diff --git a/.github/workflows/book.yml b/.github/workflows/book.yml
index 919f41802f..67d111f5f1 100644
--- a/.github/workflows/book.yml
+++ b/.github/workflows/book.yml
@@ -64,6 +64,10 @@ jobs:
 
       - name: Install toolchain
         uses: dtolnay/rust-toolchain@nightly
+      # `Build docs` below passes `-Zunstable-options`, so this job needs nightly rather than the
+      # toolchain `rust-toolchain.toml` pins.
+      - name: Use nightly for this job
+        run: echo "RUSTUP_TOOLCHAIN=nightly" >> "$GITHUB_ENV"
       - uses: Swatinem/rust-cache@v2
         with:
           cache-on-failure: true
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dd65e75198..180b87d1fe 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,6 +23,11 @@ jobs:
       matrix:
         rust: ["1.88", "stable", "nightly"]
         flags: ["--no-default-features", "", "--all-features"]
+    # This matrix is the one place that deliberately compiles on something other than the pinned
+    # toolchain, so it has to outrank `rust-toolchain.toml`. `RUSTUP_TOOLCHAIN` does; the toolchain
+    # the install step selects does not.
+    env:
+      RUSTUP_TOOLCHAIN: ${{ matrix.rust }}
     steps:
       - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@master
diff --git a/rust-toolchain.toml b/rust-toolchain.toml
new file mode 100644
index 0000000000..c8fb1836dc
--- /dev/null
+++ b/rust-toolchain.toml
@@ -0,0 +1,14 @@
+# The Rust version this fork builds and tests with. It matches the pin in `smr-moonshot`, the
+# consumer of these crates, rather than the workspace `rust-version` (1.88.0) that upstream `revm`
+# declares as its MSRV: this fork only ever has to compile under Supra's toolchain, and the range of
+# compilers that actually build it is narrower than the MSRV suggests.
+#
+# This file is the only functional pin: rustup prefers it over `rustup default`, so it also decides
+# the toolchain in CI, whatever channel a toolchain-install step there names. The jobs that
+# genuinely need a different compiler — the MSRV/nightly test matrix, and the book's nightly
+# rustdoc — override it with `RUSTUP_TOOLCHAIN`, which rustup ranks above this file.
+[toolchain]
+channel = "1.97.1"
+components = ["clippy", "rust-docs", "rustfmt"]
+# `check-no-std` cross-compiles to this target.
+targets = ["riscv32imac-unknown-none-elf"]