diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9c13aa272..3e05fb588 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -96,3 +96,72 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: crate-ci/typos@v1.44.0
+
+  field_separation:
+    name: Field-backend separation guard
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup rust toolchain, cache and bins
+        uses: moonrepo/setup-rust@v1
+        with:
+          channel: nightly-2026-03-04
+          cache-base: main
+      - name: Check field-backend dependency separation
+        run: ./scripts/check-field-separation.sh
+
+  goldilocks_standalone:
+    name: Goldilocks backend (standalone, no --all-features)
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup rust toolchain, cache and bins
+        uses: moonrepo/setup-rust@v1
+        with:
+          channel: nightly-2026-03-04
+          cache-base: main
+      - name: Build goldilocks backend in isolation
+        run: cargo build --all-targets -p provekit-backend-goldilocks
+      - name: Test goldilocks backend in isolation
+        run: cargo test --no-fail-fast --lib --tests --bins -p provekit-backend-goldilocks
+
+  bn254_standalone:
+    name: BN254 backend (standalone, no --all-features)
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup rust toolchain, cache and bins
+        uses: moonrepo/setup-rust@v1
+        with:
+          channel: nightly-2026-03-04
+          cache-base: main
+      - name: Build bn254 backend in isolation
+        run: cargo build --all-targets -p provekit-backend-bn254 --features witness-generation,provekit_ntt
+      - name: Build bn254-welded tooling in isolation
+        run: >
+          cargo build --all-targets
+          -p provekit-r1cs-compiler -p provekit-cli -p provekit-ffi -p provekit-gnark
+          -p provekit-bench -p verifier-server -p passport-input-gen
+      - name: Test bn254 backend in isolation
+        run: cargo test --no-fail-fast --lib --tests --bins -p provekit-backend-bn254 --features witness-generation,provekit_ntt
+      - name: Test bn254-welded tooling in isolation
+        run: >
+          cargo test --no-fail-fast --lib --tests --bins
+          -p provekit-r1cs-compiler -p provekit-cli -p provekit-ffi -p provekit-gnark
+          -p provekit-bench -p verifier-server -p passport-input-gen
+
+  fixtures:
+    name: Cross-field fixtures (both backends)
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup rust toolchain, cache and bins
+        uses: moonrepo/setup-rust@v1
+        with:
+          channel: nightly-2026-03-04
+          cache-base: main
+      - name: Test fixtures (both fields)
+        run: >
+          cargo test -p provekit-fixtures
+          --test goldilocks_roundtrip --test goldilocks_soundness
+          --test bn254_roundtrip --test bn254_soundness
diff --git a/scripts/check-field-separation.sh b/scripts/check-field-separation.sh
new file mode 100755
index 000000000..289cc4910
--- /dev/null
+++ b/scripts/check-field-separation.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+#
+# Field-backend dependency-separation guard (fail-closed): the 64-bit Goldilocks
+# backend must never pull a bn254/256-bit crate, and neither backend may depend on
+# the other. A --all-features build cannot catch this; this can.
+set -euo pipefail
+
+# Workspace-local crates the Goldilocks graph may contain (field-agnostic spine +
+# the backend itself). Every bn254-specific workspace crate is a local path dep,
+# so any one leaking in trips this allowlist with zero list maintenance.
+ALLOWED_LOCAL='provekit-backend-goldilocks provekit-common provekit-prover provekit-verifier'
+
+# External bn254/256-bit families the allowlist can't see (crates.io / git deps):
+# curve stacks and the Noir/Mavros frontend. Defense-in-depth; extend as needed.
+FORBIDDEN_EXTERNAL='^(ark-bn254|ark-ec|ark-poly|ark-grumpkin|grumpkin|k256|p256|ecdsa|elliptic-curve|crypto-bigint|primefield|primeorder|sec1|rfc6979|acir|acir_field|acvm|acvm_blackbox_solver|brillig|brillig_vm|nargo|noirc_abi|noirc_printable_type|noirc_span|mavros-vm|mavros-artifacts|mavros-opcode-gen) '
+
+backend_tree() {
+  # Fail closed: a cargo-tree error or empty graph means we never inspected it.
+  local pkg="$1" out
+  if ! out=$(cargo tree -p "$pkg" -e normal --prefix none); then
+    echo "ERROR: 'cargo tree -p $pkg' failed; cannot verify field separation." >&2
+    exit 1
+  fi
+  if [[ -z "$out" ]]; then
+    echo "ERROR: 'cargo tree -p $pkg' returned an empty graph." >&2
+    exit 1
+  fi
+  printf '%s\n' "$out"
+}
+
+gold=$(backend_tree provekit-backend-goldilocks)
+bn254=$(backend_tree provekit-backend-bn254)
+
+fail=0
+
+# (1) Allowlist: local path deps render as "name vX.Y.Z (/abs/path)".
+while read -r name; do
+  [[ -z "$name" ]] && continue
+  if [[ " $ALLOWED_LOCAL " != *" $name "* ]]; then
+    echo "ERROR: goldilocks backend pulls a non-spine workspace crate: $name" >&2
+    fail=1
+  fi
+done < <(grep -E ' \(/' <<<"$gold" | awk '{print $1}' | sort -u)
+
+# (2) Denylist: external bn254/256-bit families.
+leak=$(sort -u <<<"$gold" | grep -E "$FORBIDDEN_EXTERNAL" || true)
+if [[ -n "$leak" ]]; then
+  echo "ERROR: goldilocks backend leaked bn254/256-bit external crates:" >&2
+  printf '%s\n' "$leak" >&2
+  fail=1
+fi
+
+# (3) Cross-backend: neither may depend on the other.
+if grep -qE '^provekit-backend-bn254 ' <<<"$gold"; then
+  echo 'ERROR: goldilocks backend depends on the bn254 backend.' >&2
+  fail=1
+fi
+if grep -qE '^provekit-backend-goldilocks ' <<<"$bn254"; then
+  echo 'ERROR: bn254 backend depends on the goldilocks backend.' >&2
+  fail=1
+fi
+
+if [[ "$fail" -ne 0 ]]; then
+  exit 1
+fi
+echo 'OK: field-backend dependency separation holds.'