diff --git a/src/3rdparty b/src/3rdparty
index 0ea23440be..76eb27d2f5 160000
--- a/src/3rdparty
+++ b/src/3rdparty
@@ -1 +1 @@
-Subproject commit 0ea23440be639ae0d3f4269cdb55ef8a1494ed94
+Subproject commit 76eb27d2f5bea28e8e60ed0a4a29fc1ec2ec6b4e
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 2b17525860..b115055e08 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -404,6 +404,7 @@ set(MAIN_SRCS
     script_opt/ZAM/AM-Opt.cc
     script_opt/ZAM/Branches.cc
     script_opt/ZAM/BuiltIn.cc
+    script_opt/ZAM/BuiltInSupport.cc
     script_opt/ZAM/Driver.cc
     script_opt/ZAM/Expr.cc
     script_opt/ZAM/Inst-Gen.cc
diff --git a/src/script_opt/ZAM/BuiltIn.cc b/src/script_opt/ZAM/BuiltIn.cc
index 9f78df21d7..7cf0794d0a 100644
--- a/src/script_opt/ZAM/BuiltIn.cc
+++ b/src/script_opt/ZAM/BuiltIn.cc
@@ -49,6 +49,7 @@ bool ZAMCompiler::IsZAM_BuiltIn(const Expr* e)
 		{"Files::__enable_reassembly", &ZAMCompiler::BuiltIn_Files__enable_reassembly},
 		{"Files::__set_reassembly_buffer", &ZAMCompiler::BuiltIn_Files__set_reassembly_buffer},
 		{"Log::__write", &ZAMCompiler::BuiltIn_Log__write},
+		{"cat", &ZAMCompiler::BuiltIn_cat},
 		{"current_time", &ZAMCompiler::BuiltIn_current_time},
 		{"get_port_transport_proto", &ZAMCompiler::BuiltIn_get_port_etc},
 		{"network_time", &ZAMCompiler::BuiltIn_network_time},
@@ -76,7 +77,7 @@ bool ZAMCompiler::BuiltIn_Analyzer__name(const NameExpr* n, const ExprPList& arg
 
 	if ( args[0]->Tag() == EXPR_CONST )
 		// Doesn't seem worth developing a variant for this weird
-		// usage cast.
+		// usage case.
 		return false;
 
 	int nslot = Frame1Slot(n, OP1_WRITE);
@@ -199,6 +200,151 @@ bool ZAMCompiler::BuiltIn_Log__write(const NameExpr* n, const ExprPList& args)
 	return true;
 	}
 
