diff --git a/CLAUDE.md b/CLAUDE.md
index 70b35fe..733705d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1092,18 +1092,12 @@ raised anywhere inside astra's validation or resolution was reported as a
 bad target — a rerun misdiagnosing itself, at the one place nobody is
 watching.
 
-**Known violation, unfixed: a malformed `astra.yaml` tracebacks out of
-`lc status` and `lc materialize --check`** as a raw
-`yaml.scanner.ScannerError`. `plan._validate` is the gate that turns a
-bad spec into ASTRA's own errors, but it only catches what *validation*
-reports — a spec that does not parse raises inside
-`validate_analysis_schema` → `astra.helpers.load_yaml` before there is
-anything to validate. (`lc materialize` usually masks it by refusing a
-dirty tree first, which is not a fix.) The shape of the fix is a
-`yaml.YAMLError` catch in `_validate` naming the file and the line, the
-same way the validator's own errors are rendered. Pre-existing and
-independent of the one-output-one-file change; written down here because
-it contradicts the invariant directly above it.
+**Malformed YAML is a validation refusal.** `plan._validate` catches
+`yaml.YAMLError` from ASTRA's validators and includes the parser's own
+diagnostic in the existing `ProjectError`, preserving the source file,
+line and column where available. The same gate covers `astra.yaml` and
+universe files, so `lc status` and `lc materialize --check` report the
+fault cleanly. Normal `lc materialize` still refuses a dirty tree first.
 
 **`git_sha` in a manifest is the commit the run *started* at**, not the
 commit the run went on to create. It is the code that produced the output.
diff --git a/src/lightcone/engine/plan.py b/src/lightcone/engine/plan.py
index 513f410..bfab68d 100644
--- a/src/lightcone/engine/plan.py
+++ b/src/lightcone/engine/plan.py
@@ -251,13 +251,18 @@ def _validate(spec_path: Path, universes: list[Path]) -> None:
         validate_analysis_schema,
         validate_universe_file,
     )
+    from yaml import YAMLError
 
-    problems = [
-        *validate_analysis_schema(spec_path),
-        *(str(e) for e in validate_analysis_file(spec_path)),
-    ]
-    for path in universes:
-        problems += [f"{path.name}: {e}" for e in validate_universe_file(path, spec_path)]
+    try:
+        problems = [
+            *validate_analysis_schema(spec_path),
+            *(str(e) for e in validate_analysis_file(spec_path)),
+        ]
+        for path in universes:
+            problems += [f"{path.name}: {e}" for e in validate_universe_file(path, spec_path)]
+    except YAMLError as e:
+        # Parser diagnostics already name the source file and its location.
+        problems = [str(e)]
     if problems:
         listed = "\n".join(f"  {problem}" for problem in problems)
         raise ProjectError(
diff --git a/tests/test_cli_yaml.py b/tests/test_cli_yaml.py
new file mode 100644
index 0000000..1ea2049
--- /dev/null
+++ b/tests/test_cli_yaml.py
@@ -0,0 +1,59 @@
+"""Malformed YAML through the real CLI, with no engine or tool stubs."""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+from collections.abc import Callable
+from pathlib import Path
+
+import pytest
+
+_SPEC = '''version: "0.1"
+name: demo
+inputs: []
+outputs:
+  - id: fit
+    type: data
+    format: txt
+    recipe:
+      command: echo valid > {output}
+decisions: {}
+'''
+
+
+@pytest.mark.parametrize("argv", [["status"], ["materialize", "--check"]])
+@pytest.mark.parametrize("json_output", [False, True])
+@pytest.mark.parametrize("filename", ["astra.yaml", "universes/baseline.yaml"])
+def test_malformed_yaml_is_reported_without_a_traceback_or_writes(
+    analysis: Callable[..., Path], argv: list[str], json_output: bool, filename: str,
+) -> None:
+    root = analysis(_SPEC)
+    command = [sys.executable, "-c", "from lightcone.cli import main; main()", *argv]
+    if json_output:
+        command.append("--json")
+    env = dict(os.environ, PATH=f"{Path(sys.executable).parent}{os.pathsep}{os.environ['PATH']}")
+
+    valid = subprocess.run(command, cwd=root, env=env, capture_output=True, text=True)
+    assert valid.returncode == (0 if argv == ["status"] else 1), valid.stderr
+    assert valid.stderr == ""
+    assert "baseline/fit" in valid.stdout
+    assert not (root / "results/baseline/fit.txt").exists()
+
+    malformed = root / filename
+    malformed.write_text("name: a: b\n")
+    before_paths = set(root.rglob("*"))
+    before = {p.relative_to(root): p.read_bytes() for p in root.rglob("*") if p.is_file()}
+
+    result = subprocess.run(command, cwd=root, env=env, capture_output=True, text=True)
+
+    assert result.returncode == 1
+    assert result.stdout == ""
+    assert result.stderr.startswith("Error: ")
+    assert str(malformed) in result.stderr
+    assert "mapping values are not allowed here" in result.stderr
+    assert "line 1, column 8" in result.stderr
+    assert "Traceback" not in result.stderr
+    assert set(root.rglob("*")) == before_paths
+    assert {p.relative_to(root): p.read_bytes() for p in root.rglob("*") if p.is_file()} == before
diff --git a/tests/test_plan.py b/tests/test_plan.py
index 9f069d3..48b1af4 100644
--- a/tests/test_plan.py
+++ b/tests/test_plan.py
@@ -293,6 +293,55 @@ def test_no_universe_is_a_clean_error(tmp_path: Path) -> None:
         _build(tmp_path)
 
 
+@pytest.mark.parametrize(("filename", "content", "diagnostic"), [
+    ("astra.yaml", "name: a: b\n", "line 1, column 8"),
+    ("universes/baseline.yaml", "id: a: b\n", "line 1, column 6"),
+    ("astra.yaml", "outputs: [\n", "line 2, column 1"),
+    ("astra.yaml", "name: \x00\n", "unacceptable character #x0000"),
+])
+def test_malformed_yaml_is_a_clean_validation_error(
+    tmp_path: Path, filename: str, content: str, diagnostic: str,
+) -> None:
+    root = _project(tmp_path)
+    path = root / filename
+    path.write_text(content)
+
+    with pytest.raises(ProjectError, match="does not validate") as raised:
+        _build(root)
+
+    assert str(path) in str(raised.value)
+    assert diagnostic in str(raised.value)
+
+
+def test_malformed_external_analysis_names_its_own_file(tmp_path: Path) -> None:
+    root = _project(tmp_path, _SPEC + '\nanalyses:\n  child:\n    path: child\n')
+    child = root / "child" / "astra.yaml"
+    child.parent.mkdir()
+    child.write_text("name: a: b\n")
+
+    with pytest.raises(ProjectError, match="does not validate") as raised:
+        _build(root)
+
+    assert str(child) in str(raised.value)
+    assert "line 1, column 8" in str(raised.value)
+
+
+def test_validation_does_not_hide_unrelated_errors(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    from astra import validation
+
+    error = ValueError("unexpected validator defect")
+
+    def broken(path: Path) -> list[str]:
+        raise error
+
+    monkeypatch.setattr(validation, "validate_analysis_schema", broken)
+    with pytest.raises(ValueError) as raised:
+        _build(_project(tmp_path))
+    assert raised.value is error
+
+
 def test_an_output_without_a_format_is_refused_by_name(tmp_path: Path) -> None:
     """lc names the file from it, so there is nowhere to write the output.
     Every offender at once: a spec is fixed in one pass, not one run per