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Gemma 4 - OpenAIResponseChatGenerator Tool Input error

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Haystack: gemma 4 - OpenAIResponseChatGenerator Tool Input error. The verified fix is Fix: Maintainer anakin87: don't use OpenAIResponsesChatGenerator against non-OpenAI APIs - use the targeted Ollama integration (OllamaChatGenerator) instead, and for reasoning models like Gemma set think=True. Use this when you hit the symptoms above in haystack; not for unrelated haystack errors or general setup questions.

TL;DR

Fix: Maintainer anakin87: don't use OpenAIResponsesChatGenerator against non-OpenAI APIs - use the targeted Ollama integration (OllamaChatGenerator) instead, and for reasoning models like Gemma set think=True.

The error

Issue deepset-ai/haystack#11040 (closed, 14 comments): **Describe the bug** When running the ResponseChatGenerator in an Agent WITHOUT the exit conditions being a tool (So the tool feeds back into the generator) you get the following. haystack.core.errors.PipelineRuntimeError: The following component failed to run: Component name: 'Agent' Component type: 'Agent' Error: The following component failed to run: Component name: 'chat_generator' Component type: 'OpenAIResponsesChatGenerator' Error: Error code: 400 - {'error': {'message': 'input[3]: json: cannot unmarshal array into Go struct field ResponsesFunctionCallOutput.output of type string', 'typ

Fix

  1. Maintainer anakin87: don't use OpenAIResponsesChatGenerator against non-OpenAI APIs - use the targeted Ollama integration (OllamaChatGenerator) instead, and for reasoning models like Gemma set think=True.

Success check: the symptom above is gone and the original operation completes.

When to use

  • You hit this exact symptom in haystack.
  • You want the verified fix without digging through the thread.

When not to use

  • A different error in haystack, even a similar-looking one.
  • General haystack setup or credential questions.

Compatibility

  • Haystack (deepset-ai/haystack)

Variant phrasings

  • How to fix "gemma 4 - OpenAIResponseChatGenerator Tool Input error" in haystack
  • haystack: gemma 4 - OpenAIResponseChatGenerator Tool Input error
  • gemma 4 - OpenAIResponseChatGenerator Tool Input problem

Source

  • https://github.com/deepset-ai/haystack/issues/11040
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Technical Details
Use when
No specific conditions
Published by
issueatlas
Published
Skill ID
skl_g3pGrA8ccwM5gMcTVbD7tg
Version ID
skv_XTKaO0h7HiRO2sJnzi3lWg
Version History & Decisions

Version History & Decisions

Published version

Published guidance. View the version history for earlier changes.

Published recentlyPublished Oct 3, 2026. This reminder uses publication date only; it does not mean the content was verified. Review again after Apr 1, 2027.

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