diff --git a/.github/actions/authenticate-github-fetches/action.yml b/.github/actions/authenticate-github-fetches/action.yml
new file mode 100644
index 000000000..72d60e811
--- /dev/null
+++ b/.github/actions/authenticate-github-fetches/action.yml
@@ -0,0 +1,20 @@
+name: Authenticate git fetches of GitHub repos
+description: >-
+  Rewrite github.com URLs so git-CLI fetches carry the job's ephemeral token.
+  GitHub throttles anonymous git HTTPS traffic per source IP, and Warp runners
+  share egress IPs across tenants, so unauthenticated clones of public repos
+  can fail with a 401 ("could not read Username"). The rewrite covers every
+  fetch that goes through the git CLI: Lake dependency clones, Nix's eval-time
+  `builtins.fetchGit`, script clones, and cargo git dependencies when
+  CARGO_NET_GIT_FETCH_WITH_CLI is set.
+
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      env:
+        GH_TOKEN: ${{ github.token }}
+      run: >-
+        git config --global
+        url."https://x-access-token:${GH_TOKEN}@github.com/".insteadOf
+        "https://github.com/"
diff --git a/.github/actions/setup-rust-toolchain/action.yml b/.github/actions/setup-rust-toolchain/action.yml
index db4211710..420414345 100644
--- a/.github/actions/setup-rust-toolchain/action.yml
+++ b/.github/actions/setup-rust-toolchain/action.yml
@@ -27,6 +27,19 @@ runs:
           exit 1
         fi
 
+    - uses: ./.github/actions/authenticate-github-fetches
+
+    # Cargo's built-in git fetcher ignores `url.insteadOf`, so it would bypass
+    # the token rewrite above; fetching with the git CLI routes cargo's git
+    # dependencies through it. Set before the caching action below, which
+    # keys on the environment.
+    - name: Fetch cargo git dependencies with the git CLI
+      shell: bash
+      run: |
+        if [[ ! -v CARGO_NET_GIT_FETCH_WITH_CLI ]]; then
+          echo "CARGO_NET_GIT_FETCH_WITH_CLI=true" >> "$GITHUB_ENV"
+        fi
+
     - uses: actions-rust-lang/setup-rust-toolchain@v1
       with:
         rustflags: ${{ inputs.native-codegen == 'true' && '-Ctarget-cpu=native -Dwarnings' || '-Dwarnings' }}
diff --git a/.github/workflows/bench-main.yml b/.github/workflows/bench-main.yml
index cbedc5d1b..4312e8ec4 100644
--- a/.github/workflows/bench-main.yml
+++ b/.github/workflows/bench-main.yml
@@ -142,6 +142,8 @@ jobs:
           # - { env: FC, cache_pkg: formal_conjectures, mathlib: true }
     steps:
       - uses: actions/checkout@v7
+      # `lake build` below clones this package's Lake dependencies.
+      - uses: ./.github/actions/authenticate-github-fetches
       - uses: actions/cache/restore@v6
         with:
           path: ~/.local/bin
diff --git a/.github/workflows/bench-pr.yml b/.github/workflows/bench-pr.yml
index 8db31799e..bd5cb24b1 100644
--- a/.github/workflows/bench-pr.yml
+++ b/.github/workflows/bench-pr.yml
@@ -342,6 +342,8 @@ jobs:
           ref: ${{ needs.build.outputs.revision }}
           # The job runs PR code; never leave the token in .git.
           persist-credentials: false
+      # `lake build` below clones this package's Lake dependencies.
+      - uses: ./.github/actions/authenticate-github-fetches
       # Apply the allowlisted KEY=VALUE lines from the !benchmark comment,
       # so IX_COMPILE_* knobs reach the measured compile. Delivered via an
       # env var, not inline `${{ }}` — inlining would risk shell injection.
diff --git a/.github/workflows/nix.yml b/.github/workflows/nix.yml
index f8401b25c..51b38c363 100644
--- a/.github/workflows/nix.yml
+++ b/.github/workflows/nix.yml
@@ -27,6 +27,11 @@ jobs:
         with:
           name: argumentcomputer
           authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      # Covers `builtins.fetchGit` of the Lake manifest's `git+https`
+      # dependencies, which runs at flake eval time as this user;
+      # `install-nix-action`'s github_access_token covers only `github:`
+      # flake inputs, which fetch via the API.
+      - uses: ./.github/actions/authenticate-github-fetches
       # Ix CLI
       - run: nix build --print-build-logs --accept-flake-config
       - run: nix run .#ix -- --help
@@ -47,6 +52,7 @@ jobs:
         with:
           name: argumentcomputer
           authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      - uses: ./.github/actions/authenticate-github-fetches
       # Builds and runs tests using Lake as a Nix package
       - run: nix develop --accept-flake-config --command bash -c "lake build && lake test"
       # Realize the zkVM shells so they're verified and pushed to the cache,