diff --git a/AG/AttributeGraph.xcframework/ios-arm64-arm64e/AttributeGraph.framework/Headers/AGAttribute.h b/AG/AttributeGraph.xcframework/ios-arm64-arm64e/AttributeGraph.framework/Headers/AGAttribute.h
index caf5b79e..dd014270 100644
--- a/AG/AttributeGraph.xcframework/ios-arm64-arm64e/AttributeGraph.framework/Headers/AGAttribute.h
+++ b/AG/AttributeGraph.xcframework/ios-arm64-arm64e/AttributeGraph.framework/Headers/AGAttribute.h
@@ -64,7 +64,11 @@ const AGAttributeInfo AGGraphGetAttributeInfo(AGAttribute attribute) AG_SWIFT_NA
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
-void AGGraphMutateAttribute(AGAttribute attribute, const AGTypeID type, bool invalidating/*, closure*/);
+void AGGraphMutateAttribute(AGAttribute attribute,
+                            const AGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context AG_SWIFT_CONTEXT, void *body) AG_SWIFT_CC(swift),
+                            const void * _Nullable context);
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
diff --git a/AG/AttributeGraph.xcframework/ios-arm64-x86_64-simulator/AttributeGraph.framework/Headers/AGAttribute.h b/AG/AttributeGraph.xcframework/ios-arm64-x86_64-simulator/AttributeGraph.framework/Headers/AGAttribute.h
index caf5b79e..dd014270 100644
--- a/AG/AttributeGraph.xcframework/ios-arm64-x86_64-simulator/AttributeGraph.framework/Headers/AGAttribute.h
+++ b/AG/AttributeGraph.xcframework/ios-arm64-x86_64-simulator/AttributeGraph.framework/Headers/AGAttribute.h
@@ -64,7 +64,11 @@ const AGAttributeInfo AGGraphGetAttributeInfo(AGAttribute attribute) AG_SWIFT_NA
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
-void AGGraphMutateAttribute(AGAttribute attribute, const AGTypeID type, bool invalidating/*, closure*/);
+void AGGraphMutateAttribute(AGAttribute attribute,
+                            const AGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context AG_SWIFT_CONTEXT, void *body) AG_SWIFT_CC(swift),
+                            const void * _Nullable context);
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
diff --git a/AG/AttributeGraph.xcframework/macos-arm64e-arm64-x86_64/AttributeGraph.framework/Headers/AGAttribute.h b/AG/AttributeGraph.xcframework/macos-arm64e-arm64-x86_64/AttributeGraph.framework/Headers/AGAttribute.h
index caf5b79e..dd014270 100644
--- a/AG/AttributeGraph.xcframework/macos-arm64e-arm64-x86_64/AttributeGraph.framework/Headers/AGAttribute.h
+++ b/AG/AttributeGraph.xcframework/macos-arm64e-arm64-x86_64/AttributeGraph.framework/Headers/AGAttribute.h
@@ -64,7 +64,11 @@ const AGAttributeInfo AGGraphGetAttributeInfo(AGAttribute attribute) AG_SWIFT_NA
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
-void AGGraphMutateAttribute(AGAttribute attribute, const AGTypeID type, bool invalidating/*, closure*/);
+void AGGraphMutateAttribute(AGAttribute attribute,
+                            const AGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context AG_SWIFT_CONTEXT, void *body) AG_SWIFT_CC(swift),
+                            const void * _Nullable context);
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
diff --git a/AG/Sources/Headers/AGAttribute.h b/AG/Sources/Headers/AGAttribute.h
index caf5b79e..dd014270 100644
--- a/AG/Sources/Headers/AGAttribute.h
+++ b/AG/Sources/Headers/AGAttribute.h
@@ -64,7 +64,11 @@ const AGAttributeInfo AGGraphGetAttributeInfo(AGAttribute attribute) AG_SWIFT_NA
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
-void AGGraphMutateAttribute(AGAttribute attribute, const AGTypeID type, bool invalidating/*, closure*/);
+void AGGraphMutateAttribute(AGAttribute attribute,
+                            const AGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context AG_SWIFT_CONTEXT, void *body) AG_SWIFT_CC(swift),
+                            const void * _Nullable context);
 
