diff --git a/packages/compiler/src/backend/llvm/expr-primitives.ts b/packages/compiler/src/backend/llvm/expr-primitives.ts
index 82d2229c2..9bf48ab5d 100644
--- a/packages/compiler/src/backend/llvm/expr-primitives.ts
+++ b/packages/compiler/src/backend/llvm/expr-primitives.ts
@@ -9,6 +9,7 @@ import type { LlvmEmitterContext, ExprOf, LlValue } from "./expr-context.js";
 
 import { BYTES_ELEM_NUM, f64Lit } from "./common.js";
 import { emitStringInputs } from "./string-lifetimes.js";
+import { emitStringParts, stringParts } from "./string-construction.js";
 import { emitBorrowedInput } from "./borrowed-inputs.js";
 import { exactInteger, widenInteger, integerNumber } from "./integer-values.js";
 import { integerArithmeticRange } from "../../ir/integer-ranges.js";
@@ -282,6 +283,9 @@ export function emitStringExpr(host: LlvmEmitterContext, e: ExprOf<"strConcat" |
     const B = host.B;
     switch (e.kind) {
       case "strConcat": {
+        if (e.left.kind === "strConcat" || e.right.kind === "strConcat") {
+          return emitStringParts(host, stringParts(e));
+        }
         // Concat may append to a uniquely owned left temporary. A retained
         // read protects an existing binding from that in-place operation.
         const l = host.emitExpr(e.left);
diff --git a/packages/compiler/src/backend/llvm/string-construction.test.ts b/packages/compiler/src/backend/llvm/string-construction.test.ts
new file mode 100644
index 000000000..b226b676c
--- /dev/null
+++ b/packages/compiler/src/backend/llvm/string-construction.test.ts
@@ -0,0 +1,66 @@
+import { expect, test } from "vitest";
+import { STRING, VOID, type IrExpr, type IrModule } from "../../ir/ir.js";
+import { validateModule } from "../../ir/validate.js";
+import { emitLlvmModule } from "./emitter.js";
+import { stringParts } from "./string-construction.js";
+
+const loc = { file: "construction.ts", start: 0, end: 1 };
+const text = (value: string): IrExpr => ({ kind: "strLit", value, type: STRING, loc });
+const ref = (localId: string): IrExpr => ({ kind: "varRef", localId, type: STRING, loc });
+const concat = (left: IrExpr, right: IrExpr): IrExpr => ({ kind: "strConcat", left, right, type: STRING, loc });
+
+function module(value: IrExpr): IrModule {
+  return { irVersion: 13, sourceFile: loc.file, entry: "main", functions: [
+    { name: "main", loc, params: [], locals: [], returnType: VOID, body: [] },
+    { name: "assemble", loc, params: [{ name: "value", localId: "value", type: STRING }],
+      locals: [{ id: "value", name: "value", type: STRING, mutable: true }],
+      returnType: STRING, body: [{ kind: "return", value, loc }] },
+  ] };
+}
+
+test("concat grouping bounds each stack vector and preserves leaf order", () => {
+  const leaves = Array.from({ length: 100 }, (_, i) => text(String(i)));
+  for (const tree of [leaves.reduce(concat), leaves.reduceRight((right, left) => concat(left, right))]) {
+    const seen: IrExpr[] = [];
+    const walk = (value: IrExpr): void => {
+      if (value.kind !== "strConcat") { seen.push(value); return; }
+      const parts = stringParts(value);
+      expect(parts.length).toBeGreaterThanOrEqual(2);
+      expect(parts.length).toBeLessThanOrEqual(16);
+      for (const part of parts) walk(part);
+    };
+    walk(tree);
+    expect(seen).toEqual(leaves);
+  }
+});
+
+test.each([32, 64] as const)("concat uses a borrowed stack vector with %i-bit size_t", (bits) => {
+  const mod = module(concat(concat(text("<"), ref("value")), text(">")));
+  expect(validateModule(mod)).toEqual([]);
+  const ir = emitLlvmModule(mod, { pointerBits: bits });
+  const body = /^define internal [^\n]*@sc_bf_assemble\([^]*?^}/m.exec(ir)![0];
+  expect(body).toContain("alloca [3 x ptr]");
+  expect(body).toMatch(new RegExp(`call ptr @scr_str_concat_parts\\(ptr %[^,]+, i${bits} 3\\)`));
+  expect(body).not.toContain("@scr_str_retain_v");
+  expect(body).not.toContain("@scr_str_release");
+});
+
+test("later writes preserve an owned snapshot of earlier string parts", () => {
+  const changed: IrExpr = { kind: "assignExpr", localId: "value", value: text("new"), type: STRING, loc };
+  const mod = module(concat(concat(ref("value"), changed), ref("value")));
+  expect(validateModule(mod)).toEqual([]);
+  const body = /^define internal [^\n]*@sc_f_assemble\([^]*?^}/m.exec(emitLlvmModule(mod))![0];
+  const retain = body.indexOf("@scr_str_retain_v");
+  const release = body.indexOf("@scr_str_release");
+  const assemble = body.indexOf("@scr_str_concat_parts");
+  expect(retain).toBeGreaterThanOrEqual(0);
+  expect(retain).toBeLessThan(release);
+  expect(release).toBeLessThan(assemble);
+  expect(body.slice(assemble)).toContain("@scr_str_release");
+});
+
+test("two-part concatenation keeps the existing append path", () => {
