diff --git a/internal/sidekick/swift/generate_deprecated_message_test.go b/internal/sidekick/swift/generate_deprecated_message_test.go
index bf674c06a0..5148677f3b 100644
--- a/internal/sidekick/swift/generate_deprecated_message_test.go
+++ b/internal/sidekick/swift/generate_deprecated_message_test.go
@@ -162,8 +162,8 @@ func TestGenerateMessage_Diagnose(t *testing.T) {
 			}
 			contentStr := string(content)
 
-			checkDiagnose(t, contentStr, "  ", "public init(from decoder: Decoder) throws {", test.want)
-			checkDiagnose(t, contentStr, "  ", "public func encode(to encoder: Encoder) throws {", test.want)
+			checkDiagnose(t, contentStr, "  ", "public init(from decoder: any Decoder) throws {", test.want)
+			checkDiagnose(t, contentStr, "  ", "public func encode(to encoder: any Encoder) throws {", test.want)
 		})
 	}
 }
diff --git a/internal/sidekick/swift/generate_deprecated_method_test.go b/internal/sidekick/swift/generate_deprecated_method_test.go
index fd3f1de15c..3137a0c37a 100644
--- a/internal/sidekick/swift/generate_deprecated_method_test.go
+++ b/internal/sidekick/swift/generate_deprecated_method_test.go
@@ -124,8 +124,8 @@ func TestGenerateService_DeprecatedMethods(t *testing.T) {
 				},
 				{
 					start: "  @available(*, deprecated)\n  public func paginationMethodByItems(\n    request: Request",
-					end:   "-> some AsyncSequence<Item, Swift.Error> & Sendable",
-					want:  "  @available(*, deprecated)\n  public func paginationMethodByItems(\n    request: Request, options: GoogleGax.RequestOptions\n) -> some AsyncSequence<Item, Swift.Error> & Sendable",
+					end:   "-> some AsyncSequence<Item, any Swift.Error> & Sendable",
+					want:  "  @available(*, deprecated)\n  public func paginationMethodByItems(\n    request: Request, options: GoogleGax.RequestOptions\n) -> some AsyncSequence<Item, any Swift.Error> & Sendable",
 				},
 			},
 		},
diff --git a/internal/sidekick/swift/generate_enum_swift_test.go b/internal/sidekick/swift/generate_enum_swift_test.go
index f67c5e7ab8..640aa23691 100644
--- a/internal/sidekick/swift/generate_enum_swift_test.go
+++ b/internal/sidekick/swift/generate_enum_swift_test.go
@@ -103,8 +103,8 @@ func TestGenerateEnum_UniqueNumbers(t *testing.T) {
 		t.Errorf("mismatch (-want +got):\n%s", diff)
 	}
 
-	gotEncode := extractBlock(t, string(contentsB), "public func encode(to encoder: Encoder) throws {", "\n  }")
-	wantEncode := `public func encode(to encoder: Encoder) throws {
+	gotEncode := extractBlock(t, string(contentsB), "public func encode(to encoder: any Encoder) throws {", "\n  }")
+	wantEncode := `public func encode(to encoder: any Encoder) throws {
     var container = encoder.singleValueContainer()
     switch self {
     case .test: return try container.encode("KIND_TEST")
@@ -321,18 +321,18 @@ func TestGenerateEnum_Discovery(t *testing.T) {
 	}
 
 	// Verify decoder
-	wantDecoder := `  public init(from decoder: Decoder) throws {
+	wantDecoder := `  public init(from decoder: any Decoder) throws {
     let container = try decoder.singleValueContainer()
     let s = try container.decode(Swift.String.self)
     self.init(stringValue: s)
   }`
-	gotDecoder := extractBlock(t, contentStr, "  public init(from decoder: Decoder) throws {", "\n  }")
+	gotDecoder := extractBlock(t, contentStr, "  public init(from decoder: any Decoder) throws {", "\n  }")
 	if diff := cmp.Diff(wantDecoder, gotDecoder); diff != "" {
 		t.Errorf("mismatch (-want +got):\n%s", diff)
 	}
 
