fix(ai): produce new object references in tool-call message updaters

[braden-w]

I ran into this while building a Svelte 5 chat UI with tool-call approvals. Svelte 5 uses proxies for reactivity, and $derived(part.state === 'approval-requested') never fired because the part object identity didn't change.

The root cause is four functions in message-updaters.ts that mutate tool-call parts in-place after [...msg.parts]. The shallow copy creates a new array but the elements are the same references, so toolCallPart.state = 'approval-requested' changes the original object in both arrays. This affects any framework using proxy-based reactivity (Svelte 5, Vue 3).

The other updaters (updateToolCallPart, updateTextPart, updateThinkingPart) already produce new objects via spread. This PR aligns the four remaining functions to the same pattern:

const index = parts.indexOf(toolCallPart)
parts[index] = { ...toolCallPart, state: 'approval-requested', approval: { ... } }

Functions fixed: updateToolCallApproval, updateToolCallState, updateToolCallWithOutput, updateToolCallApprovalResponse. Four immutability tests added.

Checklist

• I have followed the steps in the Contributing guide.
• Changeset included.

Summary by CodeRabbit

• Bug Fixes

  • Fixed tool-call message updater behavior to ensure immutability by creating fresh object references instead of mutating existing objects in place, preventing unexpected side effects.

• Tests

  • Added test coverage to validate that tool-call updaters maintain immutability across approval, state, output, and response update scenarios.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2c4222e8-621c-4e19-8cc1-ae2347e9a4d5

📥 Commits

Reviewing files that changed from the base of the PR and between c3583e3 and e00092c.

📒 Files selected for processing (3)

• .changeset/immutable-tool-call-updates.md
• packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
• packages/typescript/ai/tests/message-updaters.test.ts

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📝 Walkthrough

Walkthrough

The PR updates tool-call message updater functions to prevent in-place object mutations. Four updater functions (updateToolCallApproval, updateToolCallState, updateToolCallWithOutput, updateToolCallApprovalResponse) now use object spread syntax to create fresh object references instead of mutating existing parts. A changeset documents the patch, and comprehensive immutability tests validate the behavior.

Changes

Cohort / File(s)	Summary
Changeset Documentation .changeset/immutable-tool-call-updates.md	Patch-level changeset documenting the immutability fix for tool-call updaters, describing the shift from in-place mutations to spread-based object replacement.
Message Updater Implementation packages/typescript/ai/src/activities/chat/stream/message-updaters.ts	Refactored four tool-call updater functions to replace array elements with new object references via spread syntax instead of mutating properties directly, ensuring immutability of tool-call parts.
Immutability Test Coverage packages/typescript/ai/tests/message-updaters.test.ts	Added four immutability-focused test cases validating that each updater function preserves the original ToolCallPart objects and returns messages with distinct, updated part instances.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 Hoppity-hop through the code so clear,
No mutations lurk, immutability's here!
Spread syntax spreads a brand new way,
Fresh objects dance in every play! 🌸

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: fixing immutability in tool-call message updaters by producing new object references instead of mutating in-place.
Description check	✅ Passed	The description follows the template structure with a comprehensive explanation of changes and includes a completed checklist; however, it omits the suggested '## 🎯 Changes' section heading that structures the template.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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