diff --git a/src/graph/executor.rs b/src/graph/executor.rs
index f9bf69c..56bc95c 100644
--- a/src/graph/executor.rs
+++ b/src/graph/executor.rs
@@ -228,15 +228,17 @@ impl GraphExecutor {
                 let step_outcome = step_result.with_context(|| format!("at node '{node_id}'"))?;
 
                 match step_outcome {
-                    StepResult::Continue(target) => {
-                        logger.routing(&node_id, &target);
+                    StepResult::Continue(targets) => {
+                        for target in &targets {
+                            logger.routing(&node_id, target);
+                        }
                         let diff = branch_state.diff_against(snapshot.as_ref());
                         branch_writes.push(BranchWrites {
                             node_id: node_id.clone(),
                             invocation_index: 0,
                             writes: diff,
                         });
-                        next_frontier.insert(target);
+                        next_frontier.extend(targets);
                     }
                     StepResult::End(output) => {
                         end_results.push((node_id.clone(), branch_state, output));
@@ -309,7 +311,12 @@ impl StepContext<'_> {
 }
 
 enum StepResult {
-    Continue(String),
+    // The set of next-node ids the executor should add to the next super-step's
+    // frontier. A `Vec` of length 1 for sequential routing (default) and the
+    // full target list for fan-out (`next: [a, b, ...]`). Dynamic single-route
+    // decisions (script `_next`, approval routes, LLM/RAG fallback) always emit
+    // a single-element vec.
+    Continue(Vec<String>),
     End(String),
 }
 
@@ -323,13 +330,8 @@ async fn step(
     match &node.node_type {
         NodeType::Agent(agent_node) => {
             AgentNodeExecutor::execute(agent_node, state, ctx).await?;
-            let next = node
-                .next_single()?
-                .ok_or_else(|| {
-                    anyhow!("agent node '{current}' has no `next` and is not an end node")
-                })?
-                .to_string();
-            Ok(StepResult::Continue(next))
+            let targets = static_next_targets(node, current, "agent")?;
+            Ok(StepResult::Continue(targets))
         }
         NodeType::Script(script_node) => {
             let dynamic = match step_ctx.script_executor.execute(script_node, state).await {
@@ -343,57 +345,96 @@ async fn step(
                             fallback,
                             e
                         );
-                        return Ok(StepResult::Continue(fallback.clone()));
+                        return Ok(StepResult::Continue(vec![fallback.clone()]));
                     }
                     return Err(e);
                 }
             };
-            let next = match dynamic {
-                Some(n) => n,
-                None => node
-                    .next_single()?
-                    .ok_or_else(|| {
-                        anyhow!(
-                            "script node '{current}' did not emit `_next` and has no static `next`"
-                        )
-                    })?
-                    .to_string(),
+            let targets = match dynamic {
+                Some(n) => vec![n],
+                None => static_next_targets(node, current, "script")?,
             };
-            Ok(StepResult::Continue(next))
+            Ok(StepResult::Continue(targets))
         }
         NodeType::Approval(approval_node) => {
             let next = ApprovalNodeExecutor::execute(approval_node, state, ctx).await?;
-            Ok(StepResult::Continue(next))
+            Ok(StepResult::Continue(vec![next]))
         }
         NodeType::Input(input_node) => {
-            let next_id = node.next_single()?;
+            let next_id = first_next_target(node);
             let next = InputNodeExecutor::execute(input_node, next_id, state, ctx).await?;
-            Ok(StepResult::Continue(next))
+            Ok(StepResult::Continue(vec![next]))
         }
         NodeType::Llm(llm_node) => {
-            let next_id = node.next_single()?;
-            let next = LlmNodeExecutor::execute(llm_node, next_id, state, ctx).await?;
-            Ok(StepResult::Continue(next))
+            let primary = first_next_target(node).map(str::to_string);
+            let llm_routing =
+                LlmNodeExecutor::execute(llm_node, primary.as_deref(), state, ctx).await?;
+            let targets = resolve_branching_next(node, &llm_routing);
+            Ok(StepResult::Continue(targets))
         }
         NodeType::Rag(rag_node) => {
-            let next_id = node.next_single()?;
-            let next = RagNodeExecutor::execute(rag_node, current, next_id, state, ctx).await?;
-            Ok(StepResult::Continue(next))
+            let primary = first_next_target(node).map(str::to_string);
+            let rag_routing =
+                RagNodeExecutor::execute(rag_node, current, primary.as_deref(), state, ctx).await?;
+            let targets = resolve_branching_next(node, &rag_routing);
+            Ok(StepResult::Continue(targets))
         }
         NodeType::End(end_node) => Ok(StepResult::End(resolve_end_output(end_node, state))),
         NodeType::Map(map_node) => {
-            let next = node
-                .next_single()?
