diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index b02cf44f3..365393326 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -1036,6 +1036,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "insp.dynS": "scr_insp_dyn_s",
   "insp.format": "scr_insp_format_pack",
   "insp.formatPrimitives": "scr_insp_format_primitives",
+  "insp.formatList": "scr_insp_format_list",
   "insp.formatWithOptions": "scr_insp_format_with_options",
   "insp.inspect": "scr_insp_inspect",
   "insp.customEnabled": "scr_insp_custom_enabled",
diff --git a/packages/compiler/src/frontend/lowering/lower-console.ts b/packages/compiler/src/frontend/lowering/lower-console.ts
index db4d39e70..21540cf47 100644
--- a/packages/compiler/src/frontend/lowering/lower-console.ts
+++ b/packages/compiler/src/frontend/lowering/lower-console.ts
@@ -21,6 +21,7 @@ import {
   lowerInspectValue,
   lowerRuntimeFormat,
   runtimeFormatString,
+  formatStringConverts,
 } from "./lower-inspect.js";
 import { streamSidesOf } from "./lower-stream.js";
 
@@ -372,7 +373,7 @@ export function lowerConsoleOutput(
     return consoleWrite(stdout, [lowerRuntimeFormat(lowerer, nodes, loc)], loc);
   const runtime =
     nodes.length > 1 && !isStringLiteralLike(first) ? runtimeFormatString(lowerer, first!) : null;
-  if (runtime === "other")
+  if (runtime === "other" && !formatStringConverts(lowerer, first!))
     return consoleWrite(stdout, [lowerRuntimeFormat(lowerer, nodes, loc)], loc);
   if (runtime !== null) {
     const held = heldFormatArgs(lowerer, nodes, loc);
diff --git a/packages/compiler/src/frontend/lowering/lower-inspect.ts b/packages/compiler/src/frontend/lowering/lower-inspect.ts
index dd950a2a2..9afe12ed4 100644
--- a/packages/compiler/src/frontend/lowering/lower-inspect.ts
+++ b/packages/compiler/src/frontend/lowering/lower-inspect.ts
@@ -35,6 +35,7 @@ import {
   canConvertToDyn,
   canDynCheckTo,
   funcOf,
+  arrayOf,
   CLASS_FIELD_ATTRIBUTES_PREFIX,
   classDynViewSupported,
   DYN_CLASS_PROPERTIES,
@@ -3205,7 +3206,8 @@ export function lowerFormatCall(
 
   if (!firstIsLiteral) {
     const runtime = argNodes.length > 1 ? runtimeFormatString(lowerer, first) : null;
-    if (runtime === "other") return lowerRuntimeFormat(lowerer, argNodes, loc);
+    if (runtime === "other" && !formatStringConverts(lowerer, first))
+      return lowerRuntimeFormat(lowerer, argNodes, loc);
     if (runtime !== null) {
       // A string that substitutes formats at run time; any other string
       // joins with the remaining arguments, as below.
@@ -3576,6 +3578,14 @@ function runtimeFenceValue(
   };
 }
 
