docs(ai-chat): add the 4.5.0-rc.8 changelog entry

[ericallam]

Summary

Adds a 4.5.0-rc.8 entry to the AI chat agents changelog covering the chat.agent changes in that release: a new apiClient option on chat.headStart and chat.createStartSessionAction for pointing chat sessions at a different project or environment, the fix for messaging chat agents deployed to a preview branch, and the fix for Head Start handovers when the agent also defines a prepareMessages hook.

All four changes are from #4018.

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Reviewing files that changed from the base of the PR and between 82bdd1f and 5321059.

📒 Files selected for processing (1)

• docs/ai-chat/changelog.mdx

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docs/**/*.mdx

📄 CodeRabbit inference engine (docs/CLAUDE.md)

docs/**/*.mdx: MDX documentation pages must include frontmatter with title (required), description (required), and sidebarTitle (optional) in YAML format
Use Mintlify components for structured content: , , , , , , /, /
Always import from @trigger.dev/sdk in code examples (never from @trigger.dev/sdk/v3)
Code examples must be complete and runnable where possible
Use language tags in code fences: typescript, bash, json

Files:

• docs/ai-chat/changelog.mdx

🧠 Learnings (4)

📚 Learning: 2026-03-10T12:44:14.176Z

Learnt from: nicktrn
Repo: triggerdotdev/trigger.dev PR: 3200
File: docs/config/config-file.mdx:353-368
Timestamp: 2026-03-10T12:44:14.176Z
Learning: In the trigger.dev repo, docs PRs are often companions to implementation PRs. When reviewing docs PRs (MDX files under docs/), check the PR description for any companion/related PR references and verify that the documented features exist in those companion PRs before flagging missing implementations. This ensures docs stay in sync with code changes across related PRs.

Applied to files:

• docs/ai-chat/changelog.mdx

📚 Learning: 2026-04-30T20:30:29.458Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3226
File: docs/ai-chat/quick-start.mdx:13-13
Timestamp: 2026-04-30T20:30:29.458Z
Learning: In this repo’s documentation MDX files (`docs/**/*.mdx`), use `ts` and `tsx` (not `typescript`) as the code-fence language tags for TypeScript/TSX snippets. Do not flag `ts`/`tsx` code-fence language tags as incorrect in any docs MDX file, since this is the site-wide Mintlify-compatible convention.

Applied to files:

• docs/ai-chat/changelog.mdx

📚 Learning: 2026-06-16T13:14:09.440Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3964
File: docs/ai-chat/reference.mdx:482-482
Timestamp: 2026-06-16T13:14:09.440Z
Learning: When documenting or reviewing usage of `ChatTurn.complete(source?)` (in `packages/trigger-sdk/src/v3/ai.ts`), note that `source` is optional (`source?: UIMessageStreamable`). Calling `complete()` with no `source` is valid specifically for a final head-start handover (`handover.isFinal`), because the warm partial already contains the response. If examples or guidance omit `source`, ensure they are in this final-hand-over context so they remain correct.

Applied to files:

• docs/ai-chat/changelog.mdx

📚 Learning: 2026-06-16T13:14:14.382Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3964
File: docs/ai-chat/reference.mdx:478-478
Timestamp: 2026-06-16T13:14:14.382Z
Learning: When reviewing RC-gated `ai-chat` docs under `docs/ai-chat/`, don’t immediately flag missing SDK type fields or implementation details just because the field isn’t present on the docs branch yet. Instead, find and cross-check the companion implementation PR that’s intended to land alongside the docs PR, and only report missing/incorrect fields if they are also absent in the companion SDK/type changes.

Applied to files:

• docs/ai-chat/changelog.mdx

🔇 Additional comments (1)

docs/ai-chat/changelog.mdx (1)

7-38: LGTM!

---

Walkthrough

Added a new June 25, 2026 changelog entry for 4.5.0-rc.8. It documents three AI chat changes: apiClient support for chat.headStart and chat.createStartSessionAction, x-trigger-branch headers for realtime preview-branch requests, and approval-tail preservation across prepareMessages, along with updated TypeScript examples.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description only includes a Summary and is missing the template's issue link, checklist, testing, changelog, and screenshots sections.	Add the missing template sections: Closes #issue, checklist items, testing steps, a changelog note, and screenshots or a clear placeholder.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: adding the 4.5.0-rc.8 AI-chat changelog entry.
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 unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/ai-chat-rc8-changelog

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