-                .ok_or_else(|| {
-                    anyhow!("map node '{current}' has no `next` and is not an end node")
-                })?
-                .to_string();
+            let targets = static_next_targets(node, current, "map")?;
             MapNodeExecutor::execute(map_node, state, ctx, step_ctx, current).await?;
-            Ok(StepResult::Continue(next))
+            Ok(StepResult::Continue(targets))
         }
     }
 }
 
+// Returns all `next:` targets from the node (handles both `One` and `Many`),
+// erroring if no `next` is set.
+fn static_next_targets(node: &Node, current: &str, kind: &str) -> Result<Vec<String>> {
+    node.next
+        .as_ref()
+        .map(|t| t.as_slice().to_vec())
+        .ok_or_else(|| anyhow!("{kind} node '{current}' has no `next` and is not an end node"))
+}
+
+// Returns the first declared `next:` target as a borrowed `&str`, or `None` if
+// no `next` is set. Used by node executors that take `Option<&str>` for their
+// primary routing argument (LLM, RAG, Input).
+fn first_next_target(node: &Node) -> Option<&str> {
+    node.next
+        .as_ref()
+        .and_then(|t| t.as_slice().first().map(|s| s.as_str()))
+}
+
+// Resolves the actual frontier-advance targets after an LLM/RAG node ran.
+//
+// LLM/RAG executors return their chosen routing as a String — either the
+// primary `next:` target (success path) or the node's `fallback:` (failure
+// path with retry exhausted). We can't tell these apart from inside step()
+// without an API refactor, so we compare strings: if the returned routing
+// matches the first declared `next` target, treat as success and (for
+// fan-out) use ALL declared targets; otherwise treat as fallback and use the
+// returned target alone.
+//
+// Known limitation: if a fan-out node's `fallback:` is set to the same node
+// id as its first `next:` target, a successful run is indistinguishable from
+// a fallback run — both look like "returned the first target". The result is
+// that the executor advances to all Many targets in the fallback case (which
+// is the OPPOSITE of the user's likely intent). Workaround: choose a
+// `fallback:` distinct from any `next:` target.