 AG_EXPORT
 AG_REFINED_FOR_SWIFT
diff --git a/Sources/OpenGraph/Attribute/Attribute/OGAttribute.swift b/Sources/OpenGraph/Attribute/Attribute/OGAttribute.swift
index 2cabafe2..bc561afa 100644
--- a/Sources/OpenGraph/Attribute/Attribute/OGAttribute.swift
+++ b/Sources/OpenGraph/Attribute/Attribute/OGAttribute.swift
@@ -43,9 +43,10 @@ extension OGAttribute {
         bodyType._visitBody(&visitor, info.body)
     }
 
-    public func mutateBody<Value>(as type: Value.Type, invalidating: Bool, _: (inout Value) -> Void) {
-        // FIXME: pass body
-        __OGGraphMutateAttribute(self, OGTypeID(type), invalidating)
+    public func mutateBody<Value>(as type: Value.Type, invalidating: Bool, _ body: (inout Value) -> Void) {
+        OGAttribute.mutateAttribute(self, type: OGTypeID(type), invalidating: invalidating) { value in
+            body(&value.assumingMemoryBound(to: Value.self).pointee)
+        }
     }
     
     public func breadthFirstSearch(options _: OGSearchOptions = [], _: (OGAttribute) -> Bool) -> Bool {
@@ -83,3 +84,14 @@ extension OGAttribute: CustomStringConvertible {
 }
 
 public typealias AttributeUpdateBlock = () -> (UnsafeMutableRawPointer, OGAttribute) -> Void
+
+// FIXME: migrate to use @_extern(c, "xx") in Swift 6
+extension OGAttribute {
+    @_silgen_name("OGGraphMutateAttribute")
+    private static func mutateAttribute(
+        _ attribute: OGAttribute,
+        type: OGTypeID,
+        invalidating: Bool,
+        body: (UnsafeMutableRawPointer) -> Void
+    )
+}
diff --git a/Sources/_OpenGraph/Attribute/AttributeID.hpp b/Sources/_OpenGraph/Attribute/AttributeID.hpp
index 533ef062..f64465f0 100644
--- a/Sources/_OpenGraph/Attribute/AttributeID.hpp
+++ b/Sources/_OpenGraph/Attribute/AttributeID.hpp
@@ -10,6 +10,7 @@
 
 #include "OGBase.h"
 #include "OGAttribute.h"
+#include "../Util/assert.hpp"
 
 namespace OG {
 class AttributeID final {
@@ -32,7 +33,7 @@ class AttributeID final {
     }
 public:
     OG_INLINE OG_CONSTEXPR
-    AttributeID(OGAttribute attribute) OG_NOEXCEPT:
+    AttributeID(OGAttribute& attribute) OG_NOEXCEPT:
     _rawValue(attribute) {}
     
     OG_INLINE OG_CONSTEXPR
@@ -61,6 +62,13 @@ class AttributeID final {
     const bool isNil() const OG_NOEXCEPT {
         return getKind() == Kind::Nil;
     }
+    
+    OG_INLINE OG_CONSTEXPR
+    const void checkIsDirect() const OG_NOEXCEPT {
+        if (!isDirect()) {
+            OG::precondition_failure("non-direct attribute id: %u", _rawValue);
+        }
+    }
 };
 static_assert(sizeof(AttributeID) == sizeof(uint32_t));
 }
diff --git a/Sources/_OpenGraph/Attribute/OGAttribute.cpp b/Sources/_OpenGraph/Attribute/OGAttribute.cpp
index 0713d80a..0b0c8e01 100644
--- a/Sources/_OpenGraph/Attribute/OGAttribute.cpp
+++ b/Sources/_OpenGraph/Attribute/OGAttribute.cpp
@@ -50,40 +50,38 @@ OGAttribute OGGraphCreateIndirectAttribute2(OGAttribute attribute, uint64_t size
 
