diff --git a/tools/build/test/config.test.ts b/tools/build/test/config.test.ts
new file mode 100644
index 00000000..e330bee1
--- /dev/null
+++ b/tools/build/test/config.test.ts
@@ -0,0 +1,25 @@
+
+import assert from 'node:assert/strict';
+import {test} from 'node:test';
+
+import {parseConfig} from '../config.ts';
+
+test('parseConfig parses values with spaces and equals', () => {
+    const cfg = parseConfig([
+        '# comment',
+        '',
+        'MODELS_OPTIPNG=-o7',
+        'MINISPRITE_ADVPNG = -z4 -i100 ',
+        'WEIRD=a=b',
+        '#DISABLED=x',
+    ].join('\n'));
+    assert.deepEqual([...cfg], [
+        ['MODELS_OPTIPNG', '-o7'],
+        ['MINISPRITE_ADVPNG', '-z4 -i100'],
+        ['WEIRD', 'a=b'],
+    ]);
+});
+
+test('parseConfig rejects malformed lines', () => {
+    assert.throws(() => parseConfig('NOVALUE'), /Invalid config line/);
+});
diff --git a/tools/build/test/graph.test.ts b/tools/build/test/graph.test.ts
new file mode 100644
index 00000000..670971bb
--- /dev/null
+++ b/tools/build/test/graph.test.ts
@@ -0,0 +1,29 @@
+
+import assert from 'node:assert/strict';
+import {test} from 'node:test';
+
+import {getRules, resetRules, rule} from '../api.ts';
+import {BuildError, checkGraph} from '../graph.ts';
+
+test('checkGraph rejects duplicate outputs', () => {
+    resetRules();
+    rule('a.png', ['c1 %f %o'], 'out/x.png');
+    rule('b.png', ['c2 %f %o'], 'out/x.png');
+    assert.throws(() => checkGraph(getRules()), BuildError);
+});
+
+test('checkGraph rejects cycles', () => {
+    resetRules();
+    rule('gen/y', ['c1 %f %o'], 'gen/x');
+    rule('gen/x', ['c2 %f %o'], 'gen/y');
+    assert.throws(() => checkGraph(getRules()), /cycle/i);
+});
+
+test('checkGraph orders producers before consumers', () => {
+    resetRules();
+    rule('gen/mid', ['consume %f %o'], 'out/final');
+    rule('src/a', ['produce %f %o'], 'gen/mid');
+    const {order, generated} = checkGraph(getRules());
+    assert.deepEqual(order.map(r => r.outputs[0]), ['gen/mid', 'out/final']);
+    assert.deepEqual([...generated].sort(), ['gen/mid', 'out/final']);
+});
diff --git a/tools/build/test/plan.test.ts b/tools/build/test/plan.test.ts
new file mode 100644
index 00000000..612d6bec
--- /dev/null
+++ b/tools/build/test/plan.test.ts
@@ -0,0 +1,150 @@
+
+import assert from 'node:assert/strict';
+import {test} from 'node:test';
+
+import {forEachRule, getRules, resetRules, rule, type RuleDecl} from '../api.ts';
+import type {StoredRule, StoredRuleOutput} from '../db.ts';
+import {computePlan, type OutputStat, ruleInputSig} from '../plan.ts';
+
+function hashOf(content : string) : Buffer {
+    return Buffer.from(content.padEnd(32, '\0'));
+}
+
+function makeForeach(input : string, outputDir : string) : RuleDecl {
+    resetRules();
+    forEachRule(input, {cmds: ['convert %f %o']}, `${outputDir}/%b`);
+    const decl = getRules()[0]!;
+    assert(decl !== undefined);
+    return decl;
+}
+
+function stored(decl : RuleDecl, hashes : Map<string, Buffer>,
+                opts : {ok? : boolean, outputs? : StoredRuleOutput[]} = {}) : StoredRule {
+    return {
+        id: 1n,
+        key: decl.key,
+        command: decl.command,
+        display: decl.display,
+        template: decl.template,
+        inputSig: ruleInputSig(decl, hashes),
+        ok: opts.ok ?? true,
+        inputs: [
+            ...decl.inputs.map(path => ({path, isDep: false, hash: hashes.get(path)!})),
+            ...decl.deps.map(path => ({path, isDep: true, hash: hashes.get(path)!})),
+        ],
+        outputs: opts.outputs ?? decl.outputs.map(path => ({path, size: 10n, mtimeNs: 100n})),
+    };
+}
+
+function statFrom(entries : Record<string, OutputStat | null>) {
+    return (path : string) : OutputStat | null => entries[path] ?? null;
+}
+
+const GOOD : OutputStat = {size: 10n, mtimeNs: 100n};
+