+/** Whether the held-argument form can test a format string whose type
+ * only may hold a string: a union that converts to a checked-dynamic value,
+ * or a value whose checker type does not map (its lowered type decides). */
+export function formatStringConverts(lowerer: Lowerer, first: ts.Expression): boolean {
+  const t = lowerer.mapTypeOf(lowerer.typeOf(first));
+  return t === null || t.kind === "dyn" || lowerer.dynConvertible(t);
+}
+
 /** How a non-literal format string is known: "string" and "dyn" values
  * test for a substitution at run time, "other" types (unions with a string
  * arm) format at run time, and null types never hold a string. */
@@ -3673,14 +3683,10 @@ export function heldFormatArgs(
         "util.format of a void call result (call it as its own statement, then pass undefined)",
       );
     }
-    if (value.type.kind !== "dyn" && !lowerer.dynConvertible(value.type)) {
-      // JavaScript keeps the space-joined branch, which printed these
-      // values before, and refuses only a string that substitutes.
-      if (!isJsSourceFile(ts.getSourceFileOfNode(node))) {
-        lowerer.noLowering(unconvertibleSurface(value.type), node, UNCONVERTIBLE_HINT);
-      }
+    // The space-joined branch prints these values as before; only a string
+    // that substitutes refuses them, where it runs.
+    if (value.type.kind !== "dyn" && !lowerer.dynConvertible(value.type))
       unconvertible ??= { node, type: value.type };
-    }
     // Unit values have no local: their effects run here and a literal
     // stands in.
     const k = value.type.kind;
@@ -3694,16 +3700,20 @@ export function heldFormatArgs(
     return { node, ref: () => varRef(held.id, value.type, loc) };
   });
   const first = args[0]!.ref();
+  // A lowered value that is neither a string nor convertible holds no
+  // string, so it never substitutes.
   const specifies: IrExpr =
     first.type.kind === "string"
       ? { kind: "libCall", fn: "insp.formatSpecifies", args: [first], type: BOOL, loc }
-      : {
-          kind: "libCall",
-          fn: "insp.formatSpecifiesDyn",
-          args: [lowerer.coerceToExpected(first, DYN)],
-          type: BOOL,
-          loc,
-        };
+      : first.type.kind === "dyn" || lowerer.dynConvertible(first.type)
+        ? {
+            kind: "libCall",
+            fn: "insp.formatSpecifiesDyn",
+            args: [lowerer.coerceToExpected(first, DYN)],
+            type: BOOL,
+            loc,
+          }
+        : { kind: "boolLit", value: false, type: BOOL, loc };
   const format = (): IrExpr => {
     if (unconvertible !== null) {
       const fenced = unconvertible;
@@ -3725,14 +3735,18 @@ export function heldFormatArgs(
       t.kind === "undefinedT" ||
       t.kind === "nullT" ||
       (t.kind === "union" && (lowerer.unions.get(t.unionId)?.arms.every(primitive) ?? false));
+    // A native list of the checked values: converting into it is a tier
+    // boundary, so the statement stays static.
     return {
       kind: "libCall",
-      fn: args.every((arg) => primitive(arg.ref().type)) ? "insp.formatPrimitives" : "insp.format",
+      fn: args.every((arg) => primitive(arg.ref().type))
+        ? "insp.formatPrimitives"
+        : "insp.formatList",
       args: [
         {
-          kind: "dynArrLit",
+          kind: "arrayLit",
           elems: args.map((arg) => lowerer.coerceToExpected(arg.ref(), DYN)),
-          type: DYN,
+          type: arrayOf(DYN),
           loc,
         },
       ],
@@ -3748,6 +3762,11 @@ export function heldFormatArgs(
     if (t.kind === "bytes") return true;
     return inspectSupport(lowerer, t, new Set(), { recursive: false }) === null;
   });
+  // Without a joined branch every call formats: TypeScript refuses the
+  // value at compile time.
+  const refused = unconvertible as { node: ts.Expression; type: IrType } | null;
+  if (!joinable && refused !== null && !isJsSourceFile(ts.getSourceFileOfNode(refused.node)))
+    lowerer.noLowering(unconvertibleSurface(refused.type), refused.node, UNCONVERTIBLE_HINT);
   return { stmts, args, specifies, format, joinable };
 }
 
diff --git a/packages/compiler/src/ir/builtin-effects.ts b/packages/compiler/src/ir/builtin-effects.ts
index d676b05dd..357992a5a 100644
--- a/packages/compiler/src/ir/builtin-effects.ts
+++ b/packages/compiler/src/ir/builtin-effects.ts
@@ -183,6 +183,7 @@ export const MAY_THROW_LIB_FNS: ReadonlySet<IrLibFn> = new Set([
   // The runtime formatter calls toString, valueOf and toJSON, and refuses
   // renderings it has no lowering for.
   "insp.format",
+  "insp.formatList",
   "insp.formatWithOptions",
   "insp.inspect",
   // The options object reads the caller's options (getters included), and
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index 6e07e1467..c127af93c 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -4376,7 +4376,8 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
   "insp.format": { argTypes: [DYN], result: STRING },
   /** insp.format over arguments that are all strings, numbers, booleans,
    * null or undefined, without the checked-dynamic walk. */
-  "insp.formatPrimitives": { argTypes: [DYN], result: STRING },
+  "insp.formatPrimitives": { argTypes: [arrayOf(DYN)], result: STRING },
+  "insp.formatList": { argTypes: [arrayOf(DYN)], result: STRING },
   "insp.formatWithOptions": { argTypes: [DYN, DYN], result: STRING },
   /** util.inspect(value, options, depth, colors) with the call's argument
    * count: the runtime reads the options and renders the checked-dynamic
diff --git a/packages/compiler/src/ir/int32-slots.ts b/packages/compiler/src/ir/int32-slots.ts
index ac0747276..6a30eb92b 100644
--- a/packages/compiler/src/ir/int32-slots.ts
+++ b/packages/compiler/src/ir/int32-slots.ts
@@ -407,6 +407,18 @@ const DYN_READ_ONLY = new Set<string>([
   "fs.utimesSync",
   "insp.dyn",
   "insp.dynS",
+  // util.format and util.inspect over checked values: the inspect walk and
+  // the format conversions read them; insp.registerCustom keeps a class's
+  // custom inspect thunk in the runtime's registry.
+  "insp.customIsSelf",
+  "insp.customResult",
+  "insp.format",
+  "insp.formatList",
+  "insp.formatPrimitives",
+  "insp.formatSpecifiesDyn",
+  "insp.formatWithOptions",
+  "insp.inspect",
+  "insp.registerCustom",
   // The dynamic-tier operators run their dyn twins (scr_js_value.c); the
   // conversions move a value between dyn and jsval, and js.toF64 is
   // dynCheck's number check.
diff --git a/packages/runtime/src/scr_inspect.c b/packages/runtime/src/scr_inspect.c
index 2c7951103..da49804c1 100644
--- a/packages/runtime/src/scr_inspect.c
+++ b/packages/runtime/src/scr_inspect.c
@@ -2575,11 +2575,21 @@ static ScrStr *insp_prim_number(ScrDyn *v, int spec) {
 static const InspFmtOps insp_fmt_primitive_ops = {insp_prim_s, insp_prim_j, insp_prim_number,
                                                   insp_prim_inspect};
 
