diff --git a/apps/figcraft-server/src/config.rs b/apps/figcraft-server/src/config.rs
index bd587b76..101c7815 100644
--- a/apps/figcraft-server/src/config.rs
+++ b/apps/figcraft-server/src/config.rs
@@ -135,9 +135,23 @@ impl Config {
     /// abuse contact, or one whose clients would all count as its proxy).
     pub fn warnings(&self) -> Vec<String> {
         let mut out = Vec::new();
+        let client_max = NetLimits::default().max_server_message >> 20;
+        if usize::try_from(self.file_mb()).is_ok_and(|mb| mb >= client_max) {
+            out.push(format!(
+                "max_file_mb is {}: the app opens files of up to {} MiB, so a file that large can't be opened",
+                self.file_mb(),
+                client_max - 1
+            ));
+        }
         if !self.open_publish {
             return out;
         }
+        if self.file_mb() > 64 {
+            out.push(format!(
+                "open_publish is on with max_file_mb = {}: anyone may send messages that large, and the memory set aside for reading and queueing them grows to eight times it; keep it at 64 or less on a public server",
+                self.file_mb()
+            ));
+        }
         if self.operator_contact.trim().is_empty() {
             out.push("open_publish is on but operator_contact is empty: people can't reach you about abuse; set it (and terms_url)".into());
         }
@@ -253,8 +267,9 @@ impl Config {
         }
     }
 
-    /// Largest message a client may send when open to the public: a whole file (or plugin bundle)
-    /// and a margin, not the core's 64 MiB.
+    /// Largest message a client may send: a whole file (or plugin bundle) and a margin. Open to the
+    /// public this is also the cap (strangers' files stay small); a private server never goes below
+    /// the core's 64 MiB, and a `max_file_mb` above it raises the cap with it.
     fn open_message(&self) -> usize {
         usize::try_from(self.file_mb().max(self.max_plugin_mb).saturating_add(1).saturating_mul(1 << 20)).unwrap_or(usize::MAX)
     }
@@ -265,9 +280,9 @@ impl Config {
     fn core(&self) -> figcraft_collab::Limits {
         let base = figcraft_collab::Limits { max_peers: self.max_people, ..figcraft_collab::Limits::default() };
         if !self.open_publish {
-            return base;
+            return figcraft_collab::Limits { max_message_bytes: base.max_message_bytes.max(self.open_message()), ..base };
         }
-        let max = self.open_message().min(base.max_message_bytes);
+        let max = self.open_message();
         let file = usize::try_from(self.file_mb().saturating_mul(1 << 20)).unwrap_or(usize::MAX);
         figcraft_collab::Limits { max_message_bytes: max, max_log_bytes: (file / 4).max(256 << 10), ..base }
     }
@@ -279,21 +294,24 @@ impl Config {
     pub fn net(&self) -> NetLimits {
         let base = NetLimits { max_connections: self.max_connections, trusted_proxy: self.proxy().ok().flatten(), ..NetLimits::default() };
         if !self.open_publish {
-            return base;
+            let max = base.max_client_message.max(self.open_message());
+            return NetLimits { max_client_message: max, max_queued_bytes: base.max_queued_bytes.max(max.saturating_add(1 << 20)), ..base };
         }
-        let max = self.open_message().min(base.max_client_message);
+        let max = self.open_message();
         // Shared budgets: one block of addresses (an IPv4 address, an IPv6 /48) gets at most an
-        // eighth of each, and its biggest consumers are dropped first.
-        let (queued, reading) = (256usize << 20, 192usize << 20);
+        // eighth of each, and its biggest consumers are dropped first. Each still fits one whole
+        // message, or a file as large as `max_file_mb` could never arrive.
