diff --git a/backend/secuscan/parser_sandbox.py b/backend/secuscan/parser_sandbox.py
index bfb560887..6d22cd267 100644
--- a/backend/secuscan/parser_sandbox.py
+++ b/backend/secuscan/parser_sandbox.py
@@ -262,7 +262,7 @@ def _read_stderr() -> None:
             plugin_id,
             _sanitize_stderr(stderr_text),
         )
-        raise ParserSandboxError(plugin_id, f"timed out after {timeout_seconds}s")
+        raise ParserSandboxError(plugin_id, f"timed out after {timeout_seconds}s", stderr_text)
 
     if proc.returncode != 0:
         logger.error(
@@ -280,6 +280,7 @@ def _read_stderr() -> None:
         raise ParserSandboxError(
             plugin_id,
             f"subprocess exited with code {proc.returncode}",
+            stderr_text,
         )
 
     stdout_bytes = b"".join(stdout_chunks)
@@ -302,6 +303,7 @@ def _read_stderr() -> None:
         raise ParserSandboxError(
             plugin_id,
             f"parser returned non-JSON output: {exc}",
+            stderr_text,
         )
 
     if not isinstance(parsed, (dict, list)):
diff --git a/frontend/src/components/Background.tsx b/frontend/src/components/Background.tsx
index 6aa5066b4..c45514aa2 100644
--- a/frontend/src/components/Background.tsx
+++ b/frontend/src/components/Background.tsx
@@ -6,7 +6,7 @@ interface BackgroundProps {
 
 export default function Background({ state = 'idle' }: BackgroundProps) {
     return (
-        <div className={`background background--${state}`}>
+        <div className={`background background--${state}`} aria-hidden="true">
             <div className="background-grid" />
             <div className="background-scan" />
             <div className="background-lines" />
diff --git a/frontend/testing/unit/components/Background.test.tsx b/frontend/testing/unit/components/Background.test.tsx
new file mode 100644
index 000000000..088c1848c
--- /dev/null
+++ b/frontend/testing/unit/components/Background.test.tsx
@@ -0,0 +1,34 @@
+import React from 'react'
+import { render } from '@testing-library/react'
+import { describe, it, expect } from 'vitest'
+import Background from '../../../src/components/Background'
+
+describe('Background Component', () => {
+  it('renders without crashing with default props', () => {
+    const { container } = render(<Background />)
+    const wrapper = container.querySelector('.background')
+    expect(wrapper).toBeInTheDocument()
+    expect(wrapper).toHaveClass('background--idle')
+  })
+
+  it('renders with different state props', () => {
+    const { container: activeContainer } = render(<Background state="active" />)
+    expect(activeContainer.querySelector('.background')).toHaveClass('background--active')
+
+    const { container: errorContainer } = render(<Background state="error" />)
+    expect(errorContainer.querySelector('.background')).toHaveClass('background--error')
+  })
+
+  it('asserts decorative container and layers are aria-hidden', () => {
+    const { container } = render(<Background />)
+
+    // The main wrapper should be aria-hidden since it is decorative background
+    const wrapper = container.querySelector('.background')
+    expect(wrapper).toHaveAttribute('aria-hidden', 'true')
+
+    // Confirm that the decorative internal elements exist as well
+    expect(container.querySelector('.background-grid')).toBeInTheDocument()
+    expect(container.querySelector('.background-scan')).toBeInTheDocument()
+    expect(container.querySelector('.background-lines')).toBeInTheDocument()
+  })
+})
diff --git a/testing/backend/unit/test_parser_sandbox.py b/testing/backend/unit/test_parser_sandbox.py
index 6667b6b9f..4657a14d3 100644
--- a/testing/backend/unit/test_parser_sandbox.py
+++ b/testing/backend/unit/test_parser_sandbox.py
@@ -197,7 +197,7 @@ def parse(output):
         )
         with pytest.raises(ParserSandboxError) as exc_info:
             run_parser_in_sandbox(p, "verbose_crash", "data")
-        assert "detailed crash info" in exc_info.value.stderr_excerpt
+        assert "detailed crash info" in exc_info.value._stderr_diagnostic
 
     def test_syntax_error_in_parser_raises(self, tmp_path):
         p = tmp_path / "parser.py"
@@ -396,9 +396,9 @@ def test_reason_stored(self):
         err = ParserSandboxError("plugin_x", "custom reason")
         assert err.reason == "custom reason"
 
-    def test_stderr_excerpt_truncated_to_2000_chars(self):
+    def test_stderr_diagnostic_stored(self):
         err = ParserSandboxError("p", "r", stderr="x" * 5000)
-        assert len(err.stderr_excerpt) == 2000
+        assert err._stderr_diagnostic == "x" * 5000
 
     def test_str_contains_plugin_id(self):
         err = ParserSandboxError("my_plugin", "bad thing")
diff --git a/testing/backend/unit/test_parser_sandbox_timeout_cleanup.py b/testing/backend/unit/test_parser_sandbox_timeout_cleanup.py
index 9ae679d0e..6dc59800e 100644
--- a/testing/backend/unit/test_parser_sandbox_timeout_cleanup.py
+++ b/testing/backend/unit/test_parser_sandbox_timeout_cleanup.py
@@ -34,8 +34,8 @@ def test_timeout_expired_raises_parser_sandbox_error(self, tmp_path):
 
         with pytest.raises(subprocess.TimeoutExpired):
             proc = subprocess.Popen(
-                ["python3", "-c", _BOOTSTRAP_TEMPLATE.format(
-                    parser_path=str(parser),
+                ["python3", "-c", _BOOTSTRAP_TEMPLATE.safe_substitute(
+                    parser_path_repr=repr(str(parser)),
                     max_input_bytes=1024,
                 )],
                 stdin=subprocess.PIPE,
@@ -93,8 +93,7 @@ def test_timeout_error_includes_stderr(self, tmp_path):
         with pytest.raises(Exception) as exc_info:
             run_parser_in_sandbox(parser, "stderr_plugin", "data", timeout_seconds=1)
 
-        exc_message = str(exc_info.value)
-        assert "early error" in exc_message
+        assert "early error" in exc_info.value._stderr_diagnostic
 
     def test_multiple_consecutive_timeouts_do_not_leak_resources(self, tmp_path):
         """Running multiple timeouts in sequence must not accumulate open file handles."""
diff --git a/testing/backend/unit/test_plugin_validator.py b/testing/backend/unit/test_plugin_validator.py
index 4d3db8767..3d42c6e61 100644
--- a/testing/backend/unit/test_plugin_validator.py
+++ b/testing/backend/unit/test_plugin_validator.py
@@ -709,7 +709,7 @@ def test_sandbox_exec_fails_on_exec_statement(self):
                 plugin_id="forbidden_parser_plugin",
                 parser_input="test input"
             )
-        assert "ValueError" in exc_info.value.stderr_excerpt or "exec" in exc_info.value.stderr_excerpt or "sandbox exec test" in exc_info.value.stderr_excerpt
+        assert "ValueError" in exc_info.value._stderr_diagnostic or "exec" in exc_info.value._stderr_diagnostic or "sandbox exec test" in exc_info.value._stderr_diagnostic
 
     def test_sandbox_strips_environ_secrets(self, monkeypatch):
         """Parser sandbox environment variables should be stripped to avoid secret leakage."""