diff --git a/.github/workflows/netsukefile-test.yml b/.github/workflows/netsukefile-test.yml
index 761f1b6b..f6a945e3 100644
--- a/.github/workflows/netsukefile-test.yml
+++ b/.github/workflows/netsukefile-test.yml
@@ -51,18 +51,28 @@ jobs:
               command: "touch unused.txt"
           MANIFEST
       - name: Build dependent and inline targets
-        run: ./target/debug/netsuke --verbose build dependent.txt inline-command.txt inline-script.txt
+        run: ./target/debug/netsuke --verbose build --emit build.ninja dependent.txt inline-command.txt inline-script.txt
       - name: Assert dependent artefacts exist
+        env:
+          NINJA_MANIFEST: build.ninja
         run: |
           scripts/assert-file-exists.sh base.txt
           scripts/assert-file-exists.sh dependent.txt
       - name: Assert inline command artefact exists
+        env:
+          NINJA_MANIFEST: build.ninja
         run: scripts/assert-file-exists.sh inline-command.txt
       - name: Assert inline script artefact exists
+        env:
+          NINJA_MANIFEST: build.ninja
         run: scripts/assert-file-exists.sh inline-script.txt
       - name: Run action target
-        run: ./target/debug/netsuke --verbose build say-hello
+        run: ./target/debug/netsuke --verbose build --emit action.ninja say-hello
       - name: Assert action artefact exists
+        env:
+          NINJA_MANIFEST: action.ninja
         run: scripts/assert-file-exists.sh action-called.txt
       - name: Assert unused action artefact absent
-        run: test ! -f unused.txt
+        env:
+          NINJA_MANIFEST: action.ninja
+        run: scripts/assert-file-absent.sh unused.txt
diff --git a/scripts/assert-file-absent.sh b/scripts/assert-file-absent.sh
new file mode 100755
index 00000000..eb3cb6c3
--- /dev/null
+++ b/scripts/assert-file-absent.sh
@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Ensures the Netsuke build did not produce an unexpected artefact.
+# If the artefact is present and `NINJA_MANIFEST` is set, the referenced
+# Ninja manifest is dumped to stderr for debugging.
+set -euo pipefail
+
+if [[ $# -ne 1 ]]; then
+  echo "Usage: $(basename "$0") <file>" >&2
+  exit 2   # usage error
+fi
+
+file="$1"
+
+if [[ -f "$file" ]]; then
+  echo "Unexpected build artefact '$file' present." >&2
+  if [[ -n "${NINJA_MANIFEST:-}" && -f "$NINJA_MANIFEST" ]]; then
+    echo "Ninja manifest '$NINJA_MANIFEST' for debugging:" >&2
+    echo "-----BEGIN NINJA MANIFEST-----" >&2
+    cat "$NINJA_MANIFEST" >&2
+    echo "-----END NINJA MANIFEST-----" >&2
+  fi
+  exit 1
+fi
diff --git a/scripts/assert-file-exists.sh b/scripts/assert-file-exists.sh
index 82c9d2ed..7c7f0c76 100755
--- a/scripts/assert-file-exists.sh
+++ b/scripts/assert-file-exists.sh
@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 # Ensures the Netsuke build produced the expected artefact.
-# Fails fast if the given file is missing.
+# If the artefact is absent and `NINJA_MANIFEST` is set, the referenced
+# Ninja manifest is dumped to stderr for debugging.
 set -euo pipefail
 
 if [[ $# -ne 1 ]]; then
@@ -12,5 +13,11 @@ file="$1"
 
 if [[ ! -f "$file" ]]; then
   echo "Expected build artefact '$file' to exist." >&2
+  if [[ -n "${NINJA_MANIFEST:-}" && -f "$NINJA_MANIFEST" ]]; then
+    echo "Ninja manifest '$NINJA_MANIFEST' for debugging:" >&2
+    echo "-----BEGIN NINJA MANIFEST-----" >&2
+    cat "$NINJA_MANIFEST" >&2
+    echo "-----END NINJA MANIFEST-----" >&2
+  fi
   exit 1
 fi