fix: compile-check svelte variants at publish time

Field failure (Codex session, 2026-07-28): the agent kept the seeded
stub style block and appended its own second top-level style element in
all three variants. Svelte forbids that, so the user saw a red Vite
compile overlay; the mount-ack loop then self-healed (failure event,
repair, republish, clean accept), but the overlay window is exactly the
kind of thing the user should never see.

The publish gate closes the class: a done reply for a component session
now compile-checks every variant with the app's own compiler BEFORE the
revision bump and the browser broadcast. Failures bounce as a 422 with
file, line, and message plus _instructions; live-poll surfaces the
details in the thrown reply error. The browser never imports a variant
that cannot compile.

Also: the stub guard comments warn that all CSS belongs in the single
existing style block, worded to never contain the literal "<style"
sequence (a mention inside a CSS comment truncates the string surgery
agents use to find the block; the fake test agent caught exactly that).
The JIT svelte instructions carry the same warning.

This work was produced with AI assistance (Claude Code).

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

skill/scripts/live-poll.mjs

@@ -119,8 +119,11 @@ export async function postReply(base, token, reply) {
   });
   if (!res.ok) {
     const body = await res.json().catch(() => ({}));
-    const parts = [body.error || res.statusText, body.reason, body.hint].filter(Boolean);
-    throw new Error(parts.join(': '));
+    const failureLines = Array.isArray(body.failures)
+      ? body.failures.map((f) => `  ${f.file}${f.line != null ? `:${f.line}` : ''} ${f.message}`).join('\n')
+      : null;
+    const parts = [body.error || res.statusText, body.reason, body.hint, failureLines, body._instructions].filter(Boolean);
+    throw new Error(parts.join('\n'));
   }
 }
 

skill/scripts/live-server.mjs

@@ -55,6 +55,7 @@ import {
 import {
   applyDeferredSvelteComponentAccepts,
   bumpSvelteComponentPreviewRevision,
+  compileCheckVariants,
   removeAllSvelteComponentSessions,
   sweepInactiveSvelteComponentSessions,
 } from './live/svelte-component.mjs';
@@ -1322,9 +1323,24 @@ function handlePollPost(req, res) {
     // variant files into a fresh revision dir before the browser is told:
     // the import path changes every publish, so no transform cache can pin a
     // stale compile of a republished module (node_modules is unwatched).
+    // Broken variants are bounced HERE, before the browser imports anything:
+    // a compile error that reaches the page is a red overlay in the user's
+    // face; bounced at publish it is a private fix with file and line.
     if (replyFileMeta.previewMode === 'svelte-component'
         && msg.id
         && (msg.type === 'done' || !msg.type)) {
+      let compileCheck = { ok: true, failures: [] };
+      try { compileCheck = compileCheckVariants(msg.id, process.cwd()); } catch { /* best-effort */ }
+      if (!compileCheck.ok) {
+        res.writeHead(422, { 'Content-Type': 'application/json' });
+        res.end(JSON.stringify({
+          error: 'variant_compile_failed',
+          id: msg.id,
+          failures: compileCheck.failures,
+          _instructions: 'The publish was NOT delivered: the listed variant file(s) do not compile, so the browser never saw them. Fix each failure at the given file and line (the most common cause is a second top-level <style> element; Svelte allows exactly one, so merge all rules into the existing block), then send the same --reply done again.',
+        }));
+        return;
+      }
       try { bumpSvelteComponentPreviewRevision(msg.id, process.cwd()); } catch { /* best-effort */ }
     }
     if (state.sessionStore && msg.id && !skipJournalReply) {

