diff --git a/core/proto/core.pb.go b/core/proto/core.pb.go
index c8acf30f91a..3dfa4c05afe 100644
--- a/core/proto/core.pb.go
+++ b/core/proto/core.pb.go
@@ -614,7 +614,6 @@ func (x *Registration) GetStatus() string {
 
 type Authorization struct {
 	state                  protoimpl.MessageState `protogen:"open.v1"`
-	Id                     string                 `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`         // TODO(#8722): reserve
 	IdInt                  int64                  `protobuf:"varint,12,opt,name=idInt,proto3" json:"idInt,omitempty"` // TODO(#8722): rename
 	RegistrationID         int64                  `protobuf:"varint,3,opt,name=registrationID,proto3" json:"registrationID,omitempty"`
 	Identifier             *Identifier            `protobuf:"bytes,11,opt,name=identifier,proto3" json:"identifier,omitempty"`
@@ -656,13 +655,6 @@ func (*Authorization) Descriptor() ([]byte, []int) {
 	return file_core_proto_rawDescGZIP(), []int{7}
 }
 
-func (x *Authorization) GetId() string {
-	if x != nil {
-		return x.Id
-	}
-	return ""
-}
-
 func (x *Authorization) GetIdInt() int64 {
 	if x != nil {
 		return x.IdInt
@@ -1018,9 +1010,8 @@ var file_core_proto_rawDesc = string([]byte{
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@@ -1039,52 +1030,53 @@ var file_core_proto_rawDesc = string([]byte{
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 })
 
 var (
diff --git a/core/proto/core.proto b/core/proto/core.proto
index 66d576f9bde..de7e876980a 100644
--- a/core/proto/core.proto
+++ b/core/proto/core.proto
@@ -94,8 +94,7 @@ message Registration {
 
 message Authorization {
   // Next unused field number: 13
-  reserved 5, 7, 8;
-  string id = 1;    // TODO(#8722): reserve
+  reserved 1, 5, 7, 8;
   int64 idInt = 12; // TODO(#8722): rename
   int64 registrationID = 3;
   // Fields specified by RFC 8555, Section 7.1.4
diff --git a/grpc/pb-marshalling.go b/grpc/pb-marshalling.go
index 18e5e4aedb4..a2909563897 100644
--- a/grpc/pb-marshalling.go
+++ b/grpc/pb-marshalling.go
@@ -8,7 +8,6 @@ package grpc
 import (
 	"fmt"
 	"net/netip"
-	"strconv"
 	"time"
 
 	"github.com/go-jose/go-jose/v4"
@@ -292,7 +291,6 @@ func AuthzToPB(authz core.Authorization) (*corepb.Authorization, error) {
 	}
 
 	return &corepb.Authorization{
-		Id:                     fmt.Sprintf("%d", authz.ID),
 		IdInt:                  authz.ID,
 		Identifier:             authz.Identifier.ToProto(),
 		RegistrationID:         authz.RegistrationID,
@@ -304,6 +302,10 @@ func AuthzToPB(authz core.Authorization) (*corepb.Authorization, error) {
 }
 
 func PBToAuthz(pb *corepb.Authorization) (core.Authorization, error) {
+	if pb.IdInt == 0 {
+		return core.Authorization{}, ErrMissingParameters
+	}
+
 	challs := make([]core.Challenge, len(pb.Challenges))
 	for i, c := range pb.Challenges {
 		chall, err := PBToChallenge(c)
@@ -318,21 +320,8 @@ func PBToAuthz(pb *corepb.Authorization) (core.Authorization, error) {
 		expires = &c
 	}
 
-	// TODO(#8722): remove this series of checks when pb.Id is int64-only
-	var authzIDInt int64
-	if pb.IdInt != 0 {
-		authzIDInt = pb.IdInt
-	} else if pb.Id != "" {
-		parsed, err := strconv.ParseInt(pb.Id, 10, 64)
-		if err != nil {
-			return core.Authorization{}, ErrInvalidParameters
-		}
-		authzIDInt = parsed
-	} else {
-		return core.Authorization{}, ErrMissingParameters
-	}
 	authz := core.Authorization{
-		ID:                     authzIDInt,
+		ID:                     pb.IdInt,
 		Identifier:             identifier.FromProto(pb.Identifier),
 		RegistrationID:         pb.RegistrationID,
 		Status:                 core.AcmeStatus(pb.Status),
diff --git a/grpc/pb-marshalling_test.go b/grpc/pb-marshalling_test.go
index 196418f077b..b4b6a42f446 100644
--- a/grpc/pb-marshalling_test.go
+++ b/grpc/pb-marshalling_test.go
@@ -242,33 +242,6 @@ func TestAuthz(t *testing.T) {
 	outAuthz2, err := PBToAuthz(pbAuthz2)
 	test.AssertNotError(t, err, "PBToAuthz failed")
 	test.AssertDeepEquals(t, inAuthzNilExpires, outAuthz2)
-
-	// Manipulate pbAuthz to test Authz marshalling with various ID combinations
-	// TODO(#8722): clean up these tests when authz IDs are int-only
-	pbAuthz3 := pbAuthz
-
-	pbAuthz3.Id = ""
-	pbAuthz3.IdInt = 0
-	_, err = PBToAuthz(pbAuthz3)
-	test.AssertError(t, err, "PBToAuthz with empty ID and empty IDInt unexpectedly succeeded")
-
-	pbAuthz3.Id = "1"
-	pbAuthz3.IdInt = 0
-	outAuthz3, err := PBToAuthz(pbAuthz3)
-	test.AssertNotError(t, err, "PBToAuthz with only string ID failed")
-	test.AssertDeepEquals(t, inAuthz, outAuthz3)
-
-	pbAuthz3.Id = "1"
-	pbAuthz3.IdInt = 1
-	outAuthz3, err = PBToAuthz(pbAuthz3)
-	test.AssertNotError(t, err, "PBToAuthz with both string ID and int IDInt failed")
-	test.AssertDeepEquals(t, inAuthz, outAuthz3)
-
-	pbAuthz3.Id = ""
-	pbAuthz3.IdInt = 1
-	outAuthz3, err = PBToAuthz(pbAuthz3)
-	test.AssertNotError(t, err, "PBToAuthz with only int IDInt failed")
-	test.AssertDeepEquals(t, inAuthz, outAuthz3)
 }
 