+test('unchanged rule is clean', () => {
+    const decl = makeForeach('src/a.png', 'build/out');
+    const hashes = new Map([['src/a.png', hashOf('A')]]);
+    const plan = computePlan({
+        current: [decl],
+        stored: [stored(decl, hashes)],
+        hashes,
+        statOutput: statFrom({'build/out/a.png': GOOD}),
+        adopt: false,
+    });
+    assert.deepEqual(plan.clean, [decl]);
+    assert.equal(plan.run.length, 0);
+});
+
+test('dirty reasons', () => {
+    const decl = makeForeach('src/a.png', 'build/out');
+    const hashes = new Map([['src/a.png', hashOf('A')]]);
+    const cases : [StoredRule, (p : string) => OutputStat | null, string][] = [
+        [stored(decl, new Map([['src/a.png', hashOf('OLD')]])),
+         statFrom({'build/out/a.png': GOOD}), 'input-changed'],
+        [stored(decl, hashes, {ok: false}),
+         statFrom({'build/out/a.png': GOOD}), 'failed-last-run'],
+        [stored(decl, hashes), statFrom({}), 'output-missing'],
+        [stored(decl, hashes),
+         statFrom({'build/out/a.png': {size: 11n, mtimeNs: 100n}}), 'output-tampered'],
+    ];
+    for (const [s, statOutput, reason] of cases) {
+        const plan = computePlan({current: [decl], stored: [s], hashes, statOutput, adopt: false});
+        assert.deepEqual(plan.run.map(r => r.reason), [reason]);
+    }
+});
+
+test('unknown rule runs as new; removed rule is stale', () => {
+    const decl = makeForeach('src/a.png', 'build/out');
+    const gone = makeForeach('src/z.png', 'build/out');
+    const hashes = new Map([['src/a.png', hashOf('A')], ['src/z.png', hashOf('Z')]]);
+    const plan = computePlan({
+        current: [decl],
+        stored: [stored(gone, hashes)],
+        hashes,
+        statOutput: statFrom({}),
+        adopt: false,
+    });
+    assert.deepEqual(plan.run.map(r => r.reason), ['new']);
+    assert.deepEqual(plan.stale.map(s => s.key), [gone.key]);
+});
+
+test('renamed input with identical content matches instead of running', () => {
+    const oldDecl = makeForeach('src/a.png', 'build/out');
+    const newDecl = makeForeach('src/b.png', 'build/out');
+    const hashes = new Map([['src/a.png', hashOf('SAME')], ['src/b.png', hashOf('SAME')]]);
+    const plan = computePlan({
+        current: [newDecl],
+        stored: [stored(oldDecl, hashes)],
+        hashes,
+        statOutput: statFrom({'build/out/a.png': GOOD}),
+        adopt: false,
+    });
+    assert.equal(plan.run.length, 0);
+    assert.deepEqual(plan.renames.map(r => [r.from.outputs[0]!.path, r.decl.outputs[0]]),
+                     [['build/out/a.png', 'build/out/b.png']]);
+    // the source rule is still deleted afterward
+    assert.deepEqual(plan.stale.map(s => s.key), [oldDecl.key]);
+});
+
+test('rename does not match differing content or tampered source outputs', () => {
+    const oldDecl = makeForeach('src/a.png', 'build/out');
+    const newDecl = makeForeach('src/b.png', 'build/out');
+
+    const differing = new Map([['src/a.png', hashOf('X')], ['src/b.png', hashOf('Y')]]);
+    let plan = computePlan({
+        current: [newDecl],
+        stored: [stored(oldDecl, differing)],
+        hashes: differing,
+        statOutput: statFrom({'build/out/a.png': GOOD}),
+        adopt: false,
+    });
+    assert.deepEqual(plan.run.map(r => r.reason), ['new']);
+
+    const same = new Map([['src/a.png', hashOf('SAME')], ['src/b.png', hashOf('SAME')]]);
+    plan = computePlan({
+        current: [newDecl],
+        stored: [stored(oldDecl, same)],
+        hashes: same,
+        statOutput: statFrom({'build/out/a.png': {size: 99n, mtimeNs: 100n}}),
+        adopt: false,
+    });
+    assert.deepEqual(plan.run.map(r => r.reason), ['new']);
+});
+
+test('adopt records existing outputs instead of running', () => {
+    resetRules();
+    rule('src/a.png', {cmds: ['convert %f %o']}, 'build/present.png');
