looking for bugs — understand what SHOULD happen first, then look for where reality diverges.

Step 6: Trace state through the critical path

Once you've identified the relevant scheduling/reconciler functions, trace mutable state values through the critical time window. Focus on the last update that fails in prod — what state does React see? Compare the same state in the dev recording for the equivalent update. The divergence in state values IS the root cause.

Step 7: Explain why dev vs prod

Dev and prod React builds differ: dev is slower (extra validation, warnings, double-renders in StrictMode), prod is faster (optimized, no dev checks). This timing difference can change when asynchronous callbacks (microtasks, promise .then()) fire relative to React's render cycle. If a callback fires at a different point in the render lifecycle between dev and prod, that's likely the mechanism.

Step 8: Red-team your conclusion

Before finalizing: Does your explanation account for WHY it's prod-only? Does your fix address the root cause or just a symptom? Could there be other scenarios that trigger the same bug?

Conceptual Context: React's Scheduling System

These concepts will help you navigate the React internals you'll encounter:

• Lanes: React uses a bitmask-based priority system called "lanes." Each update is assigned a lane (e.g., SyncLane, TransitionLane, DeferredLane). useDeferredValue schedules work on deferred lanes, which are lower priority than user-initiated updates.

• Suspension and pinging: When a component throws a Promise (Suspense), React marks the update's lanes as "suspended." When the promise resolves, a "ping" fires that marks those lanes as "pinged," telling the scheduler to retry. The lifecycle is: pending → rendering → suspended → pinged → retry rendering → committed.

• workInProgress state: During rendering, React tracks the current render's state in module-scoped variables prefixed with workInProgress — things like what root is being rendered, what lanes, what the exit status is, which lanes have been pinged during this render. These are mutable and change throughout the render cycle.

• markRootSuspended****: When a render suspends, this function marks the lanes as suspended on the root. It uses workInProgressRootPingedLanes to exclude lanes that were pinged during the current render (so they don't get stuck as suspended). It also adds lanes to warmLanes which affects future scheduling.

• Concurrent rendering and yielding: In prod, React uses time-slicing — renders can yield back to the browser mid-work and resume later. This means microtasks (like promise .then() callbacks) can fire between render chunks. In dev mode, renders tend to run longer without yielding due to extra dev checks, so microtasks fire after the render completes.

• The key timing question: When a suspended promise resolves, the .then() ping callback might fire during an active render (prod, due to yielding) or after the render completes (dev, due to slower execution). If the ping handling code doesn't correctly handle the "ping arrives mid-render" case, the lane can get stuck.

Replay MCP Tool Strategy

You have access to powerful runtime inspection tools. Here's how to use them effectively for this investigation:

• RecordingOverview — Start here for each recording. Gets you error counts, render commit stats, network summary, environment info. Run on BOTH recordings first.

• ReactRenders — Your primary comparison tool. Use mode: commits to see the full commit sequence. Use mode: commit with commitIndex to drill into individual commits. Focus on the last few commits in each recording — where does prod diverge from dev?

• NetworkRequest — Verify all server action POSTs returned successfully. Use to rule out data-level issues early. Check timing of responses relative to render commits.

• SearchSources — Find functions in the recorded bundles by name. React 19 ships unminified production bundles, so function names are preserved. Search for scheduling and reconciler functions to find their locations.

• ReadSource — Read source code with execution hit counts per line. This is a built-in static profiler — you can see which branches executed and how many times. Use this to understand code flow without needing to instrument anything.

• Logpoint — Your most powerful investigation tool. Evaluates any JavaScript expression at a specific source line, every time that line executes. Returns the expression value at each hit with timestamps. This is equivalent to adding console.log statements retroactively — unlimited, no rebuild, no reproduction needed.

• DescribePoint / Evaluate — Inspect full program state at a specific execution point. Use after Logpoint identifies an interesting moment — drill in to see all variables, the call stack, and dependency chains.

