diff --git a/lib/protocol/http/headers.rb b/lib/protocol/http/headers.rb
index ddccf02..4175fbe 100644
--- a/lib/protocol/http/headers.rb
+++ b/lib/protocol/http/headers.rb
@@ -231,9 +231,9 @@ def keys
 			
 			# Extract the specified keys from the headers.
 			#
-			# @parameter keys [Array] The keys to extract.
+			# @parameter keys [Array] The keys to extract, which must be lowercase.
 			def extract(keys)
-				deleted, @fields = @fields.partition do |field|
+				deleted = remove_fields do |field|
 					keys.include?(field.first.downcase)
 				end
 				
@@ -420,12 +420,14 @@ def merge(headers)
 			# @parameter key [String] The header key.
 			# @returns [String | Array | Object] The merged header value.
 			def delete(key)
+				key = key.downcase
+				
 				# If we've indexed the headers, we can bail out early if the key is not present:
-				if @indexed && !@indexed.key?(key.downcase)
+				if @indexed && !@indexed.key?(key)
 					return nil
 				end
 				
-				deleted, @fields = @fields.partition do |field|
+				deleted = remove_fields do |field|
 					field.first.downcase == key
 				end
 				
@@ -448,6 +450,40 @@ def delete(key)
 				end
 			end
 			
+			# Remove all fields for which the block returns true, preserving the trailer boundary.
+			#
+			# Removing a field before the trailer boundary `@tail` must shift the boundary down, otherwise the first trailer would be treated as a header.
+			#
+			# Most headers never have a trailer boundary, so that case is kept on the fast `partition` path and the indexed loop only runs when a boundary exists.
+			#
+			# @yields {|field| ...} Each `[key, value]` pair; return true to remove it.
+			# @returns [Array] The removed fields, in their original order.
+			private def remove_fields(&block)
+				if tail = @tail
+					deleted = []
+					kept = []
+					
+					@fields.each_with_index do |field, index|
+						if yield(field)
+							deleted << field
+							
+							if index < @tail
+								tail -= 1
+							end
+						else
+							kept << field
+						end
+					end
+					
+					@fields = kept
+					@tail = tail
+				else
+					deleted, @fields = @fields.partition(&block)
+				end
+				
+				return deleted
+			end
+			
 			# Merge the value into the hash according to the policy for the given key.
 			# 
 			# @parameter hash [Hash] The hash to merge into.
diff --git a/releases.md b/releases.md
index 9480d0a..26fa117 100644
--- a/releases.md
+++ b/releases.md
@@ -1,5 +1,10 @@
 # Releases
 
+## Unreleased
+
+  - Preserve the trailer boundary when `Protocol::HTTP::Headers#delete` or `#extract` removes fields from the header section, so existing trailers are no longer sent as headers.
+  - `Protocol::HTTP::Headers#delete` (and therefore `#set`) now normalizes the key to lowercase, so mixed case keys remove the matching fields instead of leaving them in place.
+
 ## v0.72.0
 
   - Clarified body stream lifecycle and ownership, including the directional semantics of `Protocol::HTTP::Body::Stream#close_read`, `#close_write`, and `#close`, how premature input closure affects the associated HTTP exchange, and ownership of streams passed to `Streamable#call`.
diff --git a/test/protocol/http/headers.rb b/test/protocol/http/headers.rb
index 9b0c504..98e24b3 100644
--- a/test/protocol/http/headers.rb
+++ b/test/protocol/http/headers.rb
@@ -293,6 +293,13 @@
 			
 			expect(headers["accept-encoding"]).to be == ["gzip"]
 		end
+		
+		it "can replace an existing field using a mixed case key" do
+			headers.set("Content-Type", "text/plain")
+			
+			expect(headers.fields.count{|key, value| key.downcase == "content-type"}).to be == 1
+			expect(headers["content-type"]).to be == "text/plain"
+		end
 	end
 	
 	with "#extract" do
@@ -329,6 +336,22 @@
 		it "can delete non-existant fields" do
 			expect(headers.delete("transfer-encoding")).to be_nil
 		end
+		
+		it "can delete fields using a mixed case key" do
+			expect(headers.delete("Content-Type")).to be == "text/html"
+			
+			expect(headers.fields).to be == fields[1..-1]
+		end
+		
+		it "can delete indexed fields using a mixed case key" do
+			# Force the headers to be indexed:
+			headers.to_h
+			
+			expect(headers.delete("Content-Type")).to be == "text/html"
+			
+			expect(headers.fields).to be == fields[1..-1]
+			expect(headers["content-type"]).to be_nil
+		end
 	end
 	
