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feat(replay): Add traces_by_timestamp to replay event #18048

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803b175
feat(replay): Add `traces_by_timestamp` to replay event
billyvg Oct 28, 2025
17cb7b4
fix tests
billyvg Oct 29, 2025
483def2
keep most recent trace id, use the current trace id if no transaction…
billyvg Oct 29, 2025
10c2825
format
billyvg Oct 29, 2025
6ffdce0
fix integration tests
billyvg Oct 29, 2025
5a8c6b3
address code review
billyvg Feb 5, 2026
f50a1cb
add more tests
billyvg Feb 5, 2026
31a8e9a
bump bundle size limits
andreiborza Feb 18, 2026
b8d9763
Merge branch 'develop' into billy/replay-802-replay-sdk-needs-to-send…
andreiborza Feb 25, 2026
f47fdc3
chore: Bump size limits for replay bundles
andreiborza Feb 25, 2026
1d68e6d
Merge branch 'develop' into billy/replay-802-replay-sdk-needs-to-send…
andreiborza Feb 26, 2026
a14a86a
Merge branch 'develop' into billy/replay-802-replay-sdk-needs-to-send…
andreiborza Feb 26, 2026
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6 changes: 3 additions & 3 deletions .size-limit.js
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Original file line number Diff line number Diff line change
Expand Up @@ -82,7 +82,7 @@ module.exports = [
path: 'packages/browser/build/npm/esm/prod/index.js',
import: createImport('init', 'browserTracingIntegration', 'replayIntegration', 'replayCanvasIntegration'),
gzip: true,
limit: '86 KB',
limit: '87 KB',
},
{
name: '@sentry/browser (incl. Tracing, Replay, Feedback)',
Expand Down Expand Up @@ -262,14 +262,14 @@ module.exports = [
path: createCDNPath('bundle.replay.logs.metrics.min.js'),
gzip: false,
brotli: false,
limit: '209 KB',
limit: '210 KB',
},
{
name: 'CDN Bundle (incl. Tracing, Replay) - uncompressed',
path: createCDNPath('bundle.tracing.replay.min.js'),
gzip: false,
brotli: false,
limit: '245 KB',
limit: '246 KB',
},
{
name: 'CDN Bundle (incl. Tracing, Replay, Logs, Metrics) - uncompressed',
Expand Down
107 changes: 13 additions & 94 deletions dev-packages/browser-integration-tests/suites/replay/captureReplay/test.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
import { expect } from '@playwright/test';
import { SDK_VERSION } from '@sentry/browser';
import { sentryTest, TEST_HOST } from '../../../utils/fixtures';
import { sentryTest } from '../../../utils/fixtures';
import { getExpectedReplayEvent } from '../../../utils/replayEventTemplates';
import { getReplayEvent, shouldSkipReplayTest, waitForReplayRequest } from '../../../utils/replayHelpers';

sentryTest('should capture replays (@sentry/browser export)', async ({ getLocalTestUrl, page }) => {
Expand All @@ -20,98 +20,17 @@ sentryTest('should capture replays (@sentry/browser export)', async ({ getLocalT
const replayEvent1 = getReplayEvent(await reqPromise1);

expect(replayEvent0).toBeDefined();
expect(replayEvent0).toEqual({
type: 'replay_event',
timestamp: expect.any(Number),
error_ids: [],
trace_ids: [],
urls: [`${TEST_HOST}/index.html`],
replay_id: expect.stringMatching(/\w{32}/),
replay_start_timestamp: expect.any(Number),
segment_id: 0,
replay_type: 'session',
event_id: expect.stringMatching(/\w{32}/),
environment: 'production',
contexts: {
culture: {
locale: expect.any(String),
timezone: expect.any(String),
calendar: expect.any(String),
},
},
sdk: {
integrations: expect.arrayContaining([
'InboundFilters',
'FunctionToString',
'BrowserApiErrors',
'Breadcrumbs',
'GlobalHandlers',
'LinkedErrors',
'Dedupe',
'HttpContext',
'BrowserSession',
'Replay',
]),
version: SDK_VERSION,
name: 'sentry.javascript.browser',
settings: {
infer_ip: 'never',
},
},
request: {
url: `${TEST_HOST}/index.html`,
headers: {
'User-Agent': expect.stringContaining(''),
},
},
platform: 'javascript',
});
expect(replayEvent0).toEqual(
getExpectedReplayEvent({
segment_id: 0,
}),
);

