fix(core): keep part timestamps out of message ordering

[TylerBarnes]

This keeps message part timestamps from affecting transcript ordering. Part timestamps are still preserved as event metadata, but only the message-level createdAt advances the ordering watermark used for later messages/signals.

Before, a late part timestamp could push a later signal forward:

message.createdAt = 10:00:00
part.createdAt = 10:00:30
nextSignal.createdAt = 10:00:30.001

After, later messages/signals are ordered from the message-level timestamp:

message.createdAt = 10:00:00
part.createdAt = 10:00:30
nextSignal.createdAt = 10:00:03

Also preserves the existing message-level createdAt when merging response content into a memory-loaded assistant message, since OM uses that timestamp to decide whether a message has already been observed.

Verified with the focused message-list regression test, pnpm build:core, and pnpm --filter ./packages/core check.

ELI5

Imagine you're organizing messages by time. This PR fixes a bug where a late timestamp on a piece of a message (a "part") could reorder later messages — now only the main message timestamp controls ordering, while part timestamps are preserved as metadata and no longer affect transcript ordering.

Changes

Changeset Entry (.changeset/old-news-press.md)

• Adds a patch release note for @mastra/core describing the fix: message-part timestamps no longer advance the transcript ordering watermark.

Message Merging (packages/core/src/agent/message-list/merge/MessageMerger.ts)

• Preserve an existing message's createdAt when merging incoming content; removed logic that would update/advance createdAt from the incoming message.
• Documentation updated to clarify that createdAt must remain unchanged so previously observed messages are not reprocessed.

Message List Ordering (packages/core/src/agent/message-list/message-list.ts)

• MessageList.updateLastCreatedAt now advances the ordering watermark using only message-level createdAt.
• Removed prior logic that inspected part-level timestamps when updating the watermark.
• Comments updated to reflect that lastCreatedAt is the message-level ordering timestamp.

Tests (packages/core/src/agent/message-list/tests)

• message-list-om-multistep.test.ts:
  • Updated OM multi-step test to ensure promoted (memory-loaded) assistant responses retain the original createdAt.
  • Added a regression test asserting that a tool-part with a later timestamp does not advance subsequent signal timestamps.
• message-list-ordering.test.ts:
  • Adjusted assertions so signals are normalized using the response (message-level) timestamp rather than a later part timestamp.

Verification

• Focused regression tests added/updated.
• Build and check commands referenced: pnpm build:core (runs turbo build --filter ./packages/core) and pnpm --filter ./packages/core check (typecheck/check step used in CI).

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Project	Deployment	Actions	Updated (UTC)
mastra-docs-1.x	Ready	Preview, Comment	Jun 5, 2026 12:57am
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[changeset-bot]

🦋 Changeset detected

Latest commit: 50bbcda

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 23 packages

Name	Type
@mastra/core	Patch
mastracode	Patch
@mastra/mcp-docs-server	Patch
@internal/playground	Patch
@mastra/client-js	Patch
@mastra/opencode	Patch
@mastra/longmemeval	Patch
mastra	Patch
@mastra/deployer-cloud	Patch
@mastra/deployer-vercel	Patch
@mastra/playground-ui	Patch
@mastra/react	Patch
@mastra/server	Patch
@mastra/deployer	Patch
create-mastra	Patch
@mastra/express	Patch
@mastra/fastify	Patch
@mastra/hono	Patch
@mastra/koa	Patch
@mastra/nestjs	Patch
@mastra/deployer-cloudflare	Patch
@mastra/deployer-netlify	Patch
@mastra/temporal	Patch

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b74ae146-b99d-402b-914d-ca107d200f3f

📥 Commits

Reviewing files that changed from the base of the PR and between 96a9306 and 50bbcda.

📒 Files selected for processing (5)

• .changeset/old-news-press.md
• packages/core/src/agent/message-list/merge/MessageMerger.ts
• packages/core/src/agent/message-list/message-list.ts
• packages/core/src/agent/message-list/tests/message-list-om-multistep.test.ts
• packages/core/src/agent/message-list/tests/message-list-ordering.test.ts

✅ Files skipped from review due to trivial changes (1)

• .changeset/old-news-press.md

🚧 Files skipped from review as they are similar to previous changes (4)

• packages/core/src/agent/message-list/tests/message-list-ordering.test.ts
• packages/core/src/agent/message-list/tests/message-list-om-multistep.test.ts
• packages/core/src/agent/message-list/merge/MessageMerger.ts
• packages/core/src/agent/message-list/message-list.ts

---

Walkthrough

Fixes transcript ordering so only message-level createdAt advances the ordering watermark; merge logic no longer updates message createdAt, and tests plus a changeset validate the behavior.

Changes

Message watermark and merge timestamp semantics

Layer / File(s)	Summary
Message merge and watermark timestamp preservation packages/core/src/agent/message-list/merge/MessageMerger.ts, packages/core/src/agent/message-list/message-list.ts	MessageMerger.merge preserves existing latestMessage.createdAt (no assignment from incoming parts). MessageList.updateLastCreatedAt and generateCreatedAt use only message-level createdAt for the ordering watermark; comments updated to reflect ordering vs part metadata semantics.
Transcript ordering test coverage and release notes packages/core/src/agent/message-list/tests/message-list-om-multistep.test.ts, packages/core/src/agent/message-list/tests/message-list-ordering.test.ts, .changeset/old-news-press.md	OM multi-step test uses a shared originalCreatedAt and asserts promoted responses retain it. New test verifies late-added tool-part timestamps do not advance signal timestamps. Ordering test assertions adjusted accordingly. Changeset records the fix.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• mastra-ai/mastra#17338: Both PRs modify message list and merge logic around createdAt and ordering-watermark behavior during observational-memory replay, specifically preserving historical response timestamps to prevent part-timestamps from causing incorrect reprocessing.

Suggested labels

tests: green ✅, complexity: low

Suggested reviewers

• abhiaiyer91

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preventing part timestamps from affecting message ordering, using imperative mood and proper formatting.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/prompt-cache-timestamp-drift

---

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[dane-ai-mastra]

PR triage

Linked issue check skipped for core contributor @TylerBarnes.

---

PR complexity score

Factor	Value	Score impact
Files changed	5	+10
Lines changed	115	+6
Author merged PRs	543	-20
Test files changed	Yes	-10
Final score		-14

Applied label: complexity: low

---

Changed test gate

Changed tests failed against the base branch as expected.

Label: tests: green ✅