• Screenshot — Confirm visual state at timestamps. Use at the end of each recording to verify the bug visually.

Investigation pattern: Overview both recordings → Compare render commits → Read source to understand the algorithm → Instrument with Logpoints to trace state through the critical window → Find where dev and prod state diverges → Explain the mechanism.

Deliverable

Write a detailed bug report with:

1. Bug Description — What happens

2. Root Cause Analysis — What exactly causes the deferred value to get stuck in prod but not dev

3. Suggested Fix — What code change would fix this

4. Key Evidence Trail — Table of tools used and findings at each step

💡 Bug Report: React useDeferredValue Stuck in Production — Race Condition in pingSuspendedRoot

1. Bug Description

When using useDeferredValue with a Promise<ReactNode> (fed by a server action) inside <Suspense>, the deferred output permanently stops updating in production builds. The final typed character never renders — the deferred value gets stuck one character behind the input. This does not happen in dev mode.

In the investigated case, the user typed "hello world from prod" but the output is permanently stuck showing "hello world from pro".

2. Root Cause Analysis

The bug is a race condition in React's pingSuspendedRoot function in react-dom-client.production.js. It only manifests in production because production builds use concurrent time-slicing, which allows microtasks (promise .then() callbacks) to fire during an active render.

The Flow That Breaks

1. User types the final character ('d'). React starts rendering the deferred lane (lane 524288 = 0x80000).

2. During rendering, the component suspends (throws a Promise) because the server action response hasn't arrived yet. React sets workInProgressRootExitStatus = 4 (RootSuspended) but continues in the render context (executionContext = 2).

3. The server action response arrives at ~5155ms. The promise resolves, firing the .then() ping callback at 5169ms — while the render is still active.

4. pingSuspendedRoot fires mid-render with this state:

5. Three things fail simultaneously:

6. 5191ms — markRootSuspended runs after the render completes:

Why Dev Mode Works

In dev mode, React is slower due to extra validation, warnings, and StrictMode double-renders. It doesn't yield to the browser mid-render as often. The Replay recording confirms this: the condition workInProgressRoot === root inside pingSuspendedRoot has 0 hits in dev. The ping .then() callback always fires after the render completes, so pingSuspendedRoot takes the early-exit path (no active render) and ensureRootIsScheduled correctly reschedules the retry.

3. The Exact Bug in Code

File: packages/react-reconciler/src/ReactFiberWorkLoop.js (in the React repo)

Function: pingSuspendedRoot

Prod bundle location: react-dom-client.production.js lines 13078-13088

// Current code (simplified):  
if (workInProgressRoot === root &&  
    (workInProgressRootRenderLanes & pingedLanes) === pingedLanes) {  
  
  if (exitStatus === RootSuspended || (exitStatus === RootSuspendedWithDelay && ...)) {  
    // TRUE branch: try to restart  
    if ((executionContext & RenderContext) === 0) {  
      prepareFreshStack(root, 0);  // Can't do this mid-render!  
    }  
    // If mid-render, NOTHING happens — ping is silently dropped  
  } else {  
    // FALSE branch: record the ping for later  
    workInProgressRootPingedLanes |= pingedLanes;  // Never reached when exitStatus===4  
  }  
}  

The bug: When exitStatus === 4 (suspended) AND executionContext has the render bit set (mid-render), neither prepareFreshStack nor workInProgressRootPingedLanes |= pingedLanes executes. The ping is completely lost.

4. Suggested Fix

When the render has suspended and we're mid-render (can't restart), we should still record the ping in workInProgressRootPingedLanes so that markRootSuspended will exclude the pinged lane from being marked suspended:

// In pingSuspendedRoot, the ternary handling for exitStatus === 4:  
if (exitStatus === RootSuspended || (exitStatus === RootSuspendedWithDelay && ...)) {  
  if ((executionContext & RenderContext) === 0) {  
    prepareFreshStack(root, 0);  
  } else {  
    // FIX: Record the ping even when we can't restart mid-render.  
    // markRootSuspended will check workInProgressRootPingedLanes  
    // and exclude this lane from being marked suspended.  
    workInProgressRootPingedLanes |= pingedLanes;  
  }  
} else {  
  workInProgressRootPingedLanes |= pingedLanes;  
}  

This ensures that when markRootSuspended runs at line 12147 (suspendedLanes &= ~workInProgressRootPingedLanes), the pinged lane is excluded and won't be permanently stuck. The subsequent ensureRootIsScheduled (line 13089, which always runs) will then correctly schedule a retry render.

