diff --git a/.github/workflows/build-stream-unzip.yml b/.github/workflows/build-stream-unzip.yml
new file mode 100644
index 000000000..6a0261ca3
--- /dev/null
+++ b/.github/workflows/build-stream-unzip.yml
@@ -0,0 +1,172 @@
+# SPDX-FileCopyrightText: 2026 The RISE Project
+# SPDX-License-Identifier: MIT
+---
+# This workflow is based on: https://github.com/uktrade/stream-unzip/blob/v0.0.101/.github/workflows/build-source-package-and-wheels.yml
+name: Build stream-unzip wheels (riscv64)
+
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'stream-unzip version to build (git tag without leading v, e.g. 0.0.101)'
+        required: true
+        default: '0.0.101'
+  pull_request:
+    paths:
+      - '.github/workflows/build-stream-unzip.yml'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ inputs.version || '0.0.101' }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read  # to fetch code (actions/checkout)
+
+env:
+  # `inputs.version` is empty on pull_request events; default to 0.0.101 there.
+  STREAM_UNZIP_VERSION: ${{ inputs.version || '0.0.101' }}
+  MANYLINUX_RISCV64_IMAGE: quay.io/pypa/manylinux_2_39_riscv64
+
+jobs:
+  setup:
+    uses: $/.github/workflows/_setup.yml
+
+  python_sdist:
+    needs: [setup]
+    runs-on: ubuntu-latest
+    outputs:
+      sdist_artifact_name: ${{ steps.build_sdist.outputs.sdist_artifact_name }}
+      package_version: ${{ steps.build_sdist.outputs.package_version }}
+    steps:
+      - name: Checkout stream-unzip v${{ env.STREAM_UNZIP_VERSION }}
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
+        with:
+          repository: uktrade/stream-unzip
+          ref: v${{ env.STREAM_UNZIP_VERSION }}
+          persist-credentials: false
+
+      - name: Install Python
+        uses: astral-sh/setup-uv@20cfd1bf945f4377ade1205e4dbc17946fc9a30d  # v10.0.1
+        with:
+          python-version: '3.12'
+          activate-environment: true
+          enable-cache: false
+
+      - name: Update version placeholder and build sdist
+        id: build_sdist
+        run: |
+          set -euo pipefail
+          rm -rf dist
+
+          # Upstream's own release CI does this same sed on tag push (see
+          # build-source-package-and-wheels.yml); the checked-out pyproject.toml
+          # still carries the 0.0.0.dev0 placeholder.
+          sed -i "s/0\\.0\\.0\\.dev0/${STREAM_UNZIP_VERSION}/g" pyproject.toml
+
+          uv pip install 'maturin>=1.7.4,<2.0.0' build
+          python -m build --sdist --outdir dist
+
+          sdist_name="$(ls dist)"
+          {
+            echo "sdist_artifact_name=${sdist_name}"
+            echo "package_version=$(echo "${sdist_name}" | sed -En 's/stream_unzip-(.+)\.tar\.gz/\1/p')"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Upload sdist artifact
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1
+        with:
+          name: ${{ steps.build_sdist.outputs.sdist_artifact_name }}
+          path: dist/${{ steps.build_sdist.outputs.sdist_artifact_name }}
+          if-no-files-found: error
+
+  build_wheels:
+    needs: [setup, python_sdist]
+    name: Build stream-unzip ${{ inputs.version || '0.0.101' }} ${{ matrix.python }}-manylinux_riscv64
+    runs-on: ubuntu-24.04-riscv
+    timeout-minutes: 60
+    strategy:
+      fail-fast: false
+      matrix:
+        # stream-unzip's _zipcrypto extension is a PyO3 module built per-interpreter
+        # (not abi3 - upstream ships cp3x-cp3x wheels), so every Python version needs
+        # its own build. Repo-standard riscv64 target set. Upstream doesn't build a
+        # 3.14t wheel yet, so skip it for now.
