mamba run fails in non-TTY runners with "Undefined error: 0"

[sghng]

Summary

In non-interactive process runners (no TTY on stdin/stdout/stderr), mamba run fails immediately and prints Undefined error: 0 to stderr. The same command works in interactive shells (fish/zsh/bash). Detached mode (-d) succeeds in the same non-TTY runner, which suggests a stdio/attach handling issue in mamba run when no PTY is available.

Potentially related to #3344.

This is important because the use of AI agents is extensive nowadays. It would be immensely helpful to make Mamba agent-friendly.

Context

This shows up in AI agent tool runners (e.g., Zed, OpenCode), which execute commands without a TTY. The runner captures stdout/stderr programmatically; mamba run should still work, but it fails with the error above.

Reproduction (non-TTY)

1. Run in a non-TTY context (stdio is not a TTY):

   • Verified in this runner by uv run python -c "import sys; print(sys.stdin.isatty(), sys.stdout.isatty(), sys.stderr.isatty())" -> False False False

2. Command:
   mamba run -n ds python -c "print('hi')"

3. Result:

   • stdout: empty
   • stderr: "Undefined error: 0"

Capturing output with redirects:

   mamba run -n ds python -c "print('hi')" 1> /tmp/mamba_stdout.txt 2> /tmp/mamba_stderr.txt
   /tmp/mamba_stdout.txt => empty
   /tmp/mamba_stderr.txt =>
     Undefined error: 0

Expected

mamba run should execute the target process and write to stdout normally, even without a TTY.

Actual

mamba run immediately fails with stderr: Undefined error: 0.

What works

• Detached mode succeeds in the same non-TTY runner:
  mamba run -n ds -d python -c "print('hi')"
  Output: "Running wrapped script exec python -c print('hi') in the background"

What does NOT work

• Default attach behavior:
  mamba run -n ds python -c "print('hi')"
  -> Undefined error: 0

• Explicit attach variants:
  mamba run -n ds -a "" python -c "print('hi')" -> Undefined error: 0
  mamba run -n ds -a stdin python -c "print('hi')" -> Undefined error: 0
  mamba run -n ds -a stderr python -c "print('hi')"-> Undefined error: 0
  mamba run -n ds -a stdout -a stderr python -c "print('hi')" -> Undefined error: 0

Environment

• macOS 26.3 (arm64)
• Kernel: Darwin 25.3.0
• mamba 2.5.0 / libmamba 2.5.0
• MAMBA_ROOT_PREFIX=/Users/sghuang/mamba
• env name: ds (exists and works interactively)

Command outputs

  uname -a:
    Darwin samuel-mbp 25.3.0 Darwin Kernel Version 25.3.0: Wed Jan 28 20:54:22 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6030 arm64

  sw_vers:
    ProductName: macOS
    ProductVersion: 26.3
    BuildVersion: 25D125

mamba info:
  libmamba version : 2.5.0
  mamba version    : 2.5.0
  envs directories : /Users/sghuang/mamba/envs
  base environment : /Users/sghuang/mamba
  platform         : osx-arm64

Non-TTY verification:

  uv run python -c "import sys; print('isatty',sys.stdin.isatty(),sys.stdout.isatty(),sys.stderr.isatty())"
  -> isatty False False False

Hypothesis

mamba run appears to require a PTY or fails when attaching to stdio in non-interactive environments. Detached mode avoids stdio attach and therefore succeeds. This breaks compatibility with non-interactive command runners used by AI agents and some CI contexts.