5. Key Evidence Trail

Step	Tool	Recording	Finding
1	RecordingOverview	Both	Dev: 125 commits, 63 POSTs. Prod: 38 commits, 21 POSTs. 3x fewer renders in prod.
2	Screenshot	Both	Dev shows "hello world from dev" (complete). Prod shows "hello world from pro" (stuck).
3	ReactRenders (commits)	Prod	Last commit #37 is SetState for final keystroke. No SuspenseResumed follows — missing final commit.
4	ReactRenders (commits)	Dev	Last commits #123→#124 show SetState→SuspenseResumed. Pattern completes.
5	NetworkRequest (detail)	Prod	POST #36 at 5149ms sends "hello world from prod", gets 200 OK. Response arrives at 5155ms with valid RSC data. Data is fine.
6	SearchSources + ReadSource	Prod	Located pingSuspendedRoot (L13073), markRootSuspended (L12141), prepareFreshStack (L12184).
7	Logpoint + Evaluate	Prod	At Point:114 (5169ms ping): exitStatus=4, executionContext=2, root.suspendedLanes=0, workInProgressRootPingedLanes=0. Confirmed ping is lost.
8	Logpoint + Evaluate	Prod	At Point:118 (5191ms markRootSuspended): workInProgressRootPingedLanes=0, lane 524288 permanently marked suspended+warm.
9	ReadSource (hit counts)	Dev	pingSuspendedRoot L10047: workInProgressRoot === root has 0 hits. Dev pings never fire mid-render. Confirms timing difference is the mechanism.
10	ReadSource (hit counts)	Prod	pingSuspendedRoot L13087: 4 hits on the mid-render path. L13085 (prepareFreshStack): 0 hits. Confirmed: entered the ternary 4 times but prepareFreshStack never executed.

6. Red-Team Analysis

Does this explain why it's prod-only? Yes. The race requires time-slicing (concurrent rendering with yields), which only happens in production. Dev mode's extra overhead means renders don't yield, so promise callbacks fire after renders complete.

Is this a React bug or an app bug? React bug. The app correctly uses useDeferredValue + Suspense + server actions. The race is entirely within React's scheduler.

Could this affect other patterns? Yes. Any pattern where a promise resolves during an active concurrent render of the same lane — useDeferredValue, lazy(), data fetching with Suspense — could theoretically hit this. The useDeferredValue + server action pattern is particularly susceptible because server action responses arrive asynchronously at unpredictable times.

Is the fix complete? The suggested fix handles the specific case where exitStatus === 4 && (executionContext & 2) !== 0. An alternative fix could be in markRootSuspended itself (checking root.pingedLanes as a fallback), but fixing pingSuspendedRoot at the point where the ping is dropped is cleaner and more direct.

Here’s some of the actual MCP tool calls the agents ran during this investigation:

RecordingOverview

# Recording Overview

## Recording Info & Environment

- **URL:** http://localhost:3000/
- **Duration:** 10.3s
- **React:** v19.3.0-canary-f93b9fd4-20251217 (production)
- **Libraries:** NextJS, React

## Console Errors & Warnings

App healthy — no console errors or warnings detected.

## Screenshot

**Final recording state** — screenshot available at timestamp 9955ms.
Use the **Screenshot** tool with `timestamp: 9955` to view it.