 func TestOrderValid(t *testing.T) {
diff --git a/ra/ra.go b/ra/ra.go
index e6a8651598b..ce6dc74cc16 100644
--- a/ra/ra.go
+++ b/ra/ra.go
@@ -689,7 +689,6 @@ func (ra *RegistrationAuthorityImpl) recheckCAA(ctx context.Context, authzs []*c
 				Identifier:       authz.Identifier.ToProto(),
 				ValidationMethod: method,
 				AccountURIID:     authz.RegistrationID,
-				AuthzID:          fmt.Sprintf("%d", authz.ID),
 				AuthzIDInt:       authz.ID,
 			})
 			if err != nil {
@@ -1384,8 +1383,7 @@ func (ra *RegistrationAuthorityImpl) PerformValidation(
 	// Clock for start of PerformValidation.
 	vStart := ra.clk.Now()
 
-	// TODO(#8722): Re-add req.Authz.Id to this check once int64-only
-	if core.IsAnyNilOrZero(req.Authz, req.Authz.Identifier, req.Authz.Status, req.Authz.Expires) {
+	if core.IsAnyNilOrZero(req.Authz, req.Authz.IdInt, req.Authz.Identifier, req.Authz.Status, req.Authz.Expires) {
 		return nil, errIncompleteGRPCRequest
 	}
 
@@ -1456,14 +1454,13 @@ func (ra *RegistrationAuthorityImpl) PerformValidation(
 			&vapb.PerformValidationRequest{
 				Identifier:               authz.Identifier.ToProto(),
 				Challenge:                &corepb.Challenge{Type: string(ch.Type), Status: string(ch.Status), Token: ch.Token},
-				Authz:                    &vapb.AuthzMeta{Id: fmt.Sprintf("%d", authz.ID), RegID: authz.RegistrationID, IdInt: authz.ID},
+				Authz:                    &vapb.AuthzMeta{IdInt: authz.ID, RegID: authz.RegistrationID},
 				ExpectedKeyAuthorization: expectedKeyAuthorization,
 			},
 			&vapb.IsCAAValidRequest{
 				Identifier:       authz.Identifier.ToProto(),
 				ValidationMethod: string(ch.Type),
 				AccountURIID:     authz.RegistrationID,
-				AuthzID:          fmt.Sprintf("%d", authz.ID),
 				AuthzIDInt:       authz.ID,
 			},
 		)
@@ -1975,23 +1972,10 @@ func (ra *RegistrationAuthorityImpl) DeactivateRegistration(ctx context.Context,
 func (ra *RegistrationAuthorityImpl) DeactivateAuthorization(ctx context.Context, req *corepb.Authorization) (*emptypb.Empty, error) {
 	ident := identifier.FromProto(req.Identifier)
 
