diff --git a/engine/oop.c b/engine/oop.c
index 661f9e0..bc9a9a9 100644
--- a/engine/oop.c
+++ b/engine/oop.c
@@ -203,6 +203,101 @@ PH7_PRIVATE sxi32 PH7_ClassInstallMethod(ph7_class *pClass, ph7_class_method *pM
 	rc = SyHashInsert(&pClass->hMethod, (const void *)pName->zString, pName->nByte, pMeth);
 	return rc;
 }
+/*
+ * Validate actual declarations, not inherited hash entries. Forward-declared
+ * bases need not have executed CLASS_INIT, and virtual methods are not mounted
+ * in the VM function table. Input identity remains independent of contracts.
+ */
+static sxi32 ClassValidateMethods(ph7_vm *pVm, ph7_class *pClass) {
+	SySet sMethods;
+	SyHashEntry *pEntry;
+	ph7_class_method *pMeth, *pOther, *pImpl;
+	sxu32 i, j, k;
+	sxi32 rc = SXRET_OK;
+	SySetInit(&sMethods, &pVm->sAllocator, sizeof(ph7_class_method *));
+	for(i = 0; i < SySetUsed(&pClass->aMro); i++) {
+		ph7_class *pOwner = *(ph7_class **)SySetAt(&pClass->aMro, i);
+		SyHashResetLoopCursor(&pOwner->hMethod);
+		while((pEntry = SyHashGetNextEntry(&pOwner->hMethod)) != 0) {
+			pMeth = (ph7_class_method *)pEntry->pUserData;
+			if(pMeth->sFunc.pClass == pOwner) {
+				rc = SySetPut(&sMethods, &pMeth);
+				if(rc != SXRET_OK) {
+					goto done;
+				}
+			}
+		}
+	}
+	for(i = 0; i < SySetUsed(&sMethods); i++) {
+		pMeth = *(ph7_class_method **)SySetAt(&sMethods, i);
+		for(j = i + 1; j < SySetUsed(&sMethods); j++) {
+			pOther = *(ph7_class_method **)SySetAt(&sMethods, j);
+			if(!PH7_VmSameSignature(&pMeth->sFunc, &pOther->sFunc)) {
+				continue;
+			}
+			for(k = 0; k < SySetUsed(&pMeth->sFunc.pClass->aMro); k++) {
+				if(*(ph7_class **)SySetAt(&pMeth->sFunc.pClass->aMro, k) == pOther->sFunc.pClass) {
+					break;
+				}
+			}
+			/* An unrelated base's private method cannot shadow this declaration.
+			 * A descendant's private declaration can still narrow an override. */
+			if(pOther->iProtection == PH7_CLASS_PROT_PRIVATE ||
+				(pMeth->iProtection == PH7_CLASS_PROT_PRIVATE &&
+				 k == SySetUsed(&pMeth->sFunc.pClass->aMro))) {
+				continue;
+			}
+			if(pOther->iFlags & PH7_CLASS_ATTR_FINAL) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Cannot overwrite final method '%z:%z()' inside child class '%z'", &pOther->sFunc.pClass->sName, &pOther->sFunc.sName, &pClass->sName);
+			}
+			/* Visibility involving an unrelated virtual requirement is checked
+			 * against the effective implementation below, not a hidden method. */
+			if(pMeth->iProtection > pOther->iProtection &&
+				(k < SySetUsed(&pMeth->sFunc.pClass->aMro) ||
+				 !((pMeth->iFlags | pOther->iFlags) & PH7_CLASS_ATTR_VIRTUAL))) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Cannot narrow visibility of method '%z::%z()' inside class '%z'", &pOther->sFunc.pClass->sName, &pOther->sFunc.sName, &pClass->sName);
+			}
+			/* Check real overrides and consistency between requirements here.
+			 * Hidden unrelated implementations do not impose contracts. */
+			if((k < SySetUsed(&pMeth->sFunc.pClass->aMro) ||
+				((pMeth->iFlags & pOther->iFlags) & PH7_CLASS_ATTR_VIRTUAL)) &&
+				(pMeth->sFunc.nType != pOther->sFunc.nType ||
+				((pMeth->sFunc.iFlags ^ pOther->sFunc.iFlags) & VM_FUNC_REF_RETURN) ||
+				((pMeth->iFlags ^ pOther->iFlags) & PH7_CLASS_ATTR_STATIC))) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Incompatible method contract '%z::%z()' inside class '%z'", &pOther->sFunc.pClass->sName, &pOther->sFunc.sName, &pClass->sName);
+			}
+		}
+		if(!(pMeth->iFlags & PH7_CLASS_ATTR_VIRTUAL)) {
+			continue;
+		}
+		pImpl = 0;
+		for(j = 0; j < SySetUsed(&sMethods); j++) {
+			pOther = *(ph7_class_method **)SySetAt(&sMethods, j);
+			if(!(pOther->iFlags & PH7_CLASS_ATTR_VIRTUAL) &&
+				pOther->iProtection != PH7_CLASS_PROT_PRIVATE &&
+				PH7_VmSameSignature(&pMeth->sFunc, &pOther->sFunc)) {
+				pImpl = pOther;
+				break;
+			}
+		}
+		if(pImpl && pImpl->iProtection > pMeth->iProtection) {
+			PH7_VmThrowError(pVm, PH7_CTX_ERR, "Cannot narrow visibility of method '%z::%z()' inside class '%z'", &pMeth->sFunc.pClass->sName, &pMeth->sFunc.sName, &pClass->sName);
+		}
+		/* Every requirement uses the same first concrete C3 declaration, not
+		 * a later implementation selected for its compatible contract. */
+		if(pImpl && (pImpl->sFunc.nType != pMeth->sFunc.nType ||
+			((pImpl->sFunc.iFlags ^ pMeth->sFunc.iFlags) & VM_FUNC_REF_RETURN) ||
+			((pImpl->iFlags ^ pMeth->iFlags) & PH7_CLASS_ATTR_STATIC))) {
+			PH7_VmThrowError(pVm, PH7_CTX_ERR, "Incompatible method contract '%z::%z()' inside class '%z'", &pMeth->sFunc.pClass->sName, &pMeth->sFunc.sName, &pClass->sName);
+		}
+		if(!pImpl && !(pClass->iFlags & PH7_CLASS_VIRTUAL)) {
+			PH7_VmThrowError(pVm, PH7_CTX_ERR, "Virtual method '%z::%z()' must be defined inside child class '%z'", &pMeth->sFunc.pClass->sName, &pMeth->sFunc.sName, &pClass->sName);
+		}
+	}
+done:
+	SySetRelease(&sMethods);
+	return rc;
+}
 /*
  * Build [C] + merge(MRO(B1), ..., MRO(Bn), [B1, ..., Bn]).
  * Resolve declarations rather than hDerived: bytecode initialization may not
@@ -328,7 +423,7 @@ done:
 		SySetRelease(&sMro);
 		pClass->iMroState = 0;
 	}
-	return rc;
+	return rc == SXRET_OK ? ClassValidateMethods(pVm, pClass) : rc;
 }
 /*
  * Perform an inheritance operation.
@@ -400,20 +495,14 @@ PH7_PRIVATE sxi32 PH7_ClassInherit(ph7_vm *pVm, ph7_class *pSub, ph7_class *pBas
 	}
 	SyHashResetLoopCursor(&pBase->hMethod);
 	while((pEntry = SyHashGetNextEntry(&pBase->hMethod)) != 0) {
-		/* Make sure the final methods are not redeclared in the subclass */
+		/* Method contracts are checked against declarations across the C3 MRO. */
 		pMeth = (ph7_class_method *)pEntry->pUserData;
+		if(pMeth->iFlags & PH7_CLASS_ATTR_VIRTUAL) {
+			continue;
+		}
 		pName = &pMeth->sFunc.sName;
 		if((pEntry = SyHashGet(&pSub->hMethod, (const void *)pName->zString, pName->nByte)) != 0) {
-			if(pMeth->iFlags & PH7_CLASS_ATTR_FINAL) {
-				/* Cannot Overwrite final method */
-				PH7_VmThrowError(&(*pVm), PH7_CTX_ERR, "Cannot overwrite final method '%z:%z()' inside child class '%z'", &pBase->sName, pName, &pSub->sName);
-			}
 			continue;
-		} else {
-			if(pMeth->iFlags & PH7_CLASS_ATTR_VIRTUAL) {
-				/* Virtual method must be defined in the child class */
-				PH7_VmThrowError(&(*pVm), PH7_CTX_ERR, "Virtual method '%z::%z()' must be defined inside child class '%z'", &pBase->sName, pName, &pSub->sName);
-			}
 		}
 		/* Install the method */
 		rc = SyHashInsert(&pSub->hMethod, (const void *)pName->zString, pName->nByte, pMeth);
diff --git a/engine/vm.c b/engine/vm.c
index a813eba..6191d55 100644
--- a/engine/vm.c
+++ b/engine/vm.c
@@ -895,8 +895,7 @@ static ph7_vm_func *VmOverload(
 			for(pLink = pEntry ? (ph7_vm_func *)pEntry->pUserData : 0; pLink; pLink = pLink->pNextName) {
 				pMeth = (ph7_class_method *)pLink;
 				if((pMeth->iFlags & PH7_CLASS_ATTR_VIRTUAL) ||
-					(bStatic && !(pMeth->iFlags & PH7_CLASS_ATTR_STATIC) &&
-					 (pLookup->iFlags & (PH7_CLASS_FINAL | PH7_CLASS_VIRTUAL)) != (PH7_CLASS_FINAL | PH7_CLASS_VIRTUAL)) ||
+					(bStatic && !(pMeth->iFlags & PH7_CLASS_ATTR_STATIC)) ||
 					(!bInternal && !VmClassMemberAccess(pVm, pLink->pClass, pMeth->iProtection))) {
 					continue;
 				}