+    rule('src/b.png', {cmds: ['convert %f %o']}, 'build/absent.png');
+    const [present, absent] = getRules() as [RuleDecl, RuleDecl];
+    const hashes = new Map([['src/a.png', hashOf('A')], ['src/b.png', hashOf('B')]]);
+    const plan = computePlan({
+        current: [present, absent],
+        stored: [],
+        hashes,
+        statOutput: statFrom({'build/present.png': GOOD}),
+        adopt: true,
+    });
+    assert.deepEqual(plan.adopt, [present]);
+    assert.deepEqual(plan.run.map(r => r.decl), [absent]);
+});
diff --git a/tools/build/test/sprites.test.ts b/tools/build/test/sprites.test.ts
new file mode 100644
index 00000000..00b92510
--- /dev/null
+++ b/tools/build/test/sprites.test.ts
@@ -0,0 +1,41 @@
+
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import pathlib from 'node:path';
+import {test} from 'node:test';
+
+import {base, spritedata, spriteglob} from '../api.ts';
+
+test('spritedata parses id and flags', () => {
+    assert.deepEqual(spritedata('10080'), {id: '10080', data: {}});
+    assert.deepEqual(spritedata('10080-a'), {id: '10080', data: {a: true}});
+    assert.deepEqual(spritedata('10016-b-s'), {id: '10016', data: {b: true, s: true}});
+    assert.deepEqual(spritedata('123-xmega'), {id: '123', data: {x: 'mega'}});
+    // Lua gmatch("[^-]+") skips empty segments
+    assert.deepEqual(spritedata('12--b'), {id: '12', data: {b: true}});
+});
+
+test('base strips directory and final extension', () => {
+    assert.equal(base('src/models/6-mega-x.gif'), '6-mega-x');
+});
+
+test('spriteglob filters by flag truthiness', () => {
+    const dir = fs.mkdtempSync(pathlib.join(os.tmpdir(), 'spriteglob-'));
+    try {
+        for (const name of ['1.png', '1-a.png', '2-b.png', '2-b-s.png', '3-g.png']) {
+            fs.writeFileSync(pathlib.join(dir, name), '');
+        }
+        assert.deepEqual(
+            spriteglob(`${dir}/*.png`, {a: false}).map(p => pathlib.basename(p)),
+            ['1.png', '2-b-s.png', '2-b.png', '3-g.png']);
+        assert.deepEqual(
+            spriteglob(`${dir}/*.png`, {b: false, s: false}).map(p => pathlib.basename(p)),
+            ['1-a.png', '1.png', '3-g.png']);
+        assert.deepEqual(
+            spriteglob(`${dir}/*.png`, {b: true}).map(p => pathlib.basename(p)),
+            ['2-b-s.png', '2-b.png']);
+    } finally {
+        fs.rmSync(dir, {recursive: true, force: true});
+    }
+});
diff --git a/tools/build/test/subst.test.ts b/tools/build/test/subst.test.ts
new file mode 100644
index 00000000..31746512
--- /dev/null
+++ b/tools/build/test/subst.test.ts
@@ -0,0 +1,36 @@
+
+import assert from 'node:assert/strict';
+import {test} from 'node:test';
+
+import {basenameNoExt, flattenCmds, substitute} from '../subst.ts';
+
+test('substitute expands %f/%o/%b/%B', () => {
+    assert.equal(
+        substitute('convert %f -x %o', ['src/a.png'], ['build/a.png']),
+        'convert src/a.png -x build/a.png');
+    assert.equal(
+        substitute('%f + %b + %B', ['src/d/x.tar.gz', 'y.png'], []),
+        'src/d/x.tar.gz y.png + x.tar.gz y.png + x.tar y');
+});
+
+test('substitute handles quoted frame selector', () => {
+    assert.equal(
+        substitute('magick convert "%f[0]" -trim %o', ['src/models/a.gif'], ['out/a.png']),
+        'magick convert "src/models/a.gif[0]" -trim out/a.png');
+});
+
+test('substitute rejects unknown escapes', () => {
+    assert.throws(() => substitute('%d', ['a'], ['b']), /Unknown substitution/);
+});
+
+test('flattenCmds flattens, trims, drops empties', () => {
+    assert.deepEqual(
+        flattenCmds(['a', ['  b ', [], ['c', '']], 'd']),
+        ['a', 'b', 'c', 'd']);
+});
+
+test('basenameNoExt', () => {
+    assert.equal(basenameNoExt('src/dex/10001-b.png'), '10001-b');
+    assert.equal(basenameNoExt('noext'), 'noext');
+    assert.equal(basenameNoExt('a/.hidden'), '.hidden');
+});