docs: updated docs with a cross-branch read gotcha

Graph-Agents.md

@@ -902,6 +902,47 @@ This means:
   need that pattern, sequentialize the work or write it as a single
   multi-step subgraph
 
+### Cross-branch reads are not visible (gotcha)
+
+A consequence of independent state forks: a branch **cannot read a key
+that a sibling branch writes in the same super-step**. Each branch
+sees the state snapshot taken BEFORE the super-step started; sibling
+writes only become visible at the next super-step (after the join).
+
+**Validator catches this at load time.** If any branch's templated
+field references a state key that another sibling branch writes in
+the same super-step, you get a load-time error like:
+
+```
+node 'rag_lookup' reads state key(s) `db_result` which sibling parallel
+branch 'query_local_db' writes in the same super-step; parallel
+branches see a state snapshot taken BEFORE the super-step and cannot
+observe each other's writes. Move the dependent read to a later
+super-step (or remove the cross-branch reference).
+```
+
+**Fix options:**
+
+- **Remove the dependency.** Drop the cross-branch reference from the
+  prompt/query/etc. so the branch operates on what was available _before_
+  the super-step.
+- **Sequentialize.** Put the dependent node in a later super-step:
+  `producer → reader_that_needed_producer_output` instead of
+  `[producer, reader_that_needed_producer_output]`.
+- **Use the join.** Restructure so any node that depends on a sibling's
+  writes runs _after_ the join. The join sees the merged state.
+
+**Caveat (scripts).** The validator scans templated fields of typed
+nodes (`prompt`, `query`, `instructions`, `question`, `default`,
+`output`, `over`, and `state_updates` values). It does not scan
+script bodies, since scripts read state opaquely via `GRAPH_STATE` /
+`GRAPH_STATE_FILE` env vars. A Python script in a parallel branch
+that does `state.get("sibling_written_key")` will silently see the
+**pre**-super-step snapshot. The script's required `state_updates`
+declaration (the validator's separate "parallel scripts must declare
+writes" check) covers writes only, not reads. Be aware that script
+branches need to be designed so they don't depend on sibling writes.
+
 ## Multi-branch UX
 
 In a TTY:
@@ -1182,6 +1223,15 @@ startup.
   add explicit declarations
 - **`max_concurrency` set to 0**: either `settings.max_concurrency: 0` or
   `MapNode.max_concurrency: 0` is rejected (would deadlock the semaphore)
+- **Cross-branch read in a parallel super-step**: one branch's templated
+  field (`prompt`, `query`, `instructions`, `question`, `default`,
+  `output`, `over`, or `state_updates` value) reads a state key that
+  another sibling branch writes in the same super-step. Branches see a
+  snapshot taken _before_ the super-step started, so a sibling's writes
+  are not yet visible. The error names the reader, the reads-key, and
+  the sibling writer. Scripts are exempt because their reads are opaque
+  to static analysis. See [Cross-branch reads are not visible](#cross-branch-reads-are-not-visible-gotcha)
+  for the script caveat
 
 **Warnings (printed, execution continues)**: