diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ed5588b7d..d7f97513f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,14 +33,14 @@ jobs:
       matrix:
         stack: ["heroku-20", "heroku-22", "heroku-24"]
     env:
-      HATCHET_APP_LIMIT: 200
+      HATCHET_APP_LIMIT: 300
       HATCHET_DEFAULT_STACK: ${{ matrix.stack }}
       HATCHET_EXPENSIVE_MODE: 1
       HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}
       HEROKU_API_USER: ${{ secrets.HEROKU_API_USER }}
       HEROKU_DISABLE_AUTOUPDATE: 1
-      PARALLEL_SPLIT_TEST_PROCESSES: 60
-      RSPEC_RETRY_RETRY_COUNT: 3
+      PARALLEL_SPLIT_TEST_PROCESSES: 70
+      RSPEC_RETRY_RETRY_COUNT: 2
     steps:
       - name: Checkout
         uses: actions/checkout@v4
diff --git a/Gemfile.lock b/Gemfile.lock
index 6dc35eb1b..7b8aeedce 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -5,7 +5,7 @@ GEM
     base64 (0.2.0)
     diff-lcs (1.5.1)
     erubis (2.7.0)
-    excon (0.110.0)
+    excon (0.111.0)
     heroics (0.1.3)
       base64
       erubis (~> 2.0)
@@ -27,7 +27,7 @@ GEM
     parallel_split_test (0.10.0)
       parallel (>= 0.5.13)
       rspec-core (>= 3.9.0)
-    parser (3.3.4.2)
+    parser (3.3.5.0)
       ast (~> 2.4.1)
       racc
     platform-api (3.7.0)
@@ -39,32 +39,32 @@ GEM
     rate_throttle_client (0.1.2)
     regexp_parser (2.9.2)
     rrrretry (1.0.0)
-    rspec-core (3.13.0)
+    rspec-core (3.13.1)
       rspec-support (~> 3.13.0)
-    rspec-expectations (3.13.2)
+    rspec-expectations (3.13.3)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
     rspec-retry (0.6.2)
       rspec-core (> 3.3)
     rspec-support (3.13.1)
-    rubocop (1.66.0)
+    rubocop (1.66.1)
       json (~> 2.3)
       language_server-protocol (>= 3.17.0)
       parallel (~> 1.10)
       parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 2.4, < 3.0)
-      rubocop-ast (>= 1.32.1, < 2.0)
+      rubocop-ast (>= 1.32.2, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 2.4.0, < 3.0)
-    rubocop-ast (1.32.1)
+    rubocop-ast (1.32.3)
       parser (>= 3.3.1.0)
-    rubocop-rspec (3.0.4)
+    rubocop-rspec (3.0.5)
       rubocop (~> 1.61)
     ruby-progressbar (1.13.0)
-    thor (1.3.1)
+    thor (1.3.2)
     threaded (0.0.4)
-    unicode-display_width (2.5.0)
+    unicode-display_width (2.6.0)
     webrick (1.8.1)
 
 PLATFORMS
@@ -80,7 +80,7 @@ DEPENDENCIES
   rubocop-rspec
 
 RUBY VERSION
-   ruby 3.3.2p78
+   ruby 3.3.5p100
 
 BUNDLED WITH
-   2.5.11
+   2.5.18
diff --git a/bin/steps/pipenv b/bin/steps/pipenv
index d7a190b4b..1d97941c7 100755
--- a/bin/steps/pipenv
+++ b/bin/steps/pipenv
@@ -15,6 +15,7 @@ if [[ -f Pipfile ]]; then
     meta_set "package_manager" "pipenv"
 
     # Skip installing dependencies using pip later.
+    # TODO: Stop leaking this env var into subshells such as post_compile hooks.
     export SKIP_PIP_INSTALL=1
 
