fix(ai): handle missing start chunks in processUIMessageStream

[codewarnab]

This PR addresses a runtime error: TypeError: Cannot read properties of undefined (reading 'text') in processUIMessageStream.

Issue

The issue occurs when text-delta, reasoning-delta, or tool-input-delta chunks are received before their corresponding start chunks (e.g., text-start). This can happen in malformed streams, network issues causing packet reordering, or specific backend behaviors where a delta is emitted immediately without a start frame.

Previously, processUIMessageStream assumed that a start part (initialized in activeTextParts, etc.) always existed when a delta arrived. When it didn't, accessing properties like textPart.text caused a crash.

Fix

This change adds explicit null checks for the active parts (textPart, reasoningPart, partialToolCall). If a delta is received for a missing part, it now throws a descriptive error:

"Received text-delta for missing text part with ID "text-1". Ensure a "text-start" chunk is sent before any "text-delta" chunks."

This descriptive error aids in debugging the actual upstream issue (broken stream) rather than crashing the UI with an opaque TypeError.

Changes

• packages/ai/src/ui/process-ui-message-stream.ts: Added guarding if (part == null) checks in text-delta, reasoning-delta, and tool-input-delta cases.
• packages/ai/src/ui/process-ui-message-stream.test.ts:
  • Added test cases simulating malformed streams (delta without start) for text, reasoning, and tool-inputs.
  • Verified that specific descriptive errors are thrown.
  • Updated test harness consumeStream to properly propagate errors for validation.

[Copilot]

Pull request overview

This PR adds defensive null checks to prevent runtime errors when processing malformed UI message streams. The fix addresses cases where delta or end chunks are received before their corresponding start chunks.

Summary

The PR adds null checks for text-delta, text-end, reasoning-delta, reasoning-end, and tool-input-delta chunk types to prevent crashes when start chunks are missing. Instead of crashing with a generic TypeError, the code now throws descriptive errors that help identify the upstream issue.

Key Changes

• Added null checks with descriptive error messages for delta and end chunks in stream processing
• Added test cases to verify error handling for malformed streams (delta without start)
• Minor formatting improvements to type definitions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
packages/ai/src/ui/process-ui-message-stream.ts	Added null checks for text, reasoning, and tool-input parts with descriptive error messages; minor formatting improvements
packages/ai/src/ui/process-ui-message-stream.test.ts	Added test cases for malformed stream scenarios where delta chunks are received without corresponding start chunks

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[aayush-kapoor]

can you first address all the comments of the above bots?

[codewarnab]

can you first address all the comments of the above bots?

addressed all of them