docs(short): dónde corre una comprobación decide lo que cuesta

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

docs/shortsmith-phases-4-6.md

@@ -116,6 +116,20 @@ What actually landed, and where it differs from the plan below:
   says. It is the same finding as "el ejemplo del prompt habla, y por eso los specs
   vuelven a hablar", arriving a second time.
 
+- **A validation rule that fires too late costs a whole generation, not a retry**
+  (2026-08-13). researchowl deliberately did not replicate shortsmith's cross-field
+  `@model_validator`s — they are not in the published JSON Schema, and the reasoning was
+  that the server's 422 covers them. It does, but at the wrong moment: the 422 arrives
+  at *render* time, after the spec loop has finished, so the spec is not rewritten, it
+  is handed back to a human. Session 162 (Trans-en-Provence) died exactly there — valid
+  on the first attempt, 39 claims grounded, and no video because a `scale_bars` shot had
+  three bars and a quote. Replicated locally, the same spec cost one retry and rendered.
+  The rule to carry forward: **where a check runs decides what it costs**, and "the
+  server will catch it" is only true if the server catches it while you can still act.
+  The error strings are copied from shortsmith word for word, because they are handed to
+  the model verbatim and two wordings of one failure is how an error message stops
+  being useful.
+
 Original plan, kept for the record:
 
 ### 4b (as planned). Narration (TTS) + burned-in captions