-	if core.IsAnyNilOrZero(ident, req.Status, req.RegistrationID) {
+	if core.IsAnyNilOrZero(ident, req.Status, req.RegistrationID, req.IdInt) {
 		return nil, errIncompleteGRPCRequest
 	}
-	// TODO(#8722): Re-add req.Id to IsAnyNilOrZero check above, and cleanup following blocks when authz ids are int64-only
-	var authzIDInt int64
-	if req.IdInt != 0 {
-		authzIDInt = req.IdInt
-	} else if req.Id != "" {
-		parsed, err := strconv.ParseInt(req.Id, 10, 64)
-		if err != nil {
-			return nil, fmt.Errorf("malformed gRPC request message field: %w", err)
-		}
-		authzIDInt = parsed
-	} else {
-		return nil, errIncompleteGRPCRequest
-	}
-	if _, err := ra.SA.DeactivateAuthorization2(ctx, &sapb.AuthorizationID2{Id: authzIDInt}); err != nil {
+	if _, err := ra.SA.DeactivateAuthorization2(ctx, &sapb.AuthorizationID2{Id: req.IdInt}); err != nil {
 		return nil, err
 	}
 	if req.Status == string(core.StatusPending) {
diff --git a/ra/ra_test.go b/ra/ra_test.go
index 2b6828047e1..f1e69218972 100644
--- a/ra/ra_test.go
+++ b/ra/ra_test.go
@@ -20,7 +20,6 @@ import (
 	mrand "math/rand/v2"
 	"net/netip"
 	"regexp"
-	"strconv"
 	"sync"
 	"testing"
 	"time"
@@ -132,17 +131,15 @@ func createPendingAuthorization(t *testing.T, sa sapb.StorageAuthorityClient, re
 func createFinalizedAuthorization(t *testing.T, saClient sapb.StorageAuthorityClient, regID int64, ident identifier.ACMEIdentifier, exp time.Time, chall core.AcmeChallenge, attemptedAt time.Time) int64 {
 	t.Helper()
 	pending := createPendingAuthorization(t, saClient, regID, ident, exp)
-	pendingID, err := strconv.ParseInt(pending.Id, 10, 64)
-	test.AssertNotError(t, err, "strconv.ParseInt failed")
-	_, err = saClient.FinalizeAuthorization2(context.Background(), &sapb.FinalizeAuthorizationRequest{
-		Id:          pendingID,
+	_, err := saClient.FinalizeAuthorization2(context.Background(), &sapb.FinalizeAuthorizationRequest{
+		Id:          pending.IdInt,
 		Status:      "valid",
 		Expires:     timestamppb.New(exp),
 		Attempted:   string(chall),
 		AttemptedAt: timestamppb.New(attemptedAt),
 	})
 	test.AssertNotError(t, err, "sa.FinalizeAuthorizations2 failed")
-	return pendingID
+	return pending.IdInt
 }
 
 func getAuthorization(t *testing.T, id int64, sa sapb.StorageAuthorityClient) *corepb.Authorization {
@@ -854,16 +851,15 @@ func TestDeactivateAuthorization(t *testing.T) {
 	_, err := ra.DeactivateAuthorization(ctx, dbAuthzPB)
 	test.AssertNotError(t, err, "Could not deactivate authorization")
 	deact, err := sa.GetAuthorization2(ctx, &sapb.AuthorizationID2{Id: authzID})
-	test.AssertNotError(t, err, "Could not get deactivated authorization with ID "+dbAuthzPB.Id)
+	test.AssertNotError(t, err, "Could not get deactivated authorization by ID")
 	test.AssertEquals(t, deact.Status, string(core.StatusDeactivated))
 
 	dbAuthzPBIdChecks := dbAuthzPB
-	dbAuthzPBIdChecks.Id = fmt.Sprintf("%d", authzID)
 	dbAuthzPBIdChecks.IdInt = authzID
 	_, err = ra.DeactivateAuthorization(ctx, dbAuthzPBIdChecks)
 	test.AssertNotError(t, err, "Could not deactivate authorization")
 	deact, err = sa.GetAuthorization2(ctx, &sapb.AuthorizationID2{Id: authzID})
-	test.AssertNotError(t, err, "Could not get deactivated authorization with ID "+dbAuthzPBIdChecks.Id)
+	test.AssertNotError(t, err, "Could not get deactivated authorization by ID")
 	test.AssertEquals(t, deact.Status, string(core.StatusDeactivated))
 }
 
@@ -909,7 +905,7 @@ func TestDeactivateAuthorization_Pausing(t *testing.T) {
 	// The first deactivation of a pending authz should work and nothing should
 	// get paused.
 	_, err = ra.DeactivateAuthorization(ctx, &corepb.Authorization{
-		Id:             "1",
+		IdInt:          1,
 		RegistrationID: registration.Id,
 		Identifier:     identifier.NewDNS("example.com").ToProto(),
 		Status:         string(core.StatusPending),
@@ -919,7 +915,7 @@ func TestDeactivateAuthorization_Pausing(t *testing.T) {
 
 	// Deactivating a valid authz shouldn't increment any limits or pause anything.
 	_, err = ra.DeactivateAuthorization(ctx, &corepb.Authorization{
-		Id:             "2",
+		IdInt:          2,
 		RegistrationID: registration.Id,
 		Identifier:     identifier.NewDNS("example.com").ToProto(),
 		Status:         string(core.StatusValid),
@@ -930,7 +926,7 @@ func TestDeactivateAuthorization_Pausing(t *testing.T) {
 	// Deactivating a second pending authz should surpass the limit and result
 	// in a pause request.
 	_, err = ra.DeactivateAuthorization(ctx, &corepb.Authorization{
-		Id:             "3",
+		IdInt:          3,
 		RegistrationID: registration.Id,
 		Identifier:     identifier.NewDNS("example.com").ToProto(),
 		Status:         string(core.StatusPending),
diff --git a/sa/model.go b/sa/model.go
index 86fd0303347..ce19fcc5ad8 100644
--- a/sa/model.go
+++ b/sa/model.go
@@ -12,7 +12,6 @@ import (
 	"math"
 	"net/netip"
 	"net/url"
-	"strconv"
 	"strings"
 	"time"
 
