Live: stop the preflight writing source, cache the resolution

The polling-rework preflight wrote the variant scaffold into source during the
poll lease, before the agent acted. On source-preview targets (React/Vue/Vite,
everything but the svelte-component path) that write full-reloaded the
framework; a browser caught mid-reload missed the agent's variant write and the
SSE done, and sat stranded at 0/N.

Restore the 3.5 single-atomic-edit semantics: the preflight still resolves the
element location and computes the scaffold, but --defer-source-write leaves
source untouched and hands the agent the wrapper text plus the picked source
range. The agent splices variants into the wrapper and replaces the range in
one write, so the framework reloads exactly once. The svelte-component path is
untouched (it never writes route source). The missed-completion recovery stays
as defense in depth.

Also cache the resolved source file per target signature (locator + route):
the ~7.6s tree search re-ran on every generate for the same element; a hit now
points the helper straight at the file via --file, invalidated when the target
changes or a resolution fails.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skill/scripts/live-insert.mjs

@@ -131,6 +131,9 @@ Output (JSON):
   const query = argVal(args, '--query');
   const filePath = argVal(args, '--file');
   const text = argVal(args, '--text');
+  // See live-wrap.mjs: preflight computes the scaffold but leaves source
+  // untouched so the agent's single edit is the only framework reload.
+  const deferSourceWrite = args.includes('--defer-source-write');
 
   if (!id) { console.error('Missing --id'); process.exit(1); }
   if (!position) { console.error('Missing --position (before | after)'); process.exit(1); }
@@ -244,12 +247,23 @@ Output (JSON):
     isJsx,
   });
 
-  const newLines = [
-    ...lines.slice(0, spliceIndex),
-    ...wrapperLines,
-    ...lines.slice(spliceIndex),
-  ];
-  fs.writeFileSync(targetFile, newLines.join('\n'), 'utf-8');
+  let deferredWrapper = null;
+  if (deferSourceWrite) {
+    // Insert-as-empty-range: the agent inserts `wrapperBlock` (variants spliced
+    // at the marker) at spliceIndex without removing any source line.
+    deferredWrapper = {
+      block: wrapperLines.join('\n'),
+      replaceStartLine: spliceIndex + 1,
+      replaceEndLine: spliceIndex, // empty range (endLine < startLine) => insertion
+    };
+  } else {
+    const newLines = [
+      ...lines.slice(0, spliceIndex),
+      ...wrapperLines,
+      ...lines.slice(spliceIndex),
+    ];
+    fs.writeFileSync(targetFile, newLines.join('\n'), 'utf-8');
+  }
 
   const insertLine = spliceIndex + 3;
 
@@ -257,6 +271,10 @@ Output (JSON):
     mode: 'insert',
     position,
     file: relTargetFile,
+    sourceWritten: deferredWrapper ? false : undefined,
+    wrapperBlock: deferredWrapper ? deferredWrapper.block : undefined,
+    replaceStartLine: deferredWrapper ? deferredWrapper.replaceStartLine : undefined,
+    replaceEndLine: deferredWrapper ? deferredWrapper.replaceEndLine : undefined,
     insertLine: insertLine + 1,
     commentSyntax,
     styleMode: styleMode.mode,

skill/scripts/live-server.mjs

@@ -235,11 +235,12 @@ function recordAgentPhase(id, phase, details = {}) {
 /**
  * Detect a browser that missed the generation `done` broadcast.
  *
- * The preflight scaffold write triggers a framework full-reload (Astro reloads
- * the page for any .astro edit). If the agent's variant write + `done` land
- * while the browser is mid-reload, the new page misses both the second HMR
- * reload and the SSE `done` — it resumes from the scaffold-only source and
- * sits in GENERATING at 0/N forever. That resumed page always checkpoints
+ * The preflight no longer writes the scaffold into source for source-preview
+ * targets (the agent writes wrapper + variants in one atomic edit), so the old
+ * scaffold-write full-reload that opened the "stranded at 0/N" race is gone.
+ * This recovery stays as defense in depth: any framework reload that drops the
+ * agent's variant write + `done` while the browser is mid-reload leaves the new
+ * page in GENERATING at 0/N. That resumed page always checkpoints
  * (`browser_resumed`), so a checkpoint claiming "still generating, variants
  * missing" for a session whose generation already completed is direct
  * evidence of the miss. Rebuild the `done` payload from the snapshot so the

