CohereChatGenerator using command-r model doesn't work with Agent

[vblagoje]

Describe the bug
command-r (Cohere model) fails during tool calling in a Haystack Agent pipeline. Although our test suite passes when evaluating tool use with the live model, actual runs frequently trigger a PipelineRuntimeError due to the model not returning valid tool calls or responses.

Error observed:

haystack.core.errors.PipelineRuntimeError: The following component failed to run:
Component name: 'chat_generator'
Component type: 'CohereChatGenerator'
Error: status_code: 422, body: {'message': 'No valid tool call or response generated'}

To Reproduce
Run a Haystack Agent pipeline using CohereChatGenerator with the command-r model and a valid tool registered. The agent setup completes, but at runtime, the model fails to return a proper tool call.

This works with command-r-plus, and with command-a-03-2025 but fails consistently with command-r.

Describe your environment (please complete the following information):

• OS: macOS 14.4.1
• Haystack version: 2.13.0
• Integration version: cohere-haystack-3.0.0

Notes:
At first I thought this must have been related to MCP as we have tool tests in test suite but I eliminated that possibility by testing an Agent instance with single Tool used in integration tests (fake weather tool) and that doesn't work either so it is likely something related to Agent, tools and command-r combination