fix(bundle): send command errors to stderr so --json stdout stays parseable (#3235)

The bundle command group's _fail() helper is documented as printing 'to stderr', and the module contract is 'human logs go to stderr/console' while --json 'emits machine-readable data on stdout'. But it called console.print(), and the shared console writes to STDOUT, so every bundle error (every command routes through _fail) landed on stdout -- corrupting the JSON stream that --json consumers parse.

Add a stderr-bound err_console to _console.py (its documented role as the single Console source) and use it in _fail. stdout now carries only the JSON payload.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/specify_cli/_console.py

@@ -34,6 +34,10 @@ TAGLINE = "GitHub Spec Kit - Spec-Driven Development Toolkit"
 
 console = Console(highlight=False)
 
+# Stderr-bound console for error/diagnostic output, so human-facing messages
+# never contaminate stdout (which carries machine-readable ``--json`` payloads).
+err_console = Console(stderr=True, highlight=False)
+
 class StepTracker:
     """Track and render hierarchical steps without emojis, similar to Claude Code tree output.
     Supports live auto-refresh via an attached refresh callback.

src/specify_cli/commands/bundle/__init__.py

@@ -13,7 +13,7 @@ from pathlib import Path
 
 import typer
 
-from ..._console import console
+from ..._console import console, err_console
 from ...bundler import BundlerError
 from ...bundler.lib.project import (
     active_integration,
@@ -41,7 +41,9 @@ bundle_app.add_typer(bundle_catalog_app, name="catalog")
 
 def _fail(message: str) -> None:
     """Print an actionable error to stderr and exit non-zero."""
-    console.print(f"[red]Error:[/red] {message}", style=None)
+    # Use the stderr console so the error never lands on stdout, which under
+    # ``--json`` carries the machine-readable payload and must stay parseable.
+    err_console.print(f"[red]Error:[/red] {message}", style=None)
     raise typer.Exit(code=1)
 
 

tests/contract/test_bundle_cli.py

@@ -62,6 +62,21 @@ def test_commands_outside_project_fail_with_guidance(tmp_path: Path, monkeypatch
     assert "Spec Kit project" in result.output
 
 
+def test_fail_writes_error_to_stderr_not_stdout(capsys):
+    """_fail must write to stderr, not stdout: every bundle command routes errors
+    through it, and under --json the error would otherwise corrupt the JSON payload
+    that consumers read from stdout."""
+    import typer
+
+    from specify_cli.commands.bundle import _fail
+
+    with pytest.raises(typer.Exit):
+        _fail("something broke")
+    captured = capsys.readouterr()
+    assert "something broke" in captured.err
+    assert "something broke" not in captured.out
+
+
 def test_search_works_without_a_project(tmp_path: Path, monkeypatch):
     # Discovery commands fall back to the built-in/user catalog stack and must
     # not require a Spec Kit project (matches README/quickstart examples).