Streaming fails with "Cannot rewind a generator" when stream is re-iterated (PromptJsonDecoder, ReplayedChatMessageIterator)
>>> [!note] Migrated issue
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<!-- Migrated from issue #3572398. -->
Reported by: [petar_basic](https://www.drupal.org/user/3626336)
Related to !30
>>>
<h3 id="summary-problem-motivation">Problem/Motivation</h3>
<p>Streaming chat responses fail with:</p>
<p> Cannot rewind a generator that was already run in MistralChatMessageIterator->doIterate()</p>
<p> Root Cause</p>
<p> MistralChatMessageIterator::doIterate() iterates directly over $this->iterator, which is a raw PHP Generator from the Partitech SDK's SSEClient::getStream(). PHP Generators are<br>
single-pass — they cannot be rewound.</p>
<p> The AI module pipeline re-iterates the stream in two places:</p>
<p> 1. PromptJsonDecoder::decodeStreamingMessage() — iterates chunks to test for JSON (line 61), then wraps in ReplayedChatMessageIterator (line 93)<br>
2. ReplayedChatMessageIterator::getIterator() — calls $this->iterator->getIterator() (line 20), which triggers a new doIterate() against the already-consumed Generator</p>
<p>In OpenAI, on the other hand, doIterate() calls $this->iterator->getIterator() — the OpenAI PHP SDK returns an IteratorAggregate that produces fresh iterators. The Partitech Mistral SDK returns a raw Generator.</p>
<h4 id="summary-steps-reproduce">Steps to reproduce</h4>
<p>- Enable ai_provider_mistral and ai_chatbot<br>
- Configure Mistral as default chat provider<br>
- configure a block with a NON-agent assistant, enable streaming, place the block e.g. under content<br>
- Send a chat message<br>
- Observe: Cannot rewind a generator that was already run</p>
<h3 id="summary-proposed-resolution">Proposed resolution</h3>
<p>Replace foreach in doIterate() with manual valid()/current()/next() iteration. foreach implicitly calls rewind() on the iterator before starting, which crashes on an already-started Generator. Manual iteration avoids rewind() entirely, so re-entering doIterate() continues from the current position. </p>
<h3 id="summary-remaining-tasks">Remaining tasks</h3>
<h3 id="summary-ui-changes">User interface changes</h3>
<h3 id="summary-api-changes">API changes</h3>
<h3 id="summary-data-model-changes">Data model changes</h3>
> Related issue: [Issue #3571925](https://www.drupal.org/node/3571925)
issue
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