+  const ir = emitLlvmModule(module(concat(ref("value"), text("suffix"))));
+  expect(ir).toContain("call ptr @scr_str_concat(");
+  expect(ir).not.toContain("@scr_str_concat_parts");
+});
diff --git a/packages/compiler/src/backend/llvm/string-construction.ts b/packages/compiler/src/backend/llvm/string-construction.ts
new file mode 100644
index 000000000..1811b9c6c
--- /dev/null
+++ b/packages/compiler/src/backend/llvm/string-construction.ts
@@ -0,0 +1,41 @@
+import type { IrExpr } from "../../ir/ir.js";
+import type { LlValue, LlvmEmitterContext } from "./expr-context.js";
+import { emitStringInputs } from "./string-lifetimes.js";
+
+const MAX_PARTS = 16;
+
+/** Flatten only string concatenation, preserving conversion and operand
+ * order. Larger trees remain operands of bounded groups, so each call uses
+ * at most sixteen stack slots and does not need a heap argument array. */
+export function stringParts(value: IrExpr): IrExpr[] {
+  const pending = [value];
+  const parts: IrExpr[] = [];
+  while (pending.length > 0) {
+    const part = pending.pop()!;
+    if (part.kind === "strConcat" && parts.length + pending.length < MAX_PARTS - 1) {
+      pending.push(part.right, part.left);
+    } else {
+      parts.push(part);
+    }
+  }
+  return parts;
+}
+
+/** The runtime borrows every part without mutating it. Earlier operands
+ * therefore borrow only when their owners survive all later evaluations;
+ * other operands retain ordinary snapshots and exception cleanup. */
+export function emitStringParts(host: LlvmEmitterContext, parts: readonly IrExpr[]): LlValue {
+  const values = emitStringInputs(host, parts);
+  const B = host.B;
+  const storage = B.slot();
+  B.entryAllocas.push(`${storage} = alloca [${parts.length} x ptr]`);
+  for (let i = 0; i < values.length; i++) {
+    const slot = B.tmp();
+    B.line(`${slot} = getelementptr [${parts.length} x ptr], ptr ${storage}, i32 0, i32 ${i}`);
+    B.line(`store ptr ${values[i]!.name}, ptr ${slot}`);
+  }
+  host.declare(`declare ptr @scr_str_concat_parts(ptr, ${host.sizeType})`);
+  const result = B.tmp();
+  B.line(`${result} = call ptr @scr_str_concat_parts(ptr ${storage}, ${host.sizeType} ${parts.length})`);
+  return host.own({ name: result, type: parts[0]!.type });
+}
diff --git a/packages/compiler/src/frontend/lowering/lower-builtins.ts b/packages/compiler/src/frontend/lowering/lower-builtins.ts
index be4bd6d03..b6a6581b2 100644
--- a/packages/compiler/src/frontend/lowering/lower-builtins.ts
+++ b/packages/compiler/src/frontend/lowering/lower-builtins.ts
@@ -9431,6 +9431,7 @@ function lowerStoredTextCodecCall(lowerer: Lowerer, call: ts.CallExpression, acc
       }
       const subs = call.arguments.slice(1).map((a): IrExpr => {
         const v = lowerer.lowerExpr(a);
+        if (isUnitType(v.type)) return lowerer.ensureString(v, a);
         if (v.type.kind === "string") return v;
         if (v.type.kind === "f64" || v.type.kind === "bool" || v.type.kind === "record") {
           return { kind: "toString", operand: v, type: STRING, loc };
diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 95d0205c1..d9c25f260 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -6311,8 +6311,18 @@ export function ensureString(lowerer: Lowerer, e: IrExpr, node: ts.Node): IrExpr
       );
     }
     if (isUnitType(e.type)) {
-      // `${undefined}` / "" + null: Node prints "undefined"/"null", but
-      // these only arise spelled literally — reject rather than special-case.
+      if (e.kind === "unitLit") {
+        return { kind: "strLit", value: e.unit, type: STRING, loc: e.loc };
+      }
+      // Keep sequencing and branch effects even though the final unit's
+      // spelling is constant. Never turn an unknown producer into a literal.
+      if (e.kind === "seqExpr") {
+        return { ...e, result: lowerer.ensureString(e.result, node), type: STRING };
+      }
+      if (e.kind === "ternary") {
+        return { ...e, then: lowerer.ensureString(e.then, node),
+          else_: lowerer.ensureString(e.else_, node), type: STRING };
+      }
       lowerer.unsupported(
         "SC1090",
         node,
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index c09e08c44..02a200d4e 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -14210,6 +14210,12 @@
     "<repo>/tests/corpus/checked-dispatch-input-lifetimes.js"
    ],
    "diags": []
+  },
+  "<repo>/tests/corpus/string-construction-order.ts": {
+   "order": [
+    "<repo>/tests/corpus/string-construction-order.ts"
+   ],
+   "diags": []
   }
  }
 }
diff --git a/packages/runtime/src/scr_array.c b/packages/runtime/src/scr_array.c
index 38810d2fb..4420dcbba 100644
--- a/packages/runtime/src/scr_array.c
+++ b/packages/runtime/src/scr_array.c
@@ -1335,21 +1335,26 @@ bool scr_arr_includes_ref(ScrArr *a, void *v) {
 
