diff --git a/command/agent/kvs_endpoint.go b/command/agent/kvs_endpoint.go
index 4f5bccb48a..bca7bf3216 100644
--- a/command/agent/kvs_endpoint.go
+++ b/command/agent/kvs_endpoint.go
@@ -348,11 +348,13 @@ func (s *HTTPServer) KVSTxn(resp http.ResponseWriter, req *http.Request) (interf
 	s.parseToken(req, &args.Token)
 
 	// Note the body is in API format, and not the RPC format. If we can't
-	// decode it, we will return a 500 since we don't have enough context to
+	// decode it, we will return a 400 since we don't have enough context to
 	// associate the error with a given operation.
 	var txn api.KVTxn
 	if err := decodeBody(req, &txn, fixupValues); err != nil {
-		return nil, fmt.Errorf("failed to parse body: %v", err)
+		resp.WriteHeader(http.StatusBadRequest)
+		resp.Write([]byte(fmt.Sprintf("Failed to parse body: %v", err)))
+		return nil, nil
 	}
 
 	// Convert the API format into the RPC format. Note that fixupValues
diff --git a/command/agent/kvs_endpoint_test.go b/command/agent/kvs_endpoint_test.go
index ab6357ed1a..3d0343c2f4 100644
--- a/command/agent/kvs_endpoint_test.go
+++ b/command/agent/kvs_endpoint_test.go
@@ -573,3 +573,219 @@ func TestKVSEndpoint_DELETE_ConflictingFlags(t *testing.T) {
 		}
 	})
 }
+
+func TestKVSEndpoint_Txn(t *testing.T) {
+	// Bad JSON.
+	httpTest(t, func(srv *HTTPServer) {
+		buf := bytes.NewBuffer([]byte("{"))
+		req, err := http.NewRequest("PUT", "/v1/kv-txn", buf)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+
+		resp := httptest.NewRecorder()
+		if _, err := srv.KVSTxn(resp, req); err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		if resp.Code != 400 {
+			t.Fatalf("expected 400, got %d", resp.Code)
+		}
+		if !bytes.Contains(resp.Body.Bytes(), []byte("Failed to parse")) {
+			t.Fatalf("expected conflicting args error")
+		}
+	})
+
+	// Bad request.
+	httpTest(t, func(srv *HTTPServer) {
+		buf := bytes.NewBuffer([]byte("{"))
+		req, err := http.NewRequest("GET", "/v1/kv-txn", buf)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+
+		resp := httptest.NewRecorder()
+		if _, err := srv.KVSTxn(resp, req); err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		if resp.Code != 405 {
+			t.Fatalf("expected 405, got %d", resp.Code)
+		}
+	})
+
+	// Make sure all incoming fields get converted properly to the internal
+	// RPC format.
+	httpTest(t, func(srv *HTTPServer) {
+		var index uint64
+		id := makeTestSession(t, srv)
+		{
+			buf := bytes.NewBuffer([]byte(fmt.Sprintf(`
+[
+    {
+        "Op": "lock",
+        "Key": "key",
+        "Value": "aGVsbG8gd29ybGQ=",
+        "Flags": 23,
+        "Session": %q
+    },
+    {
+        "Op": "get",
+        "Key": "key"
+    }
+]
+`, id)))
+			req, err := http.NewRequest("PUT", "/v1/kv-txn", buf)
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+
+			resp := httptest.NewRecorder()
+			obj, err := srv.KVSTxn(resp, req)
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+			if resp.Code != 200 {
+				t.Fatalf("expected 200, got %d", resp.Code)
+			}
+
+			atomic, ok := obj.(structs.KVSAtomicResponse)
+			if !ok {
+				t.Fatalf("bad type: %T", obj)
+			}
+			if len(atomic.Results) != 2 {
+				t.Fatalf("bad: %v", atomic)
+			}
+			index = atomic.Results[0].ModifyIndex
+			expected := structs.KVSAtomicResponse{
+				Results: structs.DirEntries{
+					&structs.DirEntry{
+						Key:       "key",
+						Value:     nil,
+						Flags:     23,
+						Session:   id,
+						LockIndex: 1,
+						RaftIndex: structs.RaftIndex{
+							CreateIndex: index,
+							ModifyIndex: index,
+						},
+					},
+					&structs.DirEntry{
+						Key:       "key",
+						Value:     []byte("hello world"),
+						Flags:     23,
+						Session:   id,
+						LockIndex: 1,
+						RaftIndex: structs.RaftIndex{
+							CreateIndex: index,
+							ModifyIndex: index,
+						},
+					},
+				},
+			}
+			if !reflect.DeepEqual(atomic, expected) {
+				t.Fatalf("bad: %v", atomic)
+			}
+		}
+
+		// Now that we have an index we can do a CAS to make sure the
+		// index field gets translated to the RPC format.
+		{
+			buf := bytes.NewBuffer([]byte(fmt.Sprintf(`
+[
+    {
+        "Op": "cas",
+        "Key": "key",
+        "Value": "Z29vZGJ5ZSB3b3JsZA==",
+        "Index": %d
+    },
+    {
+        "Op": "get",
+        "Key": "key"
+    }
+]
+`, index)))
+			req, err := http.NewRequest("PUT", "/v1/kv-txn", buf)
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+
+			resp := httptest.NewRecorder()
+			obj, err := srv.KVSTxn(resp, req)
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+			if resp.Code != 200 {
+				t.Fatalf("expected 200, got %d", resp.Code)
+			}
+
+			atomic, ok := obj.(structs.KVSAtomicResponse)
+			if !ok {
+				t.Fatalf("bad type: %T", obj)
+			}
+			if len(atomic.Results) != 2 {
+				t.Fatalf("bad: %v", atomic)
+			}
+			modIndex := atomic.Results[0].ModifyIndex
+			expected := structs.KVSAtomicResponse{
+				Results: structs.DirEntries{
+					&structs.DirEntry{
+						Key:     "key",
+						Value:   nil,
+						Session: id,
+						RaftIndex: structs.RaftIndex{
+							CreateIndex: index,
+							ModifyIndex: modIndex,
+						},
+					},
+					&structs.DirEntry{
+						Key:     "key",
+						Value:   []byte("goodbye world"),
+						Session: id,
+						RaftIndex: structs.RaftIndex{
+							CreateIndex: index,
+							ModifyIndex: modIndex,
+						},
+					},
+				},
+			}
+			for _, r := range atomic.Results {
+				fmt.Printf("%v\n", *r)
+			}
+			if !reflect.DeepEqual(atomic, expected) {
+				t.Fatalf("bad: %v", atomic)
+			}
+		}
+	})
+
+	// Verify an error inside a transaction.
+	httpTest(t, func(srv *HTTPServer) {
+		buf := bytes.NewBuffer([]byte(`
+[
+    {
+        "Op": "lock",
+        "Key": "key",
+        "Value": "aGVsbG8gd29ybGQ=",
+        "Session": "nope"
+    },
+    {
+        "Op": "get",
+        "Key": "key"
+    }
+]
+`))
+		req, err := http.NewRequest("PUT", "/v1/kv-txn", buf)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+
+		resp := httptest.NewRecorder()
+		if _, err = srv.KVSTxn(resp, req); err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		if resp.Code != 409 {
+			t.Fatalf("expected 409, got %d", resp.Code)
+		}
+		if !bytes.Contains(resp.Body.Bytes(), []byte("failed session lookup")) {
+			t.Fatalf("bad: %s", resp.Body.String())
+		}
+	})
+}