+fn resolve_branching_next(node: &Node, returned_routing: &str) -> Vec<String> {
+    let Some(targets) = &node.next else {
+        return vec![returned_routing.to_string()];
+    };
+    let slice = targets.as_slice();
+    let first_matches = slice.first().is_some_and(|s| s == returned_routing);
+    if first_matches && slice.len() > 1 {
+        slice.to_vec()
+    } else {
+        vec![returned_routing.to_string()]
+    }
+}
+
 fn resolve_end_output(end_node: &EndNode, state: &mut StateManager) -> String {
     apply_simple_state_updates(end_node.state_updates.as_ref(), state);
     state.interpolate_lenient(&end_node.output)
@@ -490,3 +531,262 @@ mod tests {
         assert_eq!(state.state().get("k"), Some(&json!("new-old")));
     }
 }
+
+#[cfg(test)]
+mod integration_tests {
+    use super::*;
+    use crate::config::{AppState, WorkingMode};
+    use crate::utils::{create_abort_signal, temp_file};
+    use std::fs;
+
+    fn cmd_available(name: &str) -> bool {
+        which::which(name).is_ok()
+    }
+
+    struct TestWorkspace {
+        dir: PathBuf,
+    }
+
+    impl TestWorkspace {
+        fn new() -> Self {
+            let dir = temp_file("-graph-integration-", "");
+            fs::create_dir_all(&dir).unwrap();
+            Self { dir }
+        }
+
+        fn write_script(&self, name: &str, contents: &str) {
+            fs::write(self.dir.join(name), contents).unwrap();
+        }
+    }
+
+    impl Drop for TestWorkspace {
+        fn drop(&mut self) {
+            let _ = fs::remove_dir_all(&self.dir);
+        }
+    }
+
+    fn make_ctx() -> RequestContext {
+        RequestContext::new(Arc::new(AppState::test_default()), WorkingMode::Cmd)
+    }
+
+    #[tokio::test]
+    async fn static_fan_out_merges_branch_writes_via_append_reducer() {
+        if !cmd_available("bash") {
+            eprintln!("skipping: bash not available");
+            return;
+        }
+        let ws = TestWorkspace::new();
+        ws.write_script("dispatcher.sh", "#!/bin/bash\necho '{}'\n");
+        ws.write_script(
+            "worker_a.sh",
+            "#!/bin/bash\necho '{\"results\": \"alpha\"}'\n",
+        );
+        ws.write_script(
+            "worker_b.sh",
+            "#!/bin/bash\necho '{\"results\": \"beta\"}'\n",
+        );
+
+        let yaml = r#"
+name: static_fan_out_test
+start: dispatcher
+reducers:
+  results: append
+nodes:
+  dispatcher:
+    type: script
+    script: dispatcher.sh
+    state_updates: {}
+    next: [worker_a, worker_b]
+  worker_a:
+    type: script
+    script: worker_a.sh
+    state_updates: {}
+    next: join
+  worker_b:
+    type: script
+    script: worker_b.sh
+    state_updates: {}
+    next: join
+  join:
+    type: end
+    output: "{{results}}"
+"#;
+        let graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        let mut ctx = make_ctx();
+        let abort = create_abort_signal();
+        let result = GraphExecutor::new(graph, &ws.dir)
+            .execute(&mut ctx, abort)
+            .await
+            .unwrap_or_else(|e| panic!("executor failed: {e:#}"));
+
+        let parsed: Value = serde_json::from_str(&result)
+            .unwrap_or_else(|_| panic!("expected JSON array, got: {result}"));
+        let arr = parsed.as_array().expect("results should be an array");
+        assert_eq!(arr.len(), 2, "expected 2 elements, got: {result}");
+        let strs: Vec<&str> = arr.iter().filter_map(|v| v.as_str()).collect();
+        assert!(strs.contains(&"alpha"), "missing 'alpha' in {strs:?}");
+        assert!(strs.contains(&"beta"), "missing 'beta' in {strs:?}");
+    }
+
+    #[tokio::test]
+    async fn map_over_list_collects_outputs_in_input_order() {
+        if !cmd_available("python3") {
+            eprintln!("skipping: python3 not available");
+            return;
+        }
+        let ws = TestWorkspace::new();
+        ws.write_script(
+            "doubler.py",
+            r#"#!/usr/bin/env python3
+import os, json
+state = json.loads(os.environ.get("GRAPH_STATE", "{}"))
+val = state["item"]
+print(json.dumps({"output": val * 2}))
+"#,
+        );
+