skill/scripts/live-wrap.mjs

@@ -68,6 +68,13 @@ The agent should insert variant HTML at insertLine.`);
   const filePath = argVal(args, '--file');
   const text = argVal(args, '--text');
   const pageUrl = argVal(args, '--page-url');
+  // Preflight passes this for source-preview targets. It computes the scaffold
+  // (element location + wrapper text) but does NOT write it into source. The
+  // agent then writes the wrapper + all variants in one atomic edit. The
+  // premature server-side write full-reloaded the framework mid-generate and
+  // stranded the browser at 0/N (live-server.mjs missed-completion note). It is
+  // a no-op on the svelte-component path, which never writes the route source.
+  const deferSourceWrite = args.includes('--defer-source-write');
 
   if (!id) { console.error('Missing --id'); process.exit(1); }
   if (!elementId && !classes && !query) {
@@ -334,6 +341,7 @@ The agent should insert variant HTML at insertLine.`);
   let outputEndLine = startLine + wrapperLines.length + (originalLines.length - 1);
   let insertLine;
   let svelteSession = null;
+  let deferredWrapper = null;
 
   if (useSvelteComponent) {
     // Svelte/SvelteKit resets component-local state on markup HMR updates.
@@ -353,6 +361,20 @@ The agent should insert variant HTML at insertLine.`);
     outputStartLine = 1;
     outputEndLine = 1;
     insertLine = 1;
+  } else if (deferSourceWrite) {
+    // Deferred source write: compute the scaffold text but leave source
+    // untouched. The agent replaces the picked element's source range with
+    // `wrapperBlock` (variants spliced at the marker) in one edit. Writing the
+    // scaffold here first would reload the framework before the agent's write
+    // lands, and a browser caught mid-reload misses the `done` and sits at 0/N.
+    deferredWrapper = {
+      block: wrapperLines.join('\n'),
+      replaceStartLine: startLine + 1, // 1-indexed picked-element range the
+      replaceEndLine: endLine + 1,     // agent's wrapper block replaces
+    };
+    // insertLine matches the final file position the wrapper occupies once the
+    // agent replaces the picked range, so downstream consumers stay consistent.
+    insertLine = startLine + 6 + (originalLines.length - 1) + 1;
   } else {
     // Replace the original element with the wrapper
     const newLines = [
@@ -383,6 +405,13 @@ The agent should insert variant HTML at insertLine.`);
     file: outputRelFile,
     sourceFile: useFrameworkComponent ? relTargetFile : undefined,
     previewMode,
+    // Deferred source write: the wrapper is NOT yet in source. The agent
+    // replaces [replaceStartLine, replaceEndLine] with `wrapperBlock` (variants
+    // spliced at the "insert below this line" marker) in one atomic edit.
+    sourceWritten: deferredWrapper ? false : undefined,
+    wrapperBlock: deferredWrapper ? deferredWrapper.block : undefined,
+    replaceStartLine: deferredWrapper ? deferredWrapper.replaceStartLine : undefined,
+    replaceEndLine: deferredWrapper ? deferredWrapper.replaceEndLine : undefined,
     componentDir: componentSession?.componentDir,
     propContract: componentSession?.propContract,
     sourceStartLine: useFrameworkComponent ? startLine + 1 : undefined,

skill/scripts/live/generation-preflight.mjs

@@ -5,7 +5,32 @@ import { promisify } from 'node:util';
 const execFileAsync = promisify(execFile);
 const PREFLIGHT_TIMEOUT_MS = 15_000;
 
