diff --git a/src/graph/state.rs b/src/graph/state.rs
index d242273..5f8fb72 100644
--- a/src/graph/state.rs
+++ b/src/graph/state.rs
@@ -353,6 +353,24 @@ fn single_reference_key(template: &str) -> Option<&str> {
     valid.then_some(inner)
 }
 
+// Returns the root state keys referenced by any `{{...}}` expressions in the
+// given template string. The "root key" is the identifier before the first
+// `.` or `[` — i.e. for `{{user.name}}` the root is `user`, for `{{items[0]}}`
+// the root is `items`. Used by the validator to compute the static read-set of
+// a node's templated fields without depending on a runtime `StateManager`.
+pub(super) fn template_root_keys(template: &str) -> Vec<String> {
+    TEMPLATE_VAR_RE
+        .captures_iter(template)
+        .flatten()
+        .filter_map(|c| c.get(1))
+        .map(|m| {
+            let inner = m.as_str();
+            let cut = inner.find(['.', '[']).unwrap_or(inner.len());
+            inner[..cut].to_string()
+        })
+        .collect()
+}
+
 fn value_to_string(value: &Value) -> String {
     match value {
         Value::String(s) => s.clone(),
diff --git a/src/graph/validator.rs b/src/graph/validator.rs
index 8785b4a..01cd166 100644
--- a/src/graph/validator.rs
+++ b/src/graph/validator.rs
@@ -1,3 +1,4 @@
+use super::state::template_root_keys;
 use super::types::{Graph, Node, NodeType};
 use crate::client::{Model, ModelType};
 use crate::config::{Agent, AppConfig, paths};
@@ -122,6 +123,7 @@ impl GraphValidator {
         self.validate_map_branches(graph, &mut result);
         self.validate_parallel_user_interaction(graph, &mut result);
         self.validate_parallel_writes(graph, &mut result);
+        self.validate_parallel_reads(graph, &mut result);
 
         result
     }
@@ -539,6 +541,51 @@ impl GraphValidator {
             }
         }
     }
+
+    fn validate_parallel_reads(&self, graph: &Graph, result: &mut ValidationResult) {
+        for group in compute_parallel_groups(graph) {
+            let nodes: Vec<(&String, &Node)> = group
+                .iter()
+                .filter_map(|id| graph.nodes.get(id).map(|n| (id, n)))
+                .collect();
+
+            for (id_a, node_a) in &nodes {
+                let read_set_a = read_set_of(node_a);
+                if read_set_a.is_empty() {
+                    continue;
+                }
+                for (id_b, node_b) in &nodes {
+                    if id_b == id_a {
+                        continue;
+                    }
+                    let Some(write_set_b) = write_set_of(node_b) else {
+                        continue;
+                    };
+                    let mut collisions: Vec<String> =
+                        read_set_a.intersection(&write_set_b).cloned().collect();
+                    if collisions.is_empty() {
+                        continue;
+                    }
+                    collisions.sort();
+                    let keys = collisions
+                        .iter()
+                        .map(|k| format!("`{k}`"))
+                        .collect::<Vec<_>>()
+                        .join(", ");
+                    result.error(ValidationError::with_node(
+                        id_a.as_str(),
+                        format!(
+                            "node '{id_a}' reads state key(s) {keys} which sibling parallel \
+                             branch '{id_b}' 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)."
+                        ),
+                    ));
+                }
+            }
+        }
+    }
 }
 
 fn declared_targets(node: &Node) -> Vec<(String, &'static str)> {
@@ -646,6 +693,103 @@ fn write_set_of(node: &Node) -> Option<HashSet<String>> {
     Some(writes)
 }
 
+// Computes the set of root state keys this node's templated fields read from.
+//
+// "Root key" follows the same definition as `template_root_keys`: for a
+// reference like `{{user.name}}` or `{{items[0]}}`, the root is the bare
+// identifier before the first `.` or `[`.