+        python: ["cp312", "cp313", "cp314"]
+
+    steps:
+      - name: Fetch sdist artifact
+        id: fetch_sdist
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c  # v8.0.1
+        with:
+          name: ${{ needs.python_sdist.outputs.sdist_artifact_name }}
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@20cfd1bf945f4377ade1205e4dbc17946fc9a30d  # v10.0.1
+        with:
+          python-version: '3.12'
+          activate-environment: true
+          enable-cache: false
+
+      - name: Build and test wheel
+        env:
+          CIBW_ARCHS: riscv64
+          # Only manylinux: rustup.rs ships no riscv64 musl host toolchain, so
+          # musllinux can't build (same reason build-tiktoken.yml/build-fastuuid.yml
+          # skip it).
+          CIBW_BUILD: ${{ matrix.python }}-manylinux_riscv64
+          CIBW_MANYLINUX_RISCV64_IMAGE: ${{ env.MANYLINUX_RISCV64_IMAGE }}
+          # stream-unzip ships no [tool.cibuildwheel], so the Rust toolchain its
+          # maturin backend needs is installed in-container here, and Cython (a
+          # transitive build dependency of its stream-inflate runtime dependency,
+          # which has no riscv64 wheel yet) is resolved from our own registry.
+          CIBW_BEFORE_ALL_LINUX: >-
+            curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+          CIBW_ENVIRONMENT_LINUX: >-
+            PATH="$PATH:$HOME/.cargo/bin"
+            PIP_EXTRA_INDEX_URL=https://pypi.riseproject.dev/simple/
+          # Test each wheel the way upstream does - run its test suite (via pytest,
+          # which can also collect a plain unittest.TestCase, so upstream's suite
+          # runs unmodified). trio is an upstream ci-extra dev dependency used by
+          # one async test; pycryptodome and stream-inflate are pulled in
+          # automatically as the wheel's own runtime dependencies (pycryptodome
+          # already ships a riscv64 wheel on our registry; stream-inflate doesn't
+          # yet, so pip builds it from its public sdist using the Cython wheel
+          # above). Tests and fixtures live at the sdist root, not inside the
+          # importable package, so reference them via {package} (=
+          # /project/stream-unzip), NOT {project} - see gotcha 5.
+          #
+          # test_bad_deflate_data is deselected: it corrupts one byte of a raw
+          # DEFLATE stream and asserts zlib.decompressobj raises (mapped to
+          # DeflateError). manylinux_2_39_riscv64 has no dedicated zlib build step
+          # (unlike e.g. openssl/sqlite3) and links CPython's zlib module against
+          # the image's system zlib-ng-compat instead, which decodes this
+          # particular corrupted stream without raising - the corruption then only
+          # surfaces once the decompressed bytes fail the trailing CRC-32 check, as
+          # CRC32IntegrityError. Same underlying image behavior as gotchas 272/279.
+          CIBW_TEST_REQUIRES: pytest trio
+          CIBW_TEST_COMMAND: >-
+            cd {package} &&
+            python -m pytest -v test.py --deselect test.py::TestStreamUnzip::test_bad_deflate_data
+        run: |
+          set -euo pipefail
+          mkdir stream-unzip
+          tar zxf "${{ steps.fetch_sdist.outputs.download-path }}/${{ needs.python_sdist.outputs.sdist_artifact_name }}" \
+            --strip-components=1 -C stream-unzip
+          uv pip install --upgrade cibuildwheel
+          python -m cibuildwheel --output-dir wheelhouse ./stream-unzip
+
+      - uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1
+        with:
+          name: stream-unzip-${{ env.STREAM_UNZIP_VERSION }}-${{ matrix.python }}-manylinux_riscv64
+          path: ./wheelhouse/*.whl
+          if-no-files-found: error
+
+  publish:
+    name: Publish stream-unzip ${{ inputs.version || '0.0.101' }}
+    needs: [setup, build_wheels]
+    permissions:
+      contents: write
+      pull-requests: write
+    uses: $/.github/workflows/_publish-wheel.yml
+    with:
+      artifact-pattern: stream-unzip-${{ inputs.version || '0.0.101' }}-*-manylinux_riscv64