fix: keep streamed tool calls without an index apart in OpenAIChatGenerator

[chrikrah]

Related Issues

• No issue exists for this. The report is below.

Proposed Changes:

OpenAIChatGenerator raises a ValueError when an OpenAI-compatible server streams a tool call with no index, which Gemini's endpoint still does per the last report in the forum thread (2025-11). haystack/components/generators/chat/openai.py:734 copies the None into ToolCallDelta and haystack/dataclasses/streaming_chunk.py:153 rejects it. The OpenRouter and vLLM generators in haystack-core-integrations import _convert_chat_completion_chunk_to_streaming_chunk, so they crash too and cannot fix it downstream. This PR assigns the index.

Before, on c5e1335, with a local server that sends one chunk with two such calls:

$ python hs_no_index.py
ValueError: If `tool_call`, `tool_call_result` or `reasoning` is set, `index` must also be set.

After:

$ python hs_no_index.py
tool_calls: [ToolCall(tool_name='get_weather', arguments={'city': 'Paris'}, id='', extra=None), ToolCall(tool_name='get_time', arguments={'tz': 'CET'}, id='', extra=None)]

An index-less delta with a name or an id now starts a call at the next free index. One with neither continues the previous call, so a server that streams one call in fragments still works. Indexed calls are unchanged.

How did you test it?

$ pytest test/components/generators/chat/test_openai.py -m "not integration" -q
58 passed, 12 deselected in 1.14s

Baseline on c5e1335 is 56 passed. The new tests cover whole calls and fragmented calls. Reverting openai.py and keeping both gives 2 failed, 56 passed, 12 deselected. ruff 0.17.0 and mypy 2.4.0 report clean.

# not run: integration tests (they need API keys)

Notes for the reviewer

None.

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  • No issue exists for this, so the report is in this description.
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@anakin87 @sjrl (who merged #13047 in openai.py): given #13187, is a crash fix in the shared converter acceptable here?

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@chrikrah is attempting to deploy a commit to the deepset Team on Vercel.

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[0xamlab]

Verified the fix locally. pytest test/components/generators/chat/test_openai.py -m 'not integration' -q gives 58 passed, 12 deselected in 6.73s. I reproduced the main-branch crash with a synthetic Gemini-style chunk missing index and confirmed this branch converts it without the ValueError. The if index is None block at haystack/components/generators/chat/openai.py:738 cleanly separates new calls from argument fragments by checking tool_call.id and function.name.