diff --git a/tools/tool_validator.py b/tools/tool_validator.py
new file mode 100644
index 000000000..7ee632d41
--- /dev/null
+++ b/tools/tool_validator.py
@@ -0,0 +1,312 @@
+"""
+Poka-Yoke: Tool Hallucination Detection — #922.
+
+Validation firewall between LLM tool-call output and actual execution.
+
+Detects and blocks:
+1. Unknown tool names (hallucinated tools)
+2. Malformed parameters (wrong types)
+3. Missing required arguments
+4. Extra unknown parameters
+
+Poka-Yoke Type: Detection (catches errors at the boundary before harm)
+"""
+
+import json
+import logging
+import re
+from dataclasses import dataclass, field
+from enum import Enum
+from typing import Any, Dict, List, Optional, Set
+
+logger = logging.getLogger(__name__)
+
+
+class ValidationSeverity(Enum):
+    """Severity of validation failure."""
+    BLOCK = "block"      # Must block execution
+    WARN = "warn"        # Warning, may proceed
+    INFO = "info"        # Informational
+
+
+@dataclass
+class ValidationIssue:
+    """A validation issue found."""
+    severity: ValidationSeverity
+    code: str
+    message: str
+    tool_name: str
+    parameter: Optional[str] = None
+    expected: Optional[str] = None
+    actual: Optional[Any] = None
+
+
+@dataclass
+class ValidationResult:
+    """Result of tool call validation."""
+    valid: bool
+    tool_name: str
+    issues: List[ValidationIssue] = field(default_factory=list)
+    corrected_args: Optional[Dict[str, Any]] = None
+    
+    @property
+    def blocking_issues(self) -> List[ValidationIssue]:
+        return [i for i in self.issues if i.severity == ValidationSeverity.BLOCK]
+    
+    @property
+    def warnings(self) -> List[ValidationIssue]:
+        return [i for i in self.issues if i.severity == ValidationSeverity.WARN]
+
+
+class ToolHallucinationDetector:
+    """
+    Poka-yoke detector for tool hallucinations.
+    
+    Validates tool calls against registered schemas before execution.
+    """
+    
+    def __init__(self, tool_registry: Optional[Dict] = None):
+        """
+        Initialize detector.
+        
+        Args:
+            tool_registry: Dict of tool_name -> tool_schema
+        """
+        self.registry = tool_registry or {}
+        self._rejection_log: List[Dict] = []
+    
+    def register_tool(self, name: str, schema: Dict):
+        """Register a tool with its JSON Schema."""
+        self.registry[name] = schema
+    
+    def register_tools(self, tools: Dict[str, Dict]):
+        """Register multiple tools."""
+        self.registry.update(tools)
+    
+    def validate_tool_call(
+        self,
+        tool_name: str,
+        arguments: Dict[str, Any],
+        model: str = "unknown",
+    ) -> ValidationResult:
+        """
+        Validate a tool call against the registry.
+        
+        Args:
+            tool_name: Name of the tool being called
+            arguments: Arguments passed to the tool
+            model: Model that generated the call (for logging)
+            
+        Returns:
+            ValidationResult with validation status
+        """
+        issues = []
+        
+        # 1. Check if tool exists
+        if tool_name not in self.registry:
+            issue = ValidationIssue(
+                severity=ValidationSeverity.BLOCK,
+                code="UNKNOWN_TOOL",
+                message=f"Tool '{tool_name}' does not exist. Available: {', '.join(sorted(self.registry.keys())[:10])}...",
+                tool_name=tool_name,
+            )
+            issues.append(issue)
+            self._log_rejection(tool_name, arguments, model, "UNKNOWN_TOOL")
+            return ValidationResult(valid=False, tool_name=tool_name, issues=issues)
+        
+        schema = self.registry[tool_name]
+        params_schema = schema.get("parameters", {}).get("properties", {})
+        required = set(schema.get("parameters", {}).get("required", []))
+        
+        # 2. Check for missing required parameters
+        for param in required:
+            if param not in arguments:
+                issue = ValidationIssue(
+                    severity=ValidationSeverity.BLOCK,
+                    code="MISSING_REQUIRED",
+                    message=f"Missing required parameter: {param}",
+                    tool_name=tool_name,
+                    parameter=param,
+                )
+                issues.append(issue)
+        
+        # 3. Check parameter types
+        for param_name, param_value in arguments.items():
+            if param_name not in params_schema:
+                # Unknown parameter
+                issue = ValidationIssue(
+                    severity=ValidationSeverity.WARN,
+                    code="UNKNOWN_PARAM",
+                    message=f"Unknown parameter: {param_name}",
+                    tool_name=tool_name,
+                    parameter=param_name,
+                )
+                issues.append(issue)
+                continue
+            
+            param_schema = params_schema[param_name]
+            expected_type = param_schema.get("type")
+            
+            if expected_type and not self._check_type(param_value, expected_type):
+                issue = ValidationIssue(
+                    severity=ValidationSeverity.BLOCK,
+                    code="WRONG_TYPE",
+                    message=f"Parameter '{param_name}' expects {expected_type}, got {type(param_value).__name__}",
+                    tool_name=tool_name,
+                    parameter=param_name,
+                    expected=expected_type,
+                    actual=type(param_value).__name__,
+                )
+                issues.append(issue)
+        
+        # 4. Check for common hallucination patterns
+        hallucination_issues = self._detect_hallucination_patterns(tool_name, arguments)
+        issues.extend(hallucination_issues)
+        
+        # Determine validity
+        has_blocking = any(i.severity == ValidationSeverity.BLOCK for i in issues)
+        
+        if has_blocking:
+            self._log_rejection(tool_name, arguments, model, 
+                              "; ".join(i.code for i in issues if i.severity == ValidationSeverity.BLOCK))
+        
+        return ValidationResult(
+            valid=not has_blocking,
+            tool_name=tool_name,
+            issues=issues,
+        )
+    
+    def _check_type(self, value: Any, expected_type: str) -> bool:
+        """Check if value matches expected JSON Schema type."""
