🧹 fix: Skip Consumed Splat Primitives in Warn/Error Debug Traversal

When no structured metadata is attached, winston still leaves consumed
`%s` / `%d` arguments in `Symbol.for('splat')`. Previous fix preferred
the structured object but still fell back to whatever sat at `SPLAT[0]`
— so `logger.warn('failed for %s', tenantId)` emitted
`failed for tenant-7 tenant-7` in the traversal path (debug file and
`DEBUG_CONSOLE`), now regressed outside of `debug` level because
warn/error share the path.

Only accept the splat fallback when the value is a plain object or an
array (structural data worth surfacing). Primitives there are almost
certainly consumed printf args and get skipped.

Regression tests cover the single-%s case and the array-as-metadata case
(which still surfaces through splat).

Reviewed-by: Codex (P2 finding on PR #12737, commit bccbf117).

api/config/__tests__/parsers.spec.js

@@ -229,4 +229,29 @@ describe('debugTraverse', () => {
     const tenantMatches = out.match(/tenant-7/g) ?? [];
     expect(tenantMatches.length).toBeLessThanOrEqual(1);
   });
+
+  it('does not duplicate a consumed %s arg when there is no structured metadata', () => {
+    const info = {
+      level: 'warn',
+      message: 'failed for tenant-7',
+      timestamp: 'ts',
+      [SPLAT_SYMBOL]: ['tenant-7'],
+    };
+    const out = runFormatter(info);
+    const tenantMatches = out.match(/tenant-7/g) ?? [];
+    expect(tenantMatches.length).toBe(1);
+  });
+
+  it('still surfaces array metadata in SPLAT[0] when no object is extracted', () => {
+    const info = {
+      level: 'debug',
+      message: 'list',
+      timestamp: 'ts',
+      [SPLAT_SYMBOL]: [['alpha', 'beta', 'gamma']],
+    };
+    const out = runFormatter(info);
+    expect(out).toContain('alpha');
+    expect(out).toContain('beta');
+    expect(out).toContain('gamma');
+  });
 });

api/config/parsers.js

@@ -189,14 +189,17 @@ const debugTraverse = winston.format.printf(({ level, message, timestamp, ...met
     /*
      * Prefer the structured metadata object (which winston merges into info)
      * over `SPLAT_SYMBOL[0]`. The first splat entry may be a *consumed*
-     * printf arg — e.g. `logger.warn('failed for %s', tenant, { provider })`
-     * leaves `tenant` in `SPLAT[0]` after interpolation, which would then
-     * be appended again to the line instead of surfacing the real metadata
-     * object `{ provider }`.
+     * printf arg — e.g. `logger.warn('failed for %s', tenant)` leaves
+     * `tenant` in `SPLAT[0]` after interpolation, and appending it again
+     * would emit `failed for tenant tenant`. Only fall back to the splat
+     * entry when it's a non-primitive (array or plain object); a string
+     * or number there is almost certainly a consumed %s/%d arg.
      */
     const extracted = extractMetaObject(metadata);
     const splatFirst = metadata[SPLAT_SYMBOL]?.[0];
-    const debugValue = extracted ?? splatFirst;
+    const splatUsable =
+      Array.isArray(splatFirst) || (splatFirst != null && typeof splatFirst === 'object');
+    const debugValue = extracted ?? (splatUsable ? splatFirst : undefined);
 
     if (!debugValue) {
       return finalize(msg);