 /* ── join ──────────────────────────────────────────────────────────────── */
 
-static void scr_join_append(char **buf, size_t *len, size_t *cap,
-                             const char *bytes, size_t n) {
-  if (*len + n > *cap) {
-    size_t cap2 = *cap;
-    while (*len + n > cap2) {
-      if (cap2 > SIZE_MAX / 2) scr_arr_oom();
-      cap2 *= 2;
-    }
-    char *grown = realloc(*buf, cap2);
-    if (!grown) scr_arr_oom();
-    *buf = grown;
-    *cap = cap2;
+static size_t scr_join_size(size_t len, size_t extra) {
+  if (extra > SIZE_MAX - sizeof(ScrStr) - 1 - len) scr_arr_oom();
+  return len + extra;
+}
+
+/* The builder owns the final string allocation throughout construction.
+ * It has no aliases or index-cache entries, and no user callbacks run while
+ * appending primitive array values. Finish by publishing the terminator. */
+static void scr_join_append(ScrStr **out, const char *bytes, size_t n) {
+  ScrStr *s = *out;
+  size_t need = scr_join_size(s->len, n);
+  if (need > s->cap) {
+    const size_t limit = SIZE_MAX - sizeof(ScrStr) - 1;
+    size_t cap = s->cap;
+    while (cap < need) cap = cap > limit / 2 ? need : cap * 2;
+    s = scr_str_regrow(s, cap);
+    *out = s;
   }
-  memcpy(*buf + *len, bytes, n);
-  *len += n;
+  memcpy(s->data + s->len, bytes, n);
+  s->len = need;
 }
 
 /* `a.slice(start?, end?)` — a fresh shallow copy of the index range,
@@ -1392,11 +1397,24 @@ ScrArr *scr_arr_slice(ScrArr *a, double start, double end) {
 }
 
