diff --git a/nix/agent-modules/default.nix b/nix/agent-modules/default.nix
index 003e58d0..6f4f5705 100644
--- a/nix/agent-modules/default.nix
+++ b/nix/agent-modules/default.nix
@@ -40,7 +40,9 @@ let
   # store path over at container start as `HYPERHIVE_CLAUDE_CODE_PATH`, and
   # the host's system closure keeps it alive: nothing in this container
   # references it. With no pin, or a path that is not there, this container's
-  # own claude-code runs. `exec -a claude` keeps the bare name in argv[0],
+  # own claude-code runs; a pin that is set but gone (collected after a bump
+  # while this container still holds the old facts) says so on stderr, since
+  # the two builds can differ. `exec -a claude` keeps the bare name in argv[0],
   # which is how the harness finds its claude child (`find_claude_child`).
   ownClaude = import ../claude-code-no-alsa.nix lib pkgs.claude-code;
   claude = pkgs.writeShellScriptBin "claude" ''
@@ -48,6 +50,9 @@ let
     if [ -n "$pinned" ] && [ -x "$pinned/bin/claude" ]; then
       exec -a claude "$pinned/bin/claude" "$@"
     fi
+    if [ -n "$pinned" ]; then
+      echo "claude: $pinned/bin/claude, the hive's pinned claude-code, is missing; running this container's own" >&2
+    fi
     exec -a claude ${ownClaude}/bin/claude "$@"
   '';
 in
diff --git a/nix/host-modules/hive-c0re/options.nix b/nix/host-modules/hive-c0re/options.nix
index 7de352ed..17267da8 100644
--- a/nix/host-modules/hive-c0re/options.nix
+++ b/nix/host-modules/hive-c0re/options.nix
@@ -366,7 +366,8 @@
         that generation's path until its next start, and only the old
         generation roots it: collect old generations after restarting
         the agents, or an agent still running the old path falls back to
-        its own `claude-code` at its next `claude` spawn.
+        its own `claude-code` at its next `claude` spawn, saying so on
+        stderr.
 
         The swarm's `swarm-claude-login` and `swarm-review-classifier`
         containers, where this host runs them, follow the same pin and
diff --git a/nix/script-tests/agent-hive-facts.sh b/nix/script-tests/agent-hive-facts.sh
index 48768387..58f57ac7 100644
--- a/nix/script-tests/agent-hive-facts.sh
+++ b/nix/script-tests/agent-hive-facts.sh
@@ -112,18 +112,22 @@ chmod +x "$dir/own/bin/claude" "$dir/pinned/bin/claude"
 printf '%s' "$CLAUDE_WRAPPER" | sed -E "s|/nix/store/[^ ]+/bin/claude|$dir/own/bin/claude|" >"$dir/claude"
 case="claude setup"
 if ! grep -qF "$dir/own/bin/claude" "$dir/claude"; then fail "the rewrite did not reach the fallback claude"; fi
+# claude_check <description> <pin or ""> <which ran> <expected stderr>
 claude_check() {
   case=$1
   count=$((count + 1))
-  local got pin=()
+  local got said pin=()
   if [ -n "$2" ]; then pin=("HYPERHIVE_CLAUDE_CODE_PATH=$2"); fi
   # Under this shell: the shebang names the build's bash, not in this sandbox.
-  got=$(env -i "${pin[@]}" "$BASH" "$dir/claude" -p hi)
+  got=$(env -i "${pin[@]}" "$BASH" "$dir/claude" -p hi 2>"$dir/stderr")
+  said=$(cat "$dir/stderr")
   if [ "$got" != "$3 -p hi" ]; then fail "ran: $got, wanted: $3 -p hi"; fi
+  if [ "$said" != "$4" ]; then fail "said on stderr: '$said', wanted: '$4'"; fi
 }
-claude_check "the pinned claude when the hive hands one over" "$dir/pinned" pinned
-claude_check "the container's own claude with no pin" "" own
-claude_check "the container's own claude when the pinned path is gone" "$dir/gone" own
+claude_check "the pinned claude when the hive hands one over, silently" "$dir/pinned" pinned ""
+claude_check "the container's own claude with no pin, silently" "" own ""
+claude_check "the container's own claude when the pinned path is gone, saying so" "$dir/gone" own \
+  "claude: $dir/gone/bin/claude, the hive's pinned claude-code, is missing; running this container's own"
 
 echo "$count cases, $failures failed"
 [ "$failures" -eq 0 ]