diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0e8a05c..7ae274e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,6 +16,16 @@ on:
 env:
   CARGO_TERM_COLOR: always
   RUSTFLAGS: -D warnings
+  # Every blocking job pins an exact toolchain. With `-D warnings` and a
+  # floating `stable`, a Rust release turns this gate red with no repo change —
+  # which is exactly how it first failed, eight days after 1.97.1 added
+  # `float_literal_f32_fallback`. Worse, the developer machine cannot install
+  # arbitrary toolchains, so such a failure is unreproducible locally and the
+  # local gate stops meaning anything. Pinned here to the version developed
+  # against; bump it deliberately, as its own commit, with the lints it
+  # introduces fixed in that commit. The nightly soak floats on `stable` and is
+  # the drift detector.
+  PINNED_STABLE: "1.95.0"
 
 jobs:
   check:
@@ -27,7 +37,7 @@ jobs:
       - name: Install Rust (pinned MSRV)
         uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: "1.77"
+          toolchain: "1.85"
           components: rustfmt, clippy
 
       - name: Cache cargo
@@ -56,18 +66,20 @@ jobs:
           RUSTDOCFLAGS: -D warnings
         run: cargo doc --workspace --exclude epiphany-editor-gui --no-deps
 
-  # The demo GUI crate, on current stable (its eframe/resvg tree requires a newer
-  # toolchain than the workspace MSRV). Separate so a GUI build break is its own
-  # attributable signal, not a regression in the core crates.
+  # The demo GUI crate, on the pinned stable rather than the MSRV: its
+  # eframe/resvg tree pulls `image`, which declares 1.88 — three releases above
+  # the workspace floor. Separate so a GUI build break is its own attributable
+  # signal, not a regression in the core crates.
   editor-gui:
-    name: editor GUI (current stable)
+    name: editor GUI (pinned stable)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install Rust (current stable)
+      - name: Install Rust (pinned stable)
         uses: dtolnay/rust-toolchain@stable
         with:
+          toolchain: ${{ env.PINNED_STABLE }}
           components: clippy
 
       - name: Install GUI build dependencies
@@ -94,6 +106,8 @@ jobs:
 
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.PINNED_STABLE }}
 
       - name: Cache cargo
         uses: Swatinem/rust-cache@v2
@@ -131,6 +145,8 @@ jobs:
 
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.PINNED_STABLE }}
 
       - name: Cache cargo
         uses: Swatinem/rust-cache@v2
@@ -139,6 +155,10 @@ jobs:
         run: cargo test -p epiphany-testkit --test prepass
 
   # A longer soak, scheduled nightly, exploring a wider seed space at scale.
+  # This job deliberately floats on `stable` while every blocking job is pinned:
+  # it is where toolchain drift surfaces, on a schedule, instead of in a push
+  # that has nothing to do with it. A red soak against a green push means Rust
+  # moved, not that the change did.
   soak:
     name: conformance soak (nightly)
     if: github.event_name == 'schedule'
diff --git a/Cargo.toml b/Cargo.toml
index fb854e3..65a732b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,7 +26,12 @@ members = [
 
 [workspace.package]
 edition = "2021"
-rust-version = "1.77"
+# 1.85 is the floor the dependency graph actually imposes, not a preference:
+# blake3 and its `constant_time_eq` both ship edition 2024, which no Cargo
+# before 1.85 can parse. The previous 1.77 claim was untestable until CI
+# existed and was false when it ran — raise this only against a measured
+# closure (`cargo metadata --filter-platform`), never to make a build pass.
+rust-version = "1.85"
 authors = ["Epiphany contributors"]
 repository = "https://github.com/epiphany-notation/epiphany"
 
diff --git a/crates/epiphany-core/src/invariants.rs b/crates/epiphany-core/src/invariants.rs
index de15cbb..1e29086 100644
--- a/crates/epiphany-core/src/invariants.rs
+++ b/crates/epiphany-core/src/invariants.rs
@@ -1332,7 +1332,7 @@ impl<'a> GraphIndex<'a> {
                     TempoShape::Constant => flag(
                         seg.end_tempo
                             .as_ref()
-                            .map_or(true, |et| et == &seg.start_tempo),
+                            .is_none_or(|et| et == &seg.start_tempo),
                         "constant tempo segment has end_tempo != start_tempo".to_string(),
                     ),
                     TempoShape::Linear | TempoShape::Exponential | TempoShape::Curve => flag(
diff --git a/crates/epiphany-editor-gui/src/main.rs b/crates/epiphany-editor-gui/src/main.rs
index e591157..06c4345 100644
--- a/crates/epiphany-editor-gui/src/main.rs
+++ b/crates/epiphany-editor-gui/src/main.rs
@@ -484,7 +484,9 @@ impl EditorApp {
                 ui.painter().rect_stroke(
                     shape_rect(&region.shape, &vm),
                     0.0,
-                    egui::Stroke::new(2.0, egui::Color32::from_rgb(0, 120, 215)),
+                    // Suffixed: `float_literal_f32_fallback` (Rust 1.97) rejects
+                    // inferring `f32` here, and it becomes a hard error later.
+                    egui::Stroke::new(2.0_f32, egui::Color32::from_rgb(0, 120, 215)),
                 );
             }
         }
diff --git a/crates/epiphany-engrave/src/casting.rs b/crates/epiphany-engrave/src/casting.rs
index e2b048d..46b3055 100644
--- a/crates/epiphany-engrave/src/casting.rs
+++ b/crates/epiphany-engrave/src/casting.rs
@@ -1516,7 +1516,7 @@ fn stroke_fate(
         } else {
             0.0
         };
-        if best.map_or(true, |(_, d)| distance < d) {
+        if best.is_none_or(|(_, d)| distance < d) {
             best = Some((r, distance));
         }
     }
@@ -1595,7 +1595,7 @@ fn curve_fate(
     for (r, span) in region_spans.iter().enumerate() {
         let Some((rlo, rhi)) = span else { continue };
         let distance = interval_distance(lo, hi, (*rlo, *rhi));
-        if best.map_or(true, |(_, d)| distance < d) {
+        if best.is_none_or(|(_, d)| distance < d) {
             best = Some((r, distance));
         }
     }
diff --git a/crates/epiphany-ops/src/reduce.rs b/crates/epiphany-ops/src/reduce.rs
index 999cd5d..7627ab9 100644
--- a/crates/epiphany-ops/src/reduce.rs
+++ b/crates/epiphany-ops/src/reduce.rs
@@ -824,7 +824,7 @@ fn tempo_segment_shape_well_formed(segment: &TempoSegment) -> bool {
         TempoShape::Constant => segment
             .end_tempo
             .as_ref()
-            .map_or(true, |end| end == &segment.start_tempo),
+            .is_none_or(|end| end == &segment.start_tempo),
         TempoShape::Linear | TempoShape::Exponential | TempoShape::Curve => {
             segment.end_tempo.is_some() && segment.end.is_some()
         }
@@ -6620,7 +6620,7 @@ impl<'a> Reducer<'a> {
                     (0u8, signed)
                 };
                 let key = (rank, distance, direction, *event);
-                if best.as_ref().map_or(true, |current| key < *current) {
+                if best.as_ref().is_none_or(|current| key < *current) {
                     best = Some(key);
                 }
             }
@@ -7425,7 +7425,7 @@ fn first_missing_vector_predecessor(
 
         if !complete {
             let candidate = OperationId::new(replica, expected);
-            if first_missing.map_or(true, |current| candidate < current) {
+            if first_missing.is_none_or(|current| candidate < current) {
                 first_missing = Some(candidate);
             }
         }