diff --git a/internal/cli/api/client.go b/internal/cli/api/client.go
index 91222a8..78f746b 100644
--- a/internal/cli/api/client.go
+++ b/internal/cli/api/client.go
@@ -25,7 +25,10 @@ const (
 	httpErrThreshold = 400
 )
 
-var errHTTP = errors.New("HTTP error")
+var (
+	errHTTP        = errors.New("HTTP error")
+	errJoinRefused = errors.New("the server refused the join")
+)
 
 // Client wraps HTTP calls to the neoirc server API.
 type Client struct {
@@ -206,13 +209,35 @@ func (client *Client) PollMessages(
 	}, nil
 }
 
-// JoinChannel joins a channel.
+// JoinChannel joins a channel. It returns an error unless the
+// server answers that the session joined. When the server
+// refuses the join, it sends the reason to the message queue as
+// an error numeric.
 func (client *Client) JoinChannel(channel string) error {
-	return client.SendMessage(
+	data, err := client.do(
+		http.MethodPost, "/api/v1/messages",
 		&Message{ //nolint:exhaustruct // only command+to needed
 			Command: irc.CmdJoin, To: channel,
 		},
 	)
+	if err != nil {
+		return err
+	}
+
+	var resp struct {
+		Status string `json:"status"`
+	}
+
+	err = json.Unmarshal(data, &resp)
+	if err != nil {
+		return fmt.Errorf("decode join: %w", err)
+	}
+
+	if resp.Status != "joined" {
+		return errJoinRefused
+	}
+
+	return nil
 }
 
 // PartChannel leaves a channel.
diff --git a/internal/cli/app_internal_test.go b/internal/cli/app_internal_test.go
index bb9e49f..15dba69 100644
--- a/internal/cli/app_internal_test.go
+++ b/internal/cli/app_internal_test.go
@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -138,11 +139,9 @@ func TestOwnCommandShownOnce(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()
 
-			// The server accepts every command and answers nothing.
-			server := httptest.NewServer(http.HandlerFunc(
-				func(http.ResponseWriter, *http.Request) {},
-			))
-			t.Cleanup(server.Close)
+			// The server accepts every command. Only the response
+			// to a JOIN is read, and it says the session joined.
+			server := newServer(t, `{"status": "joined"}`)
 
 			app := &App{
 				ui:        NewUI(),
@@ -166,6 +165,60 @@ func TestOwnCommandShownOnce(t *testing.T) {
 	}
 }
 
+// TestJoinResponse checks that /join switches to the channel's
+// buffer and makes it the target only when the server answers
+// that the session joined.
+func TestJoinResponse(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		response string
+		// The buffer shown and the target after the join.
+		want string
+	}{
+		{
+			name:     "joined",
+			response: `{"status": "joined", "channel": "#chan"}`,
+			want:     testChannel,
+		},
+		{
+			name:     "refused",
+			response: `{"status": "error"}`,
+			want:     testOtherNick,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			app := &App{
+				ui:        NewUI(),
+				client:    api.NewClient(newServer(t, test.response).URL),
+				nick:      testNick,
+				target:    testOtherNick,
+				connected: true,
+			}
+			runUI(t, app.ui)
+			app.ui.SwitchToBuffer(testOtherNick)
+
+			app.cmdJoin(testChannel)
+
+			var shown string
+
+			app.ui.app.QueueUpdate(func() {
+				shown = app.ui.buffers[app.ui.currentBuffer].Name
+			})
+
+			if shown != test.want || app.target != test.want {
+				t.Fatalf("got buffer %s and target %s, want %s",
+					shown, app.target, test.want)
+			}
+		})
+	}
+}
+
 // TestOwnEventFromOtherClient checks that a message, notice,
 // join or part that the user's session makes from another
 // client shows as one line in the buffer it belongs to.
@@ -248,6 +301,21 @@ func bufferState(ui *UI, name string) (int, int) {
 	return lines, unread
 }
 
+// newServer starts a server that answers every request with
+// body until the test ends.
+func newServer(t *testing.T, body string) *httptest.Server {
+	t.Helper()
+
+	server := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, _ *http.Request) {
+			_, _ = io.WriteString(writer, body)
+		},
+	))
+	t.Cleanup(server.Close)
+
+	return server
+}
+
 // runUI runs ui on a simulated screen until the test ends.
 func runUI(t *testing.T, ui *UI) {
 	t.Helper()
diff --git a/web/src/app.jsx b/web/src/app.jsx
index c707e1b..cd18ff1 100644
--- a/web/src/app.jsx
+++ b/web/src/app.jsx
@@ -831,10 +831,11 @@ function App() {
       // Fresh session — join any previously saved channels.
       for (const ch of saved) {
         try {
-          await api("/messages", {
+          const res = await api("/messages", {
             method: "POST",
             body: JSON.stringify({ command: "JOIN", to: ch }),
           });
+          if (res?.status !== "joined") continue;
           setTabs((prev) => {
             if (prev.find((t) => t.type === "channel" && t.name === ch))
               return prev;
@@ -854,10 +855,13 @@ function App() {
     name = name.trim();
     if (!name.startsWith("#")) name = "#" + name;
     try {
-      await api("/messages", {
+      const res = await api("/messages", {
         method: "POST",
         body: JSON.stringify({ command: "JOIN", to: name }),
       });
+      // A refused join opens no tab. The error numeric that the
+      // server sends to the message queue tells the user why.
+      if (res?.status !== "joined") return;
       // Show the channel's tab. Preact runs this update at once on the
       // newest tab list, so the tab is found there by name: the user's
       // own JOIN event may already have added it, and other tabs may