diff --git a/src/function/agents.rs b/src/function/agents.rs
index 8807fa8..071bcb2 100644
--- a/src/function/agents.rs
+++ b/src/function/agents.rs
@@ -565,7 +565,8 @@ impl Drop for CancelOnDrop {
 
 /// Runs a child agent to completion. When the child is a graph agent,
 /// `graph_inputs` is overlaid on its `initial_state` before the graph
-/// starts; it is ignored for config-only agents.
+/// starts. Passing inputs to a config-only agent is rejected before the
+/// child runs, since nothing could consume them.
 fn run_child_agent_with_graph_inputs(
     mut child_ctx: RequestContext,
     initial_input: Input,
diff --git a/src/graph/executor.rs b/src/graph/executor.rs
index 9174698..08a293a 100644
--- a/src/graph/executor.rs
+++ b/src/graph/executor.rs
@@ -109,6 +109,13 @@ impl GraphExecutor {
         let max_iterations = graph.settings.max_loop_iterations;
         let graph_timeout = graph.settings.timeout.and_then(wall_clock);
         let max_concurrency = graph.settings.max_concurrency;
+        if max_concurrency > Semaphore::MAX_PERMITS {
+            bail!(
+                "Graph '{}': settings.max_concurrency {max_concurrency} exceeds the runtime limit of {}",
+                graph.name,
+                Semaphore::MAX_PERMITS
+            );
+        }
         let graph = Arc::new(graph);
         let start = Instant::now();
 
@@ -1362,6 +1369,30 @@ nodes:
         assert!(err.contains("sleeper"), "error should name frontier: {err}");
     }
 
+    #[tokio::test]
+    async fn settings_max_concurrency_above_semaphore_limit_errors_instead_of_panicking() {
+        let ws = TestWorkspace::new();
+        let yaml = r#"
+name: oversized_cap_test
+start: done
+settings:
+  validate_before_run: false
+nodes:
+  done:
+    type: end
+    output: "done"
+"#;
+        let mut graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        graph.settings.max_concurrency = Semaphore::MAX_PERMITS + 1;
+        let mut ctx = make_ctx();
+        let err = GraphExecutor::new(graph, &ws.dir)
+            .execute(&mut ctx, create_abort_signal())
+            .await
+            .expect_err("an oversized cap is an error, not a panic");
+        let msg = format!("{err:#}");
+        assert!(msg.contains("exceeds the runtime limit of"), "{msg}");
+    }
+
     #[tokio::test]
     async fn graph_timeout_zero_is_unbounded() {
         if !cmd_available("bash") {
diff --git a/src/graph/map.rs b/src/graph/map.rs
index 16651a5..67964fd 100644
--- a/src/graph/map.rs
+++ b/src/graph/map.rs
@@ -337,7 +337,7 @@ fn resolve_max_concurrency(
         Some(ConcurrencyCap::Fixed(0)) => {
             bail!("map node '{node_id}': max_concurrency 0; expected a positive integer")
         }
-        Some(ConcurrencyCap::Fixed(n)) => return Ok(*n),
+        Some(ConcurrencyCap::Fixed(n)) => return bounded_cap(node_id, *n),
         Some(ConcurrencyCap::Template(t)) => t,
     };
 
@@ -359,7 +359,7 @@ fn resolve_max_concurrency(
     };
 
     match resolved {
-        Some(n) if n >= 1 => Ok(n),
+        Some(n) if n >= 1 => bounded_cap(node_id, n),
         _ => Err(anyhow!(
             "map node '{node_id}': max_concurrency template \"{template}\" resolved to \
              {value} ({}); expected a positive integer",
@@ -368,6 +368,18 @@ fn resolve_max_concurrency(
     }
 }
 
+/// `Semaphore::new` panics above `MAX_PERMITS`; a cap that large is a
+/// configuration error, not a crash.
+fn bounded_cap(node_id: &str, n: usize) -> Result<usize> {
+    if n > Semaphore::MAX_PERMITS {
+        bail!(
+            "map node '{node_id}': max_concurrency {n} exceeds the runtime limit of {}",
+            Semaphore::MAX_PERMITS
+        );
+    }
+    Ok(n)
+}
+
 /// Pre-provision one teammate identity per item before any chain starts, so a
 /// fast sibling can message one still waiting on the semaphore (the message
 /// queues in the pre-created inbox). The identity lives for the item's whole
@@ -686,6 +698,32 @@ mod tests {
         assert!(msg.contains("map node 'm': max_concurrency \"4\""), "{msg}");
     }
 
