diff -Nru ray-2.3.1/debian/changelog ray-2.3.1/debian/changelog
--- ray-2.3.1/debian/changelog	2024-12-02 16:37:51.000000000 +0100
+++ ray-2.3.1/debian/changelog	2026-10-13 11:19:07.000000000 +0200
@@ -1,3 +1,15 @@
+ray (2.3.1-10) UNRELEASED; urgency=medium
+
+  * d/rules: fix the FTBFS on architectures where the default MPI is
+    MPICH (armhf, i386).
+    - Query pkg-config for "mpi" instead of "mpi-c", which only OpenMPI
+      registers, so mpi.h was not found with MPICH.
+    - Link with -lmpi (the libmpi.so alternative both implementations
+      provide) instead of "pkg-config --libs", as mpich.pc leaks MPICH's
+      own build LDFLAGS.
+
+ -- Sebastien Bacher <seb128@ubuntu.com>  Tue, 13 Oct 2026 11:19:07 +0200
+
 ray (2.3.1-9) unstable; urgency=medium
 
   [ Helmut Grohne ]
diff -Nru ray-2.3.1/debian/rules ray-2.3.1/debian/rules
--- ray-2.3.1/debian/rules	2024-12-02 16:37:51.000000000 +0100
+++ ray-2.3.1/debian/rules	2026-10-13 11:19:07.000000000 +0200
@@ -7,8 +7,10 @@
 #export DH_VERBOSE=1
 
 export DEB_BUILD_MAINT_OPTIONS = hardening=+all
-export DEB_CXXFLAGS_MAINT_APPEND = $(shell $(PKG_CONFIG) --cflags mpi-c)
-export DEB_LDFLAGS_MAINT_APPEND = $(shell $(PKG_CONFIG) --libs mpi-c)
+export DEB_CXXFLAGS_MAINT_APPEND = $(shell $(PKG_CONFIG) --cflags mpi)
+# Link through the libmpi.so alternative rather than "pkg-config --libs":
+# MPICH's .pc leaks its own build LDFLAGS.
+export DEB_LDFLAGS_MAINT_APPEND = -lmpi
 
 %:
 	dh $@