diff --git a/ccp4/README.md b/ccp4/README.md
index a7ca21a..2a5027f 100644
--- a/ccp4/README.md
+++ b/ccp4/README.md
@@ -42,12 +42,30 @@ https://www.ccp4.ac.uk/download/ and update `files/config.yaml`, or drop a
 hand-downloaded tarball into the distfiles dir under the `name:` given in
 `files/config.yaml`.
 
+## Known failure modes
+
+- **"done" but broken install**: modbuild has no errexit in hooks — errors in
+  `pbuild::install` used to scroll by and the build still reported done. The
+  build script now `std::die`s on every critical step.
+- **Truncated download reused silently**: an interrupted `curl` leaves a
+  partial file in the distfiles dir, and prep reuses any file it finds there
+  (no integrity check without `shasums:`). Symptom:
+  `gzip: stdin: unexpected end of file`. Fix:
+  ```bash
+  rm ~/.cache/Pmodules/distfiles/ccp4-9.0-shelx-arp-x86_64.tar.gz
+  modbuild build --clean-install 9.0
+  # or verify by hand before building:
+  gzip -t ~/.cache/Pmodules/distfiles/ccp4-9.0-shelx-arp-x86_64.tar.gz
+  ```
+- The tarball unpacks to `ccp4-9` (not `ccp4-9.0` as the CCP4 install doc
+  implies); the build globs `ccp4-*/` instead of assuming the name.
+
 ## Verify
 
 ```bash
 module use MX unstable && module load ccp4/9.0
 which refmac5 ccp4i2
-echo $CCP4          # should point into /opt/psi/MX/ccp4/9.0/ccp4
+echo $CCP4          # should point into /opt/psi/MX/ccp4/9.0/ccp4 (-> ccp4-9)
 ```
 
 Bundled SHELX and ARP/wARP still need their own academic registrations.
diff --git a/ccp4/build b/ccp4/build
index f82090b..faaf73e 100755
--- a/ccp4/build
+++ b/ccp4/build
@@ -15,13 +15,25 @@ pbuild::compile() {
 
 pbuild::install() {
     cd "${PREFIX}"
-    tar -xzf "${SRC_DIR}/ccp4-${V}-shelx-arp-x86_64.tar.gz"
+    # modbuild has no errexit in hooks — without the die-guards a failed step
+    # still ends in a "done" build with a broken PREFIX (happened 2026-08-18
+    # with a truncated cached tarball).
+    tar -xzf "${SRC_DIR}/ccp4-${V}-shelx-arp-x86_64.tar.gz" || \
+        std::die 42 "ccp4: untar failed — cached tarball likely truncated; rm it from the distfiles dir and re-run (fetch a fresh sid link if the download fails)"
+    # the 9-series tarball unpacks to ccp4-9 (NOT ccp4-9.0 as the install doc
+    # implies) — glob instead of guessing the name
+    local -a top=( ccp4-*/ )
+    [[ -x "${top[0]}BINARY.setup" ]] || \
+        std::die 42 "ccp4: ${top[0]}BINARY.setup not found — vendor layout changed, inspect ${PREFIX}"
     # Pre-agree the CCP4 licence so BINARY.setup skips its interactive prompt
     # (downloading from ccp4.ac.uk already required accepting it). The printf
     # answers any prompt a newer BINARY.setup might still ask; printf instead
     # of `yes` because modbuild runs with pipefail and yes dies by SIGPIPE.
     touch "${PREFIX}/.agree2ccp4v6" "${HOME}/.agree2ccp4v6"
-    printf 'y\ny\ny\n' | "./ccp4-${V_MAJOR}.${V_MINOR}/BINARY.setup"
-    # version-independent symlink so the modulefile survives 9.0.xxx updates
-    ln -sfn "ccp4-${V_MAJOR}.${V_MINOR}" ccp4
+    printf 'y\ny\ny\n' | "./${top[0]}BINARY.setup" || \
+        std::die 42 "ccp4: BINARY.setup failed"
+    # version-independent symlink so the modulefile survives 9.x updates
+    ln -sfn "${top[0]%/}" ccp4
+    [[ -r ccp4/bin/ccp4.setup-sh ]] || \
+        std::die 42 "ccp4: ccp4/bin/ccp4.setup-sh missing after setup — the modulefile would be broken"
 }