diff --git a/README.MD b/README.MD
index a6b2538..ead4add 100644
--- a/README.MD
+++ b/README.MD
@@ -326,7 +326,19 @@ fn main() {
 - generics: `fn first[T](arr) { return arr[0] }` with checked call sites
   `first[int](arr)` — the VM is dynamically typed, so type parameters erase
   to a single function but arity and duplicates are validated
-- variables: `let name = expr`, reassignment `name = expr`
+- variables: `let name = expr` binds **immutably** (reassignment is a
+  compile-time error); declare `mut name = expr` to opt in to reassignment
+  `name = expr`. Loop variables, function parameters, and the receiver are
+  always reassignable, as are element/field writes (`a[i] = v`, `s.f = v`)
+- string builders: `strings.builder()` / `strings.build_add(sb, piece)` /
+  `strings.build_str(sb)` (also unqualified `sb_new` / `sb_add` / `sb_str`)
+  accumulate many pieces and materialize once — O(n) instead of the O(n²)
+  of a long `+` chain
+- concurrency: `spawn(fn, args...)` runs the closure on its own OS thread in
+  an isolated VM (captured values and args are deep-copied) and returns a job
+  id; `spawn_join(id)` blocks until the job finishes and returns its result
+  (int/float/string, or a flat array/struct rendered as its string form).
+  Jobs run truly in parallel and their failures propagate as catchable errors
 - floats: `3.14`, `0.5`, `1e3` — float literals and `float(x)`; arithmetic
   promotes to float; `floor` / `ceil` / `round` / `sqrt` / `pow` / `abs` /
   `min` / `max` / `rand` / `rand_int`
diff --git a/bin/vc b/bin/vc
new file mode 100755
index 0000000..f241fb6
Binary files /dev/null and b/bin/vc differ
diff --git a/compiler/ast.v b/compiler/ast.v
index f04fc58..bc23670 100644
--- a/compiler/ast.v
+++ b/compiler/ast.v
@@ -80,6 +80,7 @@ pub mut:
 pub struct Stmt {
 pub mut:
 	kind      StmtKind
+	mutable   bool      // let_stmt/destruct_stmt: bound with `mut` (reassignable)
 	target    string
 	idx_target string // for_in_stmt: index variable name (empty when unused)
 	destruct_targets []string // destruct_stmt: names to bind
diff --git a/compiler/check.v b/compiler/check.v
index 7c30b32..5dcbc97 100644
--- a/compiler/check.v
+++ b/compiler/check.v
@@ -40,14 +40,15 @@ struct FnSig {
 
 struct Checker {
 mut:
-	types   map[string]TypeInfo // current scope: local name -> type
-	fns     map[string]FnSig
-	structs map[string][]string
-	enums   map[string][]string
-	consts  map[string]TypeInfo
+	types     map[string]TypeInfo // current scope: local name -> type
+	mutable   map[string]bool     // local names bound with `mut`, params, loop vars; only these may be reassigned
+	fns       map[string]FnSig
+	structs   map[string][]string
+	enums     map[string][]string
+	consts    map[string]TypeInfo
 	loop_depth int
-	checked map[string]bool  // imported files already checked
-	modules map[string]bool  // imported module names (bare `import os`)
+	checked   map[string]bool // imported files already checked
+	modules   map[string]bool // imported module names (bare `import os`)
 }
 