@@ -625,9 +624,14 @@ func newAuthzReqToModel(authz *sapb.NewAuthzRequest, profile string) (*authzMode
 // Deprecated: this function is only used as part of test setup, do not
 // introduce any new uses in production code.
 func authzPBToModel(authz *corepb.Authorization) (*authzModel, error) {
+	if authz.IdInt == 0 {
+		return nil, errors.New("authorization is missing an ID value")
+	}
+
 	ident := identifier.FromProto(authz.Identifier)
 
 	am := &authzModel{
+		ID:              authz.IdInt,
 		IdentifierType:  identifierTypeToUint[ident.ToProto().Type],
 		IdentifierValue: ident.Value,
 		RegistrationID:  authz.RegistrationID,
@@ -638,17 +642,6 @@ func authzPBToModel(authz *corepb.Authorization) (*authzModel, error) {
 		profile := authz.CertificateProfileName
 		am.CertificateProfileName = &profile
 	}
-	if authz.IdInt != 0 {
-		am.ID = authz.IdInt
-	} else if authz.Id != "" {
-		id, err := strconv.Atoi(authz.Id)
-		if err != nil {
-			return nil, err
-		}
-		am.ID = int64(id)
-	} else {
-		return nil, errors.New("authorization is missing an ID value")
-	}
 	if hasMultipleNonPendingChallenges(authz.Challenges) {
 		return nil, errors.New("multiple challenges are non-pending")
 	}
@@ -782,7 +775,6 @@ func modelToAuthzPB(am authzModel) (*corepb.Authorization, error) {
 	}
 
 	pb := &corepb.Authorization{
-		Id:                     fmt.Sprintf("%d", am.ID),
 		IdInt:                  am.ID,
 		Status:                 string(uintToStatus[am.Status]),
 		Identifier:             identifier.ACMEIdentifier{Type: identType, Value: am.IdentifierValue}.ToProto(),
diff --git a/sa/model_test.go b/sa/model_test.go
index 50901aeaae1..1846adb8d2f 100644
--- a/sa/model_test.go
+++ b/sa/model_test.go
@@ -114,41 +114,9 @@ func TestAuthzModel(t *testing.T) {
 	test.AssertDeepEquals(t, authzPB.Challenges, authzPBOut.Challenges)
 	test.AssertEquals(t, authzPBOut.CertificateProfileName, authzPB.CertificateProfileName)
 
-	// Manipulate authzPB to test marshalling between corepb.Authorization and
-	// the SA authz model
-	// TODO(#8722): clean up these tests when authz IDs are int-only
-	authzPB = newTestAuthzPB(clk.Now())
-	authzPB.Id = ""
-	authzPB.IdInt = 0
-	_, err = authzPBToModel(authzPB)
-	test.AssertError(t, err, "authzPBToModel with empty Id and empty IdInt unexpectedly succeeded")
-
-	authzPB = newTestAuthzPB(clk.Now())
-	authzPB.Id = "1"
-	authzPB.IdInt = 0
-	model, err = authzPBToModel(authzPB)
-	test.AssertNotError(t, err, "authzPBToModel with a value for string Id and empty IdInt failed")
-	authzPBOut, err = modelToAuthzPB(*model)
-	test.AssertNotError(t, err, "modelToAuthzPB failed")
-	test.AssertEquals(t, fmt.Sprintf("%d", authzPBOut.IdInt), authzPBOut.Id)
-
-	authzPB = newTestAuthzPB(clk.Now())
-	authzPB.Id = ""
-	authzPB.IdInt = 1
-	model, err = authzPBToModel(authzPB)
-	test.AssertNotError(t, err, "authzPBToModel with empty Id and an int value for IdInt failed")
-	authzPBOut, err = modelToAuthzPB(*model)
-	test.AssertNotError(t, err, "modelToAuthzPB failed")
-	test.AssertEquals(t, fmt.Sprintf("%d", authzPBOut.IdInt), authzPBOut.Id)
-
-	authzPB = newTestAuthzPB(clk.Now())
-	authzPB.Id = "1"
-	authzPB.IdInt = 1
-	model, err = authzPBToModel(authzPB)
-	test.AssertNotError(t, err, "authzPBToModel with values for both string Id and int IdInt failed")
-	authzPBOut, err = modelToAuthzPB(*model)
-	test.AssertNotError(t, err, "modelToAuthzPB failed")
-	test.AssertEquals(t, fmt.Sprintf("%d", authzPBOut.IdInt), authzPBOut.Id)
+	// Complete authz -> model -> authz round-trip should keep everything intact
+	// aside from the hostname and port exceptions tested above
+	test.AssertDeepEquals(t, authzPB, authzPBOut)
 
 	authzPB = newTestAuthzPB(clk.Now())
 
