diff --git a/interactive/src/backend/corgi.rs b/interactive/src/backend/corgi.rs
index 48b065e7a..7c09c4e3a 100644
--- a/interactive/src/backend/corgi.rs
+++ b/interactive/src/backend/corgi.rs
@@ -265,13 +265,19 @@ impl Backend for CorgiBackend {
     }
 
     fn inspect<'s>(c: Collection<'s, Time, CC>, label: String) -> Collection<'s, Time, CC> {
+        use std::fmt::Write;
         c.inner
             .unary(Pipeline, "CorgiInspect", move |_, _| {
+                let mut line = String::new();
                 move |input, output| {
                     input.for_each(|cap, data| {
                         let mut cont = std::mem::take(data);
                         for ((k, v), t, d) in cont.clone().into_updates() {
-                            eprintln!("  [{label}] (({k:?}, {v:?}), {t:?}, {d})");
+                            // Format before taking stderr's lock: nested Debug output
+                            // otherwise writes many small fragments while holding it.
+                            line.clear();
+                            writeln!(&mut line, "  [{label}] (({k:?}, {v:?}), {t:?}, {d})").unwrap();
+                            eprint!("{line}");
                         }
                         output.session(&cap).give_container(&mut cont);
                     });
diff --git a/interactive/src/backend/vec.rs b/interactive/src/backend/vec.rs
index b6e10dd43..eecd62aec 100644
--- a/interactive/src/backend/vec.rs
+++ b/interactive/src/backend/vec.rs
@@ -173,7 +173,14 @@ impl Backend for VecBackend {
         )
     }
     fn inspect<'s>(c: Collection<'s, Time, Self::Container>, label: String) -> Collection<'s, Time, Self::Container> {
-        c.inspect(move |x| eprintln!("  [{}] {:?}", label, x.clone()))
+        use std::fmt::Write;
+        let mut line = String::new();
+        c.inspect(move |x| {
+            // Keep formatting outside stderr's lock, and write one complete record.
+            line.clear();
+            writeln!(&mut line, "  [{label}] {x:?}").unwrap();
+            eprint!("{line}");
+        })
     }
     fn leave_dynamic<'s>(c: Collection<'s, Time, Self::Container>, depth: usize) -> Collection<'s, Time, Self::Container> {
         c.leave_dynamic(depth)