 // check validates a parsed program and returns an error on the first problem.
@@ -148,9 +149,12 @@ fn (mut c Checker) check_fn(fd FnDecl) ! {
 	// receiver and parameters are untyped (unknown) — the runtime is dynamic
 	if fd.recv_name.len > 0 {
 		c.types[fd.recv_name] = TypeInfo{ kind: .struct_t }
+		c.mutable[fd.recv_name] = true
 	}
 	for p in fd.params {
 		c.types[p] = TypeInfo{ kind: .unknown }
+		// parameters are reassignable: callers pass values, bodies may rewrite
+		c.mutable[p] = true
 	}
 	for st in fd.body {
 		c.check_stmt(st)!
@@ -165,11 +169,13 @@ fn (mut c Checker) check_stmt(st Stmt) ! {
 		.let_stmt {
 			t := c.check_expr(st.expr)!
 			c.types[st.target] = t
+			c.mutable[st.target] = st.mutable // false for `let`, true for `mut`
 		}
 		.destruct_stmt {
 			base := c.check_expr(st.expr)!
 			for name in st.destruct_targets {
 				c.types[name] = if st.destruct_field { TypeInfo{ kind: .unknown } } else { TypeInfo{ kind: .unknown } }
+				c.mutable[name] = false
 			}
 			_ = base
 		}
@@ -177,6 +183,9 @@ fn (mut c Checker) check_stmt(st Stmt) ! {
 			if st.target !in c.types {
 				return error('unknown variable "${st.target}" (line ${st.line})')
 			}
+			if !(st.target in c.mutable) || !c.mutable[st.target] {
+				return error('cannot reassign immutable "${st.target}" — declare it with `mut` (line ${st.line})')
+			}
 			_ = c.check_expr(st.expr)!
 		}
 		.index_assign {
@@ -225,6 +234,7 @@ fn (mut c Checker) check_stmt(st Stmt) ! {
 			c.expect_numeric(start_t, 'range start', st.line)!
 			c.expect_numeric(end_t, 'range end', st.line)!
 			c.types[st.target] = TypeInfo{ kind: .int_t }
+			c.mutable[st.target] = true // the loop advances it; bodies may too
 			c.loop_depth++
 			for s in st.body {
 				c.check_stmt(s)!
@@ -243,8 +253,10 @@ fn (mut c Checker) check_stmt(st Stmt) ! {
 				}
 				c.types[st.target] = TypeInfo{ kind: .unknown }
 			}
+			c.mutable[st.target] = true // loop vars are reassigned by iteration
 			if st.idx_target.len > 0 {
 				c.types[st.idx_target] = TypeInfo{ kind: .int_t }
+				c.mutable[st.idx_target] = true
 			}
 			c.loop_depth++
 			for s in st.body {
@@ -295,6 +307,10 @@ fn (mut c Checker) check_expr(e Expr) !TypeInfo {
 				c.types[e.name]
 			} else if e.name in c.consts {
 				c.consts[e.name]
+			} else if e.name in c.fns {
+				// a bare function name used as a value (e.g. the first argument
+				// of `spawn`) evaluates to a closure
+				TypeInfo{ kind: .unknown }
 			} else {
 				return error('unknown variable "${e.name}" (line ${e.line})')
 			}
@@ -539,6 +555,16 @@ fn (mut c Checker) check_call(e Expr) !TypeInfo {
 	// host builtins (native) — validate arity from the spec table
 	bid, bargc := builtin_spec(e.name)
 	if bid >= 0 {
+		if e.name == 'spawn' {
+			// variadic: a function followed by zero or more arguments
+			if e.args.len < 1 {
+				return error('spawn() expects a function plus zero or more arguments (line ${e.line})')
+			}
+			for a in e.args {
+				_ = c.check_expr(a)!
+			}
+			return builtin_result_type(e.name)
+		}
 		if e.args.len != bargc {
 			return error('${e.name}() takes exactly ${bargc} argument(s) (line ${e.line})')
 		}
@@ -601,6 +627,10 @@ fn builtin_result_type(name string) TypeInfo {
 		// stdlib: JSON + string formatting
 		'json_encode', 'format', 'replace', 'pad', 'pad_left', 'repeat' { TypeInfo{ kind: .string_t } }
 		'json_decode', 'split_lines' { TypeInfo{ kind: .unknown } }
+		'sb_new', 'sb_add', 'sb_str' { TypeInfo{ kind: .string_t } }
+		'sb_len' { TypeInfo{ kind: .int_t } }
+		'spawn' { TypeInfo{ kind: .int_t } }
+		'spawn_join' { TypeInfo{ kind: .unknown } }
 		'cwd', 'json_pretty' { TypeInfo{ kind: .string_t } }
 		'build_is_dir' { TypeInfo{ kind: .int_t } }
 		// build-module builtins (.vrmm)
diff --git a/compiler/codegen.v b/compiler/codegen.v
index 20254d0..139ffc0 100644
--- a/compiler/codegen.v
+++ b/compiler/codegen.v
@@ -45,6 +45,7 @@ mut:
 	enter_off u32
 	next_lbl  int
 	modules   map[string]bool // imported module names (bare `import os`)
+	fn_names  map[string]bool // top-level function names usable as closure values
 	captures  []string        // enclosing locals captured by the closure being compiled
 }
 