 ScrStr *scr_arr_join(ScrArr *a, ScrStr *sep) {
-  size_t cap = 64, len = 0;
-  char *buf = malloc(cap);
-  if (!buf) scr_arr_oom();
+  size_t cap = 64;
+  /* String widths are already known. Size once to avoid growth and copying
+   * even when the array holds holes, explicit undefined or sparse slots.
+   * Numeric formatting stays single-pass rather than formatting twice. */
+  if (a->elem == SCR_ELEM_STR) {
+    const size_t limit = SIZE_MAX - sizeof(ScrStr) - 1;
+    if (a->len > 1 && sep->len > limit / (a->len - 1)) scr_arr_oom();
+    cap = a->len > 1 ? (a->len - 1) * sep->len : 0;
+    for (size_t i = 0; i < a->len; i++) {
+      uint64_t slot;
+      if (scr_arr_state_at(a, i, &slot) != SCR_ARR_VALUE) continue;
+      const ScrStr *s = (const ScrStr *)scr_slot_to_ptr(slot);
+      if (s) cap = scr_join_size(cap, s->len);
+    }
+  }
+  ScrStr *out = scr_str_alloc_raw(0, cap);
   for (size_t i = 0; i < a->len; i++) {
-    if (i > 0) scr_join_append(&buf, &len, &cap, sep->data, sep->len);
+    if (i > 0) scr_join_append(&out, sep->data, sep->len);
     uint64_t slot;
     uint8_t state = scr_arr_state_at(a, i, &slot);
     if (state != SCR_ARR_VALUE) continue;
@@ -1404,16 +1422,16 @@ ScrStr *scr_arr_join(ScrArr *a, ScrStr *sep) {
       case SCR_ELEM_F64: {
         char nb[32];
         size_t n = scr_f64_to_str(scr_slot_to_f64(slot), nb);
-        scr_join_append(&buf, &len, &cap, nb, n);
+        scr_join_append(&out, nb, n);
         break;
       }
       case SCR_ELEM_BOOL:
-        if (slot != 0) scr_join_append(&buf, &len, &cap, "true", 4);
-        else scr_join_append(&buf, &len, &cap, "false", 5);
+        if (slot != 0) scr_join_append(&out, "true", 4);
+        else scr_join_append(&out, "false", 5);
         break;
       case SCR_ELEM_STR: {
         const ScrStr *s = (const ScrStr *)scr_slot_to_ptr(slot);
-        if (s) scr_join_append(&buf, &len, &cap, s->data, s->len);
+        if (s) scr_join_append(&out, s->data, s->len);
         break;
       }
       case SCR_ELEM_ARR:
@@ -1423,8 +1441,7 @@ ScrStr *scr_arr_join(ScrArr *a, ScrStr *sep) {
         scr_trap("scriptc: internal error: join on a ref-element array\n");
     }
   }
-  ScrStr *out = scr_str_new(buf, len);
-  free(buf);
+  out->data[out->len] = '\0';
   return out;
 }
 
@@ -1434,25 +1451,22 @@ ScrStr *scr_arr_join(ScrArr *a, ScrStr *sep) {
  * missing ones skip, exactly the spec's loop. Both arrays are
  * SCR_ELEM_STR; borrows both; +1 result. Never throws. */
 ScrStr *scr_str_raw(ScrArr *raw, ScrArr *subs) {
-  size_t cap = 64, len = 0;
-  char *buf = malloc(cap);
-  if (!buf) scr_arr_oom();
+  ScrStr *out = scr_str_alloc_raw(0, 64);
   for (size_t i = 0; i < raw->len; i++) {
     uint64_t raw_slot;
     if (scr_arr_slot_at(raw, i, &raw_slot)) {
       const ScrStr *s = (const ScrStr *)scr_slot_to_ptr(raw_slot);
-      if (s) scr_join_append(&buf, &len, &cap, s->data, s->len);
+      if (s) scr_join_append(&out, s->data, s->len);
     }
     if (i + 1 < raw->len && i < subs->len) {
       uint64_t sub_slot;
       if (scr_arr_slot_at(subs, i, &sub_slot)) {
         const ScrStr *v = (const ScrStr *)scr_slot_to_ptr(sub_slot);
-        if (v) scr_join_append(&buf, &len, &cap, v->data, v->len);
+        if (v) scr_join_append(&out, v->data, v->len);
       }
     }
   }
-  ScrStr *out = scr_str_new(buf, len);
-  free(buf);
+  out->data[out->len] = '\0';
   return out;
 }
 
