diff --git a/Sources/ContainerBuild/Builder.swift b/Sources/ContainerBuild/Builder.swift
index 796ba6b3..506f0473 100644
--- a/Sources/ContainerBuild/Builder.swift
+++ b/Sources/ContainerBuild/Builder.swift
@@ -29,6 +29,22 @@ import NIOPosix
 public struct Builder: Sendable {
     public static let builderContainerId = "buildkit"
 
+    /// The `:authority` sent with every request to the builder shim.
+    ///
+    /// The shim connection is an already-connected socket, so grpc-swift has no target
+    /// address to derive an authority from and would send none. gRPC-Go v1.82.2, v1.83.2
+    /// and v1.84.0 or later reject requests without an `:authority` (grpc-go #9365), which
+    /// leaves the builder unable to answer even the `Info` probe. The shim does not route
+    /// on the authority, so any valid host name works.
+    static let grpcAuthority = Builder.builderContainerId
+
+    /// The transport configuration for the connection to the builder shim.
+    static func transportConfig() -> HTTP2ClientTransport.WrappedChannel.Config {
+        var config = HTTP2ClientTransport.WrappedChannel.Config.defaults
+        config.http2.authority = grpcAuthority
+        return config
+    }
+
     let client: Com_Apple_Container_Build_V1_Builder.Client<HTTP2ClientTransport.WrappedChannel>
     let grpcClient: GRPCClient<HTTP2ClientTransport.WrappedChannel>
     let group: EventLoopGroup
@@ -41,7 +57,7 @@ public struct Builder: Sendable {
         try socket.setRecvBufSize(2 << 20)
 
         let transport = try await HTTP2ClientTransport.WrappedChannel.wrapping(
-            config: .defaults,
+            config: Builder.transportConfig(),
             serviceConfig: .init()
         ) { configure in
             try await withCheckedThrowingContinuation { continuation in
diff --git a/Tests/ContainerBuildTests/BuilderTransportConfigTests.swift b/Tests/ContainerBuildTests/BuilderTransportConfigTests.swift
new file mode 100644
index 00000000..437057b0
--- /dev/null
+++ b/Tests/ContainerBuildTests/BuilderTransportConfigTests.swift
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Testing
+
+@testable import ContainerBuild
+
+@Suite struct BuilderTransportConfigTests {
+    /// The builder connection wraps an already-connected socket, so grpc-swift derives no
+    /// `:authority` for it. gRPC-Go v1.84.0 and later reject requests without one, so the
+    /// configuration must set it explicitly.
+    @Test func transportConfigSetsAuthority() {
+        let authority = Builder.transportConfig().http2.authority
+        #expect(authority == Builder.grpcAuthority)
+        #expect(authority?.isEmpty == false)
+    }
+}