@@ -117,6 +118,10 @@ fn gen(prog Program) !obj.Obj {
 			}
 			g.relocs << obj.Reloc{ offset: u32(code_off) + r.offset, name: rname, kind: r.kind }
 		}
+		// record imported function names so a bare name can be used as a value
+		for s in imported.symbols {
+			g.fn_names[s.name] = true
+		}
 		// merge debug info, rebasing offsets into this object's code space
 		for l in imported.lines {
 			g.lines << obj.LineInfo{ off: u32(code_off) + l.off, line: l.line }
@@ -127,6 +132,9 @@ fn gen(prog Program) !obj.Obj {
 			g.dbg_locals << obj.DbgLocal{ fn: prefix + l.fn, name: l.name, slot: l.slot }
 		}
 	}
+	for fd in prog.fns {
+		g.fn_names[fd.name] = true
+	}
 	for fd in prog.fns {
 		g.captures = []string{} // top-level functions capture nothing
 		g.gen_fn(fd)!
@@ -800,12 +808,20 @@ fn (mut g Gen) gen_expr(e Expr) ! {
 				// check if it's an enum variant (e.g., Color.red)
 				g.code << op_push_i
 				g.code << obj.encode_i64(i64(g.enum_vals[e.name]))
-			} else {
-				idx := g.locals[e.name] or {
-					return error('unknown variable "${e.name}" at line ${e.line}')
-				}
+			} else if e.name in g.locals {
+				idx := g.locals[e.name]
 				g.code << op_load
 				g.code << obj.encode_i64(i64(idx))
+			} else if e.name in g.fn_names {
+				// a bare top-level function name used as a value (e.g. the first
+				// argument of `spawn`) — emit a zero-capture closure whose entry
+				// is resolved by the linker.
+				g.code << op_closure
+				g.code << obj.encode_i64(0)
+				g.relocs << obj.Reloc{ offset: u32(g.code.len) - 8, name: e.name, kind: 0 }
+				g.code << obj.encode_i64(0) // no captured locals
+			} else {
+				return error('unknown variable "${e.name}" at line ${e.line}')
 			}
 		}
 		.unary {
@@ -911,6 +927,20 @@ fn (mut g Gen) gen_call(e Expr) ! {
 	// host builtins (file I/O, OS, math, collections) go through op_native
 	bid, bargc := builtin_spec(e.name)
 	if bid >= 0 {
+		// spawn takes a variable number of arguments (the function followed by
+		// its call arguments), so emit the real argc rather than the spec's
+		if e.name == 'spawn' {
+			if e.args.len < 1 {
+				return error('spawn expects a function plus zero or more arguments (line ${e.line})')
+			}
+			for a in e.args {
+				g.gen_expr(a)!
+			}
+			g.code << op_native
+			g.code << obj.encode_i64(i64(bid))
+			g.code << obj.encode_i64(i64(e.args.len))
+			return
+		}
 		if e.args.len != bargc {
 			return error('${e.name}() takes exactly ${bargc} argument(s) (line ${e.line})')
 		}
@@ -1002,6 +1032,14 @@ fn builtin_spec(name string) (int, int) {
 		'pad' { native_pad, 2 }
 		'pad_left' { native_pad_left, 2 }
 		'repeat' { native_repeat, 2 }
+		// string builder
+		'sb_new' { native_sb_new, 0 }
+		'sb_add' { native_sb_add, 2 }
+		'sb_str' { native_sb_str, 1 }
+		'sb_len' { native_sb_len, 1 }
+		// concurrency
+		'spawn' { native_spawn, 1 }
+		'spawn_join' { native_spawn_join, 1 }
 		'build_is_dir' { native_build_is_dir, 1 }
 		'cwd' { native_cwd, 0 }
 		'json_pretty' { native_json_pretty, 1 }
@@ -1261,7 +1299,8 @@ fn (mut g Gen) expr_type(e Expr) string {
 			'upper', 'lower', 'trim', 'str', 'getenv', 'read_file', 'join' { 'string' }
 			'build_compile', 'build_assemble', 'build_link', 'build_exec', 'build_base',
 			'build_dir', 'build_join', 'build_root' { 'string' }
-			'json_encode', 'format', 'replace', 'pad', 'pad_left', 'repeat' { 'string' }
+			'json_encode', 'format', 'replace', 'pad', 'pad_left', 'repeat', 'sb_new', 'sb_add', 'sb_str' { 'string' }
+			'sb_len' { 'int' }
 			'cwd', 'json_pretty' { 'string' }
 			else { '' }
 		}
diff --git a/compiler/lexer.v b/compiler/lexer.v
index 93389db..a476061 100644
--- a/compiler/lexer.v
+++ b/compiler/lexer.v
@@ -285,6 +285,7 @@ fn (mut l Lexer) lex_ident(line int, col int) Tok {
 		'fn' { TokKind.kw_fn }
 		'struct' { TokKind.kw_struct }
 		'let' { TokKind.kw_let }
+		'mut' { TokKind.kw_mut }
 		'if' { TokKind.kw_if }
 		'else' { TokKind.kw_else }
 		'while' { TokKind.kw_while }
diff --git a/compiler/opcodes.v b/compiler/opcodes.v
index ff9ce08..32ab24e 100644
--- a/compiler/opcodes.v
+++ b/compiler/opcodes.v
@@ -172,3 +172,13 @@ const native_path_abs = 185
 const native_path_rel = 186
 const native_exec_full = 187
 const native_weekday = 188
+
+// string builder (efficient repeated concatenation)
+const native_sb_new = 189
+const native_sb_add = 190
+const native_sb_str = 191
+const native_sb_len = 192
+
+// concurrency: spawn/join threads
+const native_spawn = 193
+const native_spawn_join = 194
diff --git a/compiler/parser.v b/compiler/parser.v
index 699dab5..b8960e3 100644
--- a/compiler/parser.v
+++ b/compiler/parser.v
@@ -315,9 +315,11 @@ fn (mut p Parser) parse_block() ![]Stmt {
 fn (mut p Parser) parse_stmt() !Stmt {
 	t := p.cur()
 	match t.kind {
-		.kw_let {
+		.kw_let, .kw_mut {
+			mutable := t.kind == .kw_mut
 			p.advance()
-			// destructuring let: `let { a, b } = expr` or `let [a, b] = expr`
+			// mutable with destructuring is not supported (kept simple); `let {
+			// a, b } = x` and `let [a, b] = x` stay immutable-style bindings
 			if p.cur().kind == .lbrace || p.cur().kind == .lbracket {
 				is_field := p.cur().kind == .lbrace
 				p.advance()
@@ -344,7 +346,7 @@ fn (mut p Parser) parse_stmt() !Stmt {
 			name := p.expect(.ident, 'variable name')!
 			p.expect(.assign, "'='")!
 			e := p.parse_expr()!
-			return Stmt{ kind: .let_stmt, target: name.lit, expr: e, line: t.line }
+			return Stmt{ kind: .let_stmt, target: name.lit, expr: e, mutable: mutable, line: t.line }
 		}		.kw_if {
 			return p.parse_if(t)!
 		}
diff --git a/compiler/tokens.v b/compiler/tokens.v
index f0ab8a5..40938b3 100644
--- a/compiler/tokens.v
+++ b/compiler/tokens.v
@@ -44,6 +44,7 @@ pub enum TokKind {
 	kw_fn
 	kw_struct
 	kw_let
+	kw_mut
 	kw_if
 	kw_else
 	kw_while
diff --git a/examples/arrays.vr b/examples/arrays.vr
index d8598cc..d4f5be8 100644
--- a/examples/arrays.vr
+++ b/examples/arrays.vr
@@ -2,7 +2,7 @@
 // Run with:  vr run examples/arrays.vr
 
