diff --git a/apps/api/src/routes/servers.ts b/apps/api/src/routes/servers.ts
index f6dedf2..bc0ae41 100644
--- a/apps/api/src/routes/servers.ts
+++ b/apps/api/src/routes/servers.ts
@@ -121,9 +121,30 @@ export async function serverRoutes(app: FastifyInstance): Promise<void> {
       });
     } catch (err) {
       if (err instanceof SpecValidationError) {
+        // Log the actual Zod validation failure so we can see *which* field
+        // the model got wrong. Without this we can only see "422" in the
+        // logs and can't tell whether to fix the prompt, the schema, or
+        // the model output cleanup. Prompt is truncated to keep PII risk
+        // bounded.
+        app.log.warn(
+          {
+            zod_message: err.message,
+            prompt: parsed.data.prompt.slice(0, 200),
+            model: choice.displayName,
+          },
+          'preview_spec_invalid',
+        );
         return reply.code(422).send({ error: 'spec_invalid', detail: err.message });
       }
       if (err instanceof BannedPatternError) {
+        app.log.warn(
+          {
+            reason: err.message,
+            prompt: parsed.data.prompt.slice(0, 200),
+            model: choice.displayName,
+          },
+          'preview_banned_pattern',
+        );
         return reply.code(422).send({ error: 'banned_pattern', detail: err.message });
       }
       if (err instanceof SpecTimeoutError) {
diff --git a/packages/llm/src/index.ts b/packages/llm/src/index.ts
index 8c5a874..0711440 100644
--- a/packages/llm/src/index.ts
+++ b/packages/llm/src/index.ts
@@ -268,7 +268,13 @@ async function generateWithAnthropic(
     .join('');
   const json = extractJson(text);
   const parsed = GeneratorSpec.safeParse(json);
-  if (!parsed.success) throw new SpecValidationError(parsed.error.message);
+  if (!parsed.success) {
+    // Include a truncated raw preview so the caller (api log) can see whether
+    // the model returned non-JSON / a refusal / a near-miss schema, instead
+    // of just the opaque zod error.
+    const preview = text.slice(0, 400).replace(/\s+/g, ' ');
+    throw new SpecValidationError(`${parsed.error.message} :: raw="${preview}"`);
+  }
   scanForInjection(parsed.data);
   return { spec: parsed.data, source: 'claude' };
 }