diff --git a/sa/sa_test.go b/sa/sa_test.go
index 3c19459ce1a..3980125c009 100644
--- a/sa/sa_test.go
+++ b/sa/sa_test.go
@@ -21,7 +21,6 @@ import (
 	"os"
 	"reflect"
 	"slices"
-	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -2441,7 +2440,7 @@ func TestAuthzModelMapToPB(t *testing.T) {
 		if !ok {
 			t.Errorf("output had element for %q, an identifier not present in input", authzPB.Identifier.Value)
 		}
-		test.AssertEquals(t, authzPB.Id, fmt.Sprintf("%d", model.ID))
+		test.AssertEquals(t, authzPB.IdInt, model.ID)
 		test.AssertEquals(t, authzPB.Identifier.Type, string(uintToIdentifierType[model.IdentifierType]))
 		test.AssertEquals(t, authzPB.Identifier.Value, model.IdentifierValue)
 		test.AssertEquals(t, authzPB.RegistrationID, model.RegistrationID)
@@ -2531,8 +2530,8 @@ func TestGetOrderAuthorizations(t *testing.T) {
 		}
 		for _, a := range authzPBs.Authzs {
 			ident := identifier.ACMEIdentifier{Type: identifier.IdentifierType(a.Identifier.Type), Value: a.Identifier.Value}
-			if fmt.Sprintf("%d", identsToCheck[ident]) != a.Id {
-				t.Fatalf("incorrect identifier %q with id %s", a.Identifier.Value, a.Id)
+			if identsToCheck[ident] != a.IdInt {
+				t.Fatalf("incorrect identifier %q with id %d", a.Identifier.Value, a.IdInt)
 			}
 			test.AssertEquals(t, a.Expires.AsTime(), expires)
 			delete(identsToCheck, ident)
@@ -2689,11 +2688,7 @@ func TestGetValidAuthorizations2(t *testing.T) {
 
 			var gotIDs []int64
 			for _, authz := range got.Authzs {
-				id, err := strconv.Atoi(authz.Id)
-				if err != nil {
-					t.Fatalf("parsing authz id: %s", err)
-				}
-				gotIDs = append(gotIDs, int64(id))
+				gotIDs = append(gotIDs, authz.IdInt)
 			}
 
 			slices.Sort(gotIDs)
diff --git a/va/caa.go b/va/caa.go
index d126e72aa6e..ab091af6e52 100644
--- a/va/caa.go
+++ b/va/caa.go
@@ -7,7 +7,6 @@ import (
 	"log/slog"
 	"net/url"
 	"regexp"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -40,24 +39,10 @@ type caaParams struct {
 // implements the CAA portion of Multi-Perspective Issuance Corroboration as
 // defined in BRs Sections 3.2.2.9 and 5.4.1.
 func (va *ValidationAuthorityImpl) DoCAA(ctx context.Context, req *vapb.IsCAAValidRequest) (*vapb.IsCAAValidResponse, error) {
-	if core.IsAnyNilOrZero(req.Identifier, req.ValidationMethod, req.AccountURIID) {
+	if core.IsAnyNilOrZero(req.AuthzIDInt, req.Identifier, req.ValidationMethod, req.AccountURIID) {
 		return nil, berrors.InternalServerError("incomplete IsCAAValid request")
 	}
 
-	// TODO(#8722): remove this whole thing when Authz IDs are int64-only
-	var authzIDInt int64
-	if req.AuthzIDInt != 0 {
-		authzIDInt = req.AuthzIDInt
-	} else if req.AuthzID != "" {
-		parsed, err := strconv.ParseInt(req.AuthzID, 10, 64)
-		if err != nil {
-			return nil, berrors.MalformedError("Unable to parse Authz ID %q as integer: %v", req.AuthzID, err)
-		}
-		authzIDInt = parsed
-	} else {
-		return nil, berrors.MalformedError("No Authz ID value supplied in gRPC message")
-	}
-
 	ident := identifier.FromProto(req.Identifier)
 	if ident.Type != identifier.TypeDNS {
 		return nil, berrors.MalformedError("Identifier type for CAA check was not DNS")
@@ -76,7 +61,7 @@ func (va *ValidationAuthorityImpl) DoCAA(ctx context.Context, req *vapb.IsCAAVal
 	// Set the log attributes that we want to appear on all subsequent log lines
 	ctx = blog.ContextWith(ctx,
 		blog.Acct(req.AccountURIID),
-		blog.Authz(authzIDInt),
+		blog.Authz(req.AuthzIDInt),
 		blog.Idents(ident),
 		slog.String("method", string(challType)),
 	)
diff --git a/va/caa_test.go b/va/caa_test.go
index e80412f7fcb..29e4542384c 100644
--- a/va/caa_test.go
+++ b/va/caa_test.go
@@ -500,7 +500,7 @@ func TestCAALogging(t *testing.T) {
 				Identifier:       identifier.NewDNS(tc.Domain).ToProto(),
 				ValidationMethod: string(tc.ChallengeType),
 				AccountURIID:     tc.AccountURIID,
-				AuthzID:          "123",
+				AuthzIDInt:       123,
 			})
 
 			caaLogLines := mockLog.GetAllMatching(`Checked CAA records`)
@@ -526,10 +526,9 @@ func TestDoCAAErrMessage(t *testing.T) {
 	domain := "caa-timeout.com"
 	resp, err := va.DoCAA(ctx, &vapb.IsCAAValidRequest{
 		Identifier:       identifier.NewDNS(domain).ToProto(),
-		AuthzID:          "123",
+		AuthzIDInt:       123,
 		ValidationMethod: string(core.ChallengeTypeHTTP01),
 		AccountURIID:     12345,
-		AuthzIDInt:       678910,
 	})
 