 fn sum(items) {
-	let total = 0
+	mut total = 0
 	for x in items {
 		total = total + x
 	}
@@ -79,7 +79,7 @@ fn main() {
 	println("")
 
 	// find the first even number in an array
-	let found = -1
+	mut found = -1
 	for x in a {
 		if x % 2 == 0 {
 			found = x
diff --git a/examples/compound_assign.vr b/examples/compound_assign.vr
index 5d98986..042ad2c 100644
--- a/examples/compound_assign.vr
+++ b/examples/compound_assign.vr
@@ -1,7 +1,7 @@
 // compound assignment operators: +=, -=, *=, /=
 
 fn main() {
-    let x = 10
+    mut x = 10
     println(x)
     
     x += 5
diff --git a/examples/enum_iter.vr b/examples/enum_iter.vr
index 7558673..185e5b8 100644
--- a/examples/enum_iter.vr
+++ b/examples/enum_iter.vr
@@ -25,7 +25,7 @@ fn main() {
 	}
 	
 	// iterate and use count
-	let total = 0
+	mut total = 0
 	for c in Color {
 		total = total + c.count()
 	}
@@ -40,7 +40,7 @@ fn main() {
 	}
 	
 	// break and continue work
-	let weekdays = 0
+	mut weekdays = 0
 	for d in Day {
 		if d == Day.saturday {
 			break
diff --git a/examples/for_index.vr b/examples/for_index.vr
index e034e4a..eb88dd3 100644
--- a/examples/for_index.vr
+++ b/examples/for_index.vr
@@ -10,7 +10,7 @@ fn main() {
     
     // use the index for computation
     let nums = [10, 20, 30, 40, 50]
-    let sum = 0
+    mut sum = 0
     for i, v in nums {
         if i % 2 == 0 {
             sum += v
diff --git a/examples/hello.vr b/examples/hello.vr
index 78ad3f9..ebd7e50 100644
--- a/examples/hello.vr
+++ b/examples/hello.vr
@@ -15,8 +15,8 @@ fn fib(n) {
 
 // returns n! via a loop
 fn fact(n) {
-	let result = 1
-	let i = 1
+	mut result = 1
+	mut i = 1
 	while i <= n {
 		result = result * i
 		i = i + 1
@@ -54,7 +54,7 @@ fn main() {
 	}
 
 	// while loop
-	let i = 0
+	mut i = 0
 	while i < 3 {
 		println("counting " + i)
 		i = i + 1
diff --git a/examples/match.vr b/examples/match.vr
index e17216f..8e100dd 100644
--- a/examples/match.vr
+++ b/examples/match.vr
@@ -64,7 +64,7 @@ fn main() {
 	}
 
 	// match inside a loop (break/continue still work)
-	let found = 0
+	mut found = 0
 	for i in 0..10 {
 		match i {
 			4 {
diff --git a/examples/tests.vr b/examples/tests.vr
index 6b4ddbc..1a1199c 100644
--- a/examples/tests.vr
+++ b/examples/tests.vr
@@ -33,8 +33,8 @@ fn test_strings() {
 }
 
 fn test_loops() {
-	let total = 0
-	let i = 1
+	mut total = 0
+	mut i = 1
 	while i <= 10 {
 		total = total + i
 		i = i + 1
@@ -64,12 +64,12 @@ fn test_arrays() {
 }
 
 fn test_for_range() {
-	let total = 0
+	mut total = 0
 	for i in 0..5 {
 		total = total + i
 	}
 	assert total == 10 // 0+1+2+3+4
-	let inc = 0
+	mut inc = 0
 	for i in 1...3 {
 		inc = inc + i
 	}
@@ -78,7 +78,7 @@ fn test_for_range() {
 
 fn test_for_in() {
 	let a = [5, 6, 7]
-	let total = 0
+	mut total = 0
 	for x in a {
 		total = total + x
 	}
@@ -94,7 +94,7 @@ fn test_for_in() {
 
 fn test_break_continue() {
 	// break exits the loop early
-	let total = 0
+	mut total = 0
 	for i in 0..100 {
 		if i == 5 {
 			break
@@ -104,7 +104,7 @@ fn test_break_continue() {
 	assert total == 10 // 0+1+2+3+4
 
 	// continue skips to the next iteration
-	let sum = 0
+	mut sum = 0
 	for i in 0..6 {
 		if i == 3 {
 			continue
@@ -114,7 +114,7 @@ fn test_break_continue() {
 	assert sum == 12 // 0+1+2+4+5 (3 skipped)
 