skill/scripts/live/instructions.mjs

@@ -88,7 +88,7 @@ function generateInstructions(event, scriptsPath) {
 function svelteComponentInstructions(event, scaffold, scriptsPath) {
   const dir = scaffold.componentDir;
   const count = event.count;
-  return `Svelte component preview. EDIT the existing stubs ${dir}/v1.svelte ... v${count}.svelte in place; never delete or recreate them; do not read them back (the prop-substituted markup is in scaffold.componentStubMarkup). Keep the stub's control flow ({#each}, {#if}) and propContract prop names exactly; never flatten a loop into literal items. The stub <style> is seeded with the source rules that style the selection; restyle or delete freely, and know that any seeded rule you do not re-declare is REMOVED from source on accept (the preview never applied it). Semantic class selectors only: no @scope, no data-impeccable-* attributes. Params go in ${dir}/params.json keyed by variant number (never an attribute); author knob CSS against var(--p-<id>, default) and :global([data-p-<id>="..."]). Reply with --file ${scaffold.file}. Accept later merges everything into ${scaffold.sourceFile} mechanically; you have no post-accept cleanup.`;
+  return `Svelte component preview. EDIT the existing stubs ${dir}/v1.svelte ... v${count}.svelte in place; never delete or recreate them; do not read them back (the prop-substituted markup is in scaffold.componentStubMarkup). Keep the stub's control flow ({#each}, {#if}) and propContract prop names exactly; never flatten a loop into literal items. The stub <style> is seeded with the source rules that style the selection; restyle or delete freely, and know that any seeded rule you do not re-declare is REMOVED from source on accept (the preview never applied it). ALL your CSS goes inside that ONE existing <style> block: Svelte forbids a second top-level style element, and a publish with a non-compiling variant is bounced back to you with file and line. Semantic class selectors only: no @scope, no data-impeccable-* attributes. Params go in ${dir}/params.json keyed by variant number (never an attribute); author knob CSS against var(--p-<id>, default) and :global([data-p-<id>="..."]). Reply with --file ${scaffold.file}. Accept later merges everything into ${scaffold.sourceFile} mechanically; you have no post-accept cleanup.`;
 }
 
 function deferredWrapperInstructions(event, scaffold, scriptsPath) {

skill/scripts/live/svelte-component.mjs

@@ -309,9 +309,13 @@ function buildVariantStubV2(variantNum, markupWithProps, contract, seededCss) {
   const propsComment = contract.length > 0
     ? `\n<!-- Props: ${contract.map((c) => `${c.prop} (${c.kind}) <- {${c.expr}}`).join(', ')} -->\n`
     : '';
+  // The guard comments must never contain the literal "<style" character
+  // sequence: agents (and the fake test agent) locate the style block with
+  // string searches, and a mention inside a comment truncates their surgery
+  // mid-comment.
   const css = seededCss
-    ? `\n<style>\n  /* Variant ${variantNum}: seeded from the route's current rules; restyle freely */\n${seededCss.split('\n').map((l) => (l.trim() ? '  ' + l : '')).join('\n')}\n</style>\n`
-    : `\n<style>\n  /* Variant ${variantNum}: add scoped CSS here */\n</style>\n`;
+    ? `\n<style>\n  /* Variant ${variantNum}: seeded from the route's current rules; restyle or delete freely.\n     ALL rules go inside THIS block. Svelte allows exactly one top-level style\n     element per component; appending a second one is a compile error. */\n${seededCss.split('\n').map((l) => (l.trim() ? '  ' + l : '')).join('\n')}\n</style>\n`
+    : `\n<style>\n  /* Variant ${variantNum}: add all CSS inside THIS block. Svelte allows exactly\n     one top-level style element; a second one is a compile error. */\n</style>\n`;
   return `${buildPropsScriptV2(contract)}${propsComment}${markupWithProps.trim()}\n${css}`;
 }
 
@@ -1042,6 +1046,41 @@ export function removeSvelteComponentSession(id, cwd = process.cwd()) {
   } catch { /* non-fatal */ }
 }
 
