fix(preferences): deduplicate unrecognized format warning on repeated loads (#2375)

parsePreferencesMarkdown emitted a console.warn every time preferences
were loaded with an unrecognized format, spamming stderr on each call
to loadEffectiveGSDPreferences. Gate the warning behind a warn-once
flag so it prints at most once per process.

Fixes #2373

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/preferences.ts

@@ -196,6 +196,13 @@ function loadPreferencesFile(path: string, scope: "global" | "project"): LoadedG
   };
 }
 
+let _warnedUnrecognizedFormat = false;
+
+/** @internal Reset the warn-once flag — exported for testing only. */
+export function _resetParseWarningFlag(): void {
+  _warnedUnrecognizedFormat = false;
+}
+
 /** @internal Exported for testing only */
 export function parsePreferencesMarkdown(content: string): GSDPreferences | null {
   // Use indexOf instead of [\s\S]*? regex to avoid backtracking (#468)
@@ -214,7 +221,10 @@ export function parsePreferencesMarkdown(content: string): GSDPreferences | null
     return parseHeadingListFormat(content);
   }
 
-  console.warn("[parsePreferencesMarkdown] preferences.md exists but uses an unrecognized format — skipping.");
+  if (!_warnedUnrecognizedFormat) {
+    _warnedUnrecognizedFormat = true;
+    console.warn("[parsePreferencesMarkdown] preferences.md exists but uses an unrecognized format — skipping.");
+  }
   return null;
 }
 

src/resources/extensions/gsd/tests/preferences.test.ts

@@ -15,6 +15,7 @@ import {
   applyModeDefaults,
   getIsolationMode,
   parsePreferencesMarkdown,
+  _resetParseWarningFlag,
 } from "../preferences.ts";
 import type { GSDPreferences, GSDModelConfigV2, GSDPhaseModelConfig } from "../preferences.ts";
 
@@ -352,3 +353,29 @@ test("handles empty models config", () => {
   assert.notEqual(prefs, null);
   assert.equal(prefs!.models, undefined);
 });
+
+// ── Warn-once for unrecognized format (#2373) ────────────────────────────────
+
+test("unrecognized format warning is emitted at most once (#2373)", () => {
+  const warnings: string[] = [];
+  const origWarn = console.warn;
+  console.warn = (...args: unknown[]) => warnings.push(args.join(" "));
+  try {
+    // Reset internal warned flag so the test starts clean
+    _resetParseWarningFlag();
+
+    const unrecognized = "This is just plain text with no frontmatter or headings.";
+
+    // Call multiple times — simulates repeated preference loads
+    parsePreferencesMarkdown(unrecognized);
+    parsePreferencesMarkdown(unrecognized);
+    parsePreferencesMarkdown(unrecognized);
+
+    const relevant = warnings.filter(w => w.includes("unrecognized format"));
+    assert.equal(relevant.length, 1, `expected exactly 1 warning, got ${relevant.length}: ${JSON.stringify(relevant)}`);
+  } finally {
+    console.warn = origWarn;
+    // Reset so other tests aren't affected by the flag state
+    _resetParseWarningFlag();
+  }
+});