fix(agent-tools): clone run parts so StrictMode doesn't double streamed text (#1835)

[threepointone]

Summary

Fixes #1835 — useAgentToolEvents doubles every word of streamed text in React StrictMode / SSR frameworks (Next.js, TanStack Start, Remix).

Root cause

The agent-tool-event reducer is impure. In applyToRun (packages/agents/src/chat/agent-tools.ts), the chunk case did:

const parts = [...seeded.parts];

This is a shallow array copy — the new array's elements are the same object references as prev.parts. It then calls applyChunkToParts, which mutates part objects in place (e.g. lastTextPart.text += delta). So the mutation leaks back into the previous state object.

React double-invokes setState updaters in StrictMode and during dev hydration (hydrateRoot). Because useAgentToolEvents updates via setState((prev) => applyAgentToolEvent(prev, message)), each text-delta is applied twice against the same, already-mutated prev → doubled text.

Fix

Clone each part before mutating, so the reducer stays pure:

const parts = seeded.parts.map((part) => ({ ...part }) as Part);

Every mutation in applyChunkToParts targets a top-level field of a part (text, state, input, output, data, …), so a per-part shallow copy is sufficient — no deep clone needed. This matches the one-line fix proposed in the issue, with a typed cast for the generic Part.

Validity / accuracy of the report

Confirmed valid and accurate: the mechanism (shared references + in-place mutation + StrictMode double-invoke) is exactly as described, and the proposed fix resolves it.

Test plan

• Added a regression test in packages/agents/src/chat/__tests__/agent-tools.test.ts that replays a text-delta against the same prev twice (simulating StrictMode) and asserts the result is not doubled and prev is left untouched.
• vitest --project chat — all 24 tests pass.
• oxlint + oxfmt --check clean on edited files.
• tsc --noEmit clean for the agents package.
• Changeset added (agents patch).

Made with Cursor

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[changeset-bot]

🦋 Changeset detected

Latest commit: b9bbd64

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

This PR includes changesets to release 1 package

Name	Type
agents	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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commit: b9bbd64