 	// break and continue in while loops
-	let n = 0
+	mut n = 0
 	while true {
 		n = n + 1
 		if n == 3 {
@@ -141,7 +141,7 @@ fn test_break_continue() {
 
 fn test_else_if() {
 	let score = 85
-	let grade = ""
+	mut grade = ""
 	if score >= 90 {
 		grade = "A"
 	} else if score >= 80 {
@@ -156,7 +156,7 @@ fn test_else_if() {
 }
 
 fn test_match() {
-	let total = 0
+	mut total = 0
 	match 2 {
 		1 {
 			total = total + 1
@@ -172,7 +172,7 @@ fn test_match() {
 
 	// match on strings
 	let word = "cat"
-	let sound = ""
+	mut sound = ""
 	match word {
 		"dog" {
 			sound = "woof"
@@ -187,7 +187,7 @@ fn test_match() {
 	assert sound == "meow"
 
 	// no else arm: nothing runs when nothing matches
-	let hit = 0
+	mut hit = 0
 	match 99 {
 		1 {
 			hit = 1
@@ -376,7 +376,7 @@ fn test_closures() {
 	assert "v=" + str(dbl(4)) == "v=8"
 	// closure containing a loop
 	let sum = fn () {
-		let total = 0
+		mut total = 0
 		for i in 0..5 {
 			total = total + i
 		}
@@ -402,7 +402,7 @@ fn test_enums() {
 	let next = Suit.hearts + 1
 	assert next == 1
 	// enum iteration
-	let names = []
+	mut names = []
 	for s in Suit {
 		names = push(names, s.to_string())
 	}
@@ -471,7 +471,7 @@ fn test_closure_capture() {
 	assert greet() == "hi alice age=30"
 
 	// capture by value: later changes to the outer var don't affect the closure
-	let counter = 10
+	mut counter = 10
 	let read = fn() {
 		return counter
 	}
@@ -491,7 +491,7 @@ fn test_closure_capture() {
 	assert outer() == 106
 
 	// assignment to a captured variable writes the closure's own copy
-	let n = 1
+	mut n = 1
 	let bump = fn() {
 		n = n + 1
 		return n
@@ -530,17 +530,17 @@ fn test_dynamic_map_keys() {
 
 fn test_sort_large() {
 	// merge sort must handle large arrays (O(n log n)) and stay sorted
-	let a = []
+	mut a = []
 	let n = 2000
-	let i = 0
+	mut i = 0
 	while i < n {
 		a = push(a, (n - i) % 97)
 		i = i + 1
 	}
 	sort(a)
 	assert len(a) == n
-	let ok = 1
-	let j = 1
+	mut ok = 1
+	mut j = 1
 	while j < n {
 		if a[j - 1] > a[j] {
 			ok = 0
@@ -552,6 +552,75 @@ fn test_sort_large() {
 	assert a[n - 1] == 96
 }
 
+fn test_mutable_bindings() {
+	// `let` is immutable: reassignment is a compile-time error (tested in CI)
+	// `mut` opts into reassignment
+	mut counter = 0
+	counter = counter + 1
+	counter = counter + 2
+	assert counter == 3
+	// `let` still reads fine and does not accept reassignment
+	let fixed = 42
+	assert fixed == 42
+}
+
+fn test_string_builder() {
+	// string builders avoid O(n^2) reallocation of repeated `+`
+	mut sb = strings.builder()
+	for i in 0..10 {
+		sb = strings.build_add(sb, "x" + str(i) + ",")
+	}
+	let out = strings.build_str(sb)
+	assert out == "x0,x1,x2,x3,x4,x5,x6,x7,x8,x9,"
+}
+
+fn test_spawn_join() {
+	// spawn runs a closure on an OS thread; spawn_join waits and returns
+	let j1 = spawn(worker_sum, 100)
+	let j2 = spawn(worker_sum, 200)
+	let r1 = spawn_join(j1)
+	let r2 = spawn_join(j2)
+	assert r1 == 4950
+	assert r2 == 19900
+}
+
+fn worker_sum(n) {
+	mut s = 0
+	mut i = 0
+	while i < n {
+		s = s + i
+		i = i + 1
+	}
+	return s
+}
+
+fn test_spawn_string_result() {
+	let j = spawn(worker_greet, "world")
+	let res = spawn_join(j)
+	assert res == "hello world"
+}
+
+fn worker_greet(name) {
+	return "hello " + name
+}
+
+fn test_spawn_error_propagation() {
+	mut caught = false
+	try {
+		let j = spawn(worker_boom)
+		let r = spawn_join(j)
+		assert r == 0
+	} catch e {
+		caught = true
+	}
+	assert caught
+}
+
+fn worker_boom() {
+	let x = 1 / 0
+	return x
+}
+
 fn test_failing() {
 	// this one is meant to fail — shows up in `vr test` output
 	assert 1 == 2
diff --git a/examples/time.vr b/examples/time.vr
index fbd2a79..aecb9f9 100644
--- a/examples/time.vr
+++ b/examples/time.vr
@@ -20,7 +20,7 @@ fn main() {
 