+        type_map = {
+            "string": str,
+            "number": (int, float),
+            "integer": int,
+            "boolean": bool,
+            "array": list,
+            "object": dict,
+        }
+        
+        expected = type_map.get(expected_type)
+        if expected is None:
+            return True  # Unknown type, assume OK
+        
+        return isinstance(value, expected)
+    
+    def _detect_hallucination_patterns(self, tool_name: str, arguments: Dict) -> List[ValidationIssue]:
+        """Detect common hallucination patterns."""
+        issues = []
+        
+        # Pattern 1: Placeholder values
+        placeholder_patterns = [
+            r"^<.*>$",           # <placeholder>
+            r"^\[.*\]$",         # [placeholder]
+            r"^TODO$|^FIXME$",   # TODO/FIXME
+            r"^example\.com$",   # example.com
+            r"^127\.0\.0\.1$",   # localhost
+        ]
+        
+        for param_name, param_value in arguments.items():
+            if isinstance(param_value, str):
+                for pattern in placeholder_patterns:
+                    if re.match(pattern, param_value, re.IGNORECASE):
+                        issues.append(ValidationIssue(
+                            severity=ValidationSeverity.WARN,
+                            code="PLACEHOLDER_VALUE",
+                            message=f"Parameter '{param_name}' contains placeholder: {param_value}",
+                            tool_name=tool_name,
+                            parameter=param_name,
+                        ))
+        
+        # Pattern 2: Suspiciously long strings (might be hallucinated content)
+        for param_name, param_value in arguments.items():
+            if isinstance(param_value, str) and len(param_value) > 10000:
+                issues.append(ValidationIssue(
+                    severity=ValidationSeverity.WARN,
+                    code="SUSPICIOUS_LENGTH",
+                    message=f"Parameter '{param_name}' is unusually long ({len(param_value)} chars)",
+                    tool_name=tool_name,
+                    parameter=param_name,
+                ))
+        
+        return issues
+    
+    def _log_rejection(self, tool_name: str, arguments: Dict, model: str, reason: str):
+        """Log a rejected tool call for analysis."""
+        import time
+        
+        entry = {
+            "timestamp": time.time(),
+            "tool_name": tool_name,
+            "arguments": {k: str(v)[:100] for k, v in arguments.items()},
+            "model": model,
+            "reason": reason,
+        }
+        
+        self._rejection_log.append(entry)
+        
+        # Keep log bounded
+        if len(self._rejection_log) > 1000:
+            self._rejection_log = self._rejection_log[-500:]
+        
+        logger.warning(
+            "Tool hallucination blocked: tool=%s, model=%s, reason=%s",
+            tool_name, model, reason
+        )
+    
+    def get_rejection_stats(self) -> Dict:
+        """Get statistics on rejected tool calls."""
+        if not self._rejection_log:
+            return {"total": 0, "by_reason": {}, "by_tool": {}}
+        
+        by_reason = {}
+        by_tool = {}
+        
+        for entry in self._rejection_log:
+            reason = entry["reason"]
+            tool = entry["tool_name"]
+            
+            by_reason[reason] = by_reason.get(reason, 0) + 1
+            by_tool[tool] = by_tool.get(tool, 0) + 1
+        
+        return {
+            "total": len(self._rejection_log),
+            "by_reason": by_reason,
+            "by_tool": by_tool,
+        }
+    
+    def format_validation_report(self, result: ValidationResult) -> str:
+        """Format validation result as human-readable report."""
+        if result.valid:
+            return f"✅ {result.tool_name}: valid"
+        
+        lines = [f"❌ {result.tool_name}: BLOCKED"]
+        for issue in result.blocking_issues:
+            lines.append(f"   [{issue.code}] {issue.message}")
+        
+        for issue in result.warnings:
+            lines.append(f"   ⚠️ [{issue.code}] {issue.message}")
+        
+        return "\n".join(lines)
+
+
+def create_rejection_response(result: ValidationResult) -> Dict:
+    """
+    Create a tool result for a rejected tool call.
+    
+    This allows the agent to see the rejection and self-correct.
+    """
+    issues_text = "\n".join(
+        f"- [{i.code}] {i.message}" 
+        for i in result.blocking_issues
+    )
+    
+    return {
+        "role": "tool",
+        "content": f"""Tool call rejected: {result.tool_name}
+
+Issues found:
+{issues_text}
+
+Please check the tool name and parameters, then try again with valid arguments.""",
+    }