diff --git a/src/profile-logic/merge-compare.ts b/src/profile-logic/merge-compare.ts
index 3e6f600dcf..833b4be80a 100644
--- a/src/profile-logic/merge-compare.ts
+++ b/src/profile-logic/merge-compare.ts
@@ -1172,21 +1172,19 @@ function combineSamplesDiffing(
 ): RawSamplesTable {
   const [
     {
-      thread: { samples: samples1, tid: tid1 },
+      thread: { samples: samples1 },
       weightMultiplier: weightMultiplier1,
     },
     {
-      thread: { samples: samples2, tid: tid2 },
+      thread: { samples: samples2 },
       weightMultiplier: weightMultiplier2,
     },
   ] = threadsAndWeightMultipliers;
 
   const newWeight: number[] = [];
-  const newThreadId: Tid[] = [];
   const newSamples = {
     ...getEmptySamplesTableWithEventDelay(),
     weight: newWeight,
-    threadId: newThreadId,
   };
 
   const samples1Time = computeTimeColumnForRawSamplesTable(samples1);
@@ -1212,7 +1210,6 @@ function combineSamplesDiffing(
       // of eventDelay/responsiveness don't mean anything.
       newSamples.eventDelay!.push(null);
       newSamples.time!.push(samples1Time[i]);
-      newThreadId.push(samples1.threadId ? samples1.threadId[i] : tid1);
       // TODO (issue #3151): Figure out a way to diff CPU usage numbers.
       // We add the first thread with a negative weight, because this is the
       // base profile.
@@ -1228,7 +1225,6 @@ function combineSamplesDiffing(
       // of eventDelay/responsiveness don't mean anything.
       newSamples.eventDelay!.push(null);
       newSamples.time!.push(samples2Time[j]);
-      newThreadId.push(samples2.threadId ? samples2.threadId[j] : tid2);
       const sampleWeight = samples2.weight ? samples2.weight[j] : 1;
       newWeight.push(weightMultiplier2 * sampleWeight);
 
@@ -1360,13 +1356,9 @@ function combineSamplesForMerging(threads: RawThread[]): RawSamplesTable {
   const nextSampleIndexPerThread: number[] = Array(
     samplesPerThread.length
   ).fill(0);
-  // This array will contain the source thread ids. It will be added to the
-  // samples table after the loop.
-  const newThreadId: Tid[] = [];
   // Creating a new empty samples table to fill.
   const newSamples = {
     ...getEmptySamplesTableWithEventDelay(),
-    threadId: newThreadId,
   };
 
   // If every source thread has threadCPUDelta, and if the threads are
@@ -1433,11 +1425,6 @@ function combineSamplesForMerging(threads: RawThread[]): RawSamplesTable {
     // from independent threads instead.
     ensureExists(newSamples.eventDelay).push(null);
     newSamples.time!.push(sourceThreadSamplesTimeCol[sourceThreadSampleIndex]);
-    newThreadId.push(
-      sourceThreadSamples.threadId
-        ? sourceThreadSamples.threadId[sourceThreadSampleIndex]
-        : threads[sourceThreadIndex].tid
-    );
     if (newThreadCPUDelta !== undefined) {
       newThreadCPUDelta.push(
         ensureExists(sourceThreadSamples.threadCPUDelta)[
diff --git a/src/profile-logic/profile-data.ts b/src/profile-logic/profile-data.ts
index d55997a537..60b30a27f6 100644
--- a/src/profile-logic/profile-data.ts
+++ b/src/profile-logic/profile-data.ts
@@ -2155,13 +2155,6 @@ export function filterThreadSamplesToRange(
     );
   }
 
-  if (samples.threadId) {
-    newSamples.threadId = samples.threadId.slice(
-      beginSampleIndex,
-      endSampleIndex
-    );
-  }
-
   const newThread: Thread = {
     ...thread,
     samples: newSamples,
@@ -2288,13 +2281,6 @@ export function filterRawThreadSamplesToRange(
     );
   }
 
-  if (samples.threadId) {
-    newSamples.threadId = samples.threadId.slice(
-      beginSampleIndex,
-      endSampleIndex
-    );
-  }
-
   const newThread: RawThread = {
     ...thread,
     samples: newSamples,
diff --git a/src/types/profile-derived.ts b/src/types/profile-derived.ts
index 6568b3b209..dc9ba9ccd8 100644
--- a/src/types/profile-derived.ts
+++ b/src/types/profile-derived.ts
@@ -149,8 +149,9 @@ export type SamplesTable = {
   category: Uint8Array;
   // The subcategory of each sample's stack in the unfiltered thread.
   subcategory: Uint16Array | Uint8Array;
-  // This property isn't present in normal threads. However it's present for
-  // merged threads, so that we know the origin thread for these samples.
+  // Legacy: older profiles produced by merging may have a per-sample
+  // origin-thread column. Not written by current code, but tolerated on
+  // input so pre-existing profiles still parse.
   threadId?: Tid[];
   argumentValues?: Array<number | null>;
   length: number;
diff --git a/src/types/profile.ts b/src/types/profile.ts
index 8b470516e2..aa68ae82c1 100644
--- a/src/types/profile.ts
+++ b/src/types/profile.ts
@@ -142,8 +142,9 @@ export type RawSamplesTable = {
   // The first value is ignored - it's not meaningful because there is no previous
   // sample.
   threadCPUDelta?: Array<number | null>;
-  // This property isn't present in normal threads. However it's present for
-  // merged threads, so that we know the origin thread for these samples.
+  // Legacy: older profiles produced by merging may have a per-sample
+  // origin-thread column. Not written by current code, but tolerated on
+  // input so pre-existing profiles still parse.
   threadId?: Tid[];
   length: number;
 };