diff --git a/qasm/parser/parser.go b/qasm/parser/parser.go
index 34a1c88..3359753 100644
--- a/qasm/parser/parser.go
+++ b/qasm/parser/parser.go
@@ -68,6 +68,9 @@ type parser struct {
 
 	// Gate definitions: name → gatedef.
 	gates map[string]*gatedef
+
+	// Parameter substitution for gate body expansion.
+	paramValues map[string]float64
 }
 
 type gatedef struct {
@@ -375,6 +378,61 @@ func (p *parser) parseGateDecl() error {
 	return nil
 }
 
+// expandGateCall inlines a user-defined gate by re-parsing its body tokens
+// with qubit and parameter substitutions.
+func (p *parser) expandGateCall(name string, gd *gatedef, params []float64, qubits []int) error {
+	if len(params) != len(gd.params) {
+		return fmt.Errorf("gate %s requires %d parameters, got %d", name, len(gd.params), len(params))
+	}
+	if len(qubits) != len(gd.qubits) {
+		return fmt.Errorf("gate %s requires %d qubit arguments, got %d", name, len(gd.qubits), len(qubits))
+	}
+
+	// Empty body - identity gate, nothing to emit.
+	if len(gd.body) == 0 {
+		return nil
+	}
+
+	// Build sub-parser to expand body tokens.
+	bodyTokens := make([]token.Token, len(gd.body)+1)
+	copy(bodyTokens, gd.body)
+	bodyTokens[len(gd.body)] = token.Token{Type: token.EOF}
+
+	sub := &parser{
+		tokens:    bodyTokens,
+		cfg:       p.cfg,
+		gates:     p.gates,
+		qregs:     make(map[string]register),
+		cregs:     p.cregs,
+		numQubits: p.numQubits,
+		numClbits: p.numClbits,
+		metadata:  make(map[string]string),
+	}
+
+	// Map gate qubit parameter names to actual qubit indices.
+	for i, qname := range gd.qubits {
+		sub.qregs[qname] = register{start: qubits[i], size: 1}
+	}
+
+	// Map gate parameter names to actual values.
+	if len(gd.params) > 0 {
+		sub.paramValues = make(map[string]float64, len(gd.params))
+		for i, pname := range gd.params {
+			sub.paramValues[pname] = params[i]
+		}
+	}
+
+	// Parse the body.
+	for sub.peek() != token.EOF {
+		if err := sub.parseStatement(); err != nil {
+			return fmt.Errorf("in gate %s: %w", name, err)
+		}
+	}
+
+	p.ops = append(p.ops, sub.ops...)
+	return nil
+}
+
 func (p *parser) parseMeasure() error {
 	t := p.advance() // consume 'measure'
 	qubits, err := p.parseQubitArgs()
@@ -991,6 +1049,14 @@ func (p *parser) parseGateCall() error {
 		return err
 	}
 
+	// Expand user-defined gate bodies inline when no modifiers are applied.
+	totalModControls := ctrlCount + negctrlCount
+	if totalModControls == 0 && invCount == 0 && !hasPow {
+		if gd, ok := p.gates[gateName]; ok {
+			return p.expandGateCall(gateName, gd, params, qubits)
+		}
+	}
+
 	g, err := p.resolveGate(gateName, params)
 	if err != nil {
 		if p.cfg.strict {
@@ -1387,6 +1453,13 @@ func (p *parser) parsePrimary() (float64, error) {
 		_, err = p.expect(token.RPAREN)
 		return v, err
 	case token.IDENT:
+		// Check gate parameter substitution before function dispatch.
+		if p.paramValues != nil {
+			if v, ok := p.paramValues[t.Literal]; ok {
+				p.advance()
+				return v, nil
+			}
+		}
 		// Built-in functions: sin, cos, tan, sqrt, exp, log, arccos, etc.
 		p.advance()
 		fname := t.Literal
diff --git a/qasm/parser/parser_test.go b/qasm/parser/parser_test.go
index 30f9689..f590f89 100644
--- a/qasm/parser/parser_test.go
+++ b/qasm/parser/parser_test.go
@@ -140,12 +140,15 @@ c = measure q;
 	if err != nil {
 		t.Fatal(err)
 	}
-	// mygate (opaque) + 2 measurements = 3 ops
-	if len(c.Ops()) != 3 {
-		t.Errorf("len(Ops) = %d, want 3", len(c.Ops()))
+	// mygate expands to h + cx, plus 2 measurements = 4 ops
+	if len(c.Ops()) != 4 {
+		t.Errorf("len(Ops) = %d, want 4", len(c.Ops()))
 	}
-	if c.Ops()[0].Gate.Name() != "mygate" {
-		t.Errorf("Ops[0].Gate.Name() = %q, want mygate", c.Ops()[0].Gate.Name())
+	if c.Ops()[0].Gate.Name() != "H" {
+		t.Errorf("Ops[0].Gate.Name() = %q, want H", c.Ops()[0].Gate.Name())
+	}
+	if c.Ops()[1].Gate.Name() != "CNOT" {
+		t.Errorf("Ops[1].Gate.Name() = %q, want CNOT", c.Ops()[1].Gate.Name())
 	}
 }
 