 	// Verify encoder
-	wantEncoder := `  public func encode(to encoder: Encoder) throws {
+	wantEncoder := `  public func encode(to encoder: any Encoder) throws {
     var container = encoder.singleValueContainer()
     switch self {
     case .done: return try container.encode("DONE")
@@ -341,7 +341,7 @@ func TestGenerateEnum_Discovery(t *testing.T) {
     case .unknownStringValue(let v): return try container.encode(v)
     }
   }`
-	gotEncoder := extractBlock(t, contentStr, "  public func encode(to encoder: Encoder) throws {", "\n  }")
+	gotEncoder := extractBlock(t, contentStr, "  public func encode(to encoder: any Encoder) throws {", "\n  }")
 	if diff := cmp.Diff(wantEncoder, gotEncoder); diff != "" {
 		t.Errorf("mismatch (-want +got):\n%s", diff)
 	}
diff --git a/internal/sidekick/swift/generate_field_swift_test.go b/internal/sidekick/swift/generate_field_swift_test.go
index 2c6c9083d4..a1d8dfda81 100644
--- a/internal/sidekick/swift/generate_field_swift_test.go
+++ b/internal/sidekick/swift/generate_field_swift_test.go
@@ -63,9 +63,9 @@ func TestGenerateField_InitFromDecoder(t *testing.T) {
 	}
 	contentStr := string(content)
 
-	// Verify init(from decoder: Decoder) content
-	gotBlock := extractBlock(t, contentStr, "  public init(from decoder: Decoder) throws {", "\n  }")
-	wantBlock := `  public init(from decoder: Decoder) throws {
+	// Verify init(from decoder: any Decoder) content
+	gotBlock := extractBlock(t, contentStr, "  public init(from decoder: any Decoder) throws {", "\n  }")
+	wantBlock := `  public init(from decoder: any Decoder) throws {
     let container = try decoder.container(keyedBy: CodingKeys.self)
     if let value = try container.decodeIfPresent(Swift.String.self, forKey: .normalField) {
       self.normalField = value
diff --git a/internal/sidekick/swift/generate_method_signatures_test.go b/internal/sidekick/swift/generate_method_signatures_test.go
index 92909468ad..e3e2c02b18 100644
--- a/internal/sidekick/swift/generate_method_signatures_test.go
+++ b/internal/sidekick/swift/generate_method_signatures_test.go
@@ -72,7 +72,7 @@ func TestGenerateService_MethodSignatures(t *testing.T) {
 					want: `public func paginationMethodByItems(
   name: Swift.String,
   optionalField: Swift.String?,
-) -> some AsyncSequence<Item, Swift.Error> & Sendable
+) -> some AsyncSequence<Item, any Swift.Error> & Sendable
  {
     let request = Request().with {
       $0.name = name
@@ -86,7 +86,7 @@ func TestGenerateService_MethodSignatures(t *testing.T) {
 					want: `public func paginationMethodByItems(
   name: Swift.String,
   normalField: Swift.String,
-) -> some AsyncSequence<Item, Swift.Error> & Sendable
+) -> some AsyncSequence<Item, any Swift.Error> & Sendable
  {
     let request = Request().with {
       $0.name = name
diff --git a/internal/sidekick/swift/generate_oneof_test.go b/internal/sidekick/swift/generate_oneof_test.go
index e4fe3e9486..2ea9fb997b 100644
--- a/internal/sidekick/swift/generate_oneof_test.go
+++ b/internal/sidekick/swift/generate_oneof_test.go
@@ -140,7 +140,7 @@ func TestGenerateOneOf(t *testing.T) {
     ]
   }
 
-  public init(from decoder: Decoder) throws {
+  public init(from decoder: any Decoder) throws {
     let container = try decoder.container(keyedBy: CodingKeys.self)
     if let value = try container.decodeIfPresent(Swift.Int32.self, forKey: .regularInt32) {
       self.regularInt32 = value
@@ -169,7 +169,7 @@ func TestGenerateOneOf(t *testing.T) {
     }
   }
 
-  public func encode(to encoder: Encoder) throws {
+  public func encode(to encoder: any Encoder) throws {
     var container = encoder.container(keyedBy: CodingKeys.self)
     try container.encode(self.regularInt32, forKey: .regularInt32)
     try container.encode(self.regularString, forKey: .regularString)
@@ -299,7 +299,7 @@ func TestGenerateOneOfWithKeyword(t *testing.T) {
     ]
   }
 
-  public init(from decoder: Decoder) throws {
+  public init(from decoder: any Decoder) throws {
     let container = try decoder.container(keyedBy: CodingKeys.self)
 
     var ` + "`in`" + `: InOneOf? = nil
@@ -325,7 +325,7 @@ func TestGenerateOneOfWithKeyword(t *testing.T) {
     }
   }
 