+    #[test]
+    fn resolve_max_concurrency_fixed_above_semaphore_limit_errors() {
+        let state = StateManager::new(HashMap::new());
+        let cap = Some(ConcurrencyCap::Fixed(Semaphore::MAX_PERMITS + 1));
+        let msg = resolve(cap, &state).unwrap_err().to_string();
+        assert_eq!(
+            msg,
+            format!(
+                "map node 'm': max_concurrency {} exceeds the runtime limit of {}",
+                Semaphore::MAX_PERMITS + 1,
+                Semaphore::MAX_PERMITS
+            )
+        );
+        // The limit itself is fine.
+        let cap = Some(ConcurrencyCap::Fixed(Semaphore::MAX_PERMITS));
+        assert_eq!(resolve(cap, &state).unwrap(), Semaphore::MAX_PERMITS);
+    }
+
+    #[test]
+    fn resolve_max_concurrency_template_above_semaphore_limit_errors() {
+        let state = state_with("budget", Value::from((Semaphore::MAX_PERMITS as u64) + 1));
+        let cap = Some(ConcurrencyCap::Template("{{budget}}".into()));
+        let msg = resolve(cap, &state).unwrap_err().to_string();
+        assert!(msg.contains("exceeds the runtime limit of"), "{msg}");
+    }
+
     #[test]
     fn resolve_max_concurrency_accepts_padded_template() {
         let state = state_with("budget", Value::from(3));
diff --git a/src/graph/validator.rs b/src/graph/validator.rs
index c19c855..98f18c1 100644
--- a/src/graph/validator.rs
+++ b/src/graph/validator.rs
@@ -8,6 +8,7 @@ use anyhow::{Result, bail};
 use std::collections::{BTreeMap, HashSet, VecDeque};
 use std::path::PathBuf;
 use std::sync::Arc;
+use tokio::sync::Semaphore;
 
 #[derive(Debug, Clone)]
 pub struct ValidationError {
@@ -537,6 +538,12 @@ impl GraphValidator {
                  deadlock the executor",
             ));
         }
+        if graph.settings.max_concurrency > Semaphore::MAX_PERMITS {
+            result.error(ValidationError::new(format!(
+                "settings.max_concurrency must be <= {} (got {}); the runtime cannot allocate more permits",
+                Semaphore::MAX_PERMITS, graph.settings.max_concurrency
+            )));
+        }
 
         for (node_id, node) in &graph.nodes {
             if let NodeType::Map(m) = &node.node_type
@@ -548,6 +555,18 @@ impl GraphValidator {
                      would deadlock the executor",
                 ));
             }
+            if let NodeType::Map(m) = &node.node_type
+                && let Some(ConcurrencyCap::Fixed(n)) = m.max_concurrency
+                && n > Semaphore::MAX_PERMITS
+            {
+                result.error(ValidationError::with_node(
+                    node_id,
+                    format!(
+                        "map node's `max_concurrency` must be <= {} (got {n}); the runtime cannot allocate more permits",
+                        Semaphore::MAX_PERMITS
+                    ),
+                ));
+            }
         }
     }
 
@@ -3158,6 +3177,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn settings_max_concurrency_above_semaphore_limit_errors() {
+        let mut graph = graph_with(vec![("e", end_node("e"))], "e");
+        graph.settings.max_concurrency = Semaphore::MAX_PERMITS + 1;
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            result.errors.iter().any(|e| e.message.contains(&format!(
+                "settings.max_concurrency must be <= {} (got {})",
+                Semaphore::MAX_PERMITS,
+                Semaphore::MAX_PERMITS + 1
+            ))),
+            "expected graph-level max_concurrency upper-bound error: {:?}",
+            result.errors
+        );
+    }
+
     #[test]
     fn settings_max_concurrency_default_is_valid() {
         let graph = graph_with(vec![("e", end_node("e"))], "e");
@@ -3458,6 +3495,30 @@ mod tests {
         assert_eq!(w[1].node_id.as_deref(), Some("l"));
     }
 
+    #[test]
+    fn map_max_concurrency_above_semaphore_limit_errors() {
+        let mut map = map_node_basic("m", "br", Some("end"));
+        if let NodeType::Map(ref mut mm) = map.node_type {
+            mm.max_concurrency = Some(ConcurrencyCap::Fixed(Semaphore::MAX_PERMITS + 1));
+        }
+        let branch = llm_with_state_updates("br", &[("output", "{{output}}")], None);
+        let graph = graph_with(
+            vec![("m", map), ("br", branch), ("end", end_node("end"))],
+            "m",
+        );
+
+        let result = validator().validate(&graph);
+
+        assert!(
+            result.errors.iter().any(|e| e
+                .message
+                .contains(&format!("must be <= {}", Semaphore::MAX_PERMITS))
+                && e.node_id.as_deref() == Some("m")),
+            "expected map max_concurrency upper-bound error: {:?}",
+            result.errors
+        );
+    }
+
     #[test]
     fn map_max_concurrency_zero_errors() {
         let mut map = map_node_basic("m", "br", Some("end"));