diff --git a/packages/runtime/src/scr_runtime.h b/packages/runtime/src/scr_runtime.h
index df6015f6e..53efc952c 100644
--- a/packages/runtime/src/scr_runtime.h
+++ b/packages/runtime/src/scr_runtime.h
@@ -437,7 +437,7 @@ typedef struct ScrVt {
  *
  * cap is the usable byte capacity of data[] excluding the NUL (allocation
  * is sizeof(ScrStr) + cap + 1); cap == len for interned literals and plain
- * allocations. Spare capacity (cap > len) exists only on concat results so
+ * allocations. Builders and concat results may have spare capacity so
  * scr_str_concat can append in place when the left operand is uniquely
  * owned (rc == 1) — observable immutability is preserved: a string with
  * rc > 1 or rc == SIZE_MAX is never mutated.
@@ -454,7 +454,7 @@ ScrStr *scr_str_new(const char *bytes, size_t len); /* returns +1 */
  * malformed subsequences with U+FFFD. NULL with len == 0 is an empty span. */
 ScrStr *scr_str_from_utf8_lossy(const uint8_t *bytes, size_t len); /* +1 */
 
-/* Internal allocators for buffer builders (scr_json.c): a +1 string with
+/* Internal allocators for buffer builders: a +1 string with
  * UNINITIALIZED data (the builder fills bytes, then len and the NUL), and
  * an rc==1-only realloc that grows capacity in place. Both keep the RC
  * audit's live count exact. */
@@ -470,6 +470,8 @@ void scr_str_release(ScrStr *s); /* NULL-tolerant (uninitialized locals) */
 
 /* Borrow both args, return +1. */
 ScrStr *scr_str_concat(ScrStr *a, ScrStr *b);
+/* Borrow each part without mutation; return one independently owned result. */
+ScrStr *scr_str_concat_parts(ScrStr *const *parts, size_t count);
 
 bool scr_str_eq(ScrStr *a, ScrStr *b);
 
diff --git a/packages/runtime/src/scr_string.c b/packages/runtime/src/scr_string.c
index ee3c613f0..ac752c78d 100644
--- a/packages/runtime/src/scr_string.c
+++ b/packages/runtime/src/scr_string.c
@@ -271,6 +271,7 @@ ScrStr *scr_str_alloc_raw(size_t len, size_t cap) {
 }
 
 ScrStr *scr_str_regrow(ScrStr *s, size_t newcap) {
+  if (newcap < s->len || newcap > SIZE_MAX - sizeof(ScrStr) - 1) scr_oom();
   scr_short_forget(s);
   scr_sidx_purge(s); /* realloc may move; the old address may be recycled */
   ScrStr *r = realloc(s, sizeof(ScrStr) + newcap + 1);
@@ -300,6 +301,8 @@ void scr_str_release(ScrStr *s) {
 }
 
 ScrStr *scr_str_concat(ScrStr *a, ScrStr *b) {
+  if (a->len == 0) return scr_str_retain(b);
+  if (b->len == 0) return scr_str_retain(a);
   if (a->len > SIZE_MAX - b->len - sizeof(ScrStr) - 1) scr_oom();
   size_t newlen = a->len + b->len;
   /* In-place append: a is uniquely owned by the caller's borrow (rc == 1 —
@@ -343,6 +346,32 @@ ScrStr *scr_str_concat(ScrStr *a, ScrStr *b) {
   return s;
 }
 
