diff --git a/pkgs/development/libraries/capnproto/default.nix b/pkgs/development/libraries/capnproto/default.nix
index cda5f005f9a2..75b5f14d67ce 100644
--- a/pkgs/development/libraries/capnproto/default.nix
+++ b/pkgs/development/libraries/capnproto/default.nix
@@ -21,6 +21,9 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = lib.optional (!(stdenv.hostPlatform.isCompatible stdenv.buildPlatform)) "-DEXTERNAL_CAPNP";
 
+  # Upstream 77ac9154440bcc216fda1092fd5bb51da62ae09c, modified to apply to v0.9.1.  Drop on update.
+  patches = lib.optional stdenv.hostPlatform.isMusl ./musl-no-fibers.patch;
+
   meta = with lib; {
     homepage    = "https://capnproto.org/";
     description = "Cap'n Proto cerealization protocol";
diff --git a/pkgs/development/libraries/capnproto/musl-no-fibers.patch b/pkgs/development/libraries/capnproto/musl-no-fibers.patch
new file mode 100644
index 000000000000..bb7804fb0abb
--- /dev/null
+++ b/pkgs/development/libraries/capnproto/musl-no-fibers.patch
@@ -0,0 +1,244 @@
+From 3d983eff304c28574c330a52d70a60145c9e157e Mon Sep 17 00:00:00 2001
+From: Jonas Vautherin <jonas.vautherin@gmail.com>
+Date: Fri, 14 Jan 2022 00:14:26 +0100
+Subject: [PATCH] Add support for musl
+
+---
+Based on upstream 77ac9154440bcc216fda1092fd5bb51da62ae09c,
+modified slightly by dtzWill to apply to v0.9.1.
+
+(drop whitespace change to a cmake "if (WITH_OPENSSL)",
+leave the other instance of that change since it applies)
+---
+
+Co-authored-by: Guillaume Papin <guillaume.papin@epitech.eu>
+(cherry picked from commit 77ac9154440bcc216fda1092fd5bb51da62ae09c)
+---
+ .github/workflows/quick-test.yml   |  9 ++++++
+ c++/CMakeLists.txt                 | 46 ++++++++++++++++++++++++++++-
+ c++/cmake/CapnProtoConfig.cmake.in | 32 ++++++++++++++++++++
+ c++/configure.ac                   | 47 ++++++++++++++++++++++++++++--
+ c++/src/kj/CMakeLists.txt          | 11 ++++++-
+ 5 files changed, 141 insertions(+), 4 deletions(-)
+
+diff --git a/.github/workflows/quick-test.yml b/.github/workflows/quick-test.yml
+index c18ef6a6..773ff043 100644
+--- a/.github/workflows/quick-test.yml
++++ b/.github/workflows/quick-test.yml
+@@ -10,6 +10,15 @@ on:
+     - 'release-*'
+ 
+ jobs:
++  Linux-musl:
++    runs-on: ubuntu-latest
++    container: alpine:latest
++    steps:
++      - uses: actions/checkout@v2
++      - name: install dependencies
++        run: apk add autoconf automake build-base cmake libtool libucontext-dev linux-headers openssl-dev
++      - name: super-test
++        run: ./super-test.sh quick
+   Linux:
+     runs-on: ubuntu-latest
+     strategy:
+diff --git a/c++/CMakeLists.txt b/c++/CMakeLists.txt
+index 548dfd1f..5de7ab26 100644
+--- a/c++/CMakeLists.txt
++++ b/c++/CMakeLists.txt
+@@ -1,4 +1,4 @@
+-cmake_minimum_required(VERSION 3.4)
++cmake_minimum_required(VERSION 3.6)
+ project("Cap'n Proto" CXX)
+ set(VERSION 0.9.1)
+ 
+@@ -64,6 +64,50 @@ elseif (WITH_OPENSSL)
+   find_package(OpenSSL REQUIRED COMPONENTS Crypto SSL)
+ endif()
+ 
++set(WITH_FIBERS "AUTO" CACHE STRING
++  "Whether or not to build libkj-async with fibers")
++# define list of values GUI will offer for the variable
++set_property(CACHE WITH_FIBERS PROPERTY STRINGS AUTO ON OFF)
++
++# CapnProtoConfig.cmake.in needs this variable.
