diff --git a/.github/actions/assemble-test-project/default-smoke-test.js b/.github/actions/assemble-test-project/default-smoke-test.js
index 8255f9e..7493089 100644
--- a/.github/actions/assemble-test-project/default-smoke-test.js
+++ b/.github/actions/assemble-test-project/default-smoke-test.js
@@ -3,8 +3,63 @@
 // file the action drops next to this file at CI time, so this template is
 // a plain, runnable file — no placeholder substitution.
 
+const fs = require('fs')
+
 const { moduleName } = require('./harness-config.json')
 
+// `err.stack` omits `cause`, so print the chain.
+function errorChain(err) {
+  const lines = []
+  let e = err
+  for (let depth = 0; e && depth < 5; depth++) {
+    if (depth > 0) lines.push('caused by:')
+    for (const line of String(e.stack || e).split('\n')) lines.push(line)
+    e = e.cause
+  }
+  return lines
+}
+
+// require-addon raises "Cannot find addon" when every candidate fails to
+// *load* — not only when none exist. A prebuild that is present but can't
+// dlopen (built without linking libnode.so so napi_* never resolves, wrong
+// ABI, …) therefore reports exactly like a missing file, which is precisely
+// the failure this smoke test exists to catch. Its `cause` doesn't settle it
+// either: the resolver overwrites `cause` per candidate, so it ends up holding
+// the last candidate's error, normally a "Cannot find module" for a path that
+// never existed.
+//
+// The candidate list is on the error, so retry the ones actually on disk and
+// report why each failed. That separates "prebuild wasn't installed for this
+// target" from "prebuild is installed but broken".
+function addonDiagnostics(err) {
+  if (!err || err.code !== 'ADDON_NOT_FOUND') return []
+  if (!Array.isArray(err.candidates)) return []
+
+  const { fileURLToPath } = require('url')
+  const lines = []
+
+  for (const candidate of err.candidates) {
+    let file
+    try {
+      file = fileURLToPath(candidate)
+    } catch {
+      continue // non-file: candidate (e.g. `linked:`)
+    }
+    if (!fs.existsSync(file)) continue
+    try {
+      process.dlopen({ exports: {} }, file)
+      lines.push(file + ': loaded on retry (original failure was elsewhere)')
+    } catch (e) {
+      lines.push(file + ': ' + (e && e.message ? e.message : e))
+    }
+  }
+
+  if (lines.length === 0) {
+    return ['no candidate exists on disk — no prebuild was installed for this target']
+  }
+  return ['candidate(s) present but failed to load:'].concat(lines)
+}
+
 try {
   require(moduleName)
   console.log('TAP version 13')
@@ -17,8 +72,13 @@ try {
   console.log('not ok 1 - require(' + moduleName + ') threw')
   console.log('  ---')
   console.log('  message: ' + JSON.stringify(err && err.message))
+  const diagnostics = addonDiagnostics(err)
+  if (diagnostics.length > 0) {
+    console.log('  addon_diagnostics: |')
+    for (const line of diagnostics) console.log('    ' + line)
+  }
   console.log('  stack: |')
-  for (const line of String((err && err.stack) || err).split('\n')) {
+  for (const line of errorChain(err)) {
     console.log('    ' + line)
   }
   console.log('  ...')
diff --git a/.github/actions/assemble-test-project/module-tests-runner.js b/.github/actions/assemble-test-project/module-tests-runner.js
index 283fe2c..b12a190 100644
--- a/.github/actions/assemble-test-project/module-tests-runner.js
+++ b/.github/actions/assemble-test-project/module-tests-runner.js
@@ -84,8 +84,62 @@ async function main() {
   }
 }
 
+// `err.stack` omits `cause`, so print the chain.
+function formatError(err) {
+  const parts = []
+  let e = err
+  for (let depth = 0; e && depth < 5; depth++) {
+    parts.push((depth === 0 ? '' : 'caused by: ') + String(e.stack || e))
+    e = e.cause
+  }
+  return parts.join('\n')
+}
+
+// require-addon raises "Cannot find addon" when every candidate fails to
+// *load* — not only when none exist. A prebuild that is present but can't
+// dlopen (built without linking libnode.so so napi_* never resolves, wrong
+// ABI, …) therefore reports exactly like a missing file. Its `cause` doesn't
+// settle it either: the resolver overwrites `cause` per candidate, so it ends
+// up holding the last candidate's error, normally a "Cannot find module" for a
+// path that never existed.
+//
+// The candidate list is on the error, so retry the ones actually on disk and
+// report why each failed. That separates "prebuild wasn't installed for this
+// target" from "prebuild is installed but broken".
+function addonDiagnostics(err) {
+  if (!err || err.code !== 'ADDON_NOT_FOUND') return []
+  if (!Array.isArray(err.candidates)) return []
+
+  const { fileURLToPath } = require('url')
+  const lines = []
+
+  for (const candidate of err.candidates) {
+    let file
+    try {
+      file = fileURLToPath(candidate)
+    } catch {
+      continue // non-file: candidate (e.g. `linked:`)
+    }
+    if (!fs.existsSync(file)) continue
+    try {
+      process.dlopen({ exports: {} }, file)
+      lines.push('  ' + file + ': loaded on retry (original failure was elsewhere)')
+    } catch (e) {
+      lines.push('  ' + file + ': ' + (e && e.message ? e.message : e))
+    }
+  }
+
+  if (lines.length === 0) {
+    return [
+      'Addon diagnostics: no candidate exists on disk — no prebuild was installed for this target.'
+    ]
+  }
+  return ['Addon diagnostics: candidate(s) present but failed to load:'].concat(lines)
+}
+
 main().catch((err) => {
   console.error('Fatal error loading tests from ' + moduleName + ':')
-  console.error(err && err.stack ? err.stack : err)
+  console.error(formatError(err))
+  for (const line of addonDiagnostics(err)) console.error(line)
   process.exit(1)
 })
diff --git a/test-harness/android/run-test.sh b/test-harness/android/run-test.sh
index 27b4f4f..404189b 100755
--- a/test-harness/android/run-test.sh
+++ b/test-harness/android/run-test.sh
@@ -20,7 +20,20 @@ adb logcat -c
 # notices the pipe has closed when it next tries to write, and no further
 # lines are coming once the app has exited.) Started BEFORE the launch so
 # no early NODEJS-MOBILE output is missed.
-coproc LOGCAT { adb logcat -v raw -s NODEJS-MOBILE:V; }
+#
+# Tags beyond NODEJS-MOBILE (node's stdout/stderr, pumped by native-lib.cpp)
+# carry failures that are otherwise invisible here, which makes a broken run
+# look like silence:
+#   nodejs        node's own fatal-error reports, logged by nodejs-mobile's
+#                 libnode rather than through our stdout/stderr pipe — this is
+#                 where an uncaught exception's message actually lands.
+#   System.err    Java stack traces. TestActivity's asset copy catches and
+#                 prints per-file failures, so a file missing on device (and a
+#                 resulting "cannot find addon") is only explained here.
+#   AndroidRuntime  uncaught Java exceptions killing the app before node starts.
+# `-v raw` is kept so TAP output stays clean and the sentinel parsing below is
+# unchanged; extra tags interleave without a prefix.
+coproc LOGCAT { adb logcat -v raw -s NODEJS-MOBILE:V nodejs:V System.err:W AndroidRuntime:E; }
 
 # Launch the activity. The app pumps node's stdout/stderr to logcat tag
 # NODEJS-MOBILE and emits __NODE_EXIT__:<code> when done. Fire-and-forget,