diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4f01c0f..c3a75d4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -120,18 +120,64 @@ jobs:
 
     - name: Test ${{ matrix.component }} basic functionality
       run: |
-        case "${{ matrix.component }}" in
-          "attestation-agent")
-            timeout 10s ./build/${{ matrix.component }}/bin/${{ matrix.component }} || true
-            ;;
-          "rust-echo-service"|"cpp-echo-service")
-            timeout 5s ./build/${{ matrix.component }}/bin/${{ matrix.component }} || true
-            ;;
-          *)
-            ./build/${{ matrix.component }}/bin/${{ matrix.component }} --help || true
-            ;;
+        set -euo pipefail
+
+        # Disable core dumps. A crashing binary that is slow to dump core can
+        # still be dying when `timeout` fires, which would be reported as 124
+        # (the healthy code) instead of 128+signal. With core dumps off, a
+        # crash is reported immediately and honestly.
+        ulimit -c 0
+
+        component="${{ matrix.component }}"
+        bin="./build/$component/bin/$component"
+
+        test -x "$bin"
+
+        case "$component" in
+          "attestation-agent")            kind=service; limit=10 ;;
+          "rust-echo-service"|"cpp-echo-service") kind=service; limit=5 ;;
+          *)                              kind=cli ;;
         esac
 
+        if [ "$kind" = "service" ]; then
+          # These components are long-running servers. The healthy outcome is
+          # that the process is STILL ALIVE when `timeout` kills it, which GNU
+          # timeout reports as exit code 124. Any other status means the
+          # process exited on its own before the deadline -- it crashed, or it
+          # failed to start (bad port, missing library, panic) -- and that is
+          # exactly the breakage this step is supposed to catch.
+          echo "=== $component: expecting it to stay alive for ${limit}s ==="
+          if timeout "${limit}s" "$bin"; then
+            status=0
+          else
+            status=$?
+          fi
+
+          if [ "$status" -eq 124 ]; then
+            echo "OK: $component was still running after ${limit}s and was killed by timeout"
+          else
+            echo "FAIL: $component exited on its own with status $status before the ${limit}s deadline."
+            echo "      A healthy service would still have been running. Status 124 is the pass condition;"
+            echo "      128+N indicates death by signal N (139 = SIGSEGV)."
+            exit 1
+          fi
+        else
+          # These components are one-shot CLIs: --help must succeed.
+          echo "=== $component: expecting '--help' to exit 0 ==="
+          if "$bin" --help; then
+            status=0
+          else
+            status=$?
+          fi
+
+          if [ "$status" -eq 0 ]; then
+            echo "OK: $component --help exited 0"
+          else
+            echo "FAIL: $component --help exited with status $status"
+            exit 1
+          fi
+        fi
+
     - name: Upload build artifacts
       uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
       with:
@@ -173,10 +219,27 @@ jobs:
 
     - name: Generate test coverage
       run: |
+        set -euo pipefail
         cd ${{ matrix.project }}
-        cargo install cargo-tarpaulin || true
-        cargo tarpaulin --out Xml || true
 
+        # cargo-tarpaulin comes from the dev shell (flake.nix). It used to be
+        # `cargo install`ed here at runtime, which (a) fetched from crates.io
+        # in the middle of a Nix-pinned build and (b) has been failing outright
+        # -- `cargo-platform` now requires rustc 1.91 and the pinned toolchain
+        # is 1.86 -- leaving `cargo tarpaulin` as "no such command". Both
+        # failures were swallowed, so no coverage was ever produced and the
+        # uploaded number was fictional.
+        cargo tarpaulin --out Xml
+
+        # Producing the report is the point of this step; if the file is not
+        # there, the step has not done its job.
+        test -f cobertura.xml
+        echo "OK: coverage report generated for ${{ matrix.project }}"
+
+    # Deliberately advisory: this uploads to a third-party service, so an
+    # outage or a rate limit on codecov.io should not fail the build. The
+    # assertion that matters -- that coverage was actually measured and the
+    # report exists -- is made in the step above, where it belongs.
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@b9fd7d16f6d7d1b5d2bec1a2887e65ceed900238 # v4
       with:
@@ -212,8 +275,40 @@ jobs:
 
