---

This section details on the original issue you should resolve

<issue_title>Fix exit codes: validation errors return 0 instead of non-zero</issue_title>
<issue_description>Parent: #1534

Problem

Option validation errors return exit code 0 instead of a non-zero value. For example:

devproxy --ip-address "not-an-ip"
# prints: "not-an-ip is not a valid IP address"
# prints full help text
# exits with code 0

An unknown option (--bogus) correctly returns exit code 1, but option validation failures (bad IP, bad log level, bad timeout value) return 0. This is inconsistent and dangerous for automation.

AI agents branch on exit codes to determine success/failure. When a validation error returns 0, the agent proceeds as if the command succeeded, leading to cascading failures.

Rationale

Exit codes are the most basic contract between a CLI and its callers. Every CI pipeline, shell script, and AI agent relies on non-zero exit codes to detect failure. The current behavior violates this universal expectation.

Proposed solution

1. Validation errors should return exit code 1 (or consider exit code 2 to distinguish input errors from runtime errors, following conventions from curl, git, and others)
2. Ensure all validators in DevProxyCommand.ConfigureCommand() produce non-zero exits when input.AddError() is called — this may be a System.CommandLine configuration issue rather than a per-validator fix
3. Document exit codes in root --help:

   Exit codes:
     0  Success
     1  Runtime error
     2  Invalid input or usage
   

4. Verify all subcommands follow the same convention

Affected code

• DevProxy/Commands/DevProxyCommand.cs — all validators using input.AddError()
• Potentially a System.CommandLine middleware/configuration issue
  </issue_description>

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