diff --git a/internal/forker/launcher.go b/internal/forker/launcher.go
index b4f6d2f..a8de278 100644
--- a/internal/forker/launcher.go
+++ b/internal/forker/launcher.go
@@ -16,15 +16,33 @@ type LaunchConfig struct {
 }
 
 func Launch(cfg LaunchConfig) error {
-	agent := strings.ToLower(cfg.Agent)
+	launcher, err := launcherForAgent(cfg.Agent)
+	if err != nil {
+		return err
+	}
 
-	switch {
-	case strings.Contains(agent, "claude"):
+	switch launcher {
+	case "claude":
 		return launchClaude(cfg)
-	case strings.Contains(agent, "codex") || strings.Contains(agent, "o4") || strings.Contains(agent, "gpt"):
+	case "codex":
 		return launchCodex(cfg)
 	default:
-		return launchClaude(cfg)
+		panic("unreachable launcher")
+	}
+}
+
+func launcherForAgent(agent string) (string, error) {
+	agent = strings.ToLower(strings.TrimSpace(agent))
+
+	switch {
+	case strings.HasPrefix(agent, "claude"):
+		return "claude", nil
+	case strings.HasPrefix(agent, "codex"),
+		strings.HasPrefix(agent, "o4"),
+		strings.HasPrefix(agent, "gpt"):
+		return "codex", nil
+	default:
+		return "", fmt.Errorf("unsupported agent %q: expected a claude, codex, o4, or gpt agent", agent)
 	}
 }
 
diff --git a/internal/forker/launcher_test.go b/internal/forker/launcher_test.go
new file mode 100644
index 0000000..2bd93df
--- /dev/null
+++ b/internal/forker/launcher_test.go
@@ -0,0 +1,47 @@
+package forker
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLauncherForAgent(t *testing.T) {
+	tests := []struct {
+		name      string
+		agent     string
+		want      string
+		wantError string
+	}{
+		{name: "claude model", agent: "claude-sonnet-4-6", want: "claude"},
+		{name: "claude case and whitespace", agent: "  Claude-Opus-4-6  ", want: "claude"},
+		{name: "codex agent", agent: "codex", want: "codex"},
+		{name: "o4 model", agent: "o4-mini", want: "codex"},
+		{name: "gpt model", agent: "gpt-5.5", want: "codex"},
+		{name: "embedded o4 substring", agent: "proto4", wantError: "unsupported agent"},
+		{name: "embedded gpt substring", agent: "not-gpt", wantError: "unsupported agent"},
+		{name: "unknown agent", agent: "gemini-pro", wantError: "unsupported agent"},
+		{name: "empty agent", agent: "", wantError: "unsupported agent"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := launcherForAgent(tt.agent)
+			if tt.wantError != "" {
+				if err == nil {
+					t.Fatalf("launcherForAgent(%q) returned %q, want error containing %q", tt.agent, got, tt.wantError)
+				}
+				if !strings.Contains(err.Error(), tt.wantError) {
+					t.Fatalf("launcherForAgent(%q) error = %q, want substring %q", tt.agent, err, tt.wantError)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("launcherForAgent(%q) returned unexpected error: %v", tt.agent, err)
+			}
+			if got != tt.want {
+				t.Fatalf("launcherForAgent(%q) = %q, want %q", tt.agent, got, tt.want)
+			}
+		})
+	}
+}