 	// The lookup itself should not return an error
@@ -1051,10 +1050,9 @@ func TestMultiCAARechecking(t *testing.T) {
 
 			isValidRes, err := va.DoCAA(context.TODO(), &vapb.IsCAAValidRequest{
 				Identifier:       tc.ident.ToProto(),
-				AuthzID:          "123",
+				AuthzIDInt:       123,
 				ValidationMethod: string(core.ChallengeTypeDNS01),
 				AccountURIID:     1,
-				AuthzIDInt:       3,
 			})
 			test.AssertNotError(t, err, "Should not have errored, but did")
 
diff --git a/va/proto/va.pb.go b/va/proto/va.pb.go
index d7115d862e7..25e4e2bde34 100644
--- a/va/proto/va.pb.go
+++ b/va/proto/va.pb.go
@@ -29,7 +29,6 @@ type IsCAAValidRequest struct {
 	Identifier       *proto.Identifier `protobuf:"bytes,5,opt,name=identifier,proto3" json:"identifier,omitempty"`
 	ValidationMethod string            `protobuf:"bytes,2,opt,name=validationMethod,proto3" json:"validationMethod,omitempty"`
 	AccountURIID     int64             `protobuf:"varint,3,opt,name=accountURIID,proto3" json:"accountURIID,omitempty"`
-	AuthzID          string            `protobuf:"bytes,4,opt,name=authzID,proto3" json:"authzID,omitempty"`        // TODO(#8722): reserve
 	AuthzIDInt       int64             `protobuf:"varint,6,opt,name=authzIDInt,proto3" json:"authzIDInt,omitempty"` // TODO(#8722): rename
 	unknownFields    protoimpl.UnknownFields
 	sizeCache        protoimpl.SizeCache
@@ -86,13 +85,6 @@ func (x *IsCAAValidRequest) GetAccountURIID() int64 {
 	return 0
 }
 
-func (x *IsCAAValidRequest) GetAuthzID() string {
-	if x != nil {
-		return x.AuthzID
-	}
-	return ""
-}
-
 func (x *IsCAAValidRequest) GetAuthzIDInt() int64 {
 	if x != nil {
 		return x.AuthzIDInt
@@ -230,11 +222,9 @@ func (x *PerformValidationRequest) GetExpectedKeyAuthorization() string {
 }
 
 type AuthzMeta struct {
-	state protoimpl.MessageState `protogen:"open.v1"`
-	// Next unused field number: 4
-	Id            string `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"` // TODO(#8722): reserve
-	RegID         int64  `protobuf:"varint,2,opt,name=regID,proto3" json:"regID,omitempty"`
-	IdInt         int64  `protobuf:"varint,3,opt,name=idInt,proto3" json:"idInt,omitempty"` // TODO(#8722): rename
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	RegID         int64                  `protobuf:"varint,2,opt,name=regID,proto3" json:"regID,omitempty"`
+	IdInt         int64                  `protobuf:"varint,3,opt,name=idInt,proto3" json:"idInt,omitempty"` // TODO(#8722): rename
 	unknownFields protoimpl.UnknownFields
 	sizeCache     protoimpl.SizeCache
 }
@@ -269,13 +259,6 @@ func (*AuthzMeta) Descriptor() ([]byte, []int) {
 	return file_va_proto_rawDescGZIP(), []int{3}
 }
 
-func (x *AuthzMeta) GetId() string {
-	if x != nil {
-		return x.Id
-	}
-	return ""
-}
-
 func (x *AuthzMeta) GetRegID() int64 {
 	if x != nil {
 		return x.RegID
@@ -363,7 +346,7 @@ var File_va_proto protoreflect.FileDescriptor
 var file_va_proto_rawDesc = string([]byte{
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+	0x41, 0x41, 0x56, 0x61, 0x6c, 0x69, 0x64, 0x52, 0x65, 0x71, 0x75, 0x65, 0x73, 0x74, 0x1a, 0x16,
+	0x2e, 0x76, 0x61, 0x2e, 0x49, 0x73, 0x43, 0x41, 0x41, 0x56, 0x61, 0x6c, 0x69, 0x64, 0x52, 0x65,
+	0x73, 0x70, 0x6f, 0x6e, 0x73, 0x65, 0x22, 0x00, 0x42, 0x29, 0x5a, 0x27, 0x67, 0x69, 0x74, 0x68,
+	0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x6c, 0x65, 0x74, 0x73, 0x65, 0x6e, 0x63, 0x72, 0x79,
+	0x70, 0x74, 0x2f, 0x62, 0x6f, 0x75, 0x6c, 0x64, 0x65, 0x72, 0x2f, 0x76, 0x61, 0x2f, 0x70, 0x72,
+	0x6f, 0x74, 0x6f, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 })
 
 var (
diff --git a/va/proto/va.proto b/va/proto/va.proto
index 950d0b4a1f0..1148380520b 100644
--- a/va/proto/va.proto
+++ b/va/proto/va.proto
@@ -15,13 +15,12 @@ service CAA {
 
 message IsCAAValidRequest {
   // Next unused field number: 7
-  reserved 1; // Previously domain
+  reserved 1, 4; // Previously domain, authzID(string)
   // NOTE: For DNS identifiers, the value may be a wildcard domain name (e.g.
   // `*.example.com`).
   core.Identifier identifier = 5;
   string validationMethod = 2;
   int64 accountURIID = 3;
-  string authzID = 4;   // TODO(#8722): reserve
   int64 authzIDInt = 6; // TODO(#8722): rename
 }
 