+//
+// Templated fields scanned per node type:
+//   - llm:      instructions, prompt, state_updates values
+//   - agent:    prompt, state_updates values
+//   - rag:      query (defaulting to "{{initial_prompt}}"), state_updates values
+//   - approval: question, state_updates values
+//   - input:    question, default, state_updates values
+//   - end:      output, state_updates values
+//   - map:      over (its `{{...}}` IS the dynamic read of the list to fan out over)
+//   - script:   state_updates values only (the script body is opaque to static
+//               analysis; its reads via GRAPH_STATE / GRAPH_STATE_FILE can't be
+//               inferred at load time)
+//
+// Scoped variables produced by THIS node's own execution are excluded from
+// state_updates value scanning:
+//   - llm/agent/rag → "output" (the node's body output)
+//   - approval      → "choice" (the user's selected option)
+//   - input         → "input"  (the user's typed text)
+// These are bindings created inside the node, not reads from prior state, so
+// they cannot race with a sibling's writes.
+fn read_set_of(node: &Node) -> HashSet<String> {
+    let mut reads: HashSet<String> = HashSet::new();
+    let scoped: &[&str] = match &node.node_type {
+        NodeType::Llm(_) | NodeType::Agent(_) | NodeType::Rag(_) => &["output"],
+        NodeType::Approval(_) => &["choice"],
+        NodeType::Input(_) => &["input"],
+        NodeType::Script(_) | NodeType::End(_) | NodeType::Map(_) => &[],
+    };
+
+    for s in primary_templated_fields(node) {
+        for k in template_root_keys(&s) {
+            reads.insert(k);
+        }
+    }
+
+    if let Some(updates) = node_state_updates_map(node) {
+        for v in updates.values() {
+            for k in template_root_keys(v) {
+                if !scoped.contains(&k.as_str()) {
+                    reads.insert(k);
+                }
+            }
+        }
+    }
+
+    reads
+}
+
+fn primary_templated_fields(node: &Node) -> Vec<String> {
+    match &node.node_type {
+        NodeType::Llm(n) => {
+            let mut v = vec![n.prompt.clone()];
+            if let Some(i) = &n.instructions {
+                v.push(i.clone());
+            }
+            v
+        }
+        NodeType::Agent(n) => vec![n.prompt.clone()],
+        NodeType::Rag(n) => {
+            vec![
+                n.query
+                    .clone()
+                    .unwrap_or_else(|| "{{initial_prompt}}".to_string()),
+            ]
+        }
+        NodeType::Approval(n) => vec![n.question.clone()],
+        NodeType::Input(n) => {
+            let mut v = vec![n.question.clone()];
+            if let Some(d) = &n.default {
+                v.push(d.clone());
+            }
+            v
+        }
+        NodeType::End(n) => vec![n.output.clone()],
+        NodeType::Map(n) => vec![n.over.clone()],
+        NodeType::Script(_) => Vec::new(),
+    }
+}
+
+fn node_state_updates_map(node: &Node) -> Option<&std::collections::HashMap<String, String>> {
+    match &node.node_type {
+        NodeType::Llm(n) => n.state_updates.as_ref(),
+        NodeType::Agent(n) => n.state_updates.as_ref(),
+        NodeType::Rag(n) => n.state_updates.as_ref(),
+        NodeType::Approval(n) => n.state_updates.as_ref(),
+        NodeType::Input(n) => n.state_updates.as_ref(),
+        NodeType::Script(n) => n.state_updates.as_ref(),
+        NodeType::End(n) => n.state_updates.as_ref(),
+        NodeType::Map(_) => None,
+    }
+}
+
 fn node_state_updates_keys(node: &Node) -> Option<HashSet<String>> {
     let updates = match &node.node_type {
         NodeType::Agent(n) => n.state_updates.as_ref(),
@@ -2064,4 +2208,140 @@ mod tests {
             result.errors
         );
     }
+
+    fn llm_with_prompt(id: &str, prompt: &str, next: Option<&str>) -> Node {
+        let mut node = llm_node(id, None, next);
+        if let NodeType::Llm(ref mut n) = node.node_type {
+            n.prompt = prompt.into();
+        }
+        node
+    }
+
+    #[test]
+    fn parallel_read_of_sibling_write_errors() {
+        let reader = llm_with_prompt("worker_a", "Hello {{summary}}!", Some("end"));
+        let writer = llm_with_state_updates("worker_b", &[("summary", "static")], Some("end"));
+        let graph = fan_out_graph_with_two_workers(reader, writer);
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            result