+        let one = max.saturating_add(1 << 20);
+        let (queued, reading) = ((256usize << 20).max(one.saturating_mul(8)), (192usize << 20).max(one.saturating_mul(8)));
         NetLimits {
             max_per_address: self.max_connections_per_ip,
             max_client_message: max,
-            max_queued_bytes: max.saturating_add(1 << 20),
+            max_queued_bytes: one,
             max_queued_block: queued / 8,
             max_queued_total: queued,
-            max_reading_block: max.saturating_add(1 << 20).min(reading / 8),
+            max_reading_block: one.min(reading / 8),
             max_reading_total: reading,
-            max_in_flight: 128 << 20,
+            max_in_flight: (128usize << 20).max(one),
             reserve_for_sessions: self.max_connections / 8,
             request_idle: std::time::Duration::from_secs(20),
             ..base
diff --git a/apps/figcraft-server/src/public_tests.rs b/apps/figcraft-server/src/public_tests.rs
index fea2667e..978d9e8e 100644
--- a/apps/figcraft-server/src/public_tests.rs
+++ b/apps/figcraft-server/src/public_tests.rs
@@ -413,7 +413,7 @@ fn open_mode_has_safe_defaults_and_warns_the_operator() {
     assert_eq!(open.net().request_idle, Duration::from_secs(20), "idle one-shot requests go quickly");
     assert_eq!(open.net().reserve_for_sessions, 512 / 8);
     assert_eq!(open.net().max_client_message, 17 << 20);
-    assert_eq!(private.net().max_client_message, NetLimits::default().max_client_message, "unchanged when private");
+    assert_eq!(private.net().max_client_message, 65 << 20, "private: a 64 MiB file and a margin");
     let w = open.warnings().join("\n");
     assert!(w.contains("operator_contact is empty") && w.contains("trusted_proxy"), "{w}");
     let set = Config { operator_contact: "abuse@example.com".into(), trusted_proxy: Some("127.0.0.1".into()), max_file_mb: Some(8), ..open };
@@ -493,3 +493,34 @@ fn open_connections_never_reach_what_needs_the_key() {
     assert!(!format!("{info:?}").contains(&k));
     server.stop();
 }
+
+#[test]
+fn files_above_64_mib_fit_every_limit() {
+    // `max_file_mb` may go up to 4096: every cap a whole file's message meets on its way in must
+    // follow it, or the upload is dropped by the transport before the server can say why.
+    for open_publish in [false, true] {
+        let cfg = Config { open_publish, max_file_mb: Some(256), public_url: Some("wss://figcraft.example.com/".into()), ..Config::default() };
+        let (net, core, file) = (cfg.net(), cfg.store().core, 256usize << 20);
+        assert!(net.max_client_message > file, "open={open_publish}: {}", net.max_client_message);
+        assert!(net.max_queued_bytes > file && net.max_reading_block > file && net.max_in_flight > file, "open={open_publish}: {net:?}");
+        assert!(net.max_reading_total >= net.max_reading_block && net.max_queued_total >= net.max_queued_block);
+        assert!(core.max_message_bytes > file, "open={open_publish}: {}", core.max_message_bytes);
+    }
+    // A client takes such a file too, but caps what any server may make it buffer at 1 GiB.
+    assert_eq!(NetLimits::default().max_server_message, 1 << 30);
+}
+
+#[test]
+fn large_files_warn_the_operator() {
+    let public_url = Some("wss://figcraft.example.com/".into());
+    let warns = |open_publish, mb| {
+        Config { open_publish, max_file_mb: Some(mb), public_url: public_url.clone(), ..Config::default() }.warnings().join(
+            "
+",
+        )
+    };
+    assert!(!warns(true, 64).contains("max_file_mb"), "{}", warns(true, 64));
+    assert!(warns(true, 65).contains("open_publish is on with max_file_mb = 65"), "{}", warns(true, 65));
+    assert!(!warns(false, 1023).contains("max_file_mb"), "{}", warns(false, 1023));
+    assert!(warns(false, 1024).contains("can't be opened"), "{}", warns(false, 1024));
+}
diff --git a/crates/collab-net/src/limits.rs b/crates/collab-net/src/limits.rs
index c3111876..a0647fa6 100644
--- a/crates/collab-net/src/limits.rs
+++ b/crates/collab-net/src/limits.rs
@@ -12,7 +12,9 @@ pub struct NetLimits {
     /// before: its hello.
     pub max_client_message: usize,
     pub max_hello: usize,
-    /// Largest message a client accepts from the host (a welcome carries the whole document).