@@ -43,7 +42,7 @@ message PerformValidationRequest {
 
 message AuthzMeta {
   // Next unused field number: 4
-  string id = 1;   // TODO(#8722): reserve
+  reserved 1; // Previously id(string)
   int64 regID = 2;
   int64 idInt = 3; // TODO(#8722): rename
 }
diff --git a/va/va.go b/va/va.go
index c986d17b9ce..f2c1a35402e 100644
--- a/va/va.go
+++ b/va/va.go
@@ -15,7 +15,6 @@ import (
 	"os"
 	"regexp"
 	"slices"
-	"strconv"
 	"strings"
 	"syscall"
 	"time"
@@ -734,25 +733,10 @@ func (va *ValidationAuthorityImpl) doRemoteOperation(ctx context.Context, op rem
 // implements the DCV portion of Multi-Perspective Issuance Corroboration as
 // defined in BRs Sections 3.2.2.9 and 5.4.1.
 func (va *ValidationAuthorityImpl) DoDCV(ctx context.Context, req *vapb.PerformValidationRequest) (*vapb.ValidationResult, error) {
-	if core.IsAnyNilOrZero(req.Identifier, req.Challenge, req.Authz, req.Authz.RegID, req.ExpectedKeyAuthorization) {
+	if core.IsAnyNilOrZero(req.Identifier, req.Challenge, req.Authz, req.Authz.IdInt, req.Authz.RegID, req.ExpectedKeyAuthorization) {
 		return nil, berrors.InternalServerError("Incomplete validation request")
 	}
 
-	// TODO(#8722): remove this and return req.Authz.Id to isAnyNilOrZero check
-	// above when Authz IDs are int64-only
-	var authzIDInt int64
-	if req.Authz.IdInt != 0 {
-		authzIDInt = req.Authz.IdInt
-	} else if req.Authz.Id != "" {
-		parsed, err := strconv.ParseInt(req.Authz.Id, 10, 64)
-		if err != nil {
-			return nil, berrors.MalformedError("Unable to parse Authz ID %q as integer: %v", req.Authz.Id, err)
-		}
-		authzIDInt = parsed
-	} else {
-		return nil, berrors.InternalServerError("incomplete validation request")
-	}
-
 	ident := identifier.FromProto(req.Identifier)
 
 	chall, err := bgrpc.PBToChallenge(req.Challenge)
@@ -768,7 +752,7 @@ func (va *ValidationAuthorityImpl) DoDCV(ctx context.Context, req *vapb.PerformV
 	// Set the log attributes that we want to appear on all subsequent log lines
 	ctx = blog.ContextWith(ctx,
 		blog.Acct(req.Authz.RegID),
-		blog.Authz(authzIDInt),
+		blog.Authz(req.Authz.IdInt),
 		blog.Idents(ident),
 		slog.String("method", string(chall.Type)),
 		slog.String("token", chall.Token),
diff --git a/va/va_test.go b/va/va_test.go
index eb4d425077c..f0152eaac9b 100644
--- a/va/va_test.go
+++ b/va/va_test.go
@@ -339,43 +339,6 @@ func TestNewValidationAuthorityImplWithDuplicateRemotes(t *testing.T) {
 	test.AssertContains(t, err.Error(), "duplicate remote VA perspective \"dadaist\"")
 }
 
-// TODO(#8722): Remove this whole function when Authz IDs are int-only
-func TestPerformValidationWithAuthzIDMatrix(t *testing.T) {
-	t.Parallel()
-
-	va, _ := setup(nil, "", nil, &txtFakeDNS{})
-
-	// create a challenge with well known token
-	req := createValidationRequest(identifier.NewDNS("good-dns01.com"), core.ChallengeTypeDNS01)
-	// manipulate Authz ID for this validation attempt
-	req.Authz.Id = ""
-	req.Authz.IdInt = 0
-	_, err := va.DoDCV(context.Background(), req)
-	test.AssertError(t, err, "expected error upon validation request with empty authz ID fields")
-
-	// repeat
-	req = createValidationRequest(identifier.NewDNS("good-dns01.com"), core.ChallengeTypeDNS01)
-	req.Authz.Id = "1"
-	req.Authz.IdInt = 0
-	res, err := va.DoDCV(context.Background(), req)
-	test.AssertNotError(t, err, "domain validation request failed")
-	test.Assert(t, res.Problem == nil, fmt.Sprintf("validation failed: %#v", res.Problem))
-
-	req = createValidationRequest(identifier.NewDNS("good-dns01.com"), core.ChallengeTypeDNS01)
-	req.Authz.Id = ""
-	req.Authz.IdInt = 1
-	res, err = va.DoDCV(context.Background(), req)
-	test.AssertNotError(t, err, "domain validation request failed")
-	test.Assert(t, res.Problem == nil, fmt.Sprintf("validation failed: %#v", res.Problem))
-
-	req = createValidationRequest(identifier.NewDNS("good-dns01.com"), core.ChallengeTypeDNS01)
-	req.Authz.Id = "1"
-	req.Authz.IdInt = 1
-	res, err = va.DoDCV(context.Background(), req)
-	test.AssertNotError(t, err, "domain validation request failed")
-	test.Assert(t, res.Problem == nil, fmt.Sprintf("validation failed: %#v", res.Problem))
-}
-
 func TestPerformValidationWithMismatchedRemoteVAPerspectives(t *testing.T) {
 	t.Parallel()
 