 OGAttributeFlags OGGraphGetFlags(OGAttribute attribute) {
     const OG::AttributeID id = OG::AttributeID(attribute);
-    if (!id.isDirect()) {
-        OG::precondition_failure("non-direct attribute id: %u", id);
-    }
+    id.checkIsDirect();
     // TODO: data/table
     return OGAttributeFlagsDefault;
 }
 
 void OGGraphSetFlags(OGAttribute attribute, OGAttributeFlags flags) {
     const OG::AttributeID id = OG::AttributeID(attribute);
-    if (!id.isDirect()) {
-        OG::precondition_failure("non-direct attribute id: %u", id);
-    }
+    id.checkIsDirect();
     // TODO: data/table
 }
 
 void OGGraphAddInput(OGAttribute attribute1, OGAttribute attribute2, OGInputOptions options, long token) {
     const OG::AttributeID id = OG::AttributeID(attribute1);
-    if (!id.isDirect()) {
-        OG::precondition_failure("non-direct attribute id: %u", id);
-    }
+    id.checkIsDirect();
     // TODO: data/table
 }
 
 const OGAttributeInfo OGGraphGetAttributeInfo(OGAttribute attribute) {
     const OG::AttributeID id = OG::AttributeID(attribute);
-    if (!id.isDirect()) {
-        OG::precondition_failure("non-direct attribute id: %u", id);
-    }
+    id.checkIsDirect();
     // TODO
     return { nullptr, nullptr };
 }
 
-void OGGraphMutateAttribute(OGAttribute attribute, const OGTypeID type, bool invalidating) {
-    // TODO
+void OGGraphMutateAttribute(OGAttribute attribute,
+                            const OGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context OG_SWIFT_CONTEXT, void *body) OG_SWIFT_CC(swift),
+                            const void * _Nullable context) {
+    const OG::AttributeID id = OG::AttributeID(attribute);
+    id.checkIsDirect();
+    // attribute_modify(AG::data::ptr<AG::Node>, AGSwiftMetadata const*, AG::ClosureFunction<void, void*>, bool
 }
 
 OGAttribute OGGraphGetIndirectDependency(OGAttribute attribute) {
diff --git a/Sources/_OpenGraph/Attribute/OGAttribute.h b/Sources/_OpenGraph/Attribute/OGAttribute.h
index 6c8a95c2..8597f7f3 100644
--- a/Sources/_OpenGraph/Attribute/OGAttribute.h
+++ b/Sources/_OpenGraph/Attribute/OGAttribute.h
@@ -64,7 +64,11 @@ const OGAttributeInfo OGGraphGetAttributeInfo(OGAttribute attribute) OG_SWIFT_NA
 
 OG_EXPORT
 OG_REFINED_FOR_SWIFT
-void OGGraphMutateAttribute(OGAttribute attribute, const OGTypeID type, bool invalidating/*, closure*/);
+void OGGraphMutateAttribute(OGAttribute attribute,
+                            const OGTypeID type,
+                            bool invalidating,
+                            const void (*function)(const void * _Nullable context OG_SWIFT_CONTEXT, void *body) OG_SWIFT_CC(swift),
+                            const void * _Nullable context);
 
 OG_EXPORT
 OG_REFINED_FOR_SWIFT
diff --git a/Tests/OpenGraphCompatibilityTests/Attribute/Attribute/AttributeTests.swift b/Tests/OpenGraphCompatibilityTests/Attribute/Attribute/AttributeTests.swift
index dd1760ec..bcaa47ed 100644
--- a/Tests/OpenGraphCompatibilityTests/Attribute/Attribute/AttributeTests.swift
+++ b/Tests/OpenGraphCompatibilityTests/Attribute/Attribute/AttributeTests.swift
@@ -65,5 +65,13 @@ final class AttributeTests: AttributeTestBase {
         attribute.prefetchValue()
         attribute.invalidateValue()
     }
+    
+    @Test
+    func mutateBodyAPI() {
+        let attribute = Attribute(value: 5)
+        attribute.mutateBody(as: External<Int>.self, invalidating: true) { _ in
+            
+        }
+    }
 }
 #endif