+                .errors
+                .iter()
+                .any(|e| e.message.contains("reads state key(s) `summary`")
+                    && e.message.contains("'worker_b'")),
+            "expected cross-branch read error mentioning `summary` and sibling writer: {:?}",
+            result.errors
+        );
+    }
+
+    #[test]
+    fn parallel_read_of_upstream_key_passes() {
+        let reader_a = llm_with_prompt("worker_a", "Topic is {{topic}}", Some("end"));
+        let reader_b = llm_with_prompt("worker_b", "Also {{topic}}", Some("end"));
+        let graph = fan_out_graph_with_two_workers(reader_a, reader_b);
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            !result
+                .errors
+                .iter()
+                .any(|e| e.message.contains("reads state key")),
+            "upstream `topic` shouldn't trigger cross-branch read error: {:?}",
+            result.errors
+        );
+    }
+
+    #[test]
+    fn scoped_output_var_in_state_updates_not_treated_as_read() {
+        let scoped_user =
+            llm_with_state_updates("worker_a", &[("a_key", "{{output}}")], Some("end"));
+        let writes_output =
+            llm_with_state_updates("worker_b", &[("output", "{{output}}")], Some("end"));
+        let graph = fan_out_graph_with_two_workers(scoped_user, writes_output);
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            !result
+                .errors
+                .iter()
+                .any(|e| e.message.contains("reads state key(s) `output`")
+                    && e.message.contains("worker_a")),
+            "scoped `{{{{output}}}}` inside state_updates value should NOT be treated as a read: {:?}",
+            result.errors
+        );
+    }
+
+    #[test]
+    fn rag_query_reading_sibling_script_write_errors() {
+        let mut rag = rag_node("worker_a", &["./k"], true);
+        if let NodeType::Rag(ref mut n) = rag.node_type {
+            n.query = Some("codes: {{loinc_codes}}\n{{db_result}}".into());
+            if let Some(m) = n.state_updates.as_mut() {
+                m.insert("rag_ctx".into(), "{{output.context}}".into());
+            }
+        }
+        rag.next = Some("end".into());
+        let mut script = script_with_state_updates("worker_b", &[("db_result", "{{output}}")]);
+        script.next = Some("end".into());
+        let graph = fan_out_graph_with_two_workers(rag, script);
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            result
+                .errors
+                .iter()
+                .any(|e| e.message.contains("reads state key(s) `db_result`")
+                    && e.message.contains("'worker_b'")),
+            "expected cross-branch read error for rag query reading db_result: {:?}",
+            result.errors
+        );
+    }
+
+    #[test]
+    fn map_over_reading_sibling_write_errors() {
+        let map_n = Node {
+            id: "fan".into(),
+            description: String::new(),
+            node_type: NodeType::Map(MapNode {
+                over: "{{items}}".into(),
+                as_name: "item".into(),
+                branch: "branch_n".into(),
+                output_key: "output".into(),
+                collect_into: "results".into(),
+                max_concurrency: None,
+            }),
+            next: Some("end".into()),
+        };
+        let branch_n = llm_with_prompt("branch_n", "Process {{item}}", None);
+        let producer = llm_with_state_updates("producer", &[("items", "[1,2,3]")], Some("end"));
+        let mut start = end_node("start");
+        start.next = Some(NextTargets::Many(vec!["fan".into(), "producer".into()]));
+        let graph = graph_with(
+            vec![
+                ("start", start),
+                ("fan", map_n),
+                ("branch_n", branch_n),
+                ("producer", producer),
+                ("end", end_node("end")),
+            ],
+            "start",
+        );
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            result
+                .errors
+                .iter()
+                .any(|e| e.message.contains("reads state key(s) `items`")
+                    && e.message.contains("'producer'")),
+            "expected cross-branch read error for map `over` reading sibling write: {:?}",
+            result.errors
+        );
+    }
 }