Bash tool exit-code sentinel uses bash $? syntax in fish, breaking exit-code detection

[MansoorMohsin]

Describe the bug

When the user's configured shell is fish, the CLI's Bash tool wraps each command with a command-completion / exit-code sentinel that uses bash syntax ($?). fish does not support $? (it uses $status), so the sentinel line errors out in fish.

This has two harmful effects:

1. Exit-code detection breaks — the wrapper can no longer reliably parse the command's exit status (the agent sees a spurious/invalid exit code such as -1 on every command).
2. Subsequent commands get polluted — the failed sentinel evaluation can leak into the next prompt/command in the same session.

The practical result is that an agent operating in a fish session is pushed toward abandoning the user's shell entirely (e.g. running exec bash) just to get reliable exit codes — which silently overrides the user's chosen shell and any fish-specific environment/config.

Affected version

1.0.49 (session runtime that exhibited the behavior); installed binary now reports GitHub Copilot CLI 1.0.57-5. Behavior reproduces across both.

Steps to reproduce the behavior

1. Configure the shell used by the Bash tool to be fish (fish, version 3.7.1).
2. Have the agent run any simple command, e.g. echo hello.
3. Observe that the appended completion sentinel uses $? (bash) rather than $status (fish), so fish emits an error for the sentinel line and the reported exit code is invalid (e.g. -1), even though the command itself succeeded.

Expected behavior

The completion/exit-code sentinel should be shell-aware:

• Detect the active shell (or the shell configured for the Bash tool) and emit the correct exit-status expansion — $status for fish, $? for POSIX shells (bash/zsh/sh).
• Exit-code detection should work correctly in fish so the agent can stay in the user's configured shell instead of switching to bash as a workaround.

A minimal fix is to branch the sentinel template on shell type (fish → $status, otherwise → $?), or to use a shell-agnostic mechanism for capturing the exit code.

Additional context

• Operating system: Linux (Linux 6.8.0-124-generic x86_64)
• CPU architecture: x86_64
• Shell: fish 3.7.1
• Impact: while a command's stdout/stderr is still captured correctly, the invalid exit code defeats success/failure detection and nudges agents into overriding the user's shell choice.