+ScrStr *scr_str_concat_parts(ScrStr *const *parts, size_t count) {
+  const size_t limit = SIZE_MAX - sizeof(ScrStr) - 1;
+  size_t len = 0;
+  ScrStr *only = NULL;
+  size_t nonempty = 0;
+  for (size_t i = 0; i < count; i++) {
+    const size_t n = parts[i]->len;
+    if (n > limit - len) scr_oom();
+    len += n;
+    if (n != 0) {
+      only = parts[i];
+      nonempty++;
+    }
+  }
+  if (nonempty == 1) return scr_str_retain(only);
+  ScrStr *out = scr_str_alloc_raw(len, len);
+  size_t offset = 0;
+  for (size_t i = 0; i < count; i++) {
+    const ScrStr *part = parts[i];
+    memcpy(out->data + offset, part->data, part->len);
+    offset += part->len;
+  }
+  out->data[len] = '\0';
+  return out;
+}
+
 bool scr_str_eq(ScrStr *a, ScrStr *b) {
   return a == b || (a->len == b->len && memcmp(a->data, b->data, a->len) == 0);
 }
diff --git a/packages/runtime/test/test_array.c b/packages/runtime/test/test_array.c
index 5665ae069..56432c500 100644
--- a/packages/runtime/test/test_array.c
+++ b/packages/runtime/test/test_array.c
@@ -549,6 +549,36 @@ static void test_join(void) {
   scr_str_release(sep2);
   scr_arr_release(s);
 
+  s = scr_arr_new(SCR_ELEM_STR, 1);
+  ScrStr *shared = scr_str_new("x\0y", 3);
+  scr_arr_set_ref(s, 0, scr_str_retain(shared));
+  scr_arr_set_undefined(s, 1);
+  scr_arr_set_ref(s, 8192, scr_str_retain(shared));
+  scr_arr_set_ref(s, -1, scr_str_new("ignored", 7));
+  sep0 = scr_str_new("", 0);
+  strs = scr_arr_join(s, sep0);
+  check(strs->len == 6 && memcmp(strs->data, "x\0yx\0y", 6) == 0,
+        "join handles sparse values and embedded zero without properties");
+  check(shared->rc == 3 && strs->data[strs->len] == '\0',
+        "join preserves duplicate input owners and terminates final storage");
+  scr_str_release(strs);
+  strs = scr_arr_join(s, sep);
+  check(strs->len == 8198 && strs->data[3] == ',' && strs->data[8195] == 'x',
+        "join sizing includes separators for sparse holes and undefined");
+  scr_str_release(strs);
+  scr_arr_release(s);
+  scr_str_release(shared);
+  scr_str_release(sep0);
+
+  n = scr_arr_new(SCR_ELEM_F64, 0);
+  for (size_t i = 0; i < 300; i++) scr_arr_push_f64(n, 123.5);
+  nums = scr_arr_join(n, sep);
+  check(nums->len == 1799 && nums->data[1799] == '\0' &&
+        memcmp(nums->data + 1794, "123.5", 5) == 0,
+        "numeric join grows final storage without losing its suffix");
+  scr_str_release(nums);
+  scr_arr_release(n);
+
   scr_str_release(sep);
 #ifdef SCR_RC_AUDIT
   check(scr_str_live_count() == strings0, "join: no strings leaked");
diff --git a/packages/runtime/test/test_string.c b/packages/runtime/test/test_string.c
index 65c38e645..cf407db7e 100644
--- a/packages/runtime/test/test_string.c
+++ b/packages/runtime/test/test_string.c
@@ -171,6 +171,41 @@ static void handoff_append(ScrStr **binding, ScrStr *suffix) {
   scr_str_release(snapshot);
 }
 
+static void construction_asserts(void) {
+  ScrStr *empty = scr_str_new("", 0);
+  ScrStr *value = scr_str_new("a\0\xE6\x97\xA5", 5);
+  ScrStr *parts[] = {empty, value, empty, value};
+  ScrStr *out = scr_str_concat_parts(parts, 4);
+  total++;
+  if (out->len != 10 || memcmp(out->data, value->data, 5) != 0 ||
+      memcmp(out->data + 5, value->data, 5) != 0 || out->data[10] != '\0' ||
+      value->rc != 1 || value->len != 5) {
+    failed++;
+    fputs("CONSTRUCTION: duplicate inputs, embedded zero or ownership\n", stderr);
+  }
+  scr_str_release(out);
+  ScrStr *single = scr_str_concat_parts(parts, 3);
+  ScrStr *left = scr_str_concat(empty, value);
+  ScrStr *right = scr_str_concat(value, empty);
+  total++;
+  if (single != value || left != value || right != value || value->rc != 4) {
+    failed++;
+    fputs("CONSTRUCTION: empty operands must preserve independent owners\n", stderr);
+  }
+  scr_str_release(single);
+  scr_str_release(left);
+  scr_str_release(right);
+  out = scr_str_concat_parts(NULL, 0);
+  total++;
+  if (out->len != 0 || out->data[0] != '\0') {
+    failed++;
+    fputs("CONSTRUCTION: empty part list\n", stderr);
+  }
+  scr_str_release(out);
+  scr_str_release(value);
+  scr_str_release(empty);
+}
+
 static void accumulation_asserts(void) {
   ScrStr *piece = scr_str_new("x", 1);
   ScrStr *acc = scr_str_new("", 0);
@@ -733,6 +768,7 @@ int main(int argc, char **argv) {
   if (in != stdin) fclose(in);
 