diff --git a/sim/statevector/dynamic.go b/sim/statevector/dynamic.go
index 0afe2d0..ffeffa0 100644
--- a/sim/statevector/dynamic.go
+++ b/sim/statevector/dynamic.go
@@ -155,7 +155,11 @@ func readClassicalValue(clbits []int, indices []int) int {
 func (s *Sim) applyOp(op ir.Operation) error {
 	switch op.Gate.Qubits() {
 	case 1:
-		s.applyGate1(op.Qubits[0], op.Gate.Matrix())
+		m := op.Gate.Matrix()
+		if m == nil {
+			return fmt.Errorf("gate %q has no matrix representation", op.Gate.Name())
+		}
+		s.applyGate1(op.Qubits[0], m)
 	case 2:
 		s.dispatchGate2(op.Gate, op.Qubits[0], op.Qubits[1])
 	case 3:
diff --git a/sim/statevector/kernel2q.go b/sim/statevector/kernel2q.go
index d57e130..545810a 100644
--- a/sim/statevector/kernel2q.go
+++ b/sim/statevector/kernel2q.go
@@ -103,6 +103,9 @@ func (s *Sim) dispatchGate2(g gate.Gate, q0, q1 int) {
 
 	// Generic fallback.
 	m := g.Matrix()
+	if m == nil {
+		return // opaque gate with no matrix - treat as identity
+	}
 	if parallel {
 		s.kernel2qGenericParallel(q0, q1, m)
 	} else {
diff --git a/sim/statevector/kernel3q.go b/sim/statevector/kernel3q.go
index 44afb91..1906da7 100644
--- a/sim/statevector/kernel3q.go
+++ b/sim/statevector/kernel3q.go
@@ -36,6 +36,9 @@ func (s *Sim) dispatchGate3(g gate.Gate, q0, q1, q2 int) {
 
 	// Generic fallback.
 	m := g.Matrix()
+	if m == nil {
+		return // opaque gate with no matrix - treat as identity
+	}
 	if parallel {
 		s.kernel3qGenericParallel(q0, q1, q2, m)
 	} else {
diff --git a/sim/statevector/kernel_controlled.go b/sim/statevector/kernel_controlled.go
index ce23735..ef62149 100644
--- a/sim/statevector/kernel_controlled.go
+++ b/sim/statevector/kernel_controlled.go
@@ -16,6 +16,9 @@ func (s *Sim) dispatchControlled(cg gate.ControlledGate, qubits []int) {
 	switch cg.Inner().Qubits() {
 	case 1:
 		m := cg.Inner().Matrix()
+		if m == nil {
+			return // opaque gate with no matrix - treat as identity
+		}
 		if s.numQubits >= 17 {
 			s.applyControlledGate1Parallel(controls, targets[0], m)
 		} else {
@@ -23,6 +26,9 @@ func (s *Sim) dispatchControlled(cg gate.ControlledGate, qubits []int) {
 		}
 	case 2:
 		m := cg.Inner().Matrix()
+		if m == nil {
+			return // opaque gate with no matrix - treat as identity
+		}
 		if s.numQubits >= 17 {
 			s.applyControlledGate2Parallel(controls, targets[0], targets[1], m)
 		} else {
diff --git a/sim/statevector/sim.go b/sim/statevector/sim.go
index 03c438c..bbcb3f3 100644
--- a/sim/statevector/sim.go
+++ b/sim/statevector/sim.go
@@ -136,7 +136,11 @@ func (s *Sim) Apply(c *ir.Circuit) error {
 		}
 		switch op.Gate.Qubits() {
 		case 1:
-			s.applyGate1(op.Qubits[0], op.Gate.Matrix())
+			m := op.Gate.Matrix()
+			if m == nil {
+				return fmt.Errorf("gate %q has no matrix representation", op.Gate.Name())
+			}
+			s.applyGate1(op.Qubits[0], m)
 		case 2:
 			s.dispatchGate2(op.Gate, op.Qubits[0], op.Qubits[1])
 		case 3: