diff --git a/agent/user_agent_test.go b/agent/user_agent_test.go
index 8699be5..0614ea9 100644
--- a/agent/user_agent_test.go
+++ b/agent/user_agent_test.go
@@ -52,6 +52,29 @@ func TestValidateUserAgentRejectsNewline(t *testing.T) {
 	}
 }
 
+// The rule is "no control character at all" (r < 0x20 or r == 0x7f), so the
+// non-newline controls are rejected too, TAB included: a header value is one
+// printable token, not something to lay out with tabs.
+func TestValidateUserAgentRejectsControlCharacters(t *testing.T) {
+	for name, in := range map[string]string{
+		"NUL": "good/1.0\x00",
+		"US":  "good/1.0\x1f",
+		"DEL": "good/1.0\x7f",
+		"TAB": "good/1.0\tX",
+	} {
+		if _, err := ValidateUserAgent(in); err == nil {
+			t.Errorf("%s: expected a user agent containing %q to be rejected", name, in)
+		}
+	}
+}
+
+func TestValidateUserAgentAcceptsPrintableToken(t *testing.T) {
+	const want = "MyTool/1.0 (contact-free)"
+	if ua, err := ValidateUserAgent("  " + want + "  "); err != nil || ua != want {
+		t.Fatalf("printable value: got (%q, %v), want (%q, nil)", ua, err, want)
+	}
+}
+
 func TestValidateUserAgentRejectsTooLong(t *testing.T) {
 	if _, err := ValidateUserAgent(strings.Repeat("a", MaxUserAgentLen+1)); err == nil {
 		t.Fatalf("expected a user agent longer than %d to be rejected", MaxUserAgentLen)
diff --git a/server/llm_key_endpoint_test.go b/server/llm_key_endpoint_test.go
index e4af8c4..6e59826 100644
--- a/server/llm_key_endpoint_test.go
+++ b/server/llm_key_endpoint_test.go
@@ -81,6 +81,44 @@ func TestLLMTestGlobalKeyOnlyForGlobalEndpoint(t *testing.T) {
 	}
 }
 
+// The connection test applies the same User-Agent rule as saving a profile: a
+// value with a control character is a 400 before any request leaves, and a
+// printable value reaches the wire exactly as given.
+func TestLLMTestValidatesUserAgent(t *testing.T) {
+	var hits atomic.Int32
+	var gotUA atomic.Value
+	fake := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		gotUA.Store(r.Header.Get("User-Agent"))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"id":"m","type":"message","role":"assistant","model":"m","content":[{"type":"text","text":"OK"}],"stop_reason":"end_turn","usage":{"input_tokens":1,"output_tokens":1}}`))
+	}))
+	t.Cleanup(fake.Close)
+	s := &Server{}
+	post := func(ua string) *httptest.ResponseRecorder {
+		b, _ := json.Marshal(map[string]any{
+			"provider": "anthropic", "model": "m", "base_url": fake.URL, "api_key": "k", "streaming": false, "user_agent": ua,
+		})
+		req := httptest.NewRequest(http.MethodPost, "/api/llm/test", bytes.NewReader(b))
+		req.Header.Set("Content-Type", "application/json")
+		rec := httptest.NewRecorder()
+		s.testLLM(rec, req)
+		return rec
+	}
+
+	if rec := post("good/1.0\r\nX-Injected: 1"); rec.Code != http.StatusBadRequest || hits.Load() != 0 {
+		t.Fatalf("control character in user_agent: status %d hits=%d body=%s", rec.Code, hits.Load(), rec.Body.String())
+	}
+
+	const want = "MyTool/1.0 (contact-free)"
+	if rec := post("  " + want + "  "); rec.Code != http.StatusOK {
+		t.Fatalf("printable user_agent: status %d body=%s", rec.Code, rec.Body.String())
+	}
+	if ua, _ := gotUA.Load().(string); hits.Load() != 1 || ua != want {
+		t.Fatalf("test request User-Agent = %q (hits=%d), want %q", ua, hits.Load(), want)
+	}
+}
+
 // A stored profile's key goes only to that profile's base URL, for both the
 // connection test and the model listing.
 func TestStoredProfileKeyOnlyForStoredEndpoint(t *testing.T) {
diff --git a/server/server.go b/server/server.go
index 322dd80..fbc2325 100644
--- a/server/server.go
+++ b/server/server.go
@@ -1488,6 +1488,14 @@ func (s *Server) testLLM(w http.ResponseWriter, r *http.Request) {
 	if stored != nil && cfg.UserAgent == "" {
 		cfg.UserAgent = strings.TrimSpace(stored.UserAgent)
 	}
+	// Same rule as saving a profile: a value with control characters or over the
+	// length cap never reaches the wire, not even from the test button.
+	ua, err := agent.ValidateUserAgent(cfg.UserAgent)
+	if err != nil {
+		writeErr(w, 400, err.Error())
+		return
+	}
+	cfg.UserAgent = ua
 	// Keyless formats (Ollama) skip the key resolution entirely.
 	if cfg.NeedsAPIKey() {
 		// API key resolution precedence: form input > the named profile's stored key > the global config's key.