diff --git a/.github/workflows/push.yaml b/.github/workflows/push.yaml
index c7b6b68e..5005b4b0 100644
--- a/.github/workflows/push.yaml
+++ b/.github/workflows/push.yaml
@@ -11,6 +11,9 @@ env:
   # For setup-rust, see https://github.com/moonrepo/setup-rust/issues/22
   GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
   CARGO_GPU_COMMITSH: 31153a8edd3bc626d4b9fb0cd7bdb7a8b30797d3
+  # Nightly toolchain used for rustfmt in CI and in renderling-build's codegen.
+  # Bump deliberately: reformat + regenerate shaders in the same PR.
+  RUST_NIGHTLY_TOOLCHAIN: nightly-2026-07-31
 
 jobs:
   # Installs cargo deps and sets the cache directory for subsequent jobs
@@ -84,8 +87,8 @@ jobs:
               ${{ needs.install-cargo-gpu.outputs.cachepath-Linux }}
               ${{ needs.install-cargo-gpu.outputs.cachepath-Windows }}
           key: rust-gpu-cache-1-${{ matrix.os }}
-      - run: rustup install nightly
-      - run: rustup component add --toolchain nightly rustfmt
+      - run: rustup install $RUST_NIGHTLY_TOOLCHAIN
+      - run: rustup component add --toolchain $RUST_NIGHTLY_TOOLCHAIN rustfmt
       - run: cargo gpu show commitsh
       - run: rm -rf crates/renderling/src/linkage/* crates/renderling/shaders
       - run: cargo shaders
@@ -105,9 +108,9 @@ jobs:
           key: renderling-fmt-${{ env.CARGO_GPU_COMMITSH }}-ubuntu-24.04
       - run: mkdir -p $HOME/.cargo/bin
       - uses: moonrepo/setup-rust@v1
-      - run: rustup install nightly
-      - run: rustup component add --toolchain nightly rustfmt
-      - run: cargo +nightly fmt -- --check
+      - run: rustup install $RUST_NIGHTLY_TOOLCHAIN
+      - run: rustup component add --toolchain $RUST_NIGHTLY_TOOLCHAIN rustfmt
+      - run: cargo +$RUST_NIGHTLY_TOOLCHAIN fmt -- --check
 
   # BAU clippy lints
   renderling-clippy:
diff --git a/AGENTS.md b/AGENTS.md
index 29432c7c..94854df1 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -17,5 +17,7 @@
 
 Always format with `cargo +nightly fmt`.
 
+The fmt toolchain is pinned to `nightly-2026-07-31` via `RUST_NIGHTLY_TOOLCHAIN` in `.github/workflows/push.yaml`. Use that toolchain locally too; bump it deliberately (reformat + regenerate shaders in the same PR).
+
 ## Disallowed Methods (clippy.toml)
 Avoid: `Vec{2,3,4}::normalize_or_zero`, `Mat4::to_scale_rotation_translation`, `f32::signum`
diff --git a/crates/renderling-build/src/lib.rs b/crates/renderling-build/src/lib.rs
index 13ca07a5..47bc2965 100644
--- a/crates/renderling-build/src/lib.rs
+++ b/crates/renderling-build/src/lib.rs
@@ -283,8 +283,11 @@ impl RenderlingPaths {
         // Just format the whole project. I know this is less than ideal,
         // but people should be running with a formatter in their editor, and all of
         // this is temporary given the wgsl-rs re-stacking happening this year (2026)
+        let fmt_toolchain =
+            std::env::var("RUST_NIGHTLY_TOOLCHAIN").unwrap_or_else(|_| "nightly".into());
         std::process::Command::new("cargo")
-            .args(["+nightly", "fmt"])
+            .arg(format!("+{fmt_toolchain}"))
+            .arg("fmt")
             .output()
             .expect("could not format generated code");
         log::info!("...done!")