fix(ai): clamp a negative balance, refresh stale comments, test the formatters (PR #34 review)

All three review points were real.

1. creditBalance() could render a negative ("-2 left"). An overage pushes the
   remaining balance below zero in the ledger, and the dollar formatter this
   replaced clamped at $0.00 — that clamp was lost in the port. Restored.
   (The same comment also flagged creditCost() rounding sub-0.05 costs to "0";
   that half was already fixed in 7d6c711, which added the "<0.1" floor.)

2. The comment block above the helpers still described the deleted money()
   behaviour ("$0.00" / "<$0.01"), and the response-bar comment still read
   "model · $cost · $left". Both now describe credits.

3. The helpers had no tests, in a repo that already extracts and tests shipped
   chat.html functions. Added creditFormat.test.js on the narrativeUi/shHighlight
   pattern — it EXTRACTS the real functions from chat.html rather than copying
   them, and asserts MICROS_PER_CREDIT is still 10000 so the editor can't drift
   from the website's conversion. To make extraction work, toCredits and
   creditBalance became multi-line (the convention slices to "\n  }").

Verified: 23 suites, 0 failures; creditFormat is 7 cases. Both fixes are
mutation-checked — removing the clamp fails, and restoring the unary + fails.
The suite also caught an arithmetic error in my own first draft ($0.70 is 70
credits, not 7.0), which is a fair advertisement for the reviewer's point.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ndemianc

extensions/levelcode-ai/media/chat.html

@@ -2419,28 +2419,34 @@
   function modelLabel(){
     try { const t = (document.getElementById('model').textContent || 'Auto').replace('▾', '').trim(); return t || 'Auto'; } catch(e){ return 'Auto'; }
   }
-  // [LevelCode] Retail micro-$ → a human figure. Sub-cent turns are common once prompt caching kicks
-  // in, and "$0.00" would read as free/broken — so floor them at "<$0.01" instead of rounding away.
-  // Credits are the in-product spending unit: $1 = 100 credits, so 1 credit = 10_000 retail micro-$.
-  // The gateway sends RETAIL micro-$ (Levelcode.retail_micros), the same figures the account dashboard
-  // renders — so converting here with the same rule keeps the editor and the website in agreement.
+  // [LevelCode] Credits are the in-product spending unit: $1 = 100 credits, so 1 credit = 10_000
+  // retail micro-$. The gateway sends RETAIL micro-$ (Levelcode.retail_micros) — the same figures the
+  // account dashboard renders — so converting here with the same rule keeps the editor and the website
+  // showing one number. Pinned by test/creditFormat.test.js, which extracts these very functions.
   const MICROS_PER_CREDIT = 10000;
-  function toCredits(micros){ const c = Number(micros) / MICROS_PER_CREDIT; return isFinite(c) ? c : 0; }
-  // A balance reads as a whole credit count. A single run, though, can cost well under one credit (a
-  // cheap-model turn is ~0.4), and rounding that to "0" would read as free — so small costs keep one
-  // decimal until they're big enough not to need it.
-  function creditBalance(micros){ return Math.round(toCredits(micros)).toLocaleString(); }
+  function toCredits(micros){
+    const c = Number(micros) / MICROS_PER_CREDIT;
+    return isFinite(c) ? c : 0;
+  }
+  // A balance reads as a whole credit count. Clamped at zero: an overage can push the remaining
+  // balance negative, and "-2 left" is a worse thing to put in front of someone than "0 left" (the
+  // dollar formatter this replaced clamped the same way).
+  function creditBalance(micros){
+    const c = toCredits(micros);
+    return (c > 0 ? Math.round(c) : 0).toLocaleString();
+  }
+  // A single run can cost well under one credit (a cheap-model turn is ~0.4), so small costs keep one
+  // decimal, and anything under 0.05 floors to "<0.1" rather than rounding to "0" — a run that cost
+  // something must never read as free. Returns the FIXED string: wrapping it in a unary + to drop
+  // trailing zeros silently defeated both of those rules. Identical to the dashboard's rate().
   function creditCost(micros){
     const c = toCredits(micros);
     if (c <= 0) { return '0'; }
     if (c >= 10) { return Math.round(c).toLocaleString(); }
-    // Fixed string, not +String(...): the unary + dropped the decimal (7.0 -> "7") and collapsed
-    // anything under 0.05 to "0" — a run that cost something reading as free. Same fix as the
-    // dashboard's rate(), kept identical so the two surfaces render a cost the same way.
     return c < 0.05 ? '<0.1' : c.toFixed(1);
   }
 
