diff --git a/cel/cel_test.go b/cel/cel_test.go
index 9259d2a6..b05ec1b0 100644
--- a/cel/cel_test.go
+++ b/cel/cel_test.go
@@ -1438,7 +1438,7 @@ func (tc testCostEstimator) EstimateSize(element checker.AstNode) *checker.SizeE
 	return nil
 }
 
-func (tc testCostEstimator) EstimateCallCost(overloadID string, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
+func (tc testCostEstimator) EstimateCallCost(function, overloadID string, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
 	switch overloadID {
 	case overloads.TimestampToYear:
 		return &checker.CallEstimate{CostEstimate: checker.CostEstimate{Min: 7, Max: 7}}
@@ -1451,7 +1451,7 @@ type testRuntimeCostEstimator struct {
 
 var timeToYearCost uint64 = 7
 
-func (e testRuntimeCostEstimator) CallCost(overloadID string, args []ref.Val) *uint64 {
+func (e testRuntimeCostEstimator) CallCost(function, overloadID string, args []ref.Val, result ref.Val) *uint64 {
 	argsSize := make([]uint64, len(args))
 	for i, arg := range args {
 		reflectV := reflect.ValueOf(arg.Value())
diff --git a/checker/cost.go b/checker/cost.go
index b1e83020..4e1ea5aa 100644
--- a/checker/cost.go
+++ b/checker/cost.go
@@ -37,7 +37,7 @@ type CostEstimator interface {
 	EstimateSize(element AstNode) *SizeEstimate
 	// EstimateCallCost returns the estimated cost of an invocation, or nil if
 	// the estimator has no estimate to provide.
-	EstimateCallCost(overloadID string, target *AstNode, args []AstNode) *CallEstimate
+	EstimateCallCost(function, overloadID string, target *AstNode, args []AstNode) *CallEstimate
 }
 
 // CallEstimate includes a CostEstimate for the call, and an optional estimate of the result object size.
@@ -384,7 +384,7 @@ func (c *coster) costCall(e *exprpb.Expr) CostEstimate {
 	fnCost := CostEstimate{Min: uint64(math.MaxUint64), Max: 0}
 	var resultSize *SizeEstimate
 	for _, overload := range ref.GetOverloadId() {
-		overloadCost := c.functionCost(overload, &targetType, argTypes, argCosts)
+		overloadCost := c.functionCost(call.GetFunction(), overload, &targetType, argTypes, argCosts)
 		fnCost = fnCost.Union(overloadCost.CostEstimate)
 		if overloadCost.ResultSize != nil {
 			if resultSize == nil {
@@ -479,7 +479,7 @@ func (c *coster) sizeEstimate(t AstNode) SizeEstimate {
 	return SizeEstimate{Min: 0, Max: math.MaxUint64}
 }
 
-func (c *coster) functionCost(overloadID string, target *AstNode, args []AstNode, argCosts []CostEstimate) CallEstimate {
+func (c *coster) functionCost(function, overloadID string, target *AstNode, args []AstNode, argCosts []CostEstimate) CallEstimate {
 	argCostSum := func() CostEstimate {
 		var sum CostEstimate
 		for _, a := range argCosts {
@@ -488,7 +488,7 @@ func (c *coster) functionCost(overloadID string, target *AstNode, args []AstNode
 		return sum
 	}
 
-	if est := c.estimator.EstimateCallCost(overloadID, target, args); est != nil {
+	if est := c.estimator.EstimateCallCost(function, overloadID, target, args); est != nil {
 		callEst := *est
 		return CallEstimate{CostEstimate: callEst.Add(argCostSum())}
 	}
diff --git a/checker/cost_test.go b/checker/cost_test.go
index 08f8f5ff..e2118f87 100644
--- a/checker/cost_test.go
+++ b/checker/cost_test.go
@@ -411,7 +411,7 @@ func (tc testCostEstimator) EstimateSize(element AstNode) *SizeEstimate {
 	return nil
 }
 
-func (tc testCostEstimator) EstimateCallCost(overloadID string, target *AstNode, args []AstNode) *CallEstimate {
+func (tc testCostEstimator) EstimateCallCost(function, overloadID string, target *AstNode, args []AstNode) *CallEstimate {
 	switch overloadID {
 	case overloads.TimestampToYear:
 		return &CallEstimate{CostEstimate: CostEstimate{Min: 7, Max: 7}}
diff --git a/interpreter/runtimecost.go b/interpreter/runtimecost.go
index acbe5696..d8233804 100644
--- a/interpreter/runtimecost.go
+++ b/interpreter/runtimecost.go
@@ -31,7 +31,7 @@ import (
 // estimate to provide. CEL attempts to provide reasonable estimates for its standard function library, so CallCost
 // should typically not need to provide an estimate for CELs standard function.
 type ActualCostEstimator interface {
-	CallCost(overloadID string, args []ref.Val) *uint64
+	CallCost(function, overloadID string, args []ref.Val, result ref.Val) *uint64
 }
 
 // CostObserver provides an observer that tracks runtime cost.
@@ -58,7 +58,7 @@ func CostObserver(tracker *CostTracker) EvalObserver {
 			tracker.cost++
 		case InterpretableCall:
 			if argVals, ok := tracker.stack.pop(len(t.Args())); ok {
-				tracker.cost += tracker.costCall(t, argVals)
+				tracker.cost += tracker.costCall(t, argVals, val)
 			}
 		case InterpretableConstructor:
 			switch t.Type() {
@@ -93,10 +93,10 @@ func (c CostTracker) ActualCost() uint64 {
 	return c.cost
 }
 
-func (c CostTracker) costCall(call InterpretableCall, argValues []ref.Val) uint64 {
+func (c CostTracker) costCall(call InterpretableCall, argValues []ref.Val, result ref.Val) uint64 {
 	var cost uint64
 	if c.Estimator != nil {
-		callCost := c.Estimator.CallCost(call.OverloadID(), argValues)
+		callCost := c.Estimator.CallCost(call.Function(), call.OverloadID(), argValues, result)
 		if callCost != nil {
 			cost += *callCost
 			return cost
diff --git a/interpreter/runtimecost_test.go b/interpreter/runtimecost_test.go
index 2e49b9fd..633057d5 100644
--- a/interpreter/runtimecost_test.go
+++ b/interpreter/runtimecost_test.go
@@ -183,7 +183,7 @@ type testRuntimeCostEstimator struct {
 
 var timeToYearCost uint64 = 7
 
-func (e testRuntimeCostEstimator) CallCost(overloadID string, args []ref.Val) *uint64 {
+func (e testRuntimeCostEstimator) CallCost(function, overloadID string, args []ref.Val, result ref.Val) *uint64 {
 	argsSize := make([]uint64, len(args))
 	for i, arg := range args {
 		reflectV := reflect.ValueOf(arg.Value())
@@ -216,7 +216,7 @@ func (tc testCostEstimator) EstimateSize(element checker.AstNode) *checker.SizeE
 	return nil
 }
 
-func (tc testCostEstimator) EstimateCallCost(overloadID string, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
+func (tc testCostEstimator) EstimateCallCost(function, overloadID string, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
 	switch overloadID {
 	case overloads.TimestampToYear:
 		return &checker.CallEstimate{CostEstimate: checker.CostEstimate{Min: 7, Max: 7}}