diff --git a/tools/hardcoded_path_guard.py b/tools/hardcoded_path_guard.py
new file mode 100644
index 000000000..0795370e3
--- /dev/null
+++ b/tools/hardcoded_path_guard.py
@@ -0,0 +1,113 @@
+#!/usr/bin/env python3
+"""
+Hardcoded Path Guard — Poka-Yoke #921
+
+Detects and blocks hardcoded home-directory paths in tool arguments.
+These paths work on one machine but break on others, VPS deployments,
+or when HOME changes.
+
+Usage:
+    from tools.hardcoded_path_guard import check_path, validate_tool_args
+    
+    # Check a single path
+    err = check_path("/Users/apayne/.hermes/config.yaml")
+    
+    # Validate all path-like args in a tool call
+    clean_args, warnings = validate_tool_args("read_file", {"path": "/home/user/file.txt"})
+"""
+
+import os
+import re
+import json as _json
+from typing import Dict, List, Optional, Tuple, Any
+
+# Patterns that indicate hardcoded home directories
+HARDCODED_PATTERNS = [
+    (r"/Users/[\w.\-]+/", "macOS home directory (/Users/...)"),
+    (r"/home/[\w.\-]+/", "Linux home directory (/home/...)"),
+    (r"(?<![\w/])~/", "unexpanded tilde (~/)"),
+    (r"/root/", "root home directory (/root/)"),
+]
+
+_COMPILED_PATTERNS = [(re.compile(p), desc) for p, desc in HARDCODED_PATTERNS]
+_NOQA_PATTERN = re.compile(r"#\s*noqa:?\s*hardcoded-path-ok")
+
+_PATH_ARG_NAMES = frozenset({
+    "path", "file_path", "filepath", "dir", "directory", "dest", "source",
+    "input", "output", "src", "dst", "target", "location", "file",
+    "image_path", "script", "config", "log_file",
+})
+
+
+def has_hardcoded_path(text: str) -> Optional[str]:
+    if _NOQA_PATTERN.search(text):
+        return None
+    for pattern, desc in _COMPILED_PATTERNS:
+        if pattern.search(text):
+            return desc
+    return None
+
+
+def check_path(path_value: str) -> Optional[str]:
+    if not isinstance(path_value, str):
+        return None
+    match_desc = has_hardcoded_path(path_value)
+    if match_desc:
+        return (
+            f"Path contains hardcoded home directory ({match_desc}): '{path_value}'. "
+            f"Use $HOME, relative paths, or get_hermes_home(). "
+            f"Add '# noqa: hardcoded-path-ok' if intentional."
+        )
+    return None
+
+
+def validate_tool_args(tool_name: str, args: Dict[str, Any]) -> Tuple[Dict[str, Any], List[str]]:
+    warnings = []
+    for key, value in args.items():
+        if key.lower() not in _PATH_ARG_NAMES:
+            continue
+        if isinstance(value, str):
+            err = check_path(value)
+            if err:
+                warnings.append(err)
+        elif isinstance(value, list):
+            for item in value:
+                if isinstance(item, str):
+                    err = check_path(item)
+                    if err:
+                        warnings.append(err)
+    return args, warnings
+
+
+def scan_source_for_violations(source_code: str, filename: str = "") -> List[Tuple[int, str, str]]:
+    violations = []
+    lines = source_code.split("\n")
+    for i, line in enumerate(lines, 1):
+        stripped = line.strip()
+        if stripped.startswith("#"):
+            if _NOQA_PATTERN.search(line):
+                continue
+            continue
+        if stripped.startswith("import ") or stripped.startswith("from "):
+            continue
+        for pattern, desc in _COMPILED_PATTERNS:
+            match = pattern.search(line)
+            if match:
+                if _NOQA_PATTERN.search(line):
+                    continue
+                violations.append((i, line.strip(), desc))
+                break
+    return violations
+
+
+def guard_tool_dispatch(tool_name: str, args: Dict[str, Any]) -> Optional[str]:
+    _, warnings = validate_tool_args(tool_name, args)
+    if warnings:
+        return _json.dumps({
+            "error": "Hardcoded home directory path detected",
+            "details": warnings,
+            "suggestion": "Use $HOME, relative paths, or get_hermes_home() instead of hardcoded paths.",
+            "pokayoke": True,
+            "rule": "hardcoded-path-guard"
+        })
+    return None