-  // Response action bar: (Retry) (Copy) (Helpful) (Unhelpful) … model · $cost · $left.
+  // Response action bar: (Retry) (Copy) (Helpful) (Unhelpful) … model · cost · balance, in credits.
   function addAgentDone(reason, edits, credits, maxSteps, costMicros){
     closeGroup();   // the run is over — finalize any open activity group
     const bar = document.createElement('div'); bar.className = 'agentbar';

extensions/levelcode-ai/test/creditFormat.test.js

@@ -0,0 +1,104 @@
+/*---------------------------------------------------------------------------------------------
+ *  Unit tests for the response bar's credit formatting — run: node test/creditFormat.test.js
+ *
+ *  Like narrativeUi.test.js and shHighlight.test.js, the functions are EXTRACTED from the shipped
+ *  chat.html, so these exercise the real code rather than a copy that can drift from it.
+ *
+ *  Two directions are load-bearing, and both are ways of lying about money:
+ *    1. A run that cost something must never render as "0" — sub-credit turns are the common case on
+ *       cheap models, so they floor rather than round away.
+ *    2. A balance must never render negative. An overage can push it below zero, and "-2 left" is a
+ *       worse thing to show someone than "0 left" (the dollar formatter this replaced clamped too).
+ *--------------------------------------------------------------------------------------------*/
+// @ts-check
+'use strict';
+
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+const html = fs.readFileSync(path.join(__dirname, '..', 'media', 'chat.html'), 'utf8');
+
+// Same slicing convention as narrativeUi.test.js: functions sit at 2 spaces and close with "  }".
+function extract(name) {
+	const start = html.indexOf('function ' + name + '(');
+	assert.ok(start >= 0, 'chat.html no longer defines ' + name + '()');
+	const end = html.indexOf('\n  }', start);
+	assert.ok(end >= 0, 'no closing brace found for ' + name + '()');
+	return html.slice(start, end + 4);
+}
+
+// MICROS_PER_CREDIT is a const in the same scope — pull it from the source so the test can never
+// disagree with the shipped conversion rate.
+const rateMatch = /const MICROS_PER_CREDIT = (\d+);/.exec(html);
+assert.ok(rateMatch, 'chat.html no longer defines MICROS_PER_CREDIT');
+assert.strictEqual(rateMatch[1], '10000', '1 credit must stay $0.01 — the website converts identically');
+
+const sandbox = /** @type {any} */ ({});
+new Function(
+	'const MICROS_PER_CREDIT = ' + rateMatch[1] + ';\n'
+	+ extract('toCredits') + '\n' + extract('creditBalance') + '\n' + extract('creditCost') + '\n'
+	+ 'this.toCredits = toCredits; this.creditBalance = creditBalance; this.creditCost = creditCost;'
+).call(sandbox);
+
+const { toCredits, creditBalance, creditCost } = sandbox;
+
+let n = 0;
+function test(name, fn) { fn(); n++; console.log('  ok - ' + name); }
+
+const usd = (d) => Math.round(d * 1e6);   // dollars → retail micro-$, the unit the gateway sends
+
+// ---- 1. a cost must never read as free --------------------------------------------------------
+
+test('COST: a sub-credit run floors to "<0.1" instead of rounding to zero', () => {
+	// The cheap-model case, and the whole reason this is not a plain round(): a gpt-oss turn is ~0.4
+	// credits and a very cheap one is far less. "0" would tell the user the run was free.
+	assert.strictEqual(creditCost(usd(0.0002)), '<0.1');
+	assert.strictEqual(creditCost(usd(0.0004)), '<0.1');
+	assert.strictEqual(creditCost(usd(0.0036)), '0.4');   // the real gpt-oss per-turn figure
+});
+
+test('COST: keeps one decimal below 10, whole credits above', () => {
+	assert.strictEqual(creditCost(usd(0.07)), '7.0', 'the trailing .0 must survive');
+	assert.strictEqual(creditCost(usd(0.0767)), '7.7');
+	assert.strictEqual(creditCost(usd(0.46)), '46');
+	assert.strictEqual(creditCost(usd(0.5117)), '51');
+});
+
+test('COST: a genuinely zero cost is "0", not "<0.1"', () => {
+	assert.strictEqual(creditCost(0), '0');
+	assert.strictEqual(creditCost(-1), '0', 'a negative cost is nonsense — show nothing owed');
+});
+
+// ---- 2. a balance must never read negative ----------------------------------------------------
+
+test('BALANCE: whole credits, with separators', () => {
+	assert.strictEqual(creditBalance(usd(12.79)), '1,279');   // the figure the dashboard shows
+	assert.strictEqual(creditBalance(usd(100)), '10,000');    // an Ultra month
+	assert.strictEqual(creditBalance(usd(0.004)), '0');
+});
+
+test('BALANCE: an overage clamps to 0 rather than showing a negative', () => {
+	// Going over budget makes remaining negative in the ledger. "-2 left" is worse than "0 left".
+	assert.strictEqual(creditBalance(usd(-0.02)), '0');
+	assert.strictEqual(creditBalance(usd(-5)), '0');
+	assert.strictEqual(creditBalance(-1), '0');
+});
+
+// ---- 3. junk in, something sane out ------------------------------------------------------------
+
+test('ROBUST: nullish and non-numeric input never render NaN', () => {
+	for (const junk of [null, undefined, '', 'abc', {}, []]) {
+		assert.strictEqual(creditBalance(junk), '0', 'balance from ' + JSON.stringify(junk));
+		assert.strictEqual(creditCost(junk), '0', 'cost from ' + JSON.stringify(junk));
+	}
+	assert.strictEqual(toCredits(Infinity), 0, 'a non-finite figure must collapse to 0, not Infinity');
+});
+
+test('CONVERSION: $1 is 100 credits, in both directions of the bar', () => {
+	assert.strictEqual(toCredits(usd(1)), 100);
+	assert.strictEqual(creditBalance(usd(1)), '100');
+	assert.strictEqual(creditCost(usd(1)), '100');
+});
+
+console.log('\ncreditFormat (chat.html): ' + n + ' tests passed.');