diff --git a/Libraries/ConnectNIO/Internal/ConnectUnaryChannelHandler.swift b/Libraries/ConnectNIO/Internal/ConnectUnaryChannelHandler.swift
index 1d0a418d..1d5611ca 100644
--- a/Libraries/ConnectNIO/Internal/ConnectUnaryChannelHandler.swift
+++ b/Libraries/ConnectNIO/Internal/ConnectUnaryChannelHandler.swift
@@ -20,6 +20,7 @@ import NIOHTTP1
 
 /// NIO-based channel handler for unary requests made through the Connect library.
 final class ConnectUnaryChannelHandler: NIOCore.ChannelInboundHandler, @unchecked Sendable {
+    private let contentLength: NIOHTTPClient.ContentLength
     private let eventLoop: NIOCore.EventLoop
     private let request: Connect.HTTPRequest<Data?>
     private let onMetrics: (Connect.HTTPMetrics) -> Void
@@ -35,11 +36,13 @@ final class ConnectUnaryChannelHandler: NIOCore.ChannelInboundHandler, @unchecke
     init(
         request: Connect.HTTPRequest<Data?>,
         eventLoop: NIOCore.EventLoop,
+        contentLength: NIOHTTPClient.ContentLength = .sent,
         onMetrics: @escaping (Connect.HTTPMetrics) -> Void,
         onResponse: @escaping (Connect.HTTPResponse) -> Void
     ) {
         self.request = request
         self.eventLoop = eventLoop
+        self.contentLength = contentLength
         self.onMetrics = onMetrics
         self.onResponse = onResponse
     }
@@ -103,7 +106,7 @@ final class ConnectUnaryChannelHandler: NIOCore.ChannelInboundHandler, @unchecke
         }
 
         var nioHeaders = NIOHTTP1.HTTPHeaders()
-        if let messageLength = self.request.message?.count {
+        if case .sent = self.contentLength, let messageLength = self.request.message?.count {
             nioHeaders.add(name: "Content-Length", value: "\(messageLength)")
         }
         nioHeaders.add(name: "Host", value: self.request.url.host!)
diff --git a/Libraries/ConnectNIO/Public/NIOHTTPClient.swift b/Libraries/ConnectNIO/Public/NIOHTTPClient.swift
index fddc9a02..a1aac235 100644
--- a/Libraries/ConnectNIO/Public/NIOHTTPClient.swift
+++ b/Libraries/ConnectNIO/Public/NIOHTTPClient.swift
@@ -31,10 +31,18 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
     private let port: Int
     private let timeout: TimeInterval?
     private let useSSL: Bool
+    private let contentLength: ContentLength
 
     private var pendingRequests = [(NIOHTTP2.NIOHTTP2Handler.StreamMultiplexer?) -> Void]()
     private var state = State.disconnected
 
+    /// Whether unary requests should include a `Content-Length` header. It is optional over
+    /// HTTP/2, the only protocol this client speaks.
+    public enum ContentLength: Sendable {
+        case sent
+        case omitted
+    }
+
     private enum State {
         case disconnected
         case connecting
@@ -52,13 +60,21 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
     ///                   (e.g., `https://connectrpc.com:8080`), the host's port will be used.
     /// - parameter timeout: Optional timeout after which to terminate requests/streams if no
     ///                      activity has occurred in the request or response path.
-    public init(host: String, port: Int? = nil, timeout: TimeInterval? = nil) {
+    /// - parameter contentLength: Whether unary requests should include a `Content-Length`
+    ///                            header. Defaults to sending it.
+    public init(
+        host: String,
+        port: Int? = nil,
+        timeout: TimeInterval? = nil,
+        contentLength: ContentLength = .sent
+    ) {
         let baseURL = URL(string: host)!
         let useSSL = baseURL.scheme?.lowercased() == "https"
         self.host = baseURL.host!
         self.port = port ?? baseURL.port ?? (useSSL ? 443 : 80)
         self.timeout = timeout
         self.useSSL = useSSL
+        self.contentLength = contentLength
     }
 
     /// Called before the first request/stream is initialized, and the result is stored for reuse
@@ -118,6 +134,7 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
         let handler = ConnectUnaryChannelHandler(
             request: request,
             eventLoop: eventLoop,
+            contentLength: self.contentLength,
             onMetrics: onMetrics,
             onResponse: onResponse
         )
diff --git a/Package.swift b/Package.swift
index 71cd4a9c..79aebc97 100644
--- a/Package.swift
+++ b/Package.swift
@@ -96,6 +96,7 @@ let package = Package(
             dependencies: [
                 "Connect",
                 "ConnectMocks",
+                "ConnectNIO",
                 .product(name: "SwiftProtobuf", package: "swift-protobuf"),
             ],
             path: "Tests/UnitTests/ConnectLibraryTests",
diff --git a/Tests/UnitTests/ConnectLibraryTests/ConnectTests/ConnectUnaryChannelHandlerTests.swift b/Tests/UnitTests/ConnectLibraryTests/ConnectTests/ConnectUnaryChannelHandlerTests.swift
new file mode 100644
index 00000000..8bd2c17b
--- /dev/null
+++ b/Tests/UnitTests/ConnectLibraryTests/ConnectTests/ConnectUnaryChannelHandlerTests.swift
@@ -0,0 +1,75 @@
+// Copyright 2022-2025 The Connect 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
+//
+//      http://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 Connect
+@testable import ConnectNIO
+import Foundation
+import NIOCore
+import NIOEmbedded
+import NIOHTTP1
+import Testing
+
+struct ConnectUnaryChannelHandlerTests {
+    @Test
+    func sendsContentLengthByDefault() throws {
+        let headers = try Self.outboundHeaders(contentLength: .sent)
+
+        #expect(headers.first(name: "Content-Length") == "5")
+    }
+
+    @Test
+    func omitsContentLengthWhenRequested() throws {
+        let headers = try Self.outboundHeaders(contentLength: .omitted)
+
+        #expect(!headers.contains(name: "Content-Length"))
+    }
+
+    @Test(arguments: [NIOHTTPClient.ContentLength.sent, .omitted])
+    func alwaysSendsHostAndCallerHeaders(contentLength: NIOHTTPClient.ContentLength) throws {
+        let headers = try Self.outboundHeaders(contentLength: contentLength)
+
+        #expect(headers.first(name: "Host") == "connectrpc.com")
+        #expect(headers.first(name: "x-custom-header") == "value")
+    }
+
+    private static func outboundHeaders(
+        contentLength: NIOHTTPClient.ContentLength
+    ) throws -> NIOHTTP1.HTTPHeaders {
+        let channel = EmbeddedChannel()
+        let request = Connect.HTTPRequest<Data?>(
+            url: URL(string: "https://connectrpc.com/connectrpc.Service/Method")!,
+            headers: ["x-custom-header": ["value"]],
+            message: Data([0, 0, 0, 0, 0]),
+            method: .post,
+            trailers: nil,
+            idempotencyLevel: .unknown
+        )
+        let handler = ConnectUnaryChannelHandler(
+            request: request,
+            eventLoop: channel.eventLoop,
+            contentLength: contentLength,
+            onMetrics: { _ in },
+            onResponse: { _ in }
+        )
+        try channel.pipeline.syncOperations.addHandler(handler)
+        channel.pipeline.fireChannelActive()
+
+        let outbound = try channel.readOutbound(as: NIOHTTP1.HTTPClientRequestPart.self)
+        guard case .head(let head) = try #require(outbound) else {
+            Issue.record("Expected a request head to be written")
+            return NIOHTTP1.HTTPHeaders()
+        }
+        return head.headers
+    }
+}