diff --git a/tests/test_devcontainer_config.py b/tests/test_devcontainer_config.py
new file mode 100644
index 000000000..4595a34e4
--- /dev/null
+++ b/tests/test_devcontainer_config.py
@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+pytestmark = pytest.mark.unit
+
+_DEVCONTAINER_DIR = Path(__file__).resolve().parents[1] / ".devcontainer"
+
+
+def test_devcontainer_json_is_valid_json() -> None:
+    """devcontainer.json must be valid JSON; the @devcontainers/cli parses it with JSON.parse()."""
+    config_path = _DEVCONTAINER_DIR / "devcontainer.json"
+    content = config_path.read_text(encoding="utf-8")
+    try:
+        json.loads(content)
+    except json.JSONDecodeError as exc:
+        pytest.fail(
+            f".devcontainer/devcontainer.json is not valid JSON: {exc}\n"
+            "This will cause `devcontainer build` (and the build-and-test CI job) to fail."
+        )
+
+
+def test_devcontainer_lock_json_is_valid_json() -> None:
+    """devcontainer-lock.json must be valid JSON with no trailing data.
+
+    A stray closing brace after the root object caused:
+        SyntaxError: Unexpected non-whitespace character after JSON at position 2143
+    which broke the build-and-test devcontainer CI job.
+    """
+    lock_path = _DEVCONTAINER_DIR / "devcontainer-lock.json"
+    if not lock_path.exists():
+        pytest.skip("devcontainer-lock.json not present")
+    content = lock_path.read_text(encoding="utf-8")
+    try:
+        json.loads(content)
+    except json.JSONDecodeError as exc:
+        pytest.fail(
+            f".devcontainer/devcontainer-lock.json is not valid JSON: {exc}\n"
+            "Trailing or extra characters after the root object will cause "
+            "`devcontainer build` (and the build-and-test CI job) to fail."
+        )
+
+
+def _assert_no_trailing_data(path: Path, content: str) -> None:
+    """Assert that *content* contains a single JSON value with only whitespace after it.
+
+    Uses ``JSONDecoder.raw_decode()`` which returns the end-position of the
+    parsed value.  Anything remaining after that position (beyond whitespace)
+    is trailing data and indicates a corrupted file.
+    """
+    decoder = json.JSONDecoder()
+    try:
+        _parsed, end_index = decoder.raw_decode(content)
+    except json.JSONDecodeError as exc:
+        pytest.fail(f"{path.name} is not valid JSON: {exc}")
+
+    trailing = content[end_index:]
+    assert trailing.strip() == "", (
+        f"{path.name} has trailing non-whitespace data after the root JSON object "
+        f"(at offset {end_index}): {trailing!r}\n"
+        "This will cause `devcontainer build` to fail with a SyntaxError."
+    )
+
+
+def test_devcontainer_json_no_trailing_data() -> None:
+    """Ensure no extra characters appear after the root JSON object in devcontainer.json."""
+    config_path = _DEVCONTAINER_DIR / "devcontainer.json"
+    content = config_path.read_text(encoding="utf-8")
+    _assert_no_trailing_data(config_path, content)
+
+
+def test_devcontainer_lock_json_no_trailing_data() -> None:
+    """Ensure no extra characters appear after the root JSON object in devcontainer-lock.json.
+
+    Regression test for the stray ``  }`` that caused:
+        SyntaxError: Unexpected non-whitespace character after JSON at position 2143
+    in CI job 86511118286.
+    """
+    lock_path = _DEVCONTAINER_DIR / "devcontainer-lock.json"
+    if not lock_path.exists():
+        pytest.skip("devcontainer-lock.json not present")
+    content = lock_path.read_text(encoding="utf-8")
+    _assert_no_trailing_data(lock_path, content)