expect(replayEvent1).toBeDefined();
expect(replayEvent1).toEqual({
type: 'replay_event',
timestamp: expect.any(Number),
error_ids: [],
trace_ids: [],
urls: [],
replay_id: expect.stringMatching(/\w{32}/),
replay_start_timestamp: expect.any(Number),
segment_id: 1,
replay_type: 'session',
event_id: expect.stringMatching(/\w{32}/),
environment: 'production',
contexts: {
culture: {
locale: expect.any(String),
timezone: expect.any(String),
calendar: expect.any(String),
},
},
sdk: {
integrations: expect.arrayContaining([
'InboundFilters',
'FunctionToString',
'BrowserApiErrors',
'Breadcrumbs',
'GlobalHandlers',
'LinkedErrors',
'Dedupe',
'HttpContext',
'BrowserSession',
'Replay',
]),
version: SDK_VERSION,
name: 'sentry.javascript.browser',
settings: {
infer_ip: 'never',
},
},
request: {
url: `${TEST_HOST}/index.html`,
headers: {
'User-Agent': expect.stringContaining(''),
},
},
platform: 'javascript',
});
expect(replayEvent1).toEqual(
getExpectedReplayEvent({
segment_id: 1,
urls: [],
}),
);
});
107 changes: 13 additions & 94 deletions dev-packages/browser-integration-tests/suites/replay/captureReplayFromReplayPackage/test.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
import { expect } from '@playwright/test';
import { SDK_VERSION } from '@sentry/browser';
import { sentryTest, TEST_HOST } from '../../../utils/fixtures';
import { sentryTest } from '../../../utils/fixtures';
import { getExpectedReplayEvent } from '../../../utils/replayEventTemplates';
import { getReplayEvent, shouldSkipReplayTest, waitForReplayRequest } from '../../../utils/replayHelpers';

sentryTest('should capture replays (@sentry-internal/replay export)', async ({ getLocalTestUrl, page }) => {
Expand All @@ -20,98 +20,17 @@ sentryTest('should capture replays (@sentry-internal/replay export)', async ({ g
const replayEvent1 = getReplayEvent(await reqPromise1);

expect(replayEvent0).toBeDefined();
expect(replayEvent0).toEqual({
type: 'replay_event',
timestamp: expect.any(Number),
error_ids: [],
trace_ids: [],
urls: [`${TEST_HOST}/index.html`],
replay_id: expect.stringMatching(/\w{32}/),
replay_start_timestamp: expect.any(Number),
segment_id: 0,
replay_type: 'session',
event_id: expect.stringMatching(/\w{32}/),
environment: 'production',
contexts: {
culture: {
locale: expect.any(String),
timezone: expect.any(String),
calendar: expect.any(String),
},
},
sdk: {
integrations: expect.arrayContaining([
'InboundFilters',
'FunctionToString',
'BrowserApiErrors',
'Breadcrumbs',
'GlobalHandlers',
'LinkedErrors',
'Dedupe',
'HttpContext',
'BrowserSession',
'Replay',
]),
version: SDK_VERSION,
name: 'sentry.javascript.browser',
settings: {
infer_ip: 'never',
},
},
request: {
url: `${TEST_HOST}/index.html`,
headers: {
'User-Agent': expect.stringContaining(''),
},
},
platform: 'javascript',
});
expect(replayEvent0).toEqual(
getExpectedReplayEvent({
segment_id: 0,
}),
);