diff --git a/wfe2/wfe.go b/wfe2/wfe.go
index 0792763af88..74206b47a14 100644
--- a/wfe2/wfe.go
+++ b/wfe2/wfe.go
@@ -1195,7 +1195,7 @@ func (wfe *WebFrontEndImpl) Challenge(
 	}
 
 	// Ensure gRPC response is complete.
-	if core.IsAnyNilOrZero(authzPB.Id, authzPB.Identifier, authzPB.Status, authzPB.Expires) {
+	if core.IsAnyNilOrZero(authzPB.IdInt, authzPB.Identifier, authzPB.Status, authzPB.Expires) {
 		wfe.sendError(response, logEvent, probs.ServerInternal("Problem getting authorization"), errIncompleteGRPCResponse)
 		return
 	}
@@ -1394,7 +1394,7 @@ func (wfe *WebFrontEndImpl) postChallenge(
 			Authz:          authzPB,
 			ChallengeIndex: int64(challengeIndex),
 		})
-		if err != nil || core.IsAnyNilOrZero(authzPB.Id, authzPB.Identifier, authzPB.Status, authzPB.Expires) {
+		if err != nil || core.IsAnyNilOrZero(authzPB.IdInt, authzPB.Identifier, authzPB.Status, authzPB.Expires) {
 			wfe.sendError(response, logEvent, web.ProblemDetailsForError(err, "Unable to update challenge"), err)
 			return
 		}
@@ -1615,7 +1615,7 @@ func (wfe *WebFrontEndImpl) Authorization(
 	ident := identifier.FromProto(authzPB.Identifier)
 
 	// Ensure gRPC response is complete.
-	if core.IsAnyNilOrZero(authzPB.Id, ident, authzPB.Status, authzPB.Expires) {
+	if core.IsAnyNilOrZero(authzPB.IdInt, ident, authzPB.Status, authzPB.Expires) {
 		wfe.sendError(response, logEvent, probs.ServerInternal("Problem getting authorization"), errIncompleteGRPCResponse)
 		return
 	}
diff --git a/wfe2/wfe_test.go b/wfe2/wfe_test.go
index 38501994122..bd2033fbddd 100644
--- a/wfe2/wfe_test.go
+++ b/wfe2/wfe_test.go
@@ -230,7 +230,7 @@ func (ra *MockRegistrationAuthority) GetAuthorization(_ context.Context, in *rap
 	switch in.Id {
 	case 1: // Return a valid authorization with a single valid challenge.
 		return &corepb.Authorization{
-			Id:             "1",
+			IdInt:          1,
 			RegistrationID: 1,
 			Identifier:     identifier.NewDNS("not-an-example.com").ToProto(),
 			Status:         string(core.StatusValid),
@@ -241,7 +241,7 @@ func (ra *MockRegistrationAuthority) GetAuthorization(_ context.Context, in *rap
 		}, nil
 	case 2: // Return a pending authorization with three pending challenges.
 		return &corepb.Authorization{
-			Id:             "2",
+			IdInt:          2,
 			RegistrationID: 1,
 			Identifier:     identifier.NewDNS("not-an-example.com").ToProto(),
 			Status:         string(core.StatusPending),
@@ -254,7 +254,7 @@ func (ra *MockRegistrationAuthority) GetAuthorization(_ context.Context, in *rap
 		}, nil
 	case 3: // Return an expired authorization with three pending (but expired) challenges.
 		return &corepb.Authorization{
-			Id:             "3",
+			IdInt:          3,
 			RegistrationID: 1,
 			Identifier:     identifier.NewDNS("not-an-example.com").ToProto(),
 			Status:         string(core.StatusPending),
@@ -269,7 +269,7 @@ func (ra *MockRegistrationAuthority) GetAuthorization(_ context.Context, in *rap
 		return nil, fmt.Errorf("unspecified error")
 	case 5: // Return a pending authorization as above, but associated with RegID 2.
 		return &corepb.Authorization{
-			Id:             "5",
+			IdInt:          5,
 			RegistrationID: 2,
 			Identifier:     identifier.NewDNS("not-an-example.com").ToProto(),
 			Status:         string(core.StatusPending),
@@ -1871,7 +1871,7 @@ type RAWithFailedChallenge struct {
 
 func (ra *RAWithFailedChallenge) GetAuthorization(ctx context.Context, id *rapb.GetAuthorizationRequest, _ ...grpc.CallOption) (*corepb.Authorization, error) {
 	return &corepb.Authorization{
-		Id:             "6",
+		IdInt:          6,
 		RegistrationID: 1,
 		Identifier:     identifier.NewDNS("not-an-example.com").ToProto(),
 		Status:         string(core.StatusInvalid),