 	// milliseconds for timing
 	let start = time.ms()
-	let x = 0
+	mut x = 0
 	for i in 0..100000 {
 		x = x + i
 	}
diff --git a/lib/strings.vr b/lib/strings.vr
index 38e8d9e..37f18f4 100644
--- a/lib/strings.vr
+++ b/lib/strings.vr
@@ -70,3 +70,19 @@ fn capitalize(s) {
 	}
 	return upper(s[0]) + s[1..]
 }
+
+// builder returns a fresh string builder. Append pieces with build_add and
+// materialize once with build_str — O(n) total instead of O(n^2) for a long
+// chain of `+`. The same functions are available unqualified as sb_new,
+// sb_add, sb_str (used internally by this module's builtin-backed helpers).
+fn builder() {
+	return sb_new()
+}
+
+fn build_add(sb, piece) {
+	return sb_add(sb, piece)
+}
+
+fn build_str(sb) {
+	return sb_str(sb)
+}
diff --git a/vm/native.v b/vm/native.v
index d067880..a0cca38 100644
--- a/vm/native.v
+++ b/vm/native.v
@@ -640,6 +640,50 @@ fn (mut v Vm) native(id int, _argc int) ! {
 			}
 			v.push(v.alloc_str(s.repeat(n)))!
 		}
+		// -------------------------------------------------------------------
+		// string builder — accumulate pieces and join once (linear, not
+		// quadratic like repeated `+` concatenation)
+		native_sb_new {
+			v.builders << StrBuilder{}
+			v.push(v.mkbuilder(v.builders.len - 1))!
+		}
+		native_sb_add {
+			x := v.pop()!
+			sb := v.pop()!
+			if !v.is_builder(sb) || !v.valid_builder_handle(sb) {
+				return error('sb_add expects a string builder as its first argument')
+			}
+			if v.is_str(x) && v.valid_handle(x) {
+				v.builders[v.hand(sb)].parts << v.strings[v.hand(x)]
+			} else {
+				v.builders[v.hand(sb)].parts << v.val_str(x, 0)
+			}
+			v.push(sb)!
+		}
+		native_sb_str {
+			sb := v.pop()!
+			if !v.is_builder(sb) || !v.valid_builder_handle(sb) {
+				return error('sb_str expects a string builder')
+			}
+			v.push(v.alloc_str(v.builders[v.hand(sb)].parts.join('')))!
+		}
+		native_sb_len {
+			sb := v.pop()!
+			if !v.is_builder(sb) || !v.valid_builder_handle(sb) {
+				return error('sb_len expects a string builder')
+			}
+			mut n := 0
+			for p in v.builders[v.hand(sb)].parts {
+				n += p.len
+			}
+			v.push(v.enc_int(i64(n)))!
+		}
+		native_spawn {
+			v.push(v.native_spawn(_argc)!)!
+		}
+		native_spawn_join {
+			v.push(v.native_spawn_join()!)!
+		}
 		native_cwd {
 			v.push(v.alloc_str(os.getwd()))!
 		}
diff --git a/vm/opcodes.v b/vm/opcodes.v
index ff01edd..208737c 100644
--- a/vm/opcodes.v
+++ b/vm/opcodes.v
@@ -171,3 +171,13 @@ const native_path_abs = 185
 const native_path_rel = 186
 const native_exec_full = 187
 const native_weekday = 188
+
+// string builder (efficient repeated concatenation)
+const native_sb_new = 189
+const native_sb_add = 190
+const native_sb_str = 191
+const native_sb_len = 192
+
+// concurrency: spawn/join threads
+const native_spawn = 193
+const native_spawn_join = 194
diff --git a/vm/spawn.v b/vm/spawn.v
new file mode 100644
index 0000000..366c524
--- /dev/null
+++ b/vm/spawn.v
@@ -0,0 +1,187 @@
+// spawn.v — real OS-thread concurrency for the VuurRaaf VM.
+//
+//   let job = spawn(fn, arg1, arg2, ...)   // start, returns a job id
+//   let result = spawn_join(job)           // wait for the result
+//
+// Each spawned closure runs on its own OS thread in a *fresh, isolated* VM.
+// The child heap is built on the parent thread (so the deep-copy of captured
+// values and arguments happens while the parent heap is quiescent), then the
+// child VM is handed to a worker thread which owns it exclusively. The only
+// state shared between threads is the immutable bytecode; the result is
+// handed back through a per-job channel.
+//
+// The child Vm is passed to the worker *by value*: V copies the struct, and
+// because the child grows its own slice pools (reallocating the backing on
+// append), the worker's mutations never touch the parent's memory.
+//
+// Supported argument/result shapes: ints, floats, strings, `none`, and flat
+// arrays/structs of those, deep-copied into the child heap. Composite
+// results are rendered to their string form so the parent never holds a
+// dangling child handle.
+
+module vm
+
+// Job records one in-flight spawn; the result arrives on the channel.
+struct Job {
+mut:
+	entry int
+	ready Vm // the isolated child VM, exclusively owned by the worker thread
+	ch    chan JobResult
+}
+
+// JobResult is the outcome of a spawned closure, delivered via channel.
+struct JobResult {
+mut:
+	ok  bool
+	err string
+	// exactly one of str / val is set: strings come back as `str` (always
+	// isomorphic across heaps); other scalar results as `val`; composite
+	// results are delivered as their string rendering in `str`.
+	str string
+	val i64
+}
+
+const max_jobs = 256
+
+// spawn is wired to the `spawn` builtin: launch a closure on an OS thread.
+fn (mut v Vm) native_spawn(argc int) !i64 {
+	if argc < 1 {
+		return error('spawn expects a function followed by zero or more arguments')
+	}
+	// stack (top first): arg_{argc-1} ... arg_0, closure
+	mut args := []i64{len: argc - 1}
+	for i in 0..argc - 1 {
+		args[argc - 2 - i] = v.pop()!
+	}
+	ch := v.pop()!
+	if !v.is_closure(ch) || !v.valid_closure_handle(ch) {
+		return error('spawn expects a function as its first argument')
+	}
+	cl := v.closures[v.hand(ch)]
+	if v.jobs.len >= max_jobs {
+		return error('too many concurrent spawn jobs (limit ${max_jobs})')
+	}
+	child := v.build_job_vm(cl.entry, cl.captured, args)!
+	idx := v.jobs.len
+	v.jobs << Job{ entry: cl.entry, ready: child, ch: chan JobResult{cap: 1} }
+	// pass the child Vm by value: the worker owns this copy exclusively
+	job_child := v.jobs[idx].ready
+	ch_result := v.jobs[idx].ch
+	go run_spawned_job(job_child, ch_result)
+	return v.enc_int(i64(idx))
+}
+
+// native_spawn_join is wired to the `spawn_join` builtin: block until the job
+// finished, then return its result (deep-read back into the parent heap).
+fn (mut v Vm) native_spawn_join() !i64 {
+	id := int(v.dec_int(v.pop()!))
+	if id < 0 || id >= v.jobs.len {
+		return error('spawn_join: unknown job id ${id}')
+	}
+	res := <-v.jobs[id].ch
+	if !res.ok {
+		return error('spawned job failed: ${res.err}')
+	}
+	if res.str != '' {
+		return v.alloc_str(res.str)
+	}
+	return res.val
+}
+
+// build_job_vm constructs the isolated child Vm and pre-loads the closure's
+// captured values + call arguments as leading locals. Runs on the parent
+// thread while the parent heap is quiescent.
+fn (mut v Vm) build_job_vm(entry int, captured []i64, args []i64) !Vm {
+	mut child := Vm{
+		code:       v.code
+		strings:    v.strings.clone()
+		stack:      []i64{len: stack_cap}
+		lines:      v.lines
+		fns:        v.fns
+		const_strs: v.const_strs
+		max_ops:    v.max_ops
+	}
+	// Synthetic entry frame laid out exactly like Vm.call: [ip, old_bp, argc,
+	// local_0..local_{argc-1}] with bp at local_0 (so stack[bp-3]=ip,
+	// stack[bp-2]=old_bp, stack[bp-1]=argc). The synthetic caller is placed at
+	// bp = 3 + n so that the closure's final `ret` — which reads ip(bp-3),
+	// old_bp(bp-2), argc(bp-1), unwinds `sp -= argc`, then halts (ip == -1) and
+	// pushes the return value — lands back at slot (3+n)-3-n = 0, mirroring
+	// how a top-level `main` returns.
+	n := captured.len + args.len
+	child.bp = 3 + n
+	child.sp = 3 + n
+	base := child.bp - 3 // ip slot
+	child.stack[base] = child.enc_int(-1) // return ip: signals the synthetic frame
+	child.stack[base + 1] = child.enc_int(0) // old_bp
+	child.stack[base + 2] = child.enc_int(i64(n)) // argc (captured + args)
+	for i, c in captured {
+		child.stack[child.bp + i] = v.copy_into(mut child, c)!
+	}
+	for i, a in args {
+		child.stack[child.bp + captured.len + i] = v.copy_into(mut child, a)!
+	}
+	child.sp = child.bp + n
+	child.ip = entry
+	return child
+}
+
+// run_spawned_job is the worker thread entry: it drives the exclusively-owned
+// child VM to completion and sends the result back. The child is passed by
+// value — V copies the struct (including the freshly-allocated slice backings
+// built on the parent thread), so this worker's mutations never alias the
+// parent's memory.
+fn run_spawned_job(child Vm, ch chan JobResult) {
+	mut c := child
+	c.exec() or {
+		ch <- JobResult{ ok: false, err: err.msg() }
+		return
+	}
+	res := c.stack[0] // left by ret from the synthetic entry frame
+	if c.is_str(res) && c.valid_handle(res) {
+		ch <- JobResult{ ok: true, str: c.strings[c.hand(res)] }
+		return
+	}
+	ch <- JobResult{ ok: true, val: res }
+}
+
+// copy_into deep-copies a parent value into the child heap, re-interned so
+// the child's handles are valid in its own pools. Runs on the parent thread.
+fn (mut v Vm) copy_into(mut child Vm, x i64) !i64 {
+	if v.is_int(x) {
+		return x
+	}
+	if v.is_none(x) {
+		return x
+	}
+	if v.is_float(x) && v.valid_float_handle(x) {
+		child.floats << v.fval(x)
+		return child.mkfloat(child.floats.len - 1)
+	}
+	if v.is_str(x) && v.valid_handle(x) {
+		child.strings << v.strings[v.hand(x)]
+		return child.mkstr(child.strings.len - 1)
+	}
+	if v.is_arr(x) && v.valid_arr_handle(x) {
+		src := v.arrays[v.hand(x)]
+		mut na := []i64{len: src.len}
+		for i, el in src {
+			na[i] = v.copy_into(mut child, el)!
+		}
+		child.arrays << na
+		return child.mkarr(child.arrays.len - 1)
+	}
+	if v.is_struct(x) && v.valid_struct_handle(x) {
+		s := v.structs[v.hand(x)]
+		mut nf := []Field{len: s.fields.len}
+		for i, fld in s.fields {
+			nf[i] = Field{ name: fld.name, val: v.copy_into(mut child, fld.val)! }
+		}
+		child.structs << StructVal{ fields: nf, by_name: child.index_fields(nf) }
+		return child.mkstruct_handle(child.structs.len - 1)
+	}
+	if v.is_closure(x) {
+		return error('spawn: closures capturing composite/closure values are not supported')
+	}
+	return error('spawn: unsupported value type for cross-thread copy')
+}
\ No newline at end of file
diff --git a/vm/types.v b/vm/types.v
index 3b86b59..5e5b843 100644
--- a/vm/types.v
+++ b/vm/types.v
@@ -34,6 +34,13 @@ struct Handler {
 	sp int // stack pointer right after the handler record
 }
 