   divergence_asserts();
+  construction_asserts();
   accumulation_asserts();
   short_string_asserts();
 #ifdef SCR_SIDX_TEST
diff --git a/tests/corpus/string-construction-order.ts b/tests/corpus/string-construction-order.ts
new file mode 100644
index 000000000..70198190d
--- /dev/null
+++ b/tests/corpus/string-construction-order.ts
@@ -0,0 +1,87 @@
+function changingParts(): void {
+  let text = "initial";
+  const alias = text;
+  const replace = (): string => {
+    text = "replacement";
+    return "middle";
+  };
+  console.log(text + "/" + replace() + "/" + text, alias);
+  text = "again";
+  console.log(`${text}:${replace()}:${text}:${alias}`);
+  console.log((text + "-") + (replace() + "!") + text);
+}
+changingParts();
+
+function projections(): void {
+  const holder = { text: "before" };
+  const change = (): string => {
+    holder.text = "after";
+    return "changed";
+  };
+  console.log(holder.text + " / " + change() + " / " + holder.text);
+}
+projections();
+
+function throwingParts(): void {
+  let order = "";
+  const part = (name: string): string => {
+    order += name;
+    if (name === "b") throw new Error("stopped");
+    return name.repeat(3);
+  };
+  try {
+    console.log(`${part("a")}:${part("b")}:${part("c")}`);
+  } catch (error) {
+    console.log((error as Error).message, order);
+  }
+}
+throwingParts();
+
+const names = ["alpha", "日本語", "😀", "zero\0byte"];
+for (const name of names) {
+  console.log("[" + name + "]" + name + ":" + name);
+  console.log(`${name}|${-0}|${NaN}|${Infinity}|${false}|${null}|${undefined}`);
+  console.log("" + name, name + "", "" + name + "");
+}
+
+console.log(String(null), String(undefined), "null=" + null, undefined + "=missing");
+let effects = 0;
+const condition = (): boolean => { effects++; return effects % 2 === 0; };
+console.log(`${condition() ? null : null}/${condition() ? null : null}/${undefined}`, effects);
+console.log(String((condition(), null)), String((condition(), undefined)), effects);
+
+const words: string[] = ["first"];
+words[2] = "last";
+words.length = 5;
+console.log(words.join("::"), words.join(""));
+words[20] = "far";
+words[-1] = "property";
+console.log(words.join("|").length, words.join("") === "firstlastfar");
+console.log([true, false, true].join("/"));
+console.log([NaN, Infinity, -Infinity, -0, 1e-7, 1e21].join(","));
+
+const raw = ["<", ">", "</", ">"];
+console.log(String.raw({ raw }, "tag", "text", "tag"));
+console.log(String.raw({ raw }, "tag"));
+const empty: string[] = [];
+console.log(String.raw({ raw: empty }, "unused"));
+console.log(String.raw({ raw }, null, undefined, false));
+
+let acc = "";
+const retained: string[] = [];
+for (let i = 0; i < 6; i++) {
+  retained.push(acc);
+  acc = acc + ("[" + names[i % names.length]! + "]");
+}
+console.log(acc, retained.join("|"));
+
+async function suspendedParts(): Promise<void> {
+  let text = "before";
+  const change = async (): Promise<string> => {
+    await Promise.resolve();
+    text = "after";
+    return "middle";
+  };
+  console.log(`${text}/${await change()}/${text}`);
+}
+await suspendedParts();