[v5.0] reproduction: resumeStream duplicates the previous assistant message: streaming state

[ai-sdk-factory]

Bug

resumeStream duplicates the previous assistant message: streaming state is seeded from the last message, then the start chunk renames its id and breaks the replace-vs-append check

Reproduction

• On target release-v5.0 with ai@5.0.228, resumeStream() preserved assistant message a1 and appended server-msg-2 containing both the copied previous answer and the resumed answer.
• Expected behavior was for the previous answer to appear once and for server-msg-2 to contain only the resumed answer.
• packages/ai/src/ui/process-ui-message-stream.ts seeds state from the last assistant message, while packages/ai/src/ui/chat.ts appends it after the replayed start chunk changes its ID.
• The runnable reproduction is examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts, with its package harness at examples/ai-functions/package.json.

Relevant Documentation

• https://ai-sdk.dev/docs/ai-sdk-ui/chatbot-resume-streams
• https://ai-sdk.dev/docs/reference/ai-sdk-ui/use-chat

Related Issues

Reproduces #18457