 	with "#merge" do
@@ -512,6 +535,164 @@
 		end
 	end
 	
+	with "trailer boundary" do
+		let(:header_fields) {fields[1..-1]}
+		
+		before do
+			headers.trailer!
+			headers.add("etag", "abcd")
+			headers.add("digest", "sha-256=xyz")
+		end
+		
+		it "preserves trailers when deleting a header" do
+			expect(headers.delete("content-type")).to be == "text/html"
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == 4
+			expect(headers.header.to_a).to be == header_fields
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+		end
+		
+		it "preserves trailers when deleting multiple fields with the same key" do
+			headers.delete("set-cookie")
+			
+			expect(headers.tail).to be == 3
+			expect(headers.header.to_a).to be == [
+				["Content-Type", "text/html"],
+				["connection", "Keep-Alive"],
+				["Accept", "*/*"],
+			]
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+		end
+		
+		it "preserves trailers when deleting a header with an index" do
+			# Force the headers to be indexed:
+			headers.to_h
+			
+			headers.delete("content-type")
+			
+			expect(headers.tail).to be == 4
+			expect(headers.header.to_a).to be == header_fields
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+			expect(headers["etag"]).to be == "abcd"
+		end
+		
+		it "preserves trailers when extracting headers" do
+			expect(headers.extract(["content-type", "set-cookie"])).to be == [
+				["Content-Type", "text/html"],
+				["Set-Cookie", "hello=world"],
+				["set-cookie", "foo=bar"],
+			]
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == 2
+			expect(headers.header.to_a).to be == [
+				["connection", "Keep-Alive"],
+				["Accept", "*/*"],
+			]
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+		end
+		
+		it "preserves trailers when extracting all headers" do
+			headers.extract(["content-type", "connection", "set-cookie", "accept"])
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == 0
+			expect(headers.header.to_a).to be == []
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+		end
+		
+		it "does not move the boundary when deleting a trailer" do
+			expect(headers.delete("etag")).to be == "abcd"
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == 5
+			expect(headers.header.to_a).to be == fields
+			expect(headers.trailer.to_a).to be == [["digest", "sha-256=xyz"]]
+		end
+		
+		it "does not move the boundary when extracting trailers" do
+			expect(headers.extract(["etag", "digest"])).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == 5
+			expect(headers.header.to_a).to be == fields
+			expect(headers.trailer.to_a).to be == []
+		end
+		
+		it "preserves the boundary when deleting a missing header" do
+			expect(headers.delete("transfer-encoding")).to be_nil
+			
+			expect(headers.tail).to be == 5
+			expect(headers.header.to_a).to be == fields
+			expect(headers.trailer.to_a).to be == [["etag", "abcd"], ["digest", "sha-256=xyz"]]
+		end
+		
+		with "a header that is permitted as a trailer" do
+			let(:fields) do
+				[
+					["Content-Type", "text/html"],
+					["Server-Timing", "db;dur=1"],
+					["Accept", "*/*"],
+				]
+			end
+			
+			it "preserves trailers when replacing the header" do
+				headers["server-timing"] = "db;dur=2"
+				
+				expect(headers).to be(:trailer?)
+				expect(headers.tail).to be == 2
+				expect(headers.header.to_a).to be == [
+					["Content-Type", "text/html"],
+					["Accept", "*/*"],
+				]
+				expect(headers.trailer.to_a).to be == [
+					["etag", "abcd"],
+					["digest", "sha-256=xyz"],
+					["server-timing", "db;dur=2"],
+				]
+			end
+		end
+		
+		with "a key that appears in both the header and trailer sections" do
+			let(:fields) do
+				[
+					["Content-Type", "text/html"],
+					["ETag", "wxyz"],
+					["Accept", "*/*"],
+				]
+			end
+			
+			it "only moves the boundary for the field in the header section" do
+				expect(headers.delete("etag")).to be == "abcd"
+				
+				expect(headers.tail).to be == 2
+				expect(headers.header.to_a).to be == [
+					["Content-Type", "text/html"],
+					["Accept", "*/*"],
+				]
+				expect(headers.trailer.to_a).to be == [["digest", "sha-256=xyz"]]
+			end
+		end
+	end
+	
+	with "trailer boundary and no trailers" do
+		before do
+			headers.trailer!
+		end
+		
+		it "keeps the boundary at the end of the fields when deleting a header" do
+			expect(headers.tail).to be == fields.size
+			
+			headers.delete("content-type")
+			
+			expect(headers).to be(:trailer?)
+			expect(headers.tail).to be == headers.fields.size
+			expect(headers.header.to_a).to be == fields[1..-1]
+			expect(headers.trailer.to_a).to be == []
+		end
+	end
+	
 	with "#add with trailer: keyword" do
 		let(:headers) {subject.new}