/rubber-duck slash command is not recognized in GitHub Copilot CLI

[ashis-baral]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Copilot CLI

Body

Hi everyone,

I’m opening this discussion because I’m having an issue with the /rubber-duck command in GitHub Copilot CLI.

The GitHub Copilot documentation describes a /rubber-duck slash command for manually invoking the Rubber Duck agent in GitHub Copilot CLI.

According to the documentation, the command can be used like this:

/rubber-duck What edge cases are missing?

However, when I try to use /rubber-duck in GitHub Copilot CLI, it is not recognized and returns:

Unknown command: /rubber-duck

This appears to be inconsistent with the documented behavior.

What I observed

The interesting part is that the Rubber Duck agent itself appears to be available and working.

If I use the equivalent request as a natural-language prompt, for example:

Rubber duck What edge cases are missing?

the request successfully invokes the Rubber Duck agent.

So this does not appear to be a problem with the Rubber Duck agent itself. The issue seems specifically related to the /rubber-duck slash-command interface.

Previous report

I originally reported this issue in the GitHub Docs repository:

github/docs#46225

The maintainer responded that the issue would be transferred internally and explained that GitHub Support or GitHub Community Discussions would be the appropriate place for further discussion.

Therefore, I am bringing the issue here.

---

I’d like to know if anyone else has experienced the same issue.

Has anyone else seen /rubber-duck return Unknown command in Copilot CLI, or is this issue specific to my setup/version

If others are experiencing it as well, it would be helpful to understand whether this is a broader Copilot CLI issue.

I’d appreciate any clarification on whether this is a Copilot CLI bug, a configuration/version issue, or simply an outdated documentation example.

[HIRATRA]

This looks more like a command-discovery/version mismatch than an issue with the Rubber Duck agent itself.

The fact that:

/rubber-duck What edge cases are missing?
→ Unknown command

while:

Rubber duck What edge cases are missing?
→ successfully invokes the agent

works is a pretty strong signal that the agent is available but the slash-command is not registered in the CLI command parser.

I'd check the exact Copilot CLI version first and compare it with the version for which the documentation was published. If /rubber-duck was introduced or exposed recently, it's possible the documentation and the CLI release are temporarily out of sync.

It would also be useful to test the command discovery itself (for example /help or the CLI's list of available slash commands). If rubber-duck doesn't appear there, that would further confirm that this is a CLI command registration issue rather than an agent execution issue.

So I would report the bug with these three pieces of information:

1. Copilot CLI version
2. Whether rubber-duck appears in the available slash commands
3. Whether the natural-language Rubber duck ... invocation still works

If #1 is on the latest CLI and #2 still doesn't list the command, I think that would be strong evidence that the documentation is ahead of the CLI implementation (or that the slash command is gated behind a feature flag).

The fact that the underlying agent works makes this distinction particularly useful for debugging.

[ashis-baral]

@HIRATRA Hi

I tested this on the latest version of Copilot CLI . I also checked /help, and /rubber-duck does not appear in the list of available slash commands.

Are you also getting the same Unknown command error on your system when running /rubber-duck?

[HIRATRA]

Yes, I’m seeing the same behavior.

What makes it interesting is that the Rubber Duck agent itself still seems to work when invoked through natural language, while /rubber-duck is missing from /help and returns Unknown command.

That makes me lean toward this being a discrepancy between the documented slash-command interface and the currently available CLI commands, rather than an issue with the agent itself.

It would be useful to know whether the slash command was intentionally removed/renamed or whether the documentation was updated ahead of the CLI release. If someone from the Copilot team can confirm the expected behavior for the current CLI version, that would probably settle it.

[ashis-baral]

Thanks for confirming! I just wanted to check whether other users were facing the same issue.

Using the CLI extension, I created the /rubber-duck slash command manually. It’s working now, so it looks like this may be an issue on the GitHub/Copilot side rather than something specific to the user or local setup or the documentation may mention something that isn’t currently supported.

[HIRATRA]

That’s a useful finding. If manually registering /rubber-duck through the CLI extension makes it work, it seems to narrow the issue down quite a bit.

It suggests that the Rubber Duck agent itself is available and the CLI can handle the command, but the built-in slash-command registration is missing or inconsistent in the current distribution.

That makes a GitHub/Copilot-side registration or release issue seem more likely than a local configuration problem. It would be interesting to know whether /rubber-duck is supposed to be built into the current CLI or if the documentation was updated ahead of the CLI release.

[RizgarOzan]

Looks like the critic-model check. Since 1.0.85 the CLI hides /rubber-duck when there's no compatible critic model for your current session model ("The /rubber-duck command is hidden after a model refresh removes its compatible critic" in the changelog), and a hidden command just gives you Unknown command.

Easy way to confirm: run /subagents and look at the rubber-duck row. Someone in github/copilot-cli#4380 had complementary — unavailable, disabled there, which is the same state.

Things to try:

• /model and pick a model from a different family than your current one, then run /rubber-duck again
• or change the rubber-duck model mapping in /subagents away from "complementary". With "inherit" the critic is the same model as your session though, so you lose the second-opinion part. I haven't checked whether the slash command comes back after that.

Not sure the plain "Rubber duck ..." prompt actually ran the critic either. When it does, the timeline shows something like Rubber-duck(<model>). If you don't see that, the main agent probably just answered on its own.

Docs: https://docs.github.com/en/copilot/concepts/agents/copilot-cli/rubber-duck
Changelog: http://github.com/github/copilot-cli/blob/main/changelog.md