diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 572a329..dc1c7cd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -66,6 +66,91 @@ jobs:
           debug_logging: "true"
           token: ${{ secrets.GITHUB_TOKEN }}
 
+  # Proves the sdist is complete and functional, since the sdist file list is an
+  # allowlist and an omission there is otherwise invisible. The test-count check
+  # is what makes this meaningful: without it, an sdist missing half its tests
+  # would still pass the tests it did ship.
+  sdist:
+    name: sdist (functional)
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          python-version: "3.13"
+
+      - name: Count the tests in the checkout
+        run: |
+          uv sync --all-extras
+          count=$(uv run pytest --collect-only -q | grep -oE '[0-9]+ tests? collected' | grep -oE '^[0-9]+')
+          echo "The checkout collects $count tests."
+          echo "checkout_tests=$count" >> "$GITHUB_ENV"
+
+      - name: Build and unpack the sdist
+        run: |
+          uv build --sdist --out-dir dist
+          mkdir -p "$RUNNER_TEMP/sdist"
+          tar -xzf dist/*.tar.gz -C "$RUNNER_TEMP/sdist" --strip-components=1
+
+      - name: Check that the sdist ships every test
+        working-directory: ${{ runner.temp }}/sdist
+        run: |
+          uv sync --all-extras
+          count=$(uv run pytest --collect-only -q | grep -oE '[0-9]+ tests? collected' | grep -oE '^[0-9]+')
+          echo "The sdist collects $count tests; the checkout collected $checkout_tests."
+          if [ "$count" -lt "$checkout_tests" ]; then
+            echo "::error::The sdist is missing tests. Add the missing paths to the include list in [tool.hatch.build.targets.sdist]."
+            exit 1
+          fi
+
+      - name: Run the tests from the sdist
+        working-directory: ${{ runner.temp }}/sdist
+        run: make test
+
+      - name: Verify typehints from the sdist
+        working-directory: ${{ runner.temp }}/sdist
+        run: make lint
+
+      # start-contract-test-service depends on install-contract-tests-deps, and
+      # the -bg target backgrounds the whole chain. Without a blocking install
+      # first, the dependency sync races the harness connecting to the service.
+      - name: Install the contract test dependencies from the sdist
+        working-directory: ${{ runner.temp }}/sdist
+        run: make install-contract-tests-deps
+
+      - name: Start the SSE contract test service from the sdist
+        working-directory: ${{ runner.temp }}/sdist
+        run: make start-contract-test-service-bg
+
+      - name: Run SSE contract tests against the sdist
+        uses: launchdarkly/gh-actions/actions/contract-tests@contract-tests-v1
+        with:
+          repo: sse-contract-tests
+          branch: main
+          version: v2
+          test_service_port: 8000
+          enable_persistence_tests: "false"
+          debug_logging: "true"
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Start the async SSE contract test service from the sdist
+        working-directory: ${{ runner.temp }}/sdist
+        run: make start-async-contract-test-service-bg
+
+      - name: Run async SSE contract tests against the sdist
+        uses: launchdarkly/gh-actions/actions/contract-tests@contract-tests-v1
+        with:
+          repo: sse-contract-tests
+          branch: main
+          version: v2
+          test_service_port: 8001
+          enable_persistence_tests: "false"
+          debug_logging: "true"
+          token: ${{ secrets.GITHUB_TOKEN }}
+
   windows:
     runs-on: windows-latest
 
diff --git a/pyproject.toml b/pyproject.toml
index 45db6eb..f9576b9 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -74,3 +74,20 @@ build-backend = "hatchling.build"
 [tool.hatch.build.targets.wheel]
 packages = ["ld_eventsource"]
 exclude = ["ld_eventsource/testing"]
+
+# An allowlist, so that a new repository-management file cannot leak into the
+# sdist by default. The CI sdist job proves the list is complete: it builds the
+# sdist, unpacks it, and fails if fewer tests collect there than in the
+# checkout. Hatchling force-includes pyproject.toml, README.md, LICENSE,
+# PKG-INFO and .gitignore whatever this says, so the sdist always builds.
+[tool.hatch.build.targets.sdist]
+include = [
+  "/ld_eventsource",
+  "/contract-tests",
+  "/docs",
+  "/Makefile",
+  "/setup.cfg",
+  "/CHANGELOG.md",
+  "/CONTRIBUTING.md",
+  "/SECURITY.md",
+]