++set(_WITH_LIBUCONTEXT OFF)
++
++if (WITH_FIBERS OR WITH_FIBERS STREQUAL "AUTO")
++  set(_capnp_fibers_found OFF)
++  if (WIN32 OR CYGWIN)
++    set(_capnp_fibers_found ON)
++  else()
++    # Fibers need makecontext, setcontext, getcontext, swapcontext that may be in libc,
++    # or in libucontext (e.g. for musl).
++    # We assume that makecontext implies that the others are present.
++    include(CheckLibraryExists)
++    check_library_exists(c makecontext "" HAVE_UCONTEXT_LIBC)
++    if (HAVE_UCONTEXT_LIBC)
++      set(_capnp_fibers_found ON)
++    else()
++      # Try with libucontext
++      find_package(PkgConfig)
++      if (PKG_CONFIG_FOUND)
++        pkg_check_modules(libucontext IMPORTED_TARGET libucontext)
++        if (libucontext_FOUND)
++          set(_WITH_LIBUCONTEXT ON)
++          set(_capnp_fibers_found ON)
++        endif()
++      else()
++        set(_capnp_fibers_found OFF)
++      endif()
++    endif()
++  endif()
++
++  if (_capnp_fibers_found)
++    set(WITH_FIBERS ON)
++  elseif(WITH_FIBERS STREQUAL "AUTO")
++    set(WITH_FIBERS OFF)
++  else()
++    message(FATAL_ERROR "Missing 'makecontext', 'getcontext', 'setcontext' or 'swapcontext' symbol in libc and no libucontext found: KJ won't be able to build with fibers. Disable fibers (-DWITH_FIBERS=OFF).")
++  endif()
++endif()
++
+ if(MSVC)
+   # TODO(cleanup): Enable higher warning level in MSVC, but make sure to test
+   #   build with that warning level and clean out false positives.
+diff --git a/c++/cmake/CapnProtoConfig.cmake.in b/c++/cmake/CapnProtoConfig.cmake.in
+index 667f502f..0580b11a 100644
+--- a/c++/cmake/CapnProtoConfig.cmake.in
++++ b/c++/cmake/CapnProtoConfig.cmake.in
+@@ -62,6 +62,38 @@ if (@WITH_OPENSSL@)  # WITH_OPENSSL
+   endif()
+ endif()
+ 
++if (@_WITH_LIBUCONTEXT@) # _WITH_LIBUCONTEXT
++  set(forwarded_config_flags)
++  if(CapnProto_FIND_QUIETLY)
++    list(APPEND forwarded_config_flags QUIET)
++  endif()
++  if(CapnProto_FIND_REQUIRED)
++    list(APPEND forwarded_config_flags REQUIRED)
++  endif()
++  # If the consuming project called find_package(CapnProto) with the QUIET or REQUIRED flags, forward
++  # them to calls to find_package(PkgConfig) and pkg_check_modules(). Note that find_dependency()
++  # would do this for us in the former case, but there is no such forwarding wrapper for
++  # pkg_check_modules().
++
++  find_package(PkgConfig ${forwarded_config_flags})
++  if(NOT ${PkgConfig_FOUND})
++    # If we're here, the REQUIRED flag must not have been passed, else we would have had a fatal
++    # error. Nevertheless, a diagnostic for this case is probably nice.
++    if(NOT CapnProto_FIND_QUIETLY)
++      message(WARNING "pkg-config cannot be found")
++    endif()
++    set(CapnProto_FOUND OFF)
++    return()
++  endif()
++
++  if (CMAKE_VERSION VERSION_LESS 3.6)
++    # CMake >= 3.6 required due to the use of IMPORTED_TARGET
++    message(SEND_ERROR "libucontext support requires CMake >= 3.6.")
++  endif()
++
++  pkg_check_modules(libucontext IMPORTED_TARGET ${forwarded_config_flags} libucontext)
++endif()
++
+ include("${CMAKE_CURRENT_LIST_DIR}/CapnProtoTargets.cmake")
+ include("${CMAKE_CURRENT_LIST_DIR}/CapnProtoMacros.cmake")
+ 
+diff --git a/c++/configure.ac b/c++/configure.ac
+index 72fe8456..b627bec8 100644
+--- a/c++/configure.ac
++++ b/c++/configure.ac
+@@ -32,6 +32,11 @@ AC_ARG_WITH([openssl],
+     [build libkj-tls by linking against openssl @<:@default=check@:>@])],
+   [],[with_openssl=check])
+ 
++AC_ARG_WITH([fibers],
++  [AS_HELP_STRING([--with-fibers],
++    [build libkj-async with fibers @<:@default=check@:>@])],
++  [],[with_fibers=check])
++
+ AC_ARG_ENABLE([reflection], [
+   AS_HELP_STRING([--disable-reflection], [
+     compile Cap'n Proto in "lite mode", in which all reflection APIs (schema.h, dynamic.h, etc.)