+bool ZAMCompiler::BuiltIn_cat(const NameExpr* n, const ExprPList& args)
+	{
+	if ( ! n )
+		{
+		reporter->Warning("return value from built-in function ignored");
+		return true;
+		}
+
+	int nslot = Frame1Slot(n, OP1_WRITE);
+	auto& a0 = args[0];
+	ZInstI z;
+
+	if ( args.empty() )
+		{
+		// Weird, but easy enough to support.
+		z = ZInstI(OP_CAT1_VC, nslot);
+		z.t = n->GetType();
+		z.c = ZVal(val_mgr->EmptyString());
+		}
+
+	else if ( args.size() > 1 )
+		{
+		switch ( args.size() )
+			{
+			case 2:
+				z = GenInst(OP_CAT2_V, n);
+				break;
+			case 3:
+				z = GenInst(OP_CAT3_V, n);
+				break;
+			case 4:
+				z = GenInst(OP_CAT4_V, n);
+				break;
+			case 5:
+				z = GenInst(OP_CAT5_V, n);
+				break;
+			case 6:
+				z = GenInst(OP_CAT6_V, n);
+				break;
+			case 7:
+				z = GenInst(OP_CAT7_V, n);
+				break;
+			case 8:
+				z = GenInst(OP_CAT8_V, n);
+				break;
+
+			default:
+				z = GenInst(OP_CATN_V, n);
+				break;
+			}
+
+		z.aux = BuildCatAux(args);
+		}
+
+	else if ( a0->GetType()->Tag() != TYPE_STRING )
+		{
+		if ( a0->Tag() == EXPR_NAME )
+			{
+			z = GenInst(OP_CAT1FULL_VV, n, a0->AsNameExpr());
+			z.t = a0->GetType();
+			}
+		else
+			{
+			z = ZInstI(OP_CAT1_VC, nslot);
+			z.t = n->GetType();
+			z.c = ZVal(ZAM_val_cat(a0->AsConstExpr()->ValuePtr()));
+			}
+		}
+
+	else if ( a0->Tag() == EXPR_CONST )
+		{
+		z = GenInst(OP_CAT1_VC, n, a0->AsConstExpr());
+		z.t = n->GetType();
+		}
+
+	else
+		z = GenInst(OP_CAT1_VV, n, a0->AsNameExpr());
+
+	AddInst(z);
+
+	return true;
+	}
+
+ZInstAux* ZAMCompiler::BuildCatAux(const ExprPList& args)
+	{
+	auto n = args.size();
+	auto aux = new ZInstAux(n);
+	aux->cat_args = new std::unique_ptr<CatArg>[n];
+
+	for ( size_t i = 0; i < n; ++i )
+		{
+		auto& a_i = args[i];
+		auto& t = a_i->GetType();
+
+		std::unique_ptr<CatArg> ca;
+
+		if ( a_i->Tag() == EXPR_CONST )
+			{
+			auto c = a_i->AsConstExpr()->ValuePtr();
+			aux->Add(i, c); // it will be ignored
+			auto sv = ZAM_val_cat(c);
+			auto s = sv->AsString();
+			auto b = reinterpret_cast<char*>(s->Bytes());
+			ca = std::make_unique<CatArg>(std::string(b, s->Len()));
+			}
+
+		else
+			{
+			auto slot = FrameSlot(a_i->AsNameExpr());
+			aux->Add(i, slot, t);
+
+			switch ( t->Tag() )
+				{
+			TYPE_BOOL:
+			TYPE_INT:
+			TYPE_COUNT:
+			TYPE_DOUBLE:
+			TYPE_TIME:
+			TYPE_ENUM:
+			TYPE_PORT:
+			TYPE_ADDR:
+			TYPE_SUBNET:
+				ca = std::make_unique<FixedCatArg>(t);
+				break;
+
+			TYPE_STRING:
+				ca = std::make_unique<StringCatArg>();
+				break;
+
+			TYPE_PATTERN:
+				ca = std::make_unique<PatternCatArg>();
+				break;
+
+				default:
+					ca = std::make_unique<DescCatArg>(t);
+					break;
+				}
+			}
+
+		aux->cat_args[i] = std::move(ca);
+		}
+
+	return aux;
+	}
+
 bool ZAMCompiler::BuiltIn_current_time(const NameExpr* n, const ExprPList& args)
 	{
 	if ( ! n )
diff --git a/src/script_opt/ZAM/BuiltIn.h b/src/script_opt/ZAM/BuiltIn.h
index 08467dcd95..3482a2cd7f 100644
--- a/src/script_opt/ZAM/BuiltIn.h
+++ b/src/script_opt/ZAM/BuiltIn.h
@@ -17,6 +17,8 @@ bool BuiltIn_Broker__flush_logs(const NameExpr* n, const ExprPList& args);
 bool BuiltIn_Files__enable_reassembly(const NameExpr* n, const ExprPList& args);
 bool BuiltIn_Files__set_reassembly_buffer(const NameExpr* n, const ExprPList& args);
 bool BuiltIn_Log__write(const NameExpr* n, const ExprPList& args);
+bool BuiltIn_cat(const NameExpr* n, const ExprPList& args);
+ZInstAux* BuildCatAux(const ExprPList& args);
 bool BuiltIn_current_time(const NameExpr* n, const ExprPList& args);
 bool BuiltIn_get_port_etc(const NameExpr* n, const ExprPList& args);
 bool BuiltIn_network_time(const NameExpr* n, const ExprPList& args);
diff --git a/src/script_opt/ZAM/BuiltInSupport.cc b/src/script_opt/ZAM/BuiltInSupport.cc
new file mode 100644
index 0000000000..7e26a3ff6c
--- /dev/null
+++ b/src/script_opt/ZAM/BuiltInSupport.cc
@@ -0,0 +1,159 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#include "zeek/script_opt/ZAM/BuiltInSupport.h"
+
+#include "zeek/IPAddr.h"
+#include "zeek/RE.h"
+
+namespace zeek::detail
+	{
+
+FixedCatArg::FixedCatArg(const TypePtr& _t) : t(_t)
+	{
+	switch ( t->Tag() )
+		{
+	TYPE_BOOL:
+		max_size = 1;
+		break;
+
+	TYPE_INT:
+		max_size = 20; // sufficient for 64 bits
+		break;
+
+	TYPE_COUNT:
+		max_size = 20; // sufficient for 64 bits
+		break;
+
+	TYPE_DOUBLE:
+	TYPE_TIME:
+		max_size = 32; // from modp_dtoa2 documentatino
+		break;
+
+	TYPE_ENUM:
+		{
+		size_t n = 0;
+		for ( auto e : t->AsEnumType()->Names() )
+			n += e.first.size();
+		max_size = n;
+		break;
+		}
+
+	TYPE_PORT:
+		max_size = 5 + 1 + 7; // <number> + / + "unknown
+		break;
+
+	TYPE_ADDR:
+		max_size = 39; // for IPv6
+		break;
+
+	TYPE_SUBNET:
+		max_size = 39 + 1 + 3; // for IPv6 + / + <3-digits>
+		break;
+
+		default:
+			reporter->InternalError("bad type in FixedCatArg constructor");
+		}
+	}
+
+void FixedCatArg::RenderInto(ZVal* zframe, int slot, char*& res)
+	{
+	auto& z = zframe[slot];
+	int n;
+	const char* text;
+	std::string str;
+
+	switch ( t->Tag() )
+		{
+	TYPE_BOOL:
+		*(res++) = z.AsInt() ? 'T' : 'F';
+		break;
+
+	TYPE_INT:
+		n = modp_litoa10(z.AsInt(), res);
+		res += n;
+		break;
+
+	TYPE_COUNT:
+		n = modp_ulitoa10(z.AsCount(), res);
+		res += n;
+		break;
+
+	TYPE_DOUBLE:
+	TYPE_TIME:
+		n = modp_dtoa2(z.AsDouble(), res, 6);
+		res += n;
+		break;
+
+	TYPE_PATTERN:
+		text = z.AsPattern()->AsPattern()->PatternText();
+		*(res++) = '/';
+		strcpy(res, text);
+		res += strlen(text);
+		*(res++) = '/';
+		break;
+
+	TYPE_ENUM:
+		text = t->AsEnumType()->Lookup(z.AsInt());
+		strcpy(res, text);
+		res += strlen(text);
+		break;
+
+	TYPE_PORT:
+		{
+		uint32_t full_p = static_cast<uint32_t>(z.AsCount());
+		zeek_uint_t p = full_p & ~PORT_SPACE_MASK;
+		n = modp_ulitoa10(p, res);
+		res += n;
+
+		if ( (full_p & TCP_PORT_MASK) == TCP_PORT_MASK )
+			{
+			strcpy(res, "/tcp");
+			res += 4;
+			}
+
+		else if ( (full_p & UDP_PORT_MASK) == UDP_PORT_MASK )
+			{
+			strcpy(res, "/udp");
+			res += 4;
+			}
+
+		else if ( (full_p & ICMP_PORT_MASK) == ICMP_PORT_MASK )
+			{
+			strcpy(res, "/icmp");
+			res += 5;
+			}
+
+		else
+			{
+			strcpy(res, "/unknown");
+			res += 8;
+			}
+
+		break;
+		}
+
+	TYPE_ADDR:
+		str = z.AsAddr()->Get().AsString();
+		strcpy(res, str.c_str());
+		res += strlen(str.c_str());
+		break;
+
+	TYPE_SUBNET:
+		str = z.AsSubNet()->Get().AsString();
+		strcpy(res, str.c_str());
+		res += strlen(str.c_str());
+		break;
+
+		default:
+			reporter->InternalError("bad type in FixedCatArg::RenderInto");
+		}
+	}
+
+size_t PatternCatArg::ComputeMaxSize(ZVal* zframe, int slot)
+	{
+	text = zframe[slot].AsPattern()->AsPattern()->PatternText();
+	n = strlen(text);
+	return n;
+	}
+
+	} // zeek::detail
diff --git a/src/script_opt/ZAM/BuiltInSupport.h b/src/script_opt/ZAM/BuiltInSupport.h
new file mode 100644
index 0000000000..a3193eaa89
--- /dev/null
+++ b/src/script_opt/ZAM/BuiltInSupport.h
@@ -0,0 +1,119 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#include "zeek/Desc.h"
+#include "zeek/Expr.h"
+
+namespace zeek::detail
+	{
+
+// Base class for tracking information about a single cat() argument, with
+// optimizations for some common cases.
+class CatArg
+	{
+public:
+	CatArg(std::string _s) : s(std::move(_s)) { max_size = s->size(); }
+
+	virtual ~CatArg() { }
+
+	size_t MaxSize(ZVal* zframe, int slot)
+		{
+		return max_size ? *max_size : ComputeMaxSize(zframe, slot);
+		}
+
+	virtual void RenderInto(ZVal* zframe, int slot, char*& res)
+		{
+		auto n = *max_size;
+		memcpy(res, s->data(), n);
+		res += n;
+		}
+
+protected:
+	CatArg() { }
+	CatArg(size_t _max_size) : max_size(_max_size) { }
+
+	virtual size_t ComputeMaxSize(ZVal* zframe, int slot) { return 0; }
+
+	// Present if max size is known a priori.
+	std::optional<size_t> max_size;
+
+	// Present if the argument is a constant.
+	std::optional<std::string> s;
+	};
+
+class FixedCatArg : public CatArg
+	{
+public:
+	FixedCatArg(const TypePtr& t);
+
+	void RenderInto(ZVal* zframe, int slot, char*& res) override;
+
+protected:
+	const TypePtr& t;
+	char tmp[256];
+	};
+
+class StringCatArg : public CatArg
+	{
+public:
+	StringCatArg() : CatArg() { }
+
+	void RenderInto(ZVal* zframe, int slot, char*& res) override
+		{
+		auto s = zframe[slot].AsString();
+		auto n = s->Len();
+		memcpy(res, s->Bytes(), n);
+		res += n;
+		}
+
+protected:
+	size_t ComputeMaxSize(ZVal* zframe, int slot) override
+		{
+		return zframe[slot].AsString()->Len();
+		}
+	};
+
+class PatternCatArg : public CatArg
+	{
+public:
+	PatternCatArg() : CatArg() { }
+
+	void RenderInto(ZVal* zframe, int slot, char*& res) override
+		{
+		*(res++) = '/';
+		strcpy(res, text);
+		res += n;
+		*(res++) = '/';
+		}
+
+protected:
+	size_t ComputeMaxSize(ZVal* zframe, int slot) override;
+
+	const char* text;
+	size_t n = 0;
+	};
+
+class DescCatArg : public CatArg
+	{
+public:
+	DescCatArg(const TypePtr& _t) : CatArg(), t(_t) { d.SetStyle(RAW_STYLE); }
+
+	void RenderInto(ZVal* zframe, int slot, char*& res) override
+		{
+		auto n = d.Len();
+		memcpy(res, d.Bytes(), n);
+		res += n;
+		d.Clear();
+		}
+
+protected:
+	size_t ComputeMaxSize(ZVal* zframe, int slot) override
+		{
+		zframe[slot].ToVal(t)->Describe(&d);
+		return d.Len();
+		}
+
+	ODesc d;
+	TypePtr t;
+	};
+
+	} // namespace zeek::detail
diff --git a/src/script_opt/ZAM/Ops.in b/src/script_opt/ZAM/Ops.in
index 5fcb63adf2..f07328b524 100644
--- a/src/script_opt/ZAM/Ops.in
+++ b/src/script_opt/ZAM/Ops.in
@@ -2173,6 +2173,174 @@ internal-op StrStr
 type VVC
 eval	EvalStrStr(frame[z.v2], z.c)
 
+macro Cat1Op(val)
+	auto& v1 = frame[z.v1];
+	ZVal::DeleteManagedType(v1);
+	v1 = val;
+	zeek::Ref(v1.string_val);
+
+internal-op Cat1
+type VC
+eval	Cat1Op(z.c)
+
+internal-op Cat1
+type VV
+eval	Cat1Op(frame[z.v2])
+
+macro Cat1FullVal(val)
+	Cat1Op(ZVal(ZAM_val_cat(val.ToVal(z.t))))
+
+internal-op Cat1Full
+type VC
+eval	Cat1FullVal(z.c)
+
+internal-op Cat1Full
+type VV
+eval	Cat1FullVal(frame[z.v2])
+
+internal-op CatN
+type V
+eval	auto aux = z.aux;
+	auto slots = z.aux->slots;
+	auto& ca = aux->cat_args;
+	int n = aux->n;
+	size_t max_size = 0;
+	for ( int i = 0; i < n; ++i )
+		max_size += ca[i]->MaxSize(frame, slots[i]);
+	auto res = new char[max_size + /* slop */ n + 1];
+	auto res_p = res;
+	for ( int i = 0; i < n; ++i )
+		ca[i]->RenderInto(frame, slots[i], res_p);
+	*res_p = '\0';
+	auto s = new String(true, reinterpret_cast<byte_vec>(res), res_p - res);
+	Cat1Op(ZVal(new StringVal(s)))
+
+macro CatNPre()
+	auto aux = z.aux;
+	auto slots = z.aux->slots;
+	auto& ca = aux->cat_args;
+
+macro CatNMid()
+	auto res = new char[max_size + /* slop */ 10];
+	auto res_p = res;
+
+macro CatNPost()
+	*res_p = '\0';
+	auto s = new String(true, reinterpret_cast<byte_vec>(res), res_p - res);
+	Cat1Op(ZVal(new StringVal(s)))
+
+internal-op Cat2
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	CatNPost()
+
+internal-op Cat3
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	CatNPost()
+
+internal-op Cat4
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	max_size += ca[3]->MaxSize(frame, slots[3]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	ca[3]->RenderInto(frame, slots[3], res_p);
+	CatNPost()
+
+internal-op Cat5
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	max_size += ca[3]->MaxSize(frame, slots[3]);
+	max_size += ca[4]->MaxSize(frame, slots[4]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	ca[3]->RenderInto(frame, slots[3], res_p);
+	ca[4]->RenderInto(frame, slots[4], res_p);
+	CatNPost()
+
+internal-op Cat6
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	max_size += ca[3]->MaxSize(frame, slots[3]);
+	max_size += ca[4]->MaxSize(frame, slots[4]);
+	max_size += ca[5]->MaxSize(frame, slots[5]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	ca[3]->RenderInto(frame, slots[3], res_p);
+	ca[4]->RenderInto(frame, slots[4], res_p);
+	ca[5]->RenderInto(frame, slots[5], res_p);
+	CatNPost()
+
+internal-op Cat7
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	max_size += ca[3]->MaxSize(frame, slots[3]);
+	max_size += ca[4]->MaxSize(frame, slots[4]);
+	max_size += ca[5]->MaxSize(frame, slots[5]);
+	max_size += ca[6]->MaxSize(frame, slots[6]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	ca[3]->RenderInto(frame, slots[3], res_p);
+	ca[4]->RenderInto(frame, slots[4], res_p);
+	ca[5]->RenderInto(frame, slots[5], res_p);
+	ca[6]->RenderInto(frame, slots[6], res_p);
+	CatNPost()
+
+internal-op Cat8
+type V
+eval	CatNPre()
+	size_t max_size = ca[0]->MaxSize(frame, slots[0]);
+	max_size += ca[1]->MaxSize(frame, slots[1]);
+	max_size += ca[2]->MaxSize(frame, slots[2]);
+	max_size += ca[3]->MaxSize(frame, slots[3]);
+	max_size += ca[4]->MaxSize(frame, slots[4]);
+	max_size += ca[5]->MaxSize(frame, slots[5]);
+	max_size += ca[6]->MaxSize(frame, slots[6]);
+	max_size += ca[7]->MaxSize(frame, slots[7]);
+	CatNMid()
+	ca[0]->RenderInto(frame, slots[0], res_p);
+	ca[1]->RenderInto(frame, slots[1], res_p);
+	ca[2]->RenderInto(frame, slots[2], res_p);
+	ca[3]->RenderInto(frame, slots[3], res_p);
+	ca[4]->RenderInto(frame, slots[4], res_p);
+	ca[5]->RenderInto(frame, slots[5], res_p);
+	ca[6]->RenderInto(frame, slots[6], res_p);
+	ca[7]->RenderInto(frame, slots[7], res_p);
+	CatNPost()
+
 internal-op Analyzer--Name
 type VV
 eval	auto atype = frame[z.v2].ToVal(z.t);
diff --git a/src/script_opt/ZAM/Support.cc b/src/script_opt/ZAM/Support.cc
index 81aeaad340..a9c25b2b7d 100644
--- a/src/script_opt/ZAM/Support.cc
+++ b/src/script_opt/ZAM/Support.cc
@@ -72,6 +72,20 @@ StringVal* ZAM_sub_bytes(const StringVal* s, zeek_uint_t start, zeek_int_t n)
 	return new StringVal(ss ? ss : new String(""));
 	}
 
