diff --git a/tools/build/subst.ts b/tools/build/subst.ts
index 1a3fc9fb..30f07096 100644
--- a/tools/build/subst.ts
+++ b/tools/build/subst.ts
@@ -22,8 +22,16 @@ export function basenameNoExt(path : string) : string {
 // Tup-style substitutions:
 //   %f  inputs, space-joined        %b  input basenames
 //   %o  outputs, space-joined       %B  input basenames without extension
+//   %oN (1-based) a single output, for rules with several
 export function substitute(s : string, inputs : string[], outputs : string[]) : string {
-    return s.replace(/%([a-zA-Z])/g, (match, c : string) => {
+    return s.replace(/%o(\d+)|%([a-zA-Z])/g, (match, n : string | undefined, c : string | undefined) => {
+        if (n !== undefined) {
+            const i = Number(n);
+            if (i < 1 || i > outputs.length) {
+                throw new Error(`Output index out of range (${outputs.length} outputs): ${match} in: ${s}`);
+            }
+            return outputs[i - 1]!;
+        }
         switch (c) {
             case 'f': return inputs.join(' ');
             case 'o': return outputs.join(' ');
diff --git a/tools/build/test/subst.test.ts b/tools/build/test/subst.test.ts
index 31746512..1e786b83 100644
--- a/tools/build/test/subst.test.ts
+++ b/tools/build/test/subst.test.ts
@@ -19,6 +19,18 @@ test('substitute handles quoted frame selector', () => {
         'magick convert "src/models/a.gif[0]" -trim out/a.png');
 });
 
+test('substitute expands indexed %oN', () => {
+    assert.equal(
+        substitute('tool --image %o1 --stylesheet %o2 -- %f', ['a.png'], ['x.png', 'x.css']),
+        'tool --image x.png --stylesheet x.css -- a.png');
+    assert.equal(substitute('%o10', [], ['1', '2', '3', '4', '5', '6', '7', '8', '9', 'ten']), 'ten');
+});
+
+test('substitute rejects out-of-range %oN', () => {
+    assert.throws(() => substitute('%o2', ['a'], ['b']), /out of range/);
+    assert.throws(() => substitute('%o0', ['a'], ['b']), /out of range/);
+});
+
 test('substitute rejects unknown escapes', () => {
     assert.throws(() => substitute('%d', ['a'], ['b']), /Unknown substitution/);
 });