[v6.0] 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 the release-v6.0 target (ai 6.0.243), resumeStream() preserved assistant message a1 but also appended server-msg-2 containing both the copied previous answer and the newly resumed answer.
• Expected behavior is for a1 to remain 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 and renames it on the replayed start chunk; packages/ai/src/ui/chat.ts then appends it because its ID no longer matches the last persisted message.
• The runnable reproduction is examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts; it uses an in-memory transport without network access or provider configuration.

Relevant Documentation

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

Related Issues

Reproduces #18457