diff --git a/abs.yaml b/abs.yaml
index 31c3d0cb..a467ff00 100644
--- a/abs.yaml
+++ b/abs.yaml
@@ -5,6 +5,9 @@ build_parameters:
   - "--error-overlinking"
   - "--error-overdepending"
 
-# Upload all testing artifacts on a labeled testing channel: ad-testing/label/py315
+# Upload testing artifacts: ad-testing/label/py315
 upload_channels:
-- ad-testing/label/py315
+  - ad-testing/label/py315
+
+build_env_vars:
+  ANACONDA_ROCKET_ENABLE_PY315: yes
diff --git a/recipe/build_base.sh b/recipe/build_base.sh
index f471027b..fa275284 100644
--- a/recipe/build_base.sh
+++ b/recipe/build_base.sh
@@ -209,7 +209,7 @@ if [[ ${target_platform} == osx-64 ]]; then
   # TODO: check with LLVM 12 if the following hack is needed.
   # https://reviews.llvm.org/D76461 may have fixed the need for the following hack.
   echo '#!/bin/bash' > $BUILD_PREFIX/bin/$HOST-llvm-ar
-  echo "$BUILD_PREFIX/bin/llvm-ar --format=darwin" '"$@"' >> $BUILD_PREFIX/bin/$HOST-llvm-ar
+  echo "$BUILD_PREFIX/bin/llvm-ar-21 --format=darwin" '"$@"' >> $BUILD_PREFIX/bin/$HOST-llvm-ar
   chmod +x $BUILD_PREFIX/bin/$HOST-llvm-ar
   export ARCHFLAGS="-arch x86_64"
 elif [[ ${target_platform} == osx-arm64 ]]; then
@@ -218,7 +218,7 @@ elif [[ ${target_platform} == osx-arm64 ]]; then
   export ac_sys_release=20.0.0
   export MACOSX_DEFAULT_ARCH=arm64
   echo '#!/bin/bash' > $BUILD_PREFIX/bin/$HOST-llvm-ar
-  echo "$BUILD_PREFIX/bin/llvm-ar --format=darwin" '"$@"' >> $BUILD_PREFIX/bin/$HOST-llvm-ar
+  echo "$BUILD_PREFIX/bin/llvm-ar-21 --format=darwin" '"$@"' >> $BUILD_PREFIX/bin/$HOST-llvm-ar
   chmod +x $BUILD_PREFIX/bin/$HOST-llvm-ar
   export ARCHFLAGS="-arch arm64"
   export CFLAGS="$CFLAGS $ARCHFLAGS"
diff --git a/recipe/conda_build_config.yaml b/recipe/conda_build_config.yaml
index dffd4121..89de489e 100644
--- a/recipe/conda_build_config.yaml
+++ b/recipe/conda_build_config.yaml
@@ -7,6 +7,14 @@ numpy:
 # Override aggregate global tk 8.6 — build/test against tk 9 from pkgs/main
 tk:
   - '9.0'
+# Override aggregate global clang 20 on osx. CPython 3.15 JIT requires LLVM 21
+# (Tools/jit/_llvm.py); mixing compiler('c')=20 with clang-21/llvm-tools-21
+# breaks osx-arm64 LTO+PGO (___llvm_profile_runtime) and JIT tool discovery.
+# Matches conda-forge python-feedstock osx c_compiler_version: 21.
+c_compiler_version:            # [osx]
+  - 21                         # [osx]
+cxx_compiler_version:          # [osx]
+  - 21                         # [osx]
 gil_type:
   - normal
   # Will be enabled as part of https://anaconda.atlassian.net/browse/PKG-5855
diff --git a/recipe/meta.yaml b/recipe/meta.yaml
index e1620e15..b2705962 100644
--- a/recipe/meta.yaml
+++ b/recipe/meta.yaml
@@ -1,5 +1,5 @@
 {% set version = "3.15.0" %}
-{% set dev = "b4" %}
+{% set dev = "rc1" %}
 {% set dev_ = "" %}
 {% set ver2 = '.'.join(version.split('.')[0:2]) %}
 {% set ver2nd = ''.join(version.split('.')[0:2]) %}
@@ -61,7 +61,7 @@ source:
 {% else %}
   - url: https://www.python.org/ftp/python/{{ version }}/Python-{{ version }}{{ dev }}.tar.xz
     # md5 from: https://www.python.org/downloads/release/python-{{ ver3nd }}/
-    sha256: 93efb9c88d7b6633368e7f7b8f8db6e98988f7f761c09b77849447262841ce3a
+    sha256: f84dad680ab2147417d2739355c2678f0f9acffe4ae8ef77895de1454b384b07
 {% endif %}
     patches:
       - patches/0001-Win32-Change-FD_SETSIZE-from-512-to-2048.patch
@@ -184,8 +184,6 @@ outputs:
         - make                          # [not win]
         - libtool                       # [unix]
         - pkg-config                    # [not win]
-        # configure script looks for llvm-ar for lto
-        - llvm-tools                    # [osx]
         - ld_impl_{{ target_platform }} # [linux]
         # called in build_base.sh
         - ripgrep                       # [not win]
@@ -194,6 +192,8 @@ outputs:
 {% endif %}
         # https://github.com/python/cpython/blob/main/Tools/jit/README.md#installing-llvm:
         # "LLVM version 21 is the officially supported version"
+        # Also supplies llvm-ar-21, which build_base.sh wraps as the lto archiver
+        # that configure picks up via $HOST-llvm-ar.
         - clang-21         # [not win]
         - llvm-tools-21    # [not win]
         - clang 21.*       # [win]