diff --git a/Makefile b/Makefile
index 83aa811..c2c498e 100644
--- a/Makefile
+++ b/Makefile
@@ -18,6 +18,24 @@ IOC_QT_PANEL := qt/$(IOC_NAME)_cpp_logic.ui
 IOC_QTGEN := $(CPP_LOGIC_ROOT)/utils/cpp_logic_ioc_qtgen.py
 IOC_SUBS_FILE := cfg/libmain.so_cpp_logic.subs
 
+# Python's ctypes library is built locally, independently of the IOC module.
+CXX ?= c++
+VIEWER_CXXFLAGS ?= -O2 -std=c++17 -Wall -Wextra -pedantic
+ifeq ($(shell uname -s),Darwin)
+VIEWER_LIBRARY := build/libhexapod.dylib
+VIEWER_SHARED := -dynamiclib
+else
+VIEWER_LIBRARY := build/libhexapod.so
+VIEWER_SHARED := -shared
+endif
+
+.PHONY: viewer-lib
+viewer-lib: $(VIEWER_LIBRARY)
+
+$(VIEWER_LIBRARY): src/c_api.cpp src/kinematics.hpp
+	mkdir -p $(@D)
+	$(CXX) $(VIEWER_CXXFLAGS) -fPIC $(VIEWER_SHARED) -o $@ src/c_api.cpp
+
 .PHONY: pvs ui
 
 pvs:
diff --git a/README.md b/README.md
index 0970c0d..406a79e 100644
--- a/README.md
+++ b/README.md
@@ -357,6 +357,15 @@ The quintic profile in the offline viewer is not an IOC trajectory generator.
 
 ## EPICS viewer
 
+Build the viewer's local C API library separately from the IOC's `libmain.so`:
+
+```sh
+make viewer-lib
+```
+
+This creates `build/libhexapod.so` (`.dylib` on macOS) from `src/c_api.cpp`
+without EPICS dependencies. Rebuild it after changing the kinematics code.
+
 For a running IOC, install the IOC viewer requirements and use the IOC record
 prefix, including the trailing colon:
 
diff --git a/python/viewer.py b/python/viewer.py
index 1ac34db..e68cee7 100644
--- a/python/viewer.py
+++ b/python/viewer.py
@@ -18,7 +18,7 @@ class Model:
         candidates = [ROOT / 'build' / ('libhexapod.' + ext) for ext in ('so', 'dylib')]
         path = next((p for p in candidates if p.exists()), None)
         if path is None:
-            raise RuntimeError('Build the C++ library first: make')
+            raise RuntimeError('Build the viewer C++ library first: make viewer-lib')
         self.lib = ct.CDLL(str(path))
         pointer = ct.POINTER(ct.c_double)
         self.lib.hex_inverse.argtypes = [pointer] * 4