diff --git a/.claude/hooks/check-formatting.sh b/.claude/hooks/check-formatting.sh
index 47ec82c4..c7d3e33b 100755
--- a/.claude/hooks/check-formatting.sh
+++ b/.claude/hooks/check-formatting.sh
@@ -10,15 +10,28 @@ if [ ! -f "Cargo.toml" ]; then
   exit 0
 fi
 
-# Run cargo +nightly fmt --all --check
-if cargo +nightly fmt --all --check 2>&1; then
+# Pinned nightly toolchain for rustfmt (single source of truth shared with CI,
+# .githooks/pre-push and flake.nix). Falls back to floating `nightly`.
+NIGHTLY="$(tr -d '[:space:]' < rustfmt-toolchain 2>/dev/null || echo nightly)"
+
+if command -v cargo-+nightly >/dev/null 2>&1; then
+  # Nix dev shell: `cargo +nightly fmt` is wrapped to the pinned nightly.
+  TOOLCHAIN="nightly"
+else
+  # rustup: pin by dated toolchain name; install it if missing (no-op if present).
+  TOOLCHAIN="$NIGHTLY"
+  rustup toolchain install "$NIGHTLY" --component rustfmt --profile minimal >/dev/null 2>&1 || true
+fi
+
+# Run cargo fmt with the pinned nightly
+if cargo "+$TOOLCHAIN" fmt --all --check 2>&1; then
   # Formatting is correct
-  echo '{"continue": true, "systemMessage": "✓ Code formatting verified with cargo +nightly fmt"}'
+  echo "{\"continue\": true, \"systemMessage\": \"✓ Code formatting verified with cargo +$TOOLCHAIN fmt ($NIGHTLY)\"}"
   exit 0
 else
   # Auto-fix formatting
-  cargo +nightly fmt --all 2>&1
+  cargo "+$TOOLCHAIN" fmt --all 2>&1
 
-  echo '{"continue": true, "systemMessage": "✓ Formatting applied with cargo +nightly fmt --all."}'
+  echo "{\"continue\": true, \"systemMessage\": \"✓ Formatting applied with cargo +$TOOLCHAIN fmt --all ($NIGHTLY).\"}"
   exit 0
 fi
diff --git a/.githooks/pre-push b/.githooks/pre-push
index 0b9a0460..9289c249 100755
--- a/.githooks/pre-push
+++ b/.githooks/pre-push
@@ -14,9 +14,14 @@ if ! cargo machete; then
     exit 1
 fi
 
-# Formatting check
-if ! cargo +nightly fmt --all -- --check; then
-    echo "❌ Formatting issues (run 'cargo +nightly fmt --all')"
+# Formatting check (pinned nightly — see rustfmt-toolchain)
+FMT_TOOLCHAIN="$(tr -d '[:space:]' < rustfmt-toolchain 2>/dev/null || echo nightly)"
+if command -v cargo-+nightly >/dev/null 2>&1; then
+    # Nix dev shell wraps `cargo +nightly fmt` to the pinned nightly.
+    FMT_TOOLCHAIN="nightly"
+fi
+if ! cargo "+$FMT_TOOLCHAIN" fmt --all -- --check; then
+    echo "❌ Formatting issues (run 'cargo +$FMT_TOOLCHAIN fmt --all')"
     exit 1
 fi
 
diff --git a/.github/workflows/linter.yml b/.github/workflows/linter.yml
index 52cc68fc..201f410a 100644
--- a/.github/workflows/linter.yml
+++ b/.github/workflows/linter.yml
@@ -10,6 +10,7 @@ on:
       - "rust-toolchain.toml"
       - "flake.nix"
       - "rustfmt.toml"
+      - "rustfmt-toolchain"
       - "clippy.toml"
       - ".github/workflows/linter.yml"
   pull_request:
@@ -21,6 +22,7 @@ on:
       - "rust-toolchain.toml"
       - "flake.nix"
       - "rustfmt.toml"
+      - "rustfmt-toolchain"
       - "clippy.toml"
       - ".github/workflows/linter.yml"
 