-  public func encode(to encoder: Encoder) throws {
+  public func encode(to encoder: any Encoder) throws {
     var container = encoder.container(keyedBy: CodingKeys.self)
 
     if let choice = self.` + "`in`" + ` {
diff --git a/internal/sidekick/swift/generate_package_swift_test.go b/internal/sidekick/swift/generate_package_swift_test.go
index d43220b86b..832f517e9c 100644
--- a/internal/sidekick/swift/generate_package_swift_test.go
+++ b/internal/sidekick/swift/generate_package_swift_test.go
@@ -98,6 +98,7 @@ func TestGeneratePackageSwift_WithDependencies(t *testing.T) {
 
 	gotSwiftSettings := extractBlock(t, contentStr, "      swiftSettings: [", "\n      ]")
 	wantSwiftSettings := `      swiftSettings: [
+        .enableUpcomingFeature("ExistentialAny"),
         .enableUpcomingFeature("InternalImportsByDefault"),
         .enableUpcomingFeature("MemberImportVisibility"),
         .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
diff --git a/internal/sidekick/swift/generate_pagination_swift_test.go b/internal/sidekick/swift/generate_pagination_swift_test.go
index 6e83c5b785..697686ac00 100644
--- a/internal/sidekick/swift/generate_pagination_swift_test.go
+++ b/internal/sidekick/swift/generate_pagination_swift_test.go
@@ -129,7 +129,7 @@ func verifyGeneratedMapService(t *testing.T, outDir string) {
     request: `, "\n  }")
 	wantMethodOverload := `  public func listSecretsByItems(
     request: ListSecretsRequest, options: GoogleGax.RequestOptions
-) -> some AsyncSequence<(Swift.String, Secret), Swift.Error> & Sendable
+) -> some AsyncSequence<(Swift.String, Secret), any Swift.Error> & Sendable
  {
     let listRpc = { @Sendable (token: Swift.String) async throws -> GoogleCloudSecretmanagerV1.ListSecretsResponse in
       var request = request
diff --git a/internal/sidekick/swift/generate_service_swift_test.go b/internal/sidekick/swift/generate_service_swift_test.go
index 7da452999a..11fc3b83d7 100644
--- a/internal/sidekick/swift/generate_service_swift_test.go
+++ b/internal/sidekick/swift/generate_service_swift_test.go
@@ -627,7 +627,7 @@ func verifyGeneratedService(t *testing.T, outDir string) {
     request: ListSecretsRequest, options: `, "\n  }")
 	wantMethodOverload := `  public func listSecretsByItems(
     request: ListSecretsRequest, options: GoogleGax.RequestOptions
-) -> some AsyncSequence<Secret, Swift.Error> & Sendable
+) -> some AsyncSequence<Secret, any Swift.Error> & Sendable
  {
     let listRpc = { @Sendable (token: Swift.String) async throws -> GoogleCloudSecretmanagerV1.ListSecretsResponse in
       var request = request
@@ -783,8 +783,8 @@ func TestGenerateService_LRO(t *testing.T) {
 