-export function buildGenerationPreflight(event, scriptsDir) {
+// Per-target cache of the resolved source file. The wrap search walks the whole
+// project tree and was measured at ~7.6s on a large repo; it re-ran on every
+// generate for the same picked element (re-rolls, param passes). Keyed by the
+// target signature (locator + route), so it invalidates automatically when the
+// element or route changes; a failed resolution evicts its entry (see below).
+const sourceResolutionCache = new Map();
+
+/** Test/lifecycle hook: drop all cached source resolutions. */
+export function clearSourceResolutionCache() {
+  sourceResolutionCache.clear();
+}
+
+function targetSignature(event) {
+  const isInsert = event.mode === 'insert';
+  const target = isInsert ? insertTarget(event) : replaceTarget(event);
+  return JSON.stringify({
+    mode: isInsert ? 'insert' : 'replace',
+    position: isInsert ? target.position : null,
+    elementId: target.elementId || null,
+    classes: target.classes || null,
+    tag: target.tag || null,
+    pageUrl: event.pageUrl || null,
+  });
+}
+
+export function buildGenerationPreflight(event, scriptsDir, { cache = null } = {}) {
   if (!event || event.type !== 'generate' || !event.id) return null;
 
   const isInsert = event.mode === 'insert';
@@ -14,13 +39,24 @@ export function buildGenerationPreflight(event, scriptsDir) {
 
   const script = path.join(scriptsDir, isInsert ? 'live-insert.mjs' : 'live-wrap.mjs');
   const args = [script, '--id', event.id, '--count', String(event.count || 3)];
+  // Compute the scaffold but do not write it into source for source-preview
+  // targets. The agent writes wrapper + variants atomically; a premature
+  // server-side write reloads the framework and strands the browser at 0/N.
+  // No-op on the svelte-component path, which never writes the route source.
+  args.push('--defer-source-write');
   if (isInsert) args.push('--position', target.position);
   if (target.elementId) args.push('--element-id', target.elementId);
   if (target.classes) args.push('--classes', target.classes);
   if (target.tag) args.push('--tag', target.tag);
   if (target.text) args.push('--text', target.text);
   if (!isInsert && event.pageUrl) args.push('--page-url', event.pageUrl);
-  return { script, args, mode: isInsert ? 'insert' : 'replace' };
+  const signature = targetSignature(event);
+  // A cached resolution points the helper straight at the file, skipping the
+  // tree search. The helper still reads current content, so line ranges stay
+  // fresh; only discovery is cached.
+  const cachedFile = cache ? cache.get(signature) : null;
+  if (cachedFile) args.push('--file', cachedFile);
+  return { script, args, mode: isInsert ? 'insert' : 'replace', signature };
 }
 
 /**
@@ -38,8 +74,9 @@ export async function runGenerationPreflight(event, {
   scriptsDir,
   execFileImpl = execFileAsync,
   timeoutMs = PREFLIGHT_TIMEOUT_MS,
+  cache = sourceResolutionCache,
 } = {}) {
-  const command = buildGenerationPreflight(event, scriptsDir);
+  const command = buildGenerationPreflight(event, scriptsDir, { cache });
   if (!command) {
     return { ok: false, skipped: true, reason: 'insufficient_locator' };
   }
@@ -53,13 +90,23 @@ export async function runGenerationPreflight(event, {
     });
     const line = String(stdout).trim().split('\n').filter(Boolean).pop();
     if (!line) throw new Error('preflight returned no scaffold metadata');
+    const scaffold = JSON.parse(line);
+    // Cache the resolved SOURCE file (route source, not the svelte manifest) so
+    // the next generate on this target skips the tree search.
+    const resolvedSource = scaffold.sourceFile || scaffold.file;
+    if (cache && command.signature && typeof resolvedSource === 'string') {
+      cache.set(command.signature, resolvedSource);
+    }
     return {
       ok: true,
       mode: command.mode,
       durationMs: performance.now() - startedAt,
-      scaffold: JSON.parse(line),
+      scaffold,
     };
   } catch (error) {
+    // Evict a stale/failed resolution so the next attempt does a full search
+    // (the element may have moved out of the previously cached file).
+    if (cache && command.signature) cache.delete(command.signature);
     return {
       ok: false,
       mode: command.mode,