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diff --git a/tests/backend_api/test_api_contract_edge_cases.py b/tests/backend_api/test_api_contract_edge_cases.py
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+++ b/tests/backend_api/test_api_contract_edge_cases.py
@@ -0,0 +1,80 @@
+import pytest
+import json
+from backend.src.protocol.messages import MessageRegistry
+from backend.src.protocol.serialize import Serializer, EncodingFormat
+from backend.src.protocol.validate import MessageValidator, ValidationResult
+
+@pytest.fixture
+def registry():
+    reg = MessageRegistry()
+    # Register a dummy handler for a known message type for testing
+    reg.register(0x1001, lambda payload: (payload, None))
+    return reg
+
+@pytest.fixture
+def validator():
+    val = MessageValidator()
+    # Register a simple field validator for message type 0x1001
+    def dummy_validator(payload):
+        # Expect payload to be a JSON object with a required field "instrument_ids"
+        if not isinstance(payload, dict):
+            return ValidationResult.error("payload", "type_error", "Payload must be an object")
+        if "instrument_ids" not in payload:
+            return ValidationResult.error("instrument_ids", "required", "Missing instrument_ids")
+        if not isinstance(payload["instrument_ids"], list):
+            return ValidationResult.error("instrument_ids", "type_error", "instrument_ids must be a list")
+        return ValidationResult.valid()
+    val.register_field_validator(0x1001, dummy_validator)
+    return val
+
+def test_malformed_payload_missing_required_field(registry, validator):
+    # Payload missing required field "instrument_ids"
+    payload = json.dumps({"types": ["tick"]}).encode("utf-8")
+    # Validate schema (simulate schema validation success)
+    result = validator.validate(0x1001, 1, payload)
+    assert result.has_errors()
+    assert any(e.field == "instrument_ids" for e in result.errors)
+
+def test_malformed_payload_wrong_type(registry, validator):
+    # Payload with wrong type for instrument_ids
+    payload = json.dumps({"instrument_ids": "not-a-list", "types": ["tick"]}).encode("utf-8")
+    result = validator.validate(0x1001, 1, payload)
+    assert result.has_errors()
+    assert any("instrument_ids" in e.field for e in result.errors)
+
+def test_async_wrapper_compatibility(registry):
+    # Test that async handler can be called without special pytest plugins
+    import asyncio
+
+    async def async_handler(payload):
+        await asyncio.sleep(0.01)
+        return b"ok"
+
+    registry.register(0x2001, lambda payload: asyncio.run(async_handler(payload)))
+
+    # Call the async handler via the registry handle method
+    result = registry.handle(0x2001, b"test")
+    assert result == b"ok"
+
+def test_response_status_and_body_assertions(registry):
+    # Register a handler that returns error response for a specific input
+    def error_handler(payload):
+        if payload == b"bad":
+            raise ValueError("Bad payload")
+        return b"good"
+
+    registry.register(0x3001, error_handler)
+
+    # Test good payload
+    result = registry.handle(0x3001, b"good")
+    assert result == b"good"
+
+    # Test bad payload triggers error
+    with pytest.raises(ValueError):
+        registry.handle(0x3001, b"bad")
+
+def test_missing_handler_returns_error(registry):
+    # Try to handle a message type with no registered handler
+    with pytest.raises(Exception) as excinfo:
+        registry.handle(0x9999, b"payload")
+    assert "No handler registered" in str(excinfo.value)