## Network Summary

- **Total requests:** 37
- **Failed:** 1
- **Slow:** 0

### API Patterns
- `POST /` — 21 requests
- `GET /` — 1 request

## React Render Summary

- **Total commits:** 38
- **Total render time:** 107ms

### Costliest Commits
- Commit #24: 54ms, 4 fibers — SuspenseResumed
- Commit #13: 4ms, 4 fibers — SuspenseResumed
- Commit #22: 4ms, 4 fibers — SuspenseResumed
- Commit #19: 4ms, 4 fibers — SuspenseResumed
- Commit #9: 4ms, 4 fibers — SuspenseResumed

## Suggested Next Steps

1. **NetworkRequest** — Investigate failed network requests
   _1 failed request_
2. **ReactRenders** — Analyze React render performance
   _38 commits, 107ms total render time_
3. **Screenshot** — View screenshot of the application state
   _Final recording state at 9955ms_

ReactRenders - List

# Commits

|  # |  Time | Duration | Fibers | Mutating | Wasted | Waste% | Trigger         |
| -: | ----: | -------: | -----: | -------: | -----: | -----: | --------------- |
| 28 | 4.52s |    3.3ms |      4 |        1 |      3 |  75.0% | SuspenseResumed |
| 29 | 4.56s |     15µs |      2 |        1 |      1 |  50.0% | SetState        |
| 30 | 4.66s |    1.6ms |      3 |        1 |      2 |  66.7% | SetState        |
| 31 | 4.67s |    3.3ms |      4 |        1 |      3 |  75.0% | SuspenseResumed |
| 32 | 4.76s |      4µs |      2 |        1 |      1 |  50.0% | SetState        |
| 33 | 4.77s |    3.3ms |      4 |        1 |      3 |  75.0% | SuspenseResumed |
| 34 | 4.83s |      4µs |      2 |        1 |      1 |  50.0% | SetState        |
| 35 | 5.07s |    3.3ms |      4 |        1 |      3 |  75.0% | SetState        |
| 36 | 5.08s |    3.3ms |      4 |        1 |      3 |  75.0% | SuspenseResumed |
| 37 | 5.15s |      4µs |      2 |        1 |      1 |  50.0% | SetState        |

Showing 29–38 of 38

## Next Steps

- Use mode=commit&commitIndex=N to inspect a specific commit
- Use mode=summary to see the recording overview

ReactRenders - Commit

# Commit #37

**Timestamp:** 5.15s
**Render duration:** 4µs
**Fibers rendered:** 2 (1 mutating, 1 wasted — 50.0%)

## Triggers

- **SetState**
  - Source: TestPreviewClient (client.tsx:38:25)
  - Point: 11682667932122379955827658622763010

## Components

| Component         | Instances | Renders | Wasted | Duration | DOM Muts |
| ----------------- | --------: | ------: | -----: | -------: | -------: |
| Resolved          |         1 |       1 |      1 |      4µs |        0 |
| TestPreviewClient |         1 |       1 |      0 |      0µs |        1 |

## DOM Mutations (1 total)

- TestPreviewClient: 1

## Next Steps

- Use mode=component&componentName=X to investigate a specific component
- Use mode=commits to browse other commits
- Use mode=summary to see the recording overview

