diff --git a/src/core/fuzzer.py b/src/core/fuzzer.py
new file mode 100644
index 0000000..01c9e4f
--- /dev/null
+++ b/src/core/fuzzer.py
@@ -0,0 +1,361 @@
+"""Fuzzer for generating edge case responses."""
+
+import random
+from typing import Any
+
+
+class FuzzerError(Exception):
+    """Base exception for fuzzer errors."""
+
+    pass
+
+
+class Fuzzer:
+    """Generates edge case data for resilience testing."""
+
+    NULL_VALUES = [None, "", [], {}, "null", "NULL", "Null"]
+
+    INVALID_STRINGS = [
+        "",
+        "   ",
+        "\n\t",
+        "undefined",
+        "NaN",
+        "Infinity",
+        "-Infinity",
+        "true",
+        "false",
+        "123abc",
+        "<script>alert('xss')</script>",
+        "' OR '1'='1",
+        "../../../etc/passwd",
+        "\x00\x00\x00",
+    ]
+
+    INVALID_NUMBERS = [
+        0,
+        -1,
+        -9999999,
+        9999999,
+        float("inf"),
+        float("-inf"),
+        float("nan"),
+        1e308,
+        -1e308,
+    ]
+
+    BOUNDARY_VALUES = [
+        -2147483649,
+        -2147483648,
+        2147483647,
+        2147483648,
+        -32769,
+        -32768,
+        32767,
+        32768,
+        -1,
+        0,
+        1,
+    ]
+
+    ARRAY_BOUNDARIES = [
+        [],
+        [None],
+        [None] * 100,
+        [{}] * 50,
+        [{"a": None}] * 25,
+    ]
+
+    def __init__(self, seed: int | None = None) -> None:
+        """Initialize the fuzzer.
+
+        Args:
+            seed: Optional seed for reproducible fuzzing.
+        """
+        if seed is not None:
+            random.seed(seed)
+
+    def fuzz_string(self, schema: dict[str, Any]) -> Any:
+        """Fuzz a string field with edge cases.
+
+        Args:
+            schema: String schema definition.
+
+        Returns:
+            Fuzzed string value.
+        """
+        pattern = schema.get("pattern")
+
+        if pattern and random.random() < 0.3:
+            return random.choice(self.INVALID_STRINGS)
+
+        min_length = schema.get("minLength", 0)
+        max_length = schema.get("maxLength", 100)
+
+        if min_length > 0 and random.random() < 0.2:
+            return ""
+
+        if random.random() < 0.1:
+            return random.choice(self.INVALID_STRINGS)
+
+        if random.random() < 0.05:
+            return "a" * (max_length + 1)
+
+        length = max(0, min_length)
+        return "x" * length
+
+    def fuzz_integer(self, schema: dict[str, Any]) -> int:
+        """Fuzz an integer field with boundary values.
+
+        Args:
+            schema: Integer schema definition.
+
+        Returns:
+            Fuzzed integer value.
+        """
+        minimum = schema.get("minimum", -2147483648)
+        maximum = schema.get("maximum", 2147483647)
+        multiple_of = schema.get("multipleOf", 1)
+
+        if random.random() < 0.2:
+            if random.random() < 0.5:
+                return minimum - 1
+            else:
+                return maximum + 1
+
+        if random.random() < 0.15:
+            return 0
+
+        if random.random() < 0.1:
+            return random.choice(self.BOUNDARY_VALUES)
+
+        value = random.randint(minimum, min(maximum, minimum + 1000))
+
+        if multiple_of > 1:
+            value = (value // multiple_of) * multiple_of
+
+        return value
+
+    def fuzz_number(self, schema: dict[str, Any]) -> float:
+        """Fuzz a number field with special values.
+
+        Args:
+            schema: Number schema definition.
+
+        Returns:
+            Fuzzed number value.
+        """
+        if random.random() < 0.2:
+            return random.choice(self.INVALID_NUMBERS)
+
+        minimum = schema.get("minimum", 0.0)
+        maximum = schema.get("maximum", 1000.0)
+
+        value = random.uniform(minimum, maximum)
+
+        if random.random() < 0.1:
+            value = round(value, 10)
+
+        return value
+
+    def fuzz_boolean(self, schema: dict[str, Any]) -> Any:
+        """Fuzz a boolean field with invalid values.
+
+        Args:
+            schema: Boolean schema definition.
+
+        Returns:
+            Fuzzed value (not necessarily boolean).
+        """
+        if random.random() < 0.3:
+            return random.choice([0, 1, "true", "false", "yes", "no", ""])
+
+        return random.choice([True, False])
+
+    def fuzz_array(self, schema: dict[str, Any]) -> Any:
+        """Fuzz an array field with edge cases.
+
+        Args:
+            schema: Array schema definition.
+
+        Returns:
+            Fuzzed array value.
+        """
+        min_items = schema.get("minItems", 0)
+        max_items = schema.get("maxItems", 10)
+
+        if random.random() < 0.2:
+            return random.choice(self.ARRAY_BOUNDARIES)
+
+        if min_items > 0 and random.random() < 0.2:
+            return []
+
+        count = random.randint(0, max(min_items, max_items + 5))
+        return [None] * count
+
+    def fuzz_object(self, schema: dict[str, Any]) -> dict[str, Any] | None:
+        """Fuzz an object field with edge cases.
+
+        Args:
+            schema: Object schema definition.
+
+        Returns:
+            Fuzzed object value.
+        """
+        properties = schema.get("properties", {})
+        required_fields = schema.get("required", [])
+        additional_properties = schema.get("additionalProperties", True)
+
+        result: dict[str, Any] = {}
+
+        for prop_name, prop_schema in properties.items():
+            if prop_name in required_fields:
+                result[prop_name] = self.fuzz_schema(prop_schema)
+            elif additional_properties and random.random() < 0.5:
+                result[prop_name] = self.fuzz_schema(prop_schema)
+
+        if random.random() < 0.2:
+            result["__fuzz__"] = "injected"
+
+        if random.random() < 0.1:
+            return None
+
+        return result
+
+    def fuzz_null(self, schema: dict[str, Any]) -> Any:
+        """Fuzz a null field.
+
+        Args:
+            schema: Null schema definition.
+
+        Returns:
+            Null value or injected null.
+        """
+        if random.random() < 0.7:
+            return None
+        return random.choice(self.NULL_VALUES)
+
+    def fuzz_schema(self, schema: dict[str, Any]) -> Any:
+        """Fuzz a complete schema with edge cases.
+
+        Args:
+            schema: JSON Schema definition.
+
+        Returns:
+            Fuzzed value matching the schema type.
+        """
+        if not schema:
+            return None
+
+        if "$ref" in schema:
+            return self.fuzz_schema(schema.get("resolved", {}))
+
+        schema_type = schema.get("type", "string").lower()
+
+        if random.random() < 0.05:
+            return None
+
+        if schema_type == "string":
+            return self.fuzz_string(schema)
+        elif schema_type == "integer":
+            return self.fuzz_integer(schema)
+        elif schema_type == "number":
+            return self.fuzz_number(schema)
+        elif schema_type == "boolean":
+            return self.fuzz_boolean(schema)
+        elif schema_type == "array":
+            return self.fuzz_array(schema)
+        elif schema_type == "object":
+            return self.fuzz_object(schema)
+        elif schema_type == "null":
+            return self.fuzz_null(schema)
+        else:
+            return None
+
+    def fuzz_request_body(self, schema: dict[str, Any] | None) -> Any:
+        """Fuzz a request body schema.
+
+        Args:
+            schema: Request body schema.
+
+        Returns:
+            Fuzzed request body.
+        """
+        if schema is None:
+            return None
+
+        return self.fuzz_schema(schema)
+
+    def fuzz_response(
+        self, schema: dict[str, Any] | None, status_code: int = 200
+    ) -> dict[str, Any]:
+        """Generate a complete fuzzed response.
+
+        Args:
+            schema: Response schema.
+            status_code: Response status code.
+
+        Returns:
+            Fuzzed response dictionary.
+        """
+        body = self.fuzz_schema(schema) if schema else None
+
+        if random.random() < 0.1:
+            body = None
+
+        return {
+            "status_code": status_code,
+            "body": body,
+        }
+
+    def inject_null_values(self, data: Any, depth: int = 0, max_depth: int = 3) -> Any:
+        """Inject null values into data structure.
+
+        Args:
+            data: Data structure to inject nulls into.
+            depth: Current recursion depth.
+            max_depth: Maximum recursion depth.
+
+        Returns:
+            Data with injected nulls.
+        """
+        if depth >= max_depth:
+            return data
+
+        if isinstance(data, dict):
+            result = {}
+            for key, value in data.items():
+                if random.random() < 0.1:
+                    result[key] = None
+                else:
+                    result[key] = self.inject_null_values(value, depth + 1, max_depth)
+            return result
+
+        elif isinstance(data, list):
+            return [
+                self.inject_null_values(item, depth + 1, max_depth)
+                for item in data
+            ]
+
+        return data
+
+    def create_edge_case_response(
+        self, schema: dict[str, Any] | None = None
+    ) -> dict[str, Any]:
+        """Create an edge case response for testing.
+
+        Args:
+            schema: Optional schema to base the response on.
+
+        Returns:
+            Edge case response dictionary.
+        """
+        if schema:
+            body = self.fuzz_schema(schema)
+        else:
+            body = {"message": "edge case test", "data": None}
+
+        return {
+            "status_code": random.choice([200, 400, 404, 500]),
+            "body": body,
+        }