+StringValPtr ZAM_val_cat(const ValPtr& v)
+	{
+	// Quite similar to cat(), but for only one value.
+	zeek::ODesc d;
+	d.SetStyle(RAW_STYLE);
+
+	v->Describe(&d);
+
+	String* s = new String(1, d.TakeBytes(), d.Len());
+	s->SetUseFreeToDelete(true);
+
+	return make_intrusive<StringVal>(s);
+	}
+
 void ZAM_run_time_error(const char* msg)
 	{
 	fprintf(stderr, "%s\n", msg);
diff --git a/src/script_opt/ZAM/Support.h b/src/script_opt/ZAM/Support.h
index 1660ae6277..84c3114e74 100644
--- a/src/script_opt/ZAM/Support.h
+++ b/src/script_opt/ZAM/Support.h
@@ -54,4 +54,6 @@ extern void ZAM_run_time_warning(const Location* loc, const char* msg);
 extern StringVal* ZAM_to_lower(const StringVal* sv);
 extern StringVal* ZAM_sub_bytes(const StringVal* s, zeek_uint_t start, zeek_int_t n);
 
+extern StringValPtr ZAM_val_cat(const ValPtr& v);
+
 	} // namespace zeek::detail
diff --git a/src/script_opt/ZAM/ZInst.h b/src/script_opt/ZAM/ZInst.h
index 9526511424..a27ba054c2 100644
--- a/src/script_opt/ZAM/ZInst.h
+++ b/src/script_opt/ZAM/ZInst.h
@@ -4,6 +4,8 @@
 
 #pragma once
 
+#include "zeek/Desc.h"
+#include "zeek/script_opt/ZAM/BuiltInSupport.h"
 #include "zeek/script_opt/ZAM/Support.h"
 #include "zeek/script_opt/ZAM/ZOp.h"
 
@@ -319,6 +321,7 @@ public:
 		delete[] ints;
 		delete[] constants;
 		delete[] types;
+		delete[] cat_args;
 		}
 
 	// Returns the i'th element of the parallel arrays as a ValPtr.
@@ -405,6 +408,9 @@ public:
 	ValPtr* constants = nullptr;
 	TypePtr* types = nullptr;
 
+	// A parallel array for the cat() built-in replacement.
+	std::unique_ptr<CatArg>* cat_args = nullptr;
+
 	// Used for accessing function names.
 	const ID* id_val = nullptr;