diff --git a/src/fastmcp/cli/generate.py b/src/fastmcp/cli/generate.py
index 2c9bb86fa..9629c28db 100644
--- a/src/fastmcp/cli/generate.py
+++ b/src/fastmcp/cli/generate.py
@@ -164,6 +164,7 @@ def _tool_function_source(tool: mcp.types.Tool) -> str:
     param_lines: list[str] = []
     call_args: list[str] = []
     json_params: list[tuple[str, str]] = []  # (prop_name, safe_name)
+    seen_names: dict[str, str] = {}  # safe_name -> original prop_name
 
     for prop_name, prop_schema in properties.items():
         py_type, needs_json = _schema_to_python_type(prop_schema)
@@ -171,6 +172,14 @@ def _tool_function_source(tool: mcp.types.Tool) -> str:
         is_required = prop_name in required
         safe_name = _to_python_identifier(prop_name)
 
+        # Check for name collisions after sanitization
+        if safe_name in seen_names:
+            raise ValueError(
+                f"Parameter name collision: '{prop_name}' and '{seen_names[safe_name]}' "
+                f"both sanitize to '{safe_name}'"
+            )
+        seen_names[safe_name] = prop_name
+
         # For complex types, add schema to help text
         if needs_json:
             schema_help = _format_schema_for_help(prop_schema)
@@ -191,14 +200,23 @@ def _tool_function_source(tool: mcp.types.Tool) -> str:
         else:
             default = prop_schema.get("default")
             if default is not None:
-                annotation = (
-                    f'Annotated[{py_type}, cyclopts.Parameter(help="{help_escaped}")]'
-                )
-                param_lines.append(f"    {safe_name}: {annotation} = {default!r},")
+                # For complex types with defaults, serialize to JSON string
+                if needs_json:
+                    import json
+
+                    default_str = json.dumps(default)
+                    annotation = f'Annotated[{py_type}, cyclopts.Parameter(help="{help_escaped}")]'
+                    param_lines.append(
+                        f"    {safe_name}: {annotation} = {default_str!r},"
+                    )
+                else:
+                    annotation = f'Annotated[{py_type}, cyclopts.Parameter(help="{help_escaped}")]'
+                    param_lines.append(f"    {safe_name}: {annotation} = {default!r},")
             else:
                 # For list types, default to empty list; others default to None
                 if py_type.startswith("list["):
-                    param_lines.append(f"    {safe_name}: {py_type} = [],")
+                    annotation = f'Annotated[{py_type}, cyclopts.Parameter(help="{help_escaped}")]'
+                    param_lines.append(f"    {safe_name}: {annotation} = [],")
                 else:
                     annotation = f'Annotated[{py_type} | None, cyclopts.Parameter(help="{help_escaped}")]'
                     param_lines.append(f"    {safe_name}: {annotation} = None,")
@@ -230,7 +248,7 @@ def _tool_function_source(tool: mcp.types.Tool) -> str:
         lines.append("    # Parse JSON parameters")
         for _prop_name, safe_name in json_params:
             lines.append(
-                f"    {safe_name}_parsed = json.loads({safe_name}) if {safe_name} else None"
+                f"    {safe_name}_parsed = json.loads({safe_name}) if isinstance({safe_name}, str) else {safe_name}"
             )
         lines.append("")
 
diff --git a/tests/cli/test_generate_cli.py b/tests/cli/test_generate_cli.py
index f86e2460c..c0f1aa43c 100644
--- a/tests/cli/test_generate_cli.py
+++ b/tests/cli/test_generate_cli.py
@@ -274,8 +274,9 @@ class TestToolFunctionSource:
             },
         )
         source = _tool_function_source(tool)
-        # Should use list[str] type
-        assert "tags: list[str] = []" in source
+        # Should use list[str] type with help metadata
+        assert "tags: Annotated[list[str]" in source
+        assert "= []" in source
         # Should not have JSON parsing for simple arrays
         assert "json.loads" not in source
         compile(source, "<test>", "exec")
@@ -304,8 +305,11 @@ class TestToolFunctionSource:
         # Should include JSON schema in help (with escaped quotes)
         assert "JSON Schema:" in source
         assert '\\"type\\": \\"object\\"' in source
-        # Should have JSON parsing
-        assert "metadata_parsed = json.loads(metadata) if metadata else None" in source
+        # Should have JSON parsing with isinstance check
+        assert (
+            "metadata_parsed = json.loads(metadata) if isinstance(metadata, str) else metadata"
+            in source
+        )
         # Should use parsed version in call
         assert "'metadata': metadata_parsed" in source
         compile(source, "<test>", "exec")
@@ -331,9 +335,47 @@ class TestToolFunctionSource:
         # Nested arrays need JSON parsing
         assert "batches: Annotated[str" in source
         assert "JSON Schema:" in source
-        assert "batches_parsed = json.loads(batches)" in source
+        assert (
+            "batches_parsed = json.loads(batches) if isinstance(batches, str) else batches"
+            in source
+        )
         compile(source, "<test>", "exec")
 
+    def test_complex_type_with_default(self):
+        """Test that complex types with defaults are JSON-serialized."""
+        tool = mcp.types.Tool(
+            name="configure",
+            inputSchema={
+                "properties": {
+                    "options": {
+                        "type": "object",
+                        "default": {"timeout": 30, "retry": True},
+                    },
+                },
+            },
+        )
+        source = _tool_function_source(tool)
+        # Default should be JSON string, not Python dict
+        assert '= \'{"timeout": 30, "retry": true}\'' in source
+        # Should parse safely even with default
+        assert "isinstance(options, str)" in source
+        compile(source, "<test>", "exec")
+
+    def test_name_collision_detection(self):
+        """Test that parameter name collisions are detected."""
+        tool = mcp.types.Tool(
+            name="test",
+            inputSchema={
+                "properties": {
+                    "content-type": {"type": "string"},
+                    "content_type": {"type": "string"},
+                },
+            },
+        )
+        # Should raise ValueError for collision
+        with pytest.raises(ValueError, match="both sanitize to 'content_type'"):
+            _tool_function_source(tool)
+
 
 # ---------------------------------------------------------------------------
 # _derive_server_name