-ScrStr *scr_insp_format_primitives(ScrDyn *pack) {
+/* A native list of checked values (the compiled format calls) as the
+ * runtime's argument pack. +1. */
+static ScrDyn *insp_list_pack(ScrArr *list) {
+  ScrDyn *pack = scr_dyn_new_arr();
+  for (uint32_t i = 0; i < list->len; i++) scr_dyn_arr_push(pack, scr_arr_get_ref(list, i));
+  return pack;
+}
+
+ScrStr *scr_insp_format_primitives(ScrArr *list) {
   ScrInspFormatOptions o;
   scr_insp_format_defaults(&o, "util.format");
-  if (pack->kind != SCR_DYN_ARR) return scr_str_new("", 0);
-  return insp_format_loop(pack->v.arr.items, pack->v.arr.len, &o, &insp_fmt_primitive_ops);
+  ScrDyn *pack = insp_list_pack(list);
+  ScrStr *out = insp_format_loop(pack->v.arr.items, pack->v.arr.len, &o, &insp_fmt_primitive_ops);
+  scr_dyn_release(pack);
+  return out;
 }
 
 /* The arguments of a dyn array pack (+1 text, or NULL pending). */
@@ -2594,6 +2604,13 @@ ScrStr *scr_insp_format_pack(ScrDyn *pack) {
   return insp_fmt_pack(pack, &o);
 }
 
+ScrStr *scr_insp_format_list(ScrArr *list) {
+  ScrDyn *pack = insp_list_pack(list);
+  ScrStr *out = scr_insp_format_pack(pack);
+  scr_dyn_release(pack);
+  return out;
+}
+
 /* Whether a format string can substitute: a '%' before its last character.
  * A string without one formats as its arguments joined with spaces. */
 bool scr_insp_format_specifies(ScrStr *first) {
diff --git a/packages/runtime/src/scr_inspect.h b/packages/runtime/src/scr_inspect.h
index a95ac1fde..339fe781b 100644
--- a/packages/runtime/src/scr_inspect.h
+++ b/packages/runtime/src/scr_inspect.h
@@ -76,9 +76,13 @@ ScrStr *scr_insp_format(ScrDyn *const *args, size_t argc, const ScrInspFormatOpt
 /* util.format(...pack) and util.formatWithOptions(options, ...pack) over a
  * dyn array of the arguments (+1). */
 ScrStr *scr_insp_format_pack(ScrDyn *pack);
-/* util.format(...pack) when every argument is a string, number, boolean,
+/* util.format over a native list of checked values, the compiled calls'
+ * held arguments (+1). */
+struct ScrArr;
+ScrStr *scr_insp_format_list(struct ScrArr *list);
+/* scr_insp_format_list when every argument is a string, number, boolean,
  * null or undefined, without the checked-dynamic walk (+1). */
-ScrStr *scr_insp_format_primitives(ScrDyn *pack);
+ScrStr *scr_insp_format_primitives(struct ScrArr *list);
 ScrStr *scr_insp_format_with_options(ScrDyn *options, ScrDyn *pack);
 /* util.inspect(value, options, depth, colors) over checked-dynamic
  * arguments with Node's legacy-argument rules; argc is the call's