expect(replayEvent1).toBeDefined();
expect(replayEvent1).toEqual({
type: 'replay_event',
timestamp: expect.any(Number),
error_ids: [],
trace_ids: [],
urls: [],
replay_id: expect.stringMatching(/\w{32}/),
replay_start_timestamp: expect.any(Number),
segment_id: 1,
replay_type: 'session',
event_id: expect.stringMatching(/\w{32}/),
environment: 'production',
contexts: {
culture: {
locale: expect.any(String),
timezone: expect.any(String),
calendar: expect.any(String),
},
},
sdk: {
integrations: expect.arrayContaining([
'InboundFilters',
'FunctionToString',
'BrowserApiErrors',
'Breadcrumbs',
'GlobalHandlers',
'LinkedErrors',
'Dedupe',
'HttpContext',
'BrowserSession',
'Replay',
]),
version: SDK_VERSION,
name: 'sentry.javascript.browser',
settings: {
infer_ip: 'never',
},
},
request: {
url: `${TEST_HOST}/index.html`,
headers: {
'User-Agent': expect.stringContaining(''),
},
},
platform: 'javascript',
});
expect(replayEvent1).toEqual(
getExpectedReplayEvent({
urls: [],
segment_id: 1,
}),
);
});
3 changes: 2 additions & 1 deletion dev-packages/browser-integration-tests/utils/replayEventTemplates.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,8 @@ const DEFAULT_REPLAY_EVENT = {
type: 'replay_event',
timestamp: expect.any(Number),
error_ids: [],
trace_ids: [],
trace_ids: [expect.any(String)],
traces_by_timestamp: [[expect.any(Number), expect.any(String)]],
urls: [expect.stringContaining('/index.html')],
replay_id: expect.stringMatching(/\w{32}/),
replay_start_timestamp: expect.any(Number),
Expand Down
2 changes: 2 additions & 0 deletions packages/core/src/types-hoist/replay.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,8 @@ export interface ReplayEvent extends Event {
replay_start_timestamp?: number;
error_ids: string[];
trace_ids: string[];
// Data expected is [timestamp (seconds), trace id]
traces_by_timestamp: [number, string][];
replay_id: string;
segment_id: number;
replay_type: ReplayRecordingMode;
Expand Down
4 changes: 2 additions & 2 deletions packages/replay-internal/src/coreHandlers/handleAfterSendEvent.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -37,8 +37,8 @@ function handleTransactionEvent(replay: ReplayContainer, event: TransactionEvent
// Collect traceIds in _context regardless of `recordingMode`
// In error mode, _context gets cleared on every checkout
// We limit to max. 100 transactions linked
if (event.contexts?.trace?.trace_id && replayContext.traceIds.size < 100) {
replayContext.traceIds.add(event.contexts.trace.trace_id);
if (event.contexts?.trace?.trace_id && event.start_timestamp && replayContext.traceIds.length < 100) {
replayContext.traceIds.push([event.start_timestamp, event.contexts.trace.trace_id]);
}
}

Expand Down
34 changes: 30 additions & 4 deletions packages/replay-internal/src/replay.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,13 @@
/* eslint-disable max-lines */ // TODO: We might want to split this file up
import type { ReplayRecordingMode, Span } from '@sentry/core';
import { getActiveSpan, getClient, getRootSpan, SEMANTIC_ATTRIBUTE_SENTRY_SOURCE, spanToJSON } from '@sentry/core';
import {
getActiveSpan,
getClient,
getCurrentScope,
getRootSpan,
SEMANTIC_ATTRIBUTE_SENTRY_SOURCE,
spanToJSON,
} from '@sentry/core';
import { EventType, record } from '@sentry-internal/rrweb';
import {
BUFFER_CHECKOUT_TIME,
Expand Down Expand Up @@ -192,7 +199,7 @@ export class ReplayContainer implements ReplayContainerInterface {
this._hasInitializedCoreListeners = false;
this._context = {
errorIds: new Set(),
traceIds: new Set(),
traceIds: [],
urls: [],
initialTimestamp: Date.now(),
initialUrl: '',
Expand Down Expand Up @@ -1098,7 +1105,11 @@ export class ReplayContainer implements ReplayContainerInterface {
private _clearContext(): void {
// XXX: `initialTimestamp` and `initialUrl` do not get cleared
this._context.errorIds.clear();
this._context.traceIds.clear();
// We want to preserve the most recent trace id for the next replay segment.
// This is so that we can associate replay events w/ the trace.
if (this._context.traceIds.length > 1) {
this._context.traceIds = this._context.traceIds.slice(-1);
}
this._context.urls = [];
}