@@ -55,11 +57,15 @@ jobs:
       - name: Install `oas3-gen`
         run: cargo install oas3-gen@0.24.0 --locked
 
-      - name: Install `rustfmt`
-        run: rustup +nightly component add rustfmt
+      - name: Read pinned rustfmt toolchain
+        id: rustfmt
+        run: echo "toolchain=$(cat rustfmt-toolchain)" >> "$GITHUB_OUTPUT"
+
+      - name: Install pinned nightly `rustfmt`
+        run: rustup toolchain install ${{ steps.rustfmt.outputs.toolchain }} --component rustfmt --profile minimal
 
       - name: Check formatting
-        run: cargo +nightly fmt --all -- --check
+        run: cargo +${{ steps.rustfmt.outputs.toolchain }} fmt --all -- --check
 
       - name: Run Clippy
         run: cargo clippy --locked --all-targets --all-features
diff --git a/AGENTS.md b/AGENTS.md
index a52e4dac..c90d1da9 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -50,11 +50,15 @@ Environment:
 
 - Recommended dev setup: `nix develop` (see `pluto/CONTRIBUTING.md`).
 - Rust toolchain is pinned in `pluto/rust-toolchain.toml`.
+- The exact nightly used for `rustfmt` is pinned in `pluto/rustfmt-toolchain`
+  (CI, git hooks and the Nix dev shell all read it). Inside `nix develop`,
+  `cargo +nightly fmt` is wrapped to that pinned nightly; with rustup, use the
+  dated toolchain, e.g. `cargo +"$(cat rustfmt-toolchain)" fmt`.
 
 Commands (run from `pluto/`):
 
 ```bash
-cargo +nightly fmt --all --check
+cargo +nightly fmt --all --check   # nightly pinned via rustfmt-toolchain
 cargo clippy --workspace --all-targets --all-features -- -D warnings
 cargo test --workspace --all-features
 cargo deny check --hide-inclusion-graph
diff --git a/flake.nix b/flake.nix
index 57d0f0c8..3792a1d6 100644
--- a/flake.nix
+++ b/flake.nix
@@ -22,11 +22,19 @@
       rustToolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
 
       # `cargo +nightly fmt` is the project-wide formatting command.
+      # The exact nightly is pinned in `rustfmt-toolchain` (format:
+      # `nightly-YYYY-MM-DD`) so this dev shell, CI and the git hooks all use
+      # the same formatter and a new nightly can never silently change output.
+      # NOTE: `rust-overlay` must know about this date; run
+      # `nix flake update rust-overlay` if evaluation fails for a newer pin.
+      fmtNightlyDate = pkgs.lib.removePrefix "nightly-"
+        (pkgs.lib.fileContents ./rustfmt-toolchain);
+
       # Nix does not use rustup, so we provide the following workarounds:
-      # - Expose a standalone binary that runs the nightly formatter, and
-      # - Wrap `cargo` to ensure that `cargo +nightly fmt` uses the nightly formatter.
+      # - Expose a standalone binary that runs the pinned nightly formatter, and
+      # - Wrap `cargo` to ensure that `cargo +nightly fmt` uses that formatter.
       rustfmtNightly = pkgs.writeShellScriptBin "rustfmt-nightly" ''
-        exec ${pkgs.rust-bin.selectLatestNightlyWith (toolchain: toolchain.rustfmt)}/bin/rustfmt "$@"
+        exec ${pkgs.rust-bin.nightly.${fmtNightlyDate}.rustfmt}/bin/rustfmt "$@"
       '';
       cargoNightly = pkgs.writeShellScriptBin "cargo-+nightly" ''
         shift
diff --git a/rustfmt-toolchain b/rustfmt-toolchain
new file mode 100644
index 00000000..e9c0e6d4
--- /dev/null
+++ b/rustfmt-toolchain
@@ -0,0 +1 @@
+nightly-2026-08-30