+        let yaml = r#"
+name: map_input_order_test
+start: fan_out
+initial_state:
+  items: [1, 2, 3, 4, 5]
+nodes:
+  fan_out:
+    type: map
+    over: "{{items}}"
+    as: item
+    branch: doubler
+    collect_into: doubled
+    next: done
+  doubler:
+    type: script
+    script: doubler.py
+    state_updates: {}
+  done:
+    type: end
+    output: "{{doubled}}"
+"#;
+        let graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        let mut ctx = make_ctx();
+        let abort = create_abort_signal();
+        let result = GraphExecutor::new(graph, &ws.dir)
+            .execute(&mut ctx, abort)
+            .await
+            .unwrap_or_else(|e| panic!("executor failed: {e:#}"));
+
+        let parsed: Value = serde_json::from_str(&result)
+            .unwrap_or_else(|_| panic!("expected JSON array, got: {result}"));
+        let arr = parsed.as_array().expect("doubled should be an array");
+        let nums: Vec<i64> = arr
+            .iter()
+            .map(|v| v.as_i64().expect("each item should be int"))
+            .collect();
+
+        assert_eq!(
+            nums,
+            vec![2, 4, 6, 8, 10],
+            "map outputs should be in input order, not finish order"
+        );
+    }
+
+    #[tokio::test]
+    async fn parallel_branch_error_aborts_super_step() {
+        if !cmd_available("bash") {
+            eprintln!("skipping: bash not available");
+            return;
+        }
+        let ws = TestWorkspace::new();
+        ws.write_script("dispatcher.sh", "#!/bin/bash\necho '{}'\n");
+        ws.write_script(
+            "worker_ok.sh",
+            "#!/bin/bash\necho '{\"results\": \"ok\"}'\n",
+        );
+        ws.write_script(
+            "worker_fail.sh",
+            "#!/bin/bash\necho 'simulated failure' >&2\nexit 1\n",
+        );
+
+        let yaml = r#"
+name: branch_error_test
+start: dispatcher
+reducers:
+  results: append
+nodes:
+  dispatcher:
+    type: script
+    script: dispatcher.sh
+    state_updates: {}
+    next: [worker_ok, worker_fail]
+  worker_ok:
+    type: script
+    script: worker_ok.sh
+    state_updates: {}
+    next: join
+  worker_fail:
+    type: script
+    script: worker_fail.sh
+    state_updates: {}
+    next: join
+  join:
+    type: end
+    output: "{{results}}"
+"#;
+        let graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        let mut ctx = make_ctx();
+        let abort = create_abort_signal();
+        let result = GraphExecutor::new(graph, &ws.dir)
+            .execute(&mut ctx, abort)
+            .await;
+
+        assert!(result.is_err(), "expected branch error to propagate");
+        let err = format!("{:#}", result.unwrap_err());
+        assert!(
+            err.contains("worker_fail"),
+            "error should mention failing node: {err}"
+        );
+    }
+
+    #[tokio::test]
+    async fn multi_end_in_super_step_is_rejected() {
+        if !cmd_available("bash") {
+            eprintln!("skipping: bash not available");
+            return;
+        }
+        let ws = TestWorkspace::new();
+        ws.write_script("dispatcher.sh", "#!/bin/bash\necho '{}'\n");
+
+        let yaml = r#"
+name: multi_end_test
+start: dispatcher
+nodes:
+  dispatcher:
+    type: script
+    script: dispatcher.sh
+    state_updates: {}
+    next: [end_a, end_b]
+  end_a:
+    type: end
+    output: "from a"
+  end_b:
+    type: end
+    output: "from b"
+"#;
+        let graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        let mut ctx = make_ctx();
+        let abort = create_abort_signal();
+        let result = GraphExecutor::new(graph, &ws.dir)
+            .execute(&mut ctx, abort)
+            .await;
+
+        assert!(result.is_err(), "expected multi-End to be rejected");
+        let err = format!("{:#}", result.unwrap_err());
+        assert!(
+            err.contains("multiple End targets"),
+            "error should explain multi-End cause: {err}"
+        );
+        assert!(
+            err.contains("end_a") && err.contains("end_b"),
+            "error should list both End nodes: {err}"
+        );
+    }
+}