diff --git a/internal/util/claude_model.go b/internal/util/claude_model.go
new file mode 100644
index 00000000..1534f02c
--- /dev/null
+++ b/internal/util/claude_model.go
@@ -0,0 +1,10 @@
+package util
+
+import "strings"
+
+// IsClaudeThinkingModel checks if the model is a Claude thinking model
+// that requires the interleaved-thinking beta header.
+func IsClaudeThinkingModel(model string) bool {
+	lower := strings.ToLower(model)
+	return strings.Contains(lower, "claude") && strings.Contains(lower, "thinking")
+}
diff --git a/internal/util/claude_model_test.go b/internal/util/claude_model_test.go
new file mode 100644
index 00000000..17f6106e
--- /dev/null
+++ b/internal/util/claude_model_test.go
@@ -0,0 +1,41 @@
+package util
+
+import "testing"
+
+func TestIsClaudeThinkingModel(t *testing.T) {
+	tests := []struct {
+		name     string
+		model    string
+		expected bool
+	}{
+		// Claude thinking models - should return true
+		{"claude-sonnet-4-5-thinking", "claude-sonnet-4-5-thinking", true},
+		{"claude-opus-4-5-thinking", "claude-opus-4-5-thinking", true},
+		{"Claude-Sonnet-Thinking uppercase", "Claude-Sonnet-4-5-Thinking", true},
+		{"claude thinking mixed case", "Claude-THINKING-Model", true},
+
+		// Non-thinking Claude models - should return false
+		{"claude-sonnet-4-5 (no thinking)", "claude-sonnet-4-5", false},
+		{"claude-opus-4-5 (no thinking)", "claude-opus-4-5", false},
+		{"claude-3-5-sonnet", "claude-3-5-sonnet-20240620", false},
+
+		// Non-Claude models - should return false
+		{"gemini-3-pro-preview", "gemini-3-pro-preview", false},
+		{"gemini-thinking model", "gemini-3-pro-thinking", false}, // not Claude
+		{"gpt-4o", "gpt-4o", false},
+		{"empty string", "", false},
+
+		// Edge cases
+		{"thinking without claude", "thinking-model", false},
+		{"claude without thinking", "claude-model", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := IsClaudeThinkingModel(tt.model)
+			if result != tt.expected {
+				t.Errorf("IsClaudeThinkingModel(%q) = %v, expected %v", tt.model, result, tt.expected)
+			}
+		})
+	}
+}