Describe the bug
My custom build-agent, which worked up through yesterday, (and likely any custom subagent whose /subagents config pins model claude-haiku-4.5 ) fails every invocation with: Reasoning effort 'low' is not supported for model 'claude-haiku-4.5' . Reproduced regardless of whether reasoning_effort is omitted or explicitly set ( low / medium / minimal ) in the task tool call — the runtime appears to always inject a reasoning_effort value for this model, which the model doesn't support at all. First seen 2026-09-22; sessions from 2026-09-14 through 2026-09-21 using the same build-agent config succeeded without this error, so it looks like a recent regression. Session: e8b4c267-83c7-431f-9a10-ede4dd88f8cf .
Affected version
GitHub Copilot CLI 1.0.87
Steps to reproduce the behavior
- Invoke the sub-agent whose model is set to claude-haiku-4.5
- Fails as described above
Expected behavior
Sub-agent invocation succeeds.
Additional context
windows 11, x86_64
Describe the bug
My custom build-agent, which worked up through yesterday, (and likely any custom subagent whose /subagents config pins model claude-haiku-4.5 ) fails every invocation with: Reasoning effort 'low' is not supported for model 'claude-haiku-4.5' . Reproduced regardless of whether reasoning_effort is omitted or explicitly set ( low / medium / minimal ) in the task tool call — the runtime appears to always inject a reasoning_effort value for this model, which the model doesn't support at all. First seen 2026-09-22; sessions from 2026-09-14 through 2026-09-21 using the same build-agent config succeeded without this error, so it looks like a recent regression. Session: e8b4c267-83c7-431f-9a10-ede4dd88f8cf .
Affected version
GitHub Copilot CLI 1.0.87
Steps to reproduce the behavior
Expected behavior
Sub-agent invocation succeeds.
Additional context
windows 11, x86_64