fix: fix the conversion of cohere chunks to Haystack streaming chunks

[sjrl]

Related Issues

• fixes #issue-number

Proposed Changes:

• Finish Reason mapping for streaming chunks was incomplete and was incorrect TOOL_CALLS --> TOOL_CALL
• StreamingChunk.start = True was incorrect for the first non-empty tool-plan-delta chunk
• StreamingChunk.start was incorrectly set to True on tool-call-end

How did you test it?

• Refactored streaming unit tests to use real Cohere chunks
• Added test to make sure final ChatMessage created from streaming chunks is as expected

Also tested locally with the script

from haystack.components.agents import Agent
from haystack.components.generators.utils import print_streaming_chunk
from haystack.dataclasses import ChatMessage
from haystack.tools import Tool

from haystack_integrations.components.generators.cohere import CohereChatGenerator


# Tool Function
def calculate(expression: str) -> dict:
    try:
        result = eval(expression, {"__builtins__": {}})
        return {"result": result}
    except Exception as e:
        return {"error": str(e)}


# Tool Definition
calculator_tool = Tool(
    name="calculator",
    description="Evaluate basic math expressions.",
    parameters={
        "type": "object",
        "properties": {"expression": {"type": "string", "description": "Math expression to evaluate"}},
        "required": ["expression"],
    },
    function=calculate,
    outputs_to_state={"calc_result": {"source": "result"}},
)

# Agent Setup
agent = Agent(
    chat_generator=CohereChatGenerator(
        model="command-a-03-2025",
        # model="command-a-reasoning-08-2025",
        # generation_kwargs={"thinking": {"type": "enabled", "token_budget": 10}}
    ),
    streaming_callback=print_streaming_chunk,
    tools=[calculator_tool],
    state_schema={"calc_result": {"type": int}},
)

# Run the Agent
response = agent.run(
    messages=[
        ChatMessage.from_user(
            "What is 7 + 2? And what is 2 * 2? Make sure to use the calculator tool to compute the result."
        )
    ]
)

Which now produces the expected formatted streaming response

Previously looked like

Notes for the reviewer

Once this is merged I'll follow it up with a PR to resolve #2179

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

---

[davidsbatista]

LGTM

[anakin87]

@sjrl sorry, I merged and released this as an experiment to generate a new API reference, but I did not realize it does not touch docstrings. Hope it was already OK.

[sjrl]

@sjrl sorry, I merged and released this as an experiment to generate a new API reference, but I did not realize it does not touch docstrings. Hope it was already OK.

yeah no worries it was good to go!