     - name: Make binaries executable
       run: |
-        chmod +x build/bin/* || true
-        ls -la build/bin/* || true
+        set -euo pipefail
+
+        # Every component built by the build-matrix job must have arrived via
+        # the artifact download. A missing binary used to be silent here and
+        # then surfaced later as a confusing integration-test failure.
+        expected=(
+          attestation-agent
+          cpp-echo-service
+          derivation-hasher
+          rust-client
+          rust-echo-service
+        )
+
+        missing=()
+        for name in "${expected[@]}"; do
+          if [ ! -f "build/bin/$name" ]; then
+            missing+=("build/bin/$name")
+          fi
+        done
+
+        if [ "${#missing[@]}" -ne 0 ]; then
+          echo "FAIL: expected binaries are missing from the downloaded artifacts:"
+          printf '  %s\n' "${missing[@]}"
+          echo "What was actually downloaded:"
+          if [ -d build ]; then
+            find build -maxdepth 3 -mindepth 1 | sort
+          else
+            echo "  (no 'build' directory at all)"
+          fi
+          exit 1
+        fi
+
+        chmod +x build/bin/*
+        ls -la build/bin/
 
     - name: Run integration tests
       run: just test-integration
@@ -285,13 +380,9 @@ jobs:
       run: just generate-docs
 
     - name: Test reproducible builds
-      run: |
-        nix build .#rust-echo-service -o result-1
-        nix build .#attestation-agent -o result-2
-        nix build .#rust-echo-service -o result-1-rebuild
-        nix build .#attestation-agent -o result-2-rebuild
-        diff -r result-1 result-1-rebuild || echo "Build not reproducible for rust-echo-service"
-        diff -r result-2 result-2-rebuild || echo "Build not reproducible for attestation-agent"
+      # Implementation lives in scripts/check-reproducibility.sh so that this
+      # gate and the `just ci-docs-reproducibility` recipe run the same code.
+      run: ./scripts/check-reproducibility.sh
 
     - name: Upload documentation
       uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
diff --git a/Justfile b/Justfile
index fa73ec8..bbc6a06 100644
--- a/Justfile
+++ b/Justfile
@@ -209,7 +209,11 @@ ci-performance-tests: test-performance
 ci-attestation-tests: test-attestation-full
 
 # Run documentation and reproducibility tests for CI
+# Runs the same script as the `documentation-reproducibility` CI job, so a
+# reproducibility regression is catchable locally via `just ci-main`.
 ci-docs-reproducibility: check generate-docs
+    @echo "=== Running Reproducibility Check ==="
+    ./scripts/check-reproducibility.sh
 
 # Run vulnerability scanning (like CI does)
 ci-vulnerability-scan:
diff --git a/flake.nix b/flake.nix
index 78e50d2..55ac94d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -62,6 +62,7 @@
             cargo-audit
             cargo-deny
             cargo-outdated
+            cargo-tarpaulin # Coverage; pinned here instead of `cargo install` at CI time
             # Additional security tools for comprehensive auditing
             semgrep # Static analysis security scanner
             bandit # Python security linter (in case we add Python scripts)
diff --git a/scripts/check-reproducibility.sh b/scripts/check-reproducibility.sh
new file mode 100755
index 0000000..fec2bf7
--- /dev/null
+++ b/scripts/check-reproducibility.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+#
+# Reproducibility gate for BlocksenseOS.
+#
+# This is the single implementation, used by both:
+#   - .github/workflows/ci.yml, job `documentation-reproducibility`
+#   - the `ci-docs-reproducibility` Justfile recipe (so `just ci-main` runs it)
+# so that the CI gate and the local command cannot drift apart.
+#
+# For each component we build it, then re-execute the derivation with
+# `nix build --rebuild`. The `--rebuild` flag is what makes this a real check:
+# a plain second `nix build` is a no-op that resolves to the store path that is
+# already there, so the two out-links would be the same path and the comparison
+# would succeed no matter what. With `--rebuild`, Nix runs the build again and
+# compares the fresh output against the store.
+#
+# Any difference prints the full diff and exits non-zero. This gate must never
+# report success on a real difference.
+
+set -euo pipefail
+
+components=(
+  "rust-echo-service|result-1|result-1-rebuild"
+  "attestation-agent|result-2|result-2-rebuild"
+)
+
+for entry in "${components[@]}"; do
+  IFS='|' read -r component first second <<< "$entry"
+  nix build ".#$component" -o "$first"
+  nix build ".#$component" --rebuild -o "$second"
+done
+
+rc=0
+for entry in "${components[@]}"; do
+  IFS='|' read -r component first second <<< "$entry"
+  echo "=== Reproducibility check: $component ($first vs $second) ==="
+  if diff -r "$first" "$second"; then
+    echo "OK: $component is reproducible"
+  else
+    echo "FAIL: build not reproducible for $component (diff shown above)"
+    rc=1
+  fi
+done
+
+if [ "$rc" -ne 0 ]; then
+  echo "Reproducibility gate failed: at least one component produced differing outputs."
+fi
+exit "$rc"