diff --git a/src/pkg/http/httptest/server.go b/src/pkg/http/httptest/server.go
index 879f04f33c2..2ec36d04cf6 100644
--- a/src/pkg/http/httptest/server.go
+++ b/src/pkg/http/httptest/server.go
@@ -9,6 +9,7 @@ package httptest
 import (
 	"crypto/rand"
 	"crypto/tls"
+	"flag"
 	"fmt"
 	"http"
 	"net"
@@ -49,15 +50,34 @@ func newLocalListener() net.Listener {
 	return l
 }
 
+// When debugging a particular http server-based test,
+// this flag lets you run
+//	gotest -run=BrokenTest -httptest.serve=127.0.0.1:8000
+// to start the broken server so you can interact with it manually.
+var serve = flag.String("httptest.serve", "", "if non-empty, httptest.NewServer serves on this address and blocks")
+
 // NewServer starts and returns a new Server.
 // The caller should call Close when finished, to shut it down.
 func NewServer(handler http.Handler) *Server {
 	ts := new(Server)
-	l := newLocalListener()
+	var l net.Listener
+	if *serve != "" {
+		var err os.Error
+		l, err = net.Listen("tcp", *serve)
+		if err != nil {
+			panic(fmt.Sprintf("httptest: failed to listen on %v: %v", *serve, err))
+		}
+	} else {
+		l = newLocalListener()
+	}
 	ts.Listener = &historyListener{l, make([]net.Conn, 0)}
 	ts.URL = "http://" + l.Addr().String()
 	server := &http.Server{Handler: handler}
 	go server.Serve(ts.Listener)
+	if *serve != "" {
+		fmt.Println(os.Stderr, "httptest: serving on", ts.URL)
+		select {}
+	}
 	return ts
 }
 
diff --git a/src/pkg/http/server.go b/src/pkg/http/server.go
index b3fb8e101c3..f14ef8c04b9 100644
--- a/src/pkg/http/server.go
+++ b/src/pkg/http/server.go
@@ -255,9 +255,7 @@ func (w *response) WriteHeader(code int) {
 	} else {
 		// If no content type, apply sniffing algorithm to body.
 		if w.header.Get("Content-Type") == "" {
-			// NOTE(dsymonds): the sniffing mechanism in this file is currently broken.
-			//w.needSniff = true
-			w.header.Set("Content-Type", "text/html; charset=utf-8")
+			w.needSniff = true
 		}
 	}
 
@@ -364,10 +362,16 @@ func (w *response) sniff() {
 	fmt.Fprintf(w.conn.buf, "Content-Type: %s\r\n", DetectContentType(data))
 	io.WriteString(w.conn.buf, "\r\n")
 
-	if w.chunking && len(data) > 0 {
+	if len(data) == 0 {
+		return
+	}
+	if w.chunking {
 		fmt.Fprintf(w.conn.buf, "%x\r\n", len(data))
 	}
-	w.conn.buf.Write(data)
+	_, err := w.conn.buf.Write(data)
+	if w.chunking && err == nil {
+		io.WriteString(w.conn.buf, "\r\n")
+	}
 }
 
 // bodyAllowed returns true if a Write is allowed for this response type.
@@ -401,12 +405,23 @@ func (w *response) Write(data []byte) (n int, err os.Error) {
 
 	var m int
 	if w.needSniff {
+		// We need to sniff the beginning of the output to
+		// determine the content type.  Accumulate the
+		// initial writes in w.conn.body.
 		body := w.conn.body
-		m = copy(body[len(body):], data)
+		m = copy(body[len(body):cap(body)], data)
 		w.conn.body = body[:len(body)+m]
 		if m == len(data) {
+			// Copied everything into the buffer.
+			// Wait for next write.
 			return m, nil
 		}
+
+		// Filled the buffer; more data remains.
+		// Sniff the content (flushes the buffer)
+		// and then proceed with the remainder
+		// of the data as a normal Write.
+		// Calling sniff clears needSniff.
 		w.sniff()
 		data = data[m:]
 	}
diff --git a/src/pkg/http/sniff_test.go b/src/pkg/http/sniff_test.go
index 770496f4051..2d01807f695 100644
--- a/src/pkg/http/sniff_test.go
+++ b/src/pkg/http/sniff_test.go
@@ -2,16 +2,22 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package http
+package http_test
 
 import (
+	"bytes"
+	. "http"
+	"http/httptest"
+	"io/ioutil"
+	"log"
+	"strconv"
 	"testing"
 )
 
 var sniffTests = []struct {
-	desc string
-	data []byte
-	exp  string
+	desc        string
+	data        []byte
+	contentType string
 }{
 	// Some nonsense.
 	{"Empty", []byte{}, "text/plain; charset=utf-8"},
@@ -30,11 +36,41 @@ var sniffTests = []struct {
 	{"GIF 89a", []byte(`GIF89a...`), "image/gif"},
 }
 
-func TestSniffing(t *testing.T) {
-	for _, st := range sniffTests {
-		got := DetectContentType(st.data)
-		if got != st.exp {
-			t.Errorf("%v: sniffed as %v, want %v", st.desc, got, st.exp)
+func TestDetectContentType(t *testing.T) {
+	for _, tt := range sniffTests {
+		ct := DetectContentType(tt.data)
+		if ct != tt.contentType {
+			t.Errorf("%v: DetectContentType = %q, want %q", tt.desc, ct, tt.contentType)
 		}
 	}
 }
+
+func TestServerContentType(t *testing.T) {
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		i, _ := strconv.Atoi(r.FormValue("i"))
+		tt := sniffTests[i]
+		n, err := w.Write(tt.data)
+		if n != len(tt.data) || err != nil {
+			log.Fatalf("%v: Write(%q) = %v, %v want %d, nil", tt.desc, tt.data, n, err, len(tt.data))
+		}
+	}))
+	defer ts.Close()
+
+	for i, tt := range sniffTests {
+		resp, err := Get(ts.URL + "/?i=" + strconv.Itoa(i))
+		if err != nil {
+			t.Errorf("%v: %v", tt.desc, err)
+			continue
+		}
+		if ct := resp.Header.Get("Content-Type"); ct != tt.contentType {
+			t.Errorf("%v: Content-Type = %q, want %q", tt.desc, ct, tt.contentType)
+		}
+		data, err := ioutil.ReadAll(resp.Body)
+		if err != nil {
+			t.Errorf("%v: reading body: %v", tt.desc, err)
+		} else if !bytes.Equal(data, tt.data) {
+			t.Errorf("%v: data is %q, want %q", tt.desc, data, tt.data)
+		}
+		resp.Body.Close()
+	}
+}