diff --git a/async-http.gemspec b/async-http.gemspec
index 62ea379b..861da016 100644
--- a/async-http.gemspec
+++ b/async-http.gemspec
@@ -31,7 +31,7 @@ Gem::Specification.new do |spec|
 	spec.add_dependency "io-endpoint", "~> 0.18"
 	spec.add_dependency "io-stream", "~> 0.14"
 	spec.add_dependency "protocol-http", "~> 0.72"
-	spec.add_dependency "protocol-http1", "~> 0.41"
+	spec.add_dependency "protocol-http1", "~> 0.42"
 	spec.add_dependency "protocol-http2", "~> 0.28"
 	spec.add_dependency "protocol-url", "~> 0.2"
 end
diff --git a/lib/async/http/protocol/http1/client.rb b/lib/async/http/protocol/http1/client.rb
index 105a2ca8..50743ace 100644
--- a/lib/async/http/protocol/http1/client.rb
+++ b/lib/async/http/protocol/http1/client.rb
@@ -69,7 +69,7 @@ def call(request, task: Task.current)
 							else
 								task.async(annotation: "Streaming request...") do
 									# Once we start writing the body, we can't recover if the request fails. That's because the body might be generated dynamically, streaming, etc.
-									write_body(@version, body, false, trailer)
+									write_body(@version, body, false, trailer, true)
 								rescue => error
 									self.close(error)
 								end
@@ -77,7 +77,7 @@ def call(request, task: Task.current)
 						elsif protocol = request.protocol
 							write_upgrade_body(protocol)
 						else
-							write_body(@version, request.body, false, trailer)
+							write_body(@version, request.body, false, trailer, true)
 						end
 						
 						return Response.read(self, request)
diff --git a/releases.md b/releases.md
index f56fb22f..41f7f95a 100644
--- a/releases.md
+++ b/releases.md
@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - `Async::HTTP::Protocol::HTTP1::Client` passes `request = true` to `Protocol::HTTP1::Connection#write_body` so an HTTP/1.0 request body of unknown length raises instead of being written close-delimited, where the server would parse it as the next request (requires `protocol-http1` >= 0.42).
+
 ## v0.105.0
 
   - Respond with `400 Bad Request` for any `Protocol::HTTP::BadRequest` raised while parsing HTTP/1 or HTTP/2 requests, include the exception class name without reflecting request data, and avoid reporting them as unhandled server errors.
diff --git a/test/async/http/protocol/http10.rb b/test/async/http/protocol/http10.rb
index 6770e699..2a58a897 100644
--- a/test/async/http/protocol/http10.rb
+++ b/test/async/http/protocol/http10.rb
@@ -6,6 +6,45 @@
 require "async/http/protocol/http10"
 require "async/http/a_protocol"
 
+require "protocol/http/body/writable"
+
 describe Async::HTTP::Protocol::HTTP10 do
 	it_behaves_like Async::HTTP::AProtocol
+	
+	with "request body of unknown length" do
+		include Sus::Fixtures::Async::HTTP::ServerContext
+		
+		let(:protocol) {subject}
+		
+		let(:paths) {[]}
+		
+		let(:app) do
+			::Protocol::HTTP::Middleware.for do |request|
+				paths << request.path
+				
+				::Protocol::HTTP::Response[200, {}, [request.path]]
+			end
+		end
+		
+		it "fails rather than writing the body where the server expects the next request" do
+			# HTTP/1.0 has no chunked encoding, and a request body cannot be delimited by closing the connection, so without a content length the server would parse these bytes as a second request:
+			body = ::Protocol::HTTP::Body::Writable.new
+			body.write "GET /smuggled HTTP/1.1\r\nhost: localhost\r\n\r\n"
+			body.close_write
+			
+			response = nil
+			
+			# The body is written in a child task which closes the connection on failure, so the error is observed when reading the response:
+			expect do
+				response = client.post("/", {"connection" => "keep-alive"}, body)
+			end.to raise_exception(::Protocol::HTTP1::ProtocolError, message: be =~ /closed/)
+			
+			expect(body).to be(:closed?)
+			
+			# The server never saw a complete request, let alone a second one:
+			expect(paths).to be(:empty?)
+		ensure
+			response&.close
+		end
+	end
 end