+/**
+ * Compile-check every variant component of a session with the app's own
+ * compiler, BEFORE the browser ever imports them. A variant that does not
+ * compile (the classic: a second top-level <style> appended next to the
+ * seeded one) used to surface as a red Vite overlay in the user's page plus
+ * a mount-failure round trip; bounced at publish time it is a private
+ * agent-side fix with the exact file and line.
+ */
+export function compileCheckVariants(id, cwd = process.cwd()) {
+  const manifest = findSvelteComponentManifest(id, cwd);
+  if (!manifest || !manifest.manifestPath) return { ok: true, failures: [], checked: 0 };
+  const compiler = loadSvelteCompiler(cwd);
+  if (!compiler || typeof compiler.compile !== 'function') return { ok: true, failures: [], checked: 0 };
+  const sessionDir = path.dirname(manifest.manifestPath);
+  const failures = [];
+  let checked = 0;
+  let entries = [];
+  try { entries = fs.readdirSync(sessionDir); } catch { return { ok: true, failures: [], checked: 0 }; }
+  for (const name of entries) {
+    if (!/^v\d+\.svelte$/.test(name)) continue;
+    checked++;
+    try {
+      compiler.compile(fs.readFileSync(path.join(sessionDir, name), 'utf-8'), { generate: false });
+    } catch (err) {
+      failures.push({
+        file: `${manifest.componentDir}/${name}`,
+        line: err?.start?.line ?? null,
+        column: err?.start?.column ?? null,
+        message: String(err?.message || err).split('\n')[0].slice(0, 300),
+      });
+    }
+  }
+  return { ok: failures.length === 0, failures, checked };
+}
+
 /**
  * Snapshot the agent-authored variant files into a fresh revision directory
  * and stamp the manifest. Called by the server on every publish (`done`

tests/live-svelte-component-accept.test.mjs

@@ -5,6 +5,7 @@ import { join, dirname } from 'node:path';
 import { tmpdir } from 'node:os';
 import { fileURLToPath } from 'node:url';
 import {
+  compileCheckVariants,
   extractMatchingSourceCss,
   removeSelectorsFromSvelteSource,
   findSvelteComponentManifest,
@@ -353,3 +354,70 @@ describe('review regressions: preview-truth supersession (the Pitch mangle)', ()
     assert.deepEqual(removed, ['.b']);
   });
 });
+
+describe('review regressions: publish-time compile gate', () => {
+  it('flags a variant with a duplicate top-level style block, passes after the fix', () => {
+    const tmp3 = realpathSync(mkdtempSync(join(tmpdir(), 'impeccable-compile-gate-')));
+    try {
+      mkdirSync(join(tmp3, 'node_modules'), { recursive: true });
+      try {
+        symlinkSync(join(REPO_NODE_MODULES, 'svelte'), join(tmp3, 'node_modules', 'svelte'), 'dir');
+      } catch {
+        cpSync(join(REPO_NODE_MODULES, 'svelte'), join(tmp3, 'node_modules', 'svelte'), { recursive: true });
+      }
+      write(tmp3, 'package.json', JSON.stringify({ name: 'app' }));
+      write(tmp3, 'src/routes/+page.svelte', '<main>\n  <div class="pick">hi</div>\n</main>\n');
+
+      const session = scaffoldSvelteComponentSession({
+        id: 'gate0001',
+        count: 1,
+        sourceFile: 'src/routes/+page.svelte',
+        sourceStartLine: 2,
+        sourceEndLine: 2,
+        originalLines: ['  <div class="pick">hi</div>'],
+        cwd: tmp3,
+      });
+      assert.equal(session.fallback, undefined, session.reason);
+
+      // The exact field failure: the agent kept the seeded block and
+      // appended its own second top-level <style>.
+      write(tmp3, join(session.componentDir, 'v1.svelte'), `<script>
+  let {} = $props();
+</script>
+
+<div class="pick board">hi</div>
+
+<style>
+  .pick { color: red; }
+</style>
+
+<style>
+  .board { margin-top: 86px; }
+</style>
+`);
+      const broken = compileCheckVariants('gate0001', tmp3);
+      assert.equal(broken.ok, false);
+      assert.equal(broken.checked, 1);
+      assert.match(broken.failures[0].message, /single top-level/);
+      assert.match(broken.failures[0].file, /gate0001\/v1\.svelte/);
+      assert.equal(typeof broken.failures[0].line, 'number');
+
+      // Merged into one block: the gate opens.
+      write(tmp3, join(session.componentDir, 'v1.svelte'), `<script>
+  let {} = $props();
+</script>
+
+<div class="pick board">hi</div>
+
+<style>
+  .pick { color: red; }
+  .board { margin-top: 86px; }
+</style>
+`);
+      const fixed = compileCheckVariants('gate0001', tmp3);
+      assert.equal(fixed.ok, true, JSON.stringify(fixed.failures));
+    } finally {
+      rmSync(tmp3, { recursive: true, force: true });
+    }
+  });
+});