Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

reasoning_effort binding to claude-haiku-4.5 model fails on sub-agent invocation

Open
#4,941 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
shell
Domain
ai, cli

Research direction

Look for the sub-agent invocation logic and where the reasoning_effort parameter is injected for the claude-haiku-4.5 model. Check the model configuration or validation code to see if this model supports reasoning_effort. The error suggests a recent change; compare recent commits around model handling. Test by running a sub-agent with the specified model and see if the parameter can be omitted conditionally.

Written by the indexing model from the issue text.

Description

area:agents area:models
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
  1. Invoke the sub-agent whose model is set to claude-haiku-4.5
  2. Fails as described above
Expected behavior

Sub-agent invocation succeeds.

Additional context

windows 11, x86_64

Dominant language
Shell
Stars
11.2k
Forks
1.9k
Avg merge
14h 16m
Merged PRs (30d)
6

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from github/copilot-cli

All issues in github/copilot-cli

Similar issues

More Shell/Bash issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.