 	wantContains := []string{
 		"public import GoogleCloudLongrunningV1",
-		"let pollingErrorPolicy: GoogleGax.PollingErrorPolicy",
-		"let pollingBackoffPolicy: GoogleGax.PollingBackoffPolicy",
+		"let pollingErrorPolicy: any GoogleGax.PollingErrorPolicy",
+		"let pollingBackoffPolicy: any GoogleGax.PollingBackoffPolicy",
 		"public func createWorkflowPollingUntilDone(request: CreateWorkflowRequest) async throws -> Workflow",
 		"let extractStatus = { @Sendable (op: GoogleCloudLongrunningV1.Operation) throws -> GoogleGax._PollableOperationImpl<Workflow>.State in",
 		"let poll = { @Sendable () async throws -> GoogleGax._PollableOperationImpl<Workflow>.State in",
diff --git a/internal/sidekick/swift/generate_storage_test.go b/internal/sidekick/swift/generate_storage_test.go
index 249c43409c..21d88cc0ec 100644
--- a/internal/sidekick/swift/generate_storage_test.go
+++ b/internal/sidekick/swift/generate_storage_test.go
@@ -202,7 +202,7 @@ func TestGenerateStorage_MultiModel(t *testing.T) {
 		t.Errorf("StorageControlProtocol.swift missing unified methods:\n%s", protocolStr)
 	}
 	if !strings.Contains(protocolStr, "request: ListBucketsRequest, options: GoogleGax.RequestOptions") ||
-		!strings.Contains(protocolStr, "some AsyncSequence<Bucket, Swift.Error> & Sendable") ||
+		!strings.Contains(protocolStr, "some AsyncSequence<Bucket, any Swift.Error> & Sendable") ||
 		!strings.Contains(protocolStr, "return GoogleGax.PaginatedResponseSequence(listRpc: listRpc, initialPageToken: request.pageToken)") {
 		t.Errorf("StorageControlProtocol.swift missing paginated helper method:\n%s", protocolStr)
 	}
diff --git a/internal/sidekick/swift/templates/common/enum.gotmpl b/internal/sidekick/swift/templates/common/enum.gotmpl
index 1e78cc12aa..a47110cd6c 100644
--- a/internal/sidekick/swift/templates/common/enum.gotmpl
+++ b/internal/sidekick/swift/templates/common/enum.gotmpl
@@ -119,7 +119,7 @@ public enum {{ .Codec.Name }}: Codable, Equatable, Hashable, Sendable {
     }
   }
 
-  public init(from decoder: Decoder) throws {
+  public init(from decoder: any Decoder) throws {
     let container = try decoder.singleValueContainer()
     if let v = try? container.decode(Int.self) {
       self.init(intValue: v)
@@ -137,7 +137,7 @@ public enum {{ .Codec.Name }}: Codable, Equatable, Hashable, Sendable {
       in: container, debugDescription: "Expected enum value, must be integer or string.")
   }
 
-  public func encode(to encoder: Encoder) throws {
+  public func encode(to encoder: any Encoder) throws {
     var container = encoder.singleValueContainer()
     switch self {
     {{- range .UniqueNumberValues }}
diff --git a/internal/sidekick/swift/templates/common/message.gotmpl b/internal/sidekick/swift/templates/common/message.gotmpl
index cdcab8ce69..a498e1e876 100644
--- a/internal/sidekick/swift/templates/common/message.gotmpl
+++ b/internal/sidekick/swift/templates/common/message.gotmpl
@@ -101,7 +101,7 @@ public struct {{ .Codec.Name }}: Codable, Equatable, {{ .Codec.Model.WktPackage
   {{- if .Codec.DiagnoseCodable }}
   {{- include "templates/common/diagnose" . | indent 2 }}
   {{- end }}
-  public init(from decoder: Decoder) throws {
+  public init(from decoder: any Decoder) throws {
     let container = try decoder.container(keyedBy: CodingKeys.self)
     {{- range .Fields }}
     {{- if not .IsOneOf }}
@@ -137,7 +137,7 @@ public struct {{ .Codec.Name }}: Codable, Equatable, {{ .Codec.Model.WktPackage
   {{- if .Codec.DiagnoseCodable }}
   {{- include "templates/common/diagnose" . | indent 2 }}
   {{- end }}
-  public func encode(to encoder: Encoder) throws {
+  public func encode(to encoder: any Encoder) throws {
     var container = encoder.container(keyedBy: CodingKeys.self)
     {{- range .Fields }}
     {{- if not .IsOneOf }}
diff --git a/internal/sidekick/swift/templates/common/method_signature/pagination.gotmpl b/internal/sidekick/swift/templates/common/method_signature/pagination.gotmpl
index 310b42047f..cedf8215d9 100644
--- a/internal/sidekick/swift/templates/common/method_signature/pagination.gotmpl
+++ b/internal/sidekick/swift/templates/common/method_signature/pagination.gotmpl
@@ -15,4 +15,4 @@ limitations under the License.
 */ -}}
 {{ .Codec.Name }}ByItems(
   request: {{ .InputType.Codec.ParameterTypeName }}
-) -> some AsyncSequence<{{ .OutputType.Codec.PageableItemType }}, Swift.Error> & Sendable
+) -> some AsyncSequence<{{ .OutputType.Codec.PageableItemType }}, any Swift.Error> & Sendable
diff --git a/internal/sidekick/swift/templates/common/method_signature/pagination_options.gotmpl b/internal/sidekick/swift/templates/common/method_signature/pagination_options.gotmpl
index 49e9e4bec3..7197a99b6a 100644
--- a/internal/sidekick/swift/templates/common/method_signature/pagination_options.gotmpl
+++ b/internal/sidekick/swift/templates/common/method_signature/pagination_options.gotmpl
@@ -15,4 +15,4 @@ limitations under the License.
 */ -}}
 {{ .Codec.Name }}ByItems(
     request: {{ .InputType.Codec.ParameterTypeName }}, options: GoogleGax.RequestOptions
-) -> some AsyncSequence<{{ .OutputType.Codec.PageableItemType }}, Swift.Error> & Sendable
+) -> some AsyncSequence<{{ .OutputType.Codec.PageableItemType }}, any Swift.Error> & Sendable
diff --git a/internal/sidekick/swift/templates/common/method_signature/pagination_overload.gotmpl b/internal/sidekick/swift/templates/common/method_signature/pagination_overload.gotmpl
index 9faf86a24d..926400b49a 100644
--- a/internal/sidekick/swift/templates/common/method_signature/pagination_overload.gotmpl
+++ b/internal/sidekick/swift/templates/common/method_signature/pagination_overload.gotmpl
@@ -17,4 +17,4 @@ limitations under the License.
   {{- range .Fields }}
   {{ .Codec.Name }}: {{ .Codec.FieldType }},
   {{- end }}
-) -> some AsyncSequence<{{ .Method.OutputType.Codec.PageableItemType }}, Swift.Error> & Sendable
+) -> some AsyncSequence<{{ .Method.OutputType.Codec.PageableItemType }}, any Swift.Error> & Sendable
diff --git a/internal/sidekick/swift/templates/common/service.swift.gotmpl b/internal/sidekick/swift/templates/common/service.swift.gotmpl
index 6fa819704e..33ece6165d 100644
--- a/internal/sidekick/swift/templates/common/service.swift.gotmpl
+++ b/internal/sidekick/swift/templates/common/service.swift.gotmpl
@@ -48,8 +48,8 @@ import Foundation
 public final class {{ .Codec.ClientName }}: Clients.{{ .Codec.StubPrefix }}Protocol, Sendable {
   let inner: any Clients.{{ .Codec.StubPrefix }}Stub
   {{- if .Codec.HasLROs }}
-  let pollingErrorPolicy: GoogleGax.PollingErrorPolicy
-  let pollingBackoffPolicy: GoogleGax.PollingBackoffPolicy
+  let pollingErrorPolicy: any GoogleGax.PollingErrorPolicy
+  let pollingBackoffPolicy: any GoogleGax.PollingBackoffPolicy
   {{- end }}
 
