diff --git a/crates/engine/src/settings/session_settings.rs b/crates/engine/src/settings/session_settings.rs
index 98f34bd34..a4e800da6 100644
--- a/crates/engine/src/settings/session_settings.rs
+++ b/crates/engine/src/settings/session_settings.rs
@@ -119,6 +119,7 @@ impl Session {
         // (no file system in the browser: the disk cache stays off there)
         let Some(folder) = folder.filter(|_| d.disk_cache_enabled && !cfg!(target_arch = "wasm32")) else {
             self.disk_cache = None;
+            self.disk_watch = Default::default();
             self.layer_cache.set_disk(None);
             return;
         };
@@ -132,11 +133,17 @@ impl Session {
                 c.set_max_bytes(max);
                 c.clone()
             }
+            // A new folder: the free-space reading was for the old one (a reader still running
+            // holds the old watcher's state, so it can't write into the new one).
             _ => match DiskCache::open(&folder, max) {
-                Ok(c) => c,
+                Ok(c) => {
+                    self.disk_watch = Default::default();
+                    c
+                }
                 Err(e) => {
                     log::warn!("disk cache at {}: {e}", folder.display());
                     self.disk_cache = None;
+                    self.disk_watch = Default::default();
                     self.layer_cache.set_disk(None);
                     return;
                 }
diff --git a/crates/render/src/disk_cache.rs b/crates/render/src/disk_cache.rs
index 41be3c454..44a53f8d9 100644
--- a/crates/render/src/disk_cache.rs
+++ b/crates/render/src/disk_cache.rs
@@ -547,6 +547,8 @@ impl DiskCache {
         if data.len() as u64 > self.limit_bytes() {
             return;
         }
+        // Make room first, so the cache never exceeds its limit (or the free-space reserve).
+        self.evict_to(self.limit_bytes().saturating_sub(data.len() as u64));
         let n = self.tmp_counter.fetch_add(1, Ordering::Relaxed);
         let tmp = parent.join(format!("{key:032x}.tmp.{}.{n}", std::process::id()));
         if let Err(e) = std::fs::write(&tmp, &data).and_then(|()| std::fs::rename(&tmp, &path)) {
@@ -558,7 +560,6 @@ impl DiskCache {
         if let Ok(mut g) = self.index.lock() {
             g.insert(kind, key, data.len() as u64);
         }
-        self.evict();
     }
 
     /// Count a failed write; the first one is logged (a full disk shouldn't go unnoticed).
@@ -570,7 +571,11 @@ impl DiskCache {
 
     /// Remove least recently used entries until the cache fits its limit.
     fn evict(&self) {
-        let max = self.limit_bytes();
+        self.evict_to(self.limit_bytes());
+    }
+
+    /// Remove least recently used entries until the cache fits `max` bytes.
+    fn evict_to(&self, max: u64) {
         let victims = match self.index.lock() {
             Ok(mut g) => g.evict(max),
             Err(_) => return,
@@ -1134,7 +1139,7 @@ mod tests {
         assert_eq!((st.max_bytes, st.limit_bytes), (1 << 30, used));
         dc.put_layer_now(9, &buf(9, 64, 64));
         let st = dc.stats();
-        assert!(st.bytes <= used - used / 10 && st.evictions > 0, "{st:?}");
+        assert!(st.bytes <= st.limit_bytes && st.bytes <= used - used / 10 && st.evictions > 0, "{st:?}");
         assert!(dc.contains(Kind::Layer, 9), "the new entry is the most recently used");
         // Below the reserve the cache gives its space back, and skips writes that can't fit.
         dc.set_disk_space(Some((100 << 30, 4 << 30)));