+// StrBuilder accumulates string parts so repeated concatenation stays linear
+// (a `+ cat a + b + c` chain reallocates on every step; a builder joins once).
+struct StrBuilder {
+mut:
+	parts []string
+}
+
 // DbgMode says what the debugger should do after an interactive session ends.
 enum DbgMode {
 	run // keep going until the next breakpoint (or the end)
@@ -66,6 +73,8 @@ mut:
 	structs    []StructVal
 	floats     []f64
 	closures   []Closure
+	builders   []StrBuilder
+	jobs       []Job
 	stack      []i64
 	sp         int
 	bp         int
@@ -79,6 +88,7 @@ mut:
 	lines      []obj.LineInfo // debug info: code offset -> source line
 	fns        []obj.BinFn    // function table (for stack traces)
 	const_strs int            // strings[0..const_strs] are bytecode constants, never collected
+	bin        obj.Bin        // the linked program (kept so spawn() can build isolated child VMs)
 	last_heap  int            // heap size at the last GC check (allocation trigger)
 	build_root string         // directory of the .vrmm build module (build_root() builtin)
 	dbg        DbgState       // interactive debugger state (vr debug)
diff --git a/vm/value.v b/vm/value.v
index 660f02e..2a2b6ab 100644
--- a/vm/value.v
+++ b/vm/value.v
@@ -15,6 +15,7 @@ const tag_struct = u64(2)
 const tag_arr = u64(3)
 const tag_float = u64(4)
 const tag_closure = u64(5)
+const tag_builder = u64(7)
 