+@@ -195,8 +200,46 @@ AS_IF([test "$with_openssl" != no], [
+ ])
+ AM_CONDITIONAL([BUILD_KJ_TLS], [test "$with_openssl" != no])
+ 
+-# CapnProtoConfig.cmake.in needs this variable.
+-AC_SUBST(WITH_OPENSSL, $with_openssl)
++# Fibers need the symbols getcontext, setcontext, swapcontext and makecontext.
++# We assume that makecontext implies the rest.
++AS_IF([test "$with_fibers" != no], [
++  libc_supports_fibers=yes
++  AC_SEARCH_LIBS([makecontext], [], [], [
++    libc_supports_fibers=no
++  ])
++
++  AS_IF([test "$libc_supports_fibers" = yes], [
++      with_fibers=yes
++    ], [
++    # If getcontext does not exist in libc, try with libucontext
++    ucontext_supports_fibers=yes
++    AC_CHECK_LIB(ucontext, [makecontext], [], [
++      ucontext_supports_fibers=no
++    ])
++    AS_IF([test "$ucontext_supports_fibers" = yes], [
++      ASYNC_LIBS="$ASYNC_LIBS -lucontext"
++      with_fibers=yes
++    ], [
++      AS_IF([test "$with_fibers" = yes], [
++        AC_MSG_ERROR([Missing symbols required for fibers (makecontext, setcontext, ...). Disable fibers (--without-fibers) or install libucontext])
++      ], [
++        AC_MSG_WARN([could not find required symbols (makecontext, setcontext, ...) -- won't build with fibers])
++        with_fibers=no
++      ])
++    ])
++  ])
++])
++AS_IF([test "$with_fibers" = yes], [
++  CXXFLAGS="$CXXFLAGS -DKJ_USE_FIBERS"
++], [
++  CXXFLAGS="$CXXFLAGS -DKJ_USE_FIBERS=0"
++])
++
++# CapnProtoConfig.cmake.in needs these variables,
++# we force them to NO because we don't need the CMake dependency for them,
++# the dependencies are provided by the .pc files.
++AC_SUBST(WITH_OPENSSL, NO)
++AC_SUBST(_WITH_LIBUCONTEXT, NO)
+ 
+ AM_CONDITIONAL([HAS_FUZZING_ENGINE], [test "x$LIB_FUZZING_ENGINE" != "x"])
+ 
+diff --git a/c++/src/kj/CMakeLists.txt b/c++/src/kj/CMakeLists.txt
+index 813fac4d..f7b4dddf 100644
+--- a/c++/src/kj/CMakeLists.txt
++++ b/c++/src/kj/CMakeLists.txt
+@@ -136,6 +136,15 @@ if(NOT CAPNP_LITE)
+   add_library(kj-async ${kj-async_sources})
+   add_library(CapnProto::kj-async ALIAS kj-async)
+   target_link_libraries(kj-async PUBLIC kj)
++  if(WITH_FIBERS)
++    target_compile_definitions(kj-async PUBLIC KJ_USE_FIBERS)
++    if(_WITH_LIBUCONTEXT)
++      target_link_libraries(kj-async PUBLIC PkgConfig::libucontext)
++    endif()
++  else()
++    target_compile_definitions(kj-async PUBLIC KJ_USE_FIBERS=0)
++  endif()
++
+   if(UNIX)
+     # external clients of this library need to link to pthreads
+     target_compile_options(kj-async INTERFACE "-pthread")
+@@ -181,7 +190,7 @@ if(NOT CAPNP_LITE)
+   add_library(kj-tls ${kj-tls_sources})
+   add_library(CapnProto::kj-tls ALIAS kj-tls)
+   target_link_libraries(kj-tls PUBLIC kj-async)
+-  if (WITH_OPENSSL)
++  if(WITH_OPENSSL)
+     target_compile_definitions(kj-tls PRIVATE KJ_HAS_OPENSSL)
+     target_link_libraries(kj-tls PRIVATE OpenSSL::SSL OpenSSL::Crypto)
+   endif()
+-- 
+2.35.1
+