Expand Down Expand Up @@ -1126,11 +1137,26 @@ export class ReplayContainer implements ReplayContainerInterface {
* Return and clear _context
*/
private _popEventContext(): PopEventContext {
if (this._context.traceIds.length === 0) {
const currentTraceId = getCurrentScope().getPropagationContext().traceId;
if (currentTraceId) {
// Previously, in order to associate a replay with a trace, sdk waits
// until after a `type: transaction` event is sent successfully. it's
// possible that the replay integration is loaded after this event is
// sent and never gets associated with any trace. in this case, use the
// trace from current scope and propagation context. We associate the
// current trace w/ the earliest timestamp in event buffer.
//
// This is in seconds to be consistent with how we normally collect
// trace ids from the SDK hook event
this._context.traceIds.push([this._context.initialTimestamp / 1000, currentTraceId]);
}
}
const _context = {
initialTimestamp: this._context.initialTimestamp,
initialUrl: this._context.initialUrl,
errorIds: Array.from(this._context.errorIds),
traceIds: Array.from(this._context.traceIds),
traceIds: this._context.traceIds,
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@sentry sentry bot Feb 26, 2026

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Bug: A race condition exists because _popEventContext returns a direct reference to the traceIds array, allowing it to be mutated during the async sendReplay operation.
Severity: MEDIUM

Suggested Fix

In _popEventContext, return a shallow copy of the traceIds array instead of a direct reference. This can be achieved by using the spread syntax [...this._context.traceIds] or the slice() method.

Prompt for AI Agent 
Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: packages/replay-internal/src/replay.ts#L1159

Potential issue: The `_popEventContext` method returns a context object containing a
direct reference to the internal `_context.traceIds` array, not a copy. When a segment
flush is triggered with one or zero trace IDs, the subsequent call to `_clearContext`
does not reassign this array. This creates a race condition: during the `await` for the
asynchronous `sendReplay` operation, a new transaction event can trigger
`handleAfterSendEvent`, which pushes a new trace ID onto the shared array. As a result,
the replay segment being sent is mutated to include a trace ID that was captured after
the segment was finalized, leading to incorrect trace linkage.

Did we get this right? 👍 / 👎 to inform future reviews.

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@cursor cursor bot Feb 26, 2026

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Shared mutable array reference causes race condition during flush

Medium Severity

In _popEventContext, traceIds is assigned as a direct reference (traceIds: this._context.traceIds) instead of a copy. The old code used Array.from(this._context.traceIds). When _clearContext runs and traceIds.length <= 1, the array is not reassigned — so the returned context and this._context share the same mutable array. In _runFlush, there's an await this.eventBuffer.finish() between _popEventContext() and sendReplay(), during which new transaction events can .push() into the shared array, corrupting the data being sent.

Additional Locations (1)

Fix in Cursor Fix in Web

urls: this._context.urls,
};

Expand Down
6 changes: 3 additions & 3 deletions packages/replay-internal/src/types/replay.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -352,7 +352,7 @@ export interface PopEventContext extends CommonEventContext {
/**
* List of Sentry trace ids that have occurred during a replay segment
*/
traceIds: Array<string>;
traceIds: Array<[number, string]>;
}

/**
Expand All @@ -365,9 +365,9 @@ export interface InternalEventContext extends CommonEventContext {
errorIds: Set<string>;

/**
* Set of Sentry trace ids that have occurred during a replay segment
* List of <timestamp, trace_id> for Sentry traces that have occurred during a replay segment
*/
traceIds: Set<string>;
traceIds: Array<[number, string]>;
}

export type Sampled = false | 'session' | 'buffer';
Expand Down
4 changes: 3 additions & 1 deletion packages/replay-internal/src/util/sendReplayRequest.ts
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Expand Up @@ -37,12 +37,14 @@ export async function sendReplayRequest({
return Promise.resolve({});
}

const uniqueTraceIds = Array.from(new Set(traceIds.map(([_ts, traceId]) => traceId)));
const baseEvent: ReplayEvent = {
type: REPLAY_EVENT_NAME,
replay_start_timestamp: initialTimestamp / 1000,
timestamp: timestamp / 1000,
error_ids: errorIds,
trace_ids: traceIds,
trace_ids: uniqueTraceIds,
traces_by_timestamp: traceIds.map(([ts, traceId]) => [ts, traceId]),
urls,
replay_id: replayId,
segment_id,
Expand Down
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