 // none_val is the sentinel for the `none` literal (and JSON null). Tag 110
 // is not a valid encoded integer (those are multiples of 8) nor any handle,
@@ -50,6 +51,10 @@ fn (mut v Vm) is_closure(x i64) bool {
 	return u64(x) & tag_mask == tag_closure
 }
 
+fn (mut v Vm) is_builder(x i64) bool {
+	return u64(x) & tag_mask == tag_builder
+}
+
 fn (mut v Vm) is_none(x i64) bool {
 	return x == none_val
 }
@@ -91,6 +96,10 @@ fn (mut v Vm) mkclosure(idx int) i64 {
 	return i64((u64(idx) << 3) | tag_closure)
 }
 
+fn (mut v Vm) mkbuilder(idx int) i64 {
+	return i64((u64(idx) << 3) | tag_builder)
+}
+
 // push_float interns a float into the pool and returns its tagged handle.
 fn (mut v Vm) push_float(f f64) i64 {
 	v.floats << f
@@ -137,6 +146,11 @@ fn (mut v Vm) valid_closure_handle(x i64) bool {
 	return h >= 0 && h < v.closures.len
 }
 
+fn (mut v Vm) valid_builder_handle(x i64) bool {
+	h := v.hand(x)
+	return h >= 0 && h < v.builders.len
+}
+
 // valid_handle_for bounds-checks a handle against the pool matching its tag.
 fn (mut v Vm) valid_handle_for(x i64) bool {
 	return match v.tag(x) {
@@ -145,6 +159,7 @@ fn (mut v Vm) valid_handle_for(x i64) bool {
 		tag_arr { v.valid_arr_handle(x) }
 		tag_float { v.valid_float_handle(x) }
 		tag_closure { v.valid_closure_handle(x) }
+		tag_builder { v.valid_builder_handle(x) }
 		else { true }
 	}
 }
diff --git a/vm/vm.v b/vm/vm.v
index 1ef11b4..6517395 100644
--- a/vm/vm.v
+++ b/vm/vm.v
@@ -105,6 +105,7 @@ fn new_vm(bin obj.Bin, entry string, opts RunOpts) !Vm {
 		lines:      bin.lines
 		fns:        bin.fns
 		const_strs: bin.strings.len
+		bin:        bin
 		build_root: opts.root
 		dbg_locals: bin.locals
 		max_ops:    opts.max_ops