diff --git a/Sources/Containerization/Image/ImageStore/ImageStore+Export.swift b/Sources/Containerization/Image/ImageStore/ImageStore+Export.swift
index 24af8bd2..916285f6 100644
--- a/Sources/Containerization/Image/ImageStore/ImageStore+Export.swift
+++ b/Sources/Containerization/Image/ImageStore/ImageStore+Export.swift
@@ -78,8 +78,11 @@ extension ImageStore {
             // Lastly, we need to construct and push a new index, since we may
             // have pushed content only for specific platforms.
             let digest = SHA256.hash(data: localIndexData)
+            // The descriptor's mediaType becomes the HTTP Content-Type in
+            // RegistryClient.push and must match the mediaType field inside
+            // localIndexData. Registries reject mismatches with MANIFEST_INVALID.
             let descriptor = Descriptor(
-                mediaType: MediaTypes.index,
+                mediaType: index.mediaType,
                 digest: digest.digestString,
                 size: Int64(localIndexData.count))
             let stream = ReadStream(data: localIndexData)
diff --git a/Tests/ContainerizationTests/ImageTests/ExportOperationTests.swift b/Tests/ContainerizationTests/ImageTests/ExportOperationTests.swift
new file mode 100644
index 00000000..9120e578
--- /dev/null
+++ b/Tests/ContainerizationTests/ImageTests/ExportOperationTests.swift
@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the Containerization 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 ContainerizationExtras
+import ContainerizationOCI
+import Crypto
+import Foundation
+import NIO
+import Testing
+
+@testable import Containerization
+
+@Suite
+final class ExportOperationTests {
+    @Test(arguments: [MediaTypes.dockerManifestList, MediaTypes.index])
+    func testIndexPushMediaTypeMatchesBody(_ sourceMediaType: String) async throws {
+        let dir = FileManager.default.uniqueTemporaryDirectory(create: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+
+        let cs = try LocalContentStore(path: dir)
+
+        // Opaque child mediaType so ExportOperation's recursion stops here
+        // and we don't have to seed config/layer blobs.
+        let opaqueType = "application/vnd.test.opaque.v1+json"
+        let childData = Data("child-amd64".utf8)
+        let childDigest = SHA256.hash(data: childData).digestString
+        let childDesc = Descriptor(
+            mediaType: opaqueType,
+            digest: childDigest,
+            size: Int64(childData.count),
+            platform: Platform(arch: "amd64", os: "linux"))
+
+        let index = Index(mediaType: sourceMediaType, manifests: [childDesc])
+        let indexData = try JSONEncoder().encode(index)
+        let indexDigest = SHA256.hash(data: indexData).digestString
+
+        try await cs.ingest { ingestDir in
+            for (digest, data) in [(childDigest, childData), (indexDigest, indexData)] {
+                let path = ingestDir.appendingPathComponent(digest.trimmingDigestPrefix)
+                try data.write(to: path)
+            }
+        }
+
+        let indexDesc = Descriptor(
+            mediaType: sourceMediaType,
+            digest: indexDigest,
+            size: Int64(indexData.count))
+
+        let capture = CapturingContentClient()
+        let op = ImageStore.ExportOperation(
+            name: "test/repo", tag: "v1", contentStore: cs, client: capture)
+        let pushed = try await op.export(index: indexDesc, platforms: { _ in true })
+
+        #expect(pushed.mediaType == sourceMediaType)
+
+        let indexPush = try #require(
+            capture.pushes.first(where: { $0.descriptor.digest == pushed.digest }))
+        #expect(indexPush.descriptor.mediaType == sourceMediaType)
+        let pushedIndex = try JSONDecoder().decode(Index.self, from: indexPush.body)
+        #expect(pushedIndex.mediaType == sourceMediaType)
+    }
+}
+
+private final class CapturingContentClient: ContentClient, @unchecked Sendable {
+    struct Push: Sendable {
+        let descriptor: Descriptor
+        let body: Data
+    }
+
+    private let lock = NSLock()
+    private var _pushes: [Push] = []
+
+    var pushes: [Push] {
+        lock.withLock { _pushes }
+    }
+
+    private struct NotImplemented: Error {}
+
+    func fetch<T: Codable>(name: String, descriptor: Descriptor) async throws -> T {
+        throw NotImplemented()
+    }
+
+    func fetchBlob(name: String, descriptor: Descriptor, into file: URL, progress: ProgressHandler?) async throws -> (Int64, SHA256Digest) {
+        throw NotImplemented()
+    }
+
+    func fetchData(name: String, descriptor: Descriptor) async throws -> Data {
+        throw NotImplemented()
+    }
+
+    func push<T: Sendable & AsyncSequence>(
+        name: String,
+        ref: String,
+        descriptor: Descriptor,
+        streamGenerator: () throws -> T,
+        progress: ProgressHandler?
+    ) async throws where T.Element == ByteBuffer {
+        let stream = try streamGenerator()
+        var data = Data()
+        for try await buf in stream {
+            data.append(contentsOf: buf.readableBytesView)
+        }
+        lock.withLock {
+            _pushes.append(Push(descriptor: descriptor, body: data))
+        }
+    }
+}