diff --git a/lib/cli/parser.py b/lib/cli/parser.py
index fae8e6a4b..59d585cbe 100644
--- a/lib/cli/parser.py
+++ b/lib/cli/parser.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import os
+import stat
 import sys
 from typing import Iterable
 
@@ -84,6 +86,12 @@ def parse(self, argv: Iterable[str]) -> ParsedCommand:
     def _read_optional_stdin(self) -> str:
         if sys.stdin.isatty():
             return ''
+        try:
+            mode = os.fstat(0).st_mode
+            if stat.S_ISSOCK(mode):
+                return ''  # Unix socket stdin (e.g. Claude Code Bash tool) — never closes; don't block on read
+        except OSError:
+            pass  # Fall through to read_stdin_text on fstat failure (AC5)
         try:
             return read_stdin_text()
         except OSError:
diff --git a/test/test_cli_parser.py b/test/test_cli_parser.py
new file mode 100644
index 000000000..60a649cd6
--- /dev/null
+++ b/test/test_cli_parser.py
@@ -0,0 +1,102 @@
+"""TD-007: CLI parser stdin detection tests."""
+from __future__ import annotations
+import stat
+import sys
+from unittest.mock import MagicMock, patch
+import pytest
+
+from cli.parser import CliParser
+
+
+class TestReadOptionalStdin:
+    """Test _read_optional_stdin socket detection (TD-007)."""
+
+    def test_stdin_socket_returns_empty(self, monkeypatch):
+        """AC1: Unix socket stdin should return empty without blocking."""
+        parser = CliParser()
+        
+        # Mock isatty to return False (not TTY)
+        monkeypatch.setattr(sys.stdin, 'isatty', lambda: False)
+        
+        # Mock os.fstat to return socket mode
+        mock_stat = MagicMock()
+        mock_stat.st_mode = stat.S_IFSOCK | 0o600
+        monkeypatch.setattr('os.fstat', lambda fd: mock_stat)
+        
+        # Mock read_stdin_text to verify it's NOT called
+        mock_read = MagicMock(return_value="should not be read")
+        monkeypatch.setattr('cli.parser.read_stdin_text', mock_read)
+        
+        result = parser._read_optional_stdin()
+        
+        assert result == ''
+        mock_read.assert_not_called()  # Should not block on socket
+
+    def test_stdin_fifo_reads_normally(self, monkeypatch):
+        """AC2: FIFO (pipe) stdin should read normally."""
+        parser = CliParser()
+        
+        monkeypatch.setattr(sys.stdin, 'isatty', lambda: False)
+        
+        mock_stat = MagicMock()
+        mock_stat.st_mode = stat.S_IFIFO | 0o600
+        monkeypatch.setattr('os.fstat', lambda fd: mock_stat)
+        
+        monkeypatch.setattr('cli.parser.read_stdin_text', lambda: "piped content")
+        
+        result = parser._read_optional_stdin()
+        
+        assert result == "piped content"
+
+    def test_stdin_regular_file_reads_normally(self, monkeypatch):
+        """AC3: Regular file stdin should read normally."""
+        parser = CliParser()
+        
+        monkeypatch.setattr(sys.stdin, 'isatty', lambda: False)
+        
+        mock_stat = MagicMock()
+        mock_stat.st_mode = stat.S_IFREG | 0o644
+        monkeypatch.setattr('os.fstat', lambda fd: mock_stat)
+        
+        monkeypatch.setattr('cli.parser.read_stdin_text', lambda: "file content")
+        
+        result = parser._read_optional_stdin()
+        
+        assert result == "file content"
+
+    def test_stdin_tty_returns_empty_without_checking_mode(self, monkeypatch):
+        """AC4: TTY stdin should return empty without calling fstat."""
+        parser = CliParser()
+        
+        # Mock isatty to return True (TTY)
+        monkeypatch.setattr(sys.stdin, 'isatty', lambda: True)
+        
+        # Track if fstat is called
+        fstat_called = []
+        def mock_fstat(fd):
+            fstat_called.append(True)
+            return MagicMock()
+        monkeypatch.setattr('os.fstat', mock_fstat)
+        
+        result = parser._read_optional_stdin()
+        
+        assert result == ''
+        assert len(fstat_called) == 0  # fstat should NOT be called for TTY
+
+    def test_fstat_oserror_falls_back_to_read(self, monkeypatch):
+        """AC5: OSError from fstat should fall back to read_stdin_text."""
+        parser = CliParser()
+        
+        monkeypatch.setattr(sys.stdin, 'isatty', lambda: False)
+        
+        # Mock fstat to raise OSError
+        def mock_fstat(fd):
+            raise OSError("fstat failed")
+        monkeypatch.setattr('os.fstat', mock_fstat)
+        
+        # read_stdin_text should still be called as fallback
+        monkeypatch.setattr('cli.parser.read_stdin_text', lambda: "fallback content")
+        
+        result = parser._read_optional_stdin()
+        
+        assert result == "fallback content"