fix: increase branch retries to 3 and relax cross-reference validation on final attempt

next_node_id mismatches are a common model hallucination that the retry
prompt doesn't reliably fix. On the final (3rd) attempt, accept the branch
with strict=False so only truly fatal errors (missing fields, dead ends,
bad JSON) cause a hard failure. Cross-reference issues are minor and
fixable in the tree editor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/ai_tree_generator_service.py

@@ -199,7 +199,7 @@ async def generate_branch_detail(
     total_input = 0
     total_output = 0
 
-    for attempt in range(2):
+    for attempt in range(3):
         response = await client.messages.create(
             model=settings.AI_MODEL,
             max_tokens=8192,
@@ -223,7 +223,7 @@ async def generate_branch_detail(
         try:
             branch_tree = json.loads(raw_text)
         except json.JSONDecodeError as e:
-            if attempt == 0:
+            if attempt < 2:
                 messages.append({"role": "assistant", "content": raw_text})
                 messages.append({
                     "role": "user",
@@ -234,13 +234,18 @@ async def generate_branch_detail(
                 continue
             raise ValueError(f"AI returned invalid JSON after retry: {e}")
 
-        # Validate the branch structure
+        # On the final attempt, use strict=False to accept cross-reference
-        errors = validate_generated_tree(branch_tree)
+        # mismatches (next_node_id pointing to wrong child) — these are
+        # minor structural issues the user can fix in the editor.
+        is_final_attempt = attempt == 2
+        errors = validate_generated_tree(branch_tree, strict=not is_final_attempt)
         if not errors:
+            if is_final_attempt:
+                logger.warning("branch_detail accepted on final attempt (lenient validation): branch=%s", branch_name)
             cost = _estimate_cost(total_input, total_output)
             return branch_tree, total_input, total_output, cost
 
-        if attempt == 0:
+        if attempt < 2:
             messages.append({"role": "assistant", "content": raw_text})
             messages.append({
                 "role": "user",

backend/app/core/ai_tree_validator.py

@@ -24,7 +24,7 @@ class TreeValidationError(Exception):
         super().__init__(f"Tree validation failed: {'; '.join(errors)}")
 
 
-def validate_generated_tree(tree: dict[str, Any]) -> list[str]:
+def validate_generated_tree(tree: dict[str, Any], strict: bool = True) -> list[str]:
     """Validate an AI-generated tree structure.
 
     Returns a list of error strings. Empty list means valid.
@@ -106,7 +106,7 @@ def validate_generated_tree(tree: dict[str, Any]) -> list[str]:
                     next_id = opt.get("next_node_id")
                     if next_id:
                         all_referenced_ids.add(next_id)
-                        if child_ids and next_id not in child_ids:
+                        if strict and child_ids and next_id not in child_ids:
                             errors.append(
                                 f"Option '{opt.get('label', '?')}' in node '{node_id}' "
                                 f"references non-existent child '{next_id}'"