   /// Creates a new `{{ .Codec.ClientName }}` instance.
diff --git a/internal/sidekick/swift/templates/common/string_enum.gotmpl b/internal/sidekick/swift/templates/common/string_enum.gotmpl
index ce53993475..2c5740b316 100644
--- a/internal/sidekick/swift/templates/common/string_enum.gotmpl
+++ b/internal/sidekick/swift/templates/common/string_enum.gotmpl
@@ -72,13 +72,13 @@ public enum {{ .Codec.Name }}: Codable, Equatable, Hashable, Sendable {
     }
   }
 
-  public init(from decoder: Decoder) throws {
+  public init(from decoder: any Decoder) throws {
     let container = try decoder.singleValueContainer()
     let s = try container.decode(Swift.String.self)
     self.init(stringValue: s)
   }
 
-  public func encode(to encoder: Encoder) throws {
+  public func encode(to encoder: any Encoder) throws {
     var container = encoder.singleValueContainer()
     switch self {
     {{- range .Values }}
diff --git a/internal/sidekick/swift/templates/package/Package.swift.gotmpl b/internal/sidekick/swift/templates/package/Package.swift.gotmpl
index 08afe33dd2..ccd5aff78e 100644
--- a/internal/sidekick/swift/templates/package/Package.swift.gotmpl
+++ b/internal/sidekick/swift/templates/package/Package.swift.gotmpl
@@ -81,6 +81,7 @@ let package = Package(
       ],
       {{- end }}
       swiftSettings: [
+        .enableUpcomingFeature("ExistentialAny"),
         .enableUpcomingFeature("InternalImportsByDefault"),
         .enableUpcomingFeature("MemberImportVisibility"),
         .enableUpcomingFeature("NonisolatedNonsendingByDefault"),