ReadSource

hits | L | code 
------|------|------
   0 | 12116 |       for (var i = 0; i < tag.length; i++) { 
   0 | 12117 |         var check = tag[i], 
   0 | 12118 |           getSnapshot = check.getSnapshot; 
   0 | 12119 |         check = check.value; 
     | 12120 |         try { 
   0 | 12121 |           if (!objectIs(getSnapshot(), check)) return !1; 
     | 12122 |         } catch (error) { 
   0 | 12123 |           return !1; 
     | 12124 |         } 
     | 12125 |       } 
     | 12126 |     tag = node.child; 
  87 | 12127 |     if (node.subtreeFlags & 16384 && null !== tag) 
   0 | 12128 |       (tag.return = node), (node = tag); 
     | 12129 |     else { 
  87 | 12130 |       if (node === finishedWork) break; 
   0 | 12131 |       for (; null === node.sibling; ) { 
   0 | 12132 |         if (null === node.return || node.return === finishedWork) return !0; 
   0 | 12133 |         node = node.return; 
     | 12134 |       } 
   0 | 12135 |       node.sibling.return = node.return; 
   0 | 12136 |       node = node.sibling; 
     | 12137 |     } 
     | 12138 |   } 
  87 | 12139 |   return !0; 
  87 | 12140 | } 
     | *12141 | function markRootSuspended( 
     | 12142 |   root, 
     | 12143 |   suspendedLanes, 
     | 12144 |   spawnedLane, 
     | 12145 |   didAttemptEntireTree 
     | 12146 | ) { 
  67 | 12147 |   suspendedLanes &= ~workInProgressRootPingedLanes; 
  67 | 12148 |   suspendedLanes &= ~workInProgressRootInterleavedUpdatedLanes; 
  67 | 12149 |   root.suspendedLanes |= suspendedLanes; 
  67 | 12150 |   root.pingedLanes &= ~suspendedLanes; 
  67 | 12151 |   didAttemptEntireTree && (root.warmLanes |= suspendedLanes); 
  67 | 12152 |   didAttemptEntireTree = root.expirationTimes; 
 134 | 12153 |   for (var lanes = suspendedLanes; /*<HIT>*/0 < lanes; ) { 
  67 | 12154 |     var index$6 = 31 - clz32(lanes), 
  67 | 12155 |       lane = 1 << index$6; 
  67 | 12156 |     didAttemptEntireTree[index$6] = -1; 
  67 | 12157 |     lanes &= ~lane; 
     | 12158 |   } 
  67 | 12159 |   0 !== spawnedLane && 
   0 | 12160 |     markSpawnedDeferredLane(root, spawnedLane, suspendedLanes); 
     | 12161 | } 
     | 12162 | function flushSyncWork$1() { 
  21 | 12163 |   return 0 === (executionContext & 6) 
  21 | 12164 |     ? (flushSyncWorkAcrossRoots_impl(0, !1), !1) 
     | 12165 |     : !0; 
     | 12166 | }

Logpoint

Found 46 total hits at turbopack:///[project]/node_modules/.pnpm/next@16.1.6_react-dom@19.2.4_react@19.2.4__react@19.2.4/node_modules/next/dist/compiled/react-dom/cjs/react-dom-client.production.js:13076 (showing first 20)

Hit 1 at point Point:239 @ 2676.17ms:
  Expression: ["ping", pingedLanes, "suspended", root.suspendedLanes, "renderLanes", workInProgressRootRenderLanes, "exit", workInProgressRootExitStatus, "isRoot", workInProgressRoot === root, "execCtx", executionContext]
  Result: [
  "ping",
  524288,
  "suspended",
  524800,
  "renderLanes",
  0,
  "exit",
  4,
  "isRoot",
  false,
  "execCtx",
  0
]

Hit 2 at point Point:240 @ 2676.21ms:
  Expression: ["ping", pingedLanes, "suspended", root.suspendedLanes, "renderLanes", workInProgressRootRenderLanes, "exit", workInProgressRootExitStatus, "isRoot", workInProgressRoot === root, "execCtx", executionContext]
  Result: [
  "ping",
  512,
  "suspended",
  524800,
  "renderLanes",
  0,
  "exit",
  4,
  "isRoot",
  false,
  "execCtx",
  0
]

Hit 3 at point Point:241 @ 2841.09ms:
  Expression: ["ping", pingedLanes, "suspended", root.suspendedLanes, "renderLanes", workInProgressRootRenderLanes, "exit", workInProgressRootExitStatus, "isRoot", workInProgressRoot === root, "execCtx", executionContext]
  Result: [
  "ping",
  1048576,
  "suspended",
  1049600,
  "renderLanes",
  0,
  "exit",
  4,
  "isRoot",
  false,
  "execCtx",
  0
]

// snip