diff --git a/debug/debug_test.go b/debug/debug_test.go
new file mode 100644
index 0000000..566ab30
--- /dev/null
+++ b/debug/debug_test.go
@@ -0,0 +1,166 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && amd64
+
+package debug
+
+import "testing"
+
+func TestConditionEval(t *testing.T) {
+	regs := &Regs{
+		RAX: 42,
+		RBX: 0,
+		RCX: 100,
+		RIP: 0x1000,
+		RSP: 0x2000,
+		R8:  8,
+		R15: 15,
+	}
+
+	tests := []struct {
+		cond Condition
+		want bool
+	}{
+		{Condition{Reg: "rax", Op: "==", Value: 42}, true},
+		{Condition{Reg: "rax", Op: "==", Value: 43}, false},
+		{Condition{Reg: "rax", Op: "!=", Value: 43}, true},
+		{Condition{Reg: "rax", Op: "!=", Value: 42}, false},
+		{Condition{Reg: "rax", Op: "<", Value: 50}, true},
+		{Condition{Reg: "rax", Op: "<", Value: 40}, false},
+		{Condition{Reg: "rax", Op: ">", Value: 40}, true},
+		{Condition{Reg: "rax", Op: ">", Value: 50}, false},
+		{Condition{Reg: "rax", Op: "<=", Value: 42}, true},
+		{Condition{Reg: "rax", Op: ">=", Value: 42}, true},
+		{Condition{Reg: "rbx", Op: "==", Value: 0}, true},
+		{Condition{Reg: "rcx", Op: ">", Value: 50}, true},
+		{Condition{Reg: "rip", Op: "==", Value: 0x1000}, true},
+		{Condition{Reg: "rsp", Op: ">", Value: 0x1000}, true},
+		{Condition{Reg: "r8", Op: "==", Value: 8}, true},
+		{Condition{Reg: "r15", Op: "==", Value: 15}, true},
+		{Condition{Reg: "eax", Op: "==", Value: 42}, true},    // 32-bit alias
+		{Condition{Reg: "ax", Op: "==", Value: 42}, true},     // 16-bit alias
+		{Condition{Reg: "unknown", Op: "==", Value: 0}, true}, // unknown reg → don't block
+		{Condition{Reg: "rax", Op: "??", Value: 0}, true},     // unknown op → don't block
+	}
+
+	for _, tt := range tests {
+		got := tt.cond.Eval(regs)
+		if got != tt.want {
+			t.Errorf("Condition{%q %q %d}.Eval() = %v, want %v",
+				tt.cond.Reg, tt.cond.Op, tt.cond.Value, got, tt.want)
+		}
+	}
+}
+
+func TestLineAt(t *testing.T) {
+	lines := []SourceLine{
+		{Offset: 0, Line: 5},
+		{Offset: 5, Line: 6},
+		{Offset: 10, Line: 7},
+		{Offset: 15, Line: 8},
+	}
+
+	tests := []struct {
+		offset int
+		want   int
+	}{
+		{0, 5},
+		{1, 5},
+		{4, 5},
+		{5, 6},
+		{7, 6},
+		{10, 7},
+		{12, 7},
+		{15, 8},
+		{20, 8},
+	}
+
+	for _, tt := range tests {
+		got := lineAt(lines, tt.offset)
+		if got != tt.want {
+			t.Errorf("lineAt(lines, %d) = %d, want %d", tt.offset, got, tt.want)
+		}
+	}
+
+	// Empty table.
+	if lineAt(nil, 5) != 0 {
+		t.Error("lineAt(nil, 5) should return 0")
+	}
+}
+
+func TestOffsetForLine(t *testing.T) {
+	lines := []SourceLine{
+		{Offset: 0, Line: 5},
+		{Offset: 5, Line: 6},
+		{Offset: 10, Line: 7},
+	}
+
+	tests := []struct {
+		line int
+		want int
+	}{
+		{5, 0},
+		{6, 5},
+		{7, 10},
+		{99, -1}, // not found
+		{0, -1},  // not found
+	}
+
+	for _, tt := range tests {
+		got := offsetForLine(lines, tt.line)
+		if got != tt.want {
+			t.Errorf("offsetForLine(lines, %d) = %d, want %d", tt.line, got, tt.want)
+		}
+	}
+}
+
+func TestDecodeRflags(t *testing.T) {
+	tests := []struct {
+		flags uint64
+		want  string
+	}{
+		{0x202, "IF"},                   // only IF set (bit 9)
+		{0x246, "PF ZF IF"},             // PF(2) + ZF(6) + IF(9)
+		{0x001, "CF"},                   // carry flag
+		{0x080, "SF"},                   // sign flag
+		{0x800, "OF"},                   // overflow flag
+		{0x000, "none"},                 // no flags
+		{0x202 | 0x001, "CF IF"},        // CF + IF
+		{0x3F7, "CF PF AF ZF SF TF IF"}, // all arithmetic flags
+	}
+
+	for _, tt := range tests {
+		got := decodeRflags(tt.flags)
+		if got != tt.want {
+			t.Errorf("decodeRflags(%#x) = %q, want %q", tt.flags, got, tt.want)
+		}
+	}
+}
+
+func TestNearestLabel(t *testing.T) {
+	labels := []Label{
+		{Name: "start", Offset: 0},
+		{Name: "loop", Offset: 10},
+		{Name: "done", Offset: 20},
+	}
+
+	tests := []struct {
+		offset int
+		want   string
+	}{
+		{0, "start"},
+		{5, "start"},
+		{10, "loop"},
+		{15, "loop"},
+		{20, "done"},
+		{25, "done"},
+	}
+
+	for _, tt := range tests {
+		got := nearestLabel(labels, tt.offset)
+		if got != tt.want {
+			t.Errorf("nearestLabel(labels, %d) = %q, want %q", tt.offset, got, tt.want)
+		}
+	}
+}
diff --git a/debug/repl.go b/debug/repl.go
index 3198198..9a5768f 100644
--- a/debug/repl.go
+++ b/debug/repl.go
@@ -538,7 +538,7 @@ func decodeRflags(f uint64) string {
 		flags += "OF "
 	}
 	if flags == "" {
-		flags = "none"
+		return "none"
 	}
 	return flags[:len(flags)-1] // trim trailing space
 }