+    /// Largest message a client accepts from the host (a welcome carries the whole document): 1 GiB,
+    /// room for any file up to 1023 MiB and well past the servers' defaults, while a hostile server
+    /// (any link can name one) can't make the app or a browser tab buffer more than that.
     pub max_server_message: usize,
     /// Messages a client may send per second on average, and in a burst; beyond that it is
     /// disconnected.
@@ -61,7 +63,7 @@ impl Default for NetLimits {
             max_connections: 64,
             max_client_message: figcraft_collab::Limits::default().max_message_bytes,
             max_hello: 64 << 10,
-            max_server_message: figcraft_collab::Limits::default().max_message_bytes,
+            max_server_message: 1 << 30,
             rate: 100.0,
             burst: 400.0,
             hello_timeout: Duration::from_secs(10),
diff --git a/docs/server.md b/docs/server.md
index 021d0e3f..87e68b3d 100644
--- a/docs/server.md
+++ b/docs/server.md
@@ -282,7 +282,7 @@ are errors):
 | `data` | `figcraft-data` | The data directory |
 | `public_url` | (the listen address) | How links name the server, when people reach it another way (a reverse proxy: `wss://figcraft.example.com/`) |
 | `max_files` | 1000 | Most files kept |
-| `max_file_mb` | 64 (16 when open) | Largest file (its snapshot), in MiB; a welcome carries the whole file, and messages are at most 64 MiB |
+| `max_file_mb` | 64 (16 when open) | Largest file (its snapshot), in MiB; a welcome carries the whole file, so messages may be as large as a file and 1 MiB (at least 64 MiB on a private server). The app opens files of up to 1023 MiB; above 64 with `open_publish` the server warns |
 | `max_links` | 100 | Most share links per file |
 | `max_connections` | 512 | Most open connections |
 | `max_people` | 256 | Most people in one file at once |
@@ -490,7 +490,7 @@ are larger.
 
 | Resource | Limit |
 |---|---|
-| Message size | A file's size and 1 MiB (`max_file_mb`, 16 by default when open, or `max_plugin_mb` if larger): checked as the message is read, before anything is decoded; 64 KiB on a connection that only browses plugins (a client sending more is disconnected with the limit as the reason; the app keeps its own copy). The core's 64 MiB otherwise |
+| Message size | A file's size and 1 MiB (`max_file_mb`, 16 by default when open, or `max_plugin_mb` if larger): checked as the message is read, before anything is decoded; 64 KiB on a connection that only browses plugins (a client sending more is disconnected with the limit as the reason; the app keeps its own copy). On a private server, the larger of that and the core's 64 MiB |
 | A file as it grows | Every batch is checked first (*always*): past `max_file_mb`, past its publisher's `max_mb_per_ip` or past `max_total_mb`, it is refused with the reason and the app rolls it back. A long log is compacted to make room at most once a minute per file (a compaction rewrites the whole file). A file's snapshot also holds the recent batches kept for reconnecting clients, at most a quarter of `max_file_mb` |
 | Recent batches in memory | A quarter of `max_file_mb` (256 KiB at least) per open file; older ones are dropped (a client that missed them catches up with the whole file) |
 | Bytes queued for slow clients | The largest message and 1 MiB per connection, 32 MiB per block of addresses, 256 MiB for everyone. A message sent to many people counts once for everyone and once per block (twenty colleagues behind one office address cost their block one copy of a broadcast), and welcomes (bounded by the file's size) don't count against blocks, so a whole office can join a file at once. A client that doesn't read its answers is dropped; when everyone's budget runs out, the connection holding the most is dropped |