     # Set Pip env vars
diff --git a/spec/fixtures/ci_pipenv/Pipfile b/spec/fixtures/ci_pipenv/Pipfile
new file mode 100644
index 000000000..6d6473eb4
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/Pipfile
@@ -0,0 +1,10 @@
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+typing-extensions = "*"
+
+[dev-packages]
+pytest = "*"
diff --git a/spec/fixtures/ci_pipenv/Pipfile.lock b/spec/fixtures/ci_pipenv/Pipfile.lock
new file mode 100644
index 000000000..aa11d8ecf
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/Pipfile.lock
@@ -0,0 +1,62 @@
+{
+    "_meta": {
+        "hash": {
+            "sha256": "9ac30f761973e7bb9a0425635eb284370fede0e49a74b475c418f98bf13f3075"
+        },
+        "pipfile-spec": 6,
+        "requires": {},
+        "sources": [
+            {
+                "name": "pypi",
+                "url": "https://pypi.org/simple",
+                "verify_ssl": true
+            }
+        ]
+    },
+    "default": {
+        "typing-extensions": {
+            "hashes": [
+                "sha256:04e5ca0351e0f3f85c6853954072df659d0d13fac324d0072316b67d7794700d",
+                "sha256:1a7ead55c7e559dd4dee8856e3a88b41225abfe1ce8df57b7c13915fe121ffb8"
+            ],
+            "index": "pypi",
+            "markers": "python_version >= '3.8'",
+            "version": "==4.12.2"
+        }
+    },
+    "develop": {
+        "iniconfig": {
+            "hashes": [
+                "sha256:2d91e135bf72d31a410b17c16da610a82cb55f6b0477d1a902134b24a455b8b3",
+                "sha256:b6a85871a79d2e3b22d2d1b94ac2824226a63c6b741c88f7ae975f18b6778374"
+            ],
+            "markers": "python_version >= '3.7'",
+            "version": "==2.0.0"
+        },
+        "packaging": {
+            "hashes": [
+                "sha256:026ed72c8ed3fcce5bf8950572258698927fd1dbda10a5e981cdf0ac37f4f002",
+                "sha256:5b8f2217dbdbd2f7f384c41c628544e6d52f2d0f53c6d0c3ea61aa5d1d7ff124"
+            ],
+            "markers": "python_version >= '3.8'",
+            "version": "==24.1"
+        },
+        "pluggy": {
+            "hashes": [
+                "sha256:2cffa88e94fdc978c4c574f15f9e59b7f4201d439195c3715ca9e2486f1d0cf1",
+                "sha256:44e1ad92c8ca002de6377e165f3e0f1be63266ab4d554740532335b9d75ea669"
+            ],
+            "markers": "python_version >= '3.8'",
+            "version": "==1.5.0"
+        },
+        "pytest": {
+            "hashes": [
+                "sha256:70b98107bd648308a7952b06e6ca9a50bc660be218d53c257cc1fc94fda10181",
+                "sha256:a6853c7375b2663155079443d2e45de913a911a11d669df02a50814944db57b2"
+            ],
+            "index": "pypi",
+            "markers": "python_version >= '3.8'",
+            "version": "==8.3.3"
+        }
+    }
+}
diff --git a/spec/fixtures/ci_pipenv/app.json b/spec/fixtures/ci_pipenv/app.json
new file mode 100644
index 000000000..59617c169
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/app.json
@@ -0,0 +1,9 @@
+{
+  "environments": {
+    "test": {
+      "scripts": {
+        "test": "./bin/print-env-vars.sh && pytest --version"
+      }
+    }
+  }
+}
diff --git a/spec/fixtures/ci_pipenv/bin/compile b/spec/fixtures/ci_pipenv/bin/compile
new file mode 100755
index 000000000..fcb7055e3
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/bin/compile
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack, and tests that the environment is
+# configured as expected for buildpacks that run after the Python buildpack.
+
+set -euo pipefail
+
+BUILD_DIR="${1}"
+
+cd "${BUILD_DIR}"
+
+exec bin/print-env-vars.sh
diff --git a/spec/fixtures/ci_pipenv/bin/detect b/spec/fixtures/ci_pipenv/bin/detect
new file mode 100755
index 000000000..68cdcc4a2
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/bin/detect
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack.
+
+set -euo pipefail
+
+echo "Inline"
diff --git a/spec/fixtures/ci_pipenv/bin/post_compile b/spec/fixtures/ci_pipenv/bin/post_compile
new file mode 100644
index 000000000..15362f6b1
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/bin/post_compile
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+exec bin/print-env-vars.sh
diff --git a/spec/fixtures/ci_pipenv/bin/print-env-vars.sh b/spec/fixtures/ci_pipenv/bin/print-env-vars.sh
new file mode 100755
index 000000000..9e0bebe6b
--- /dev/null
+++ b/spec/fixtures/ci_pipenv/bin/print-env-vars.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+printenv | sort | grep -vE '^(_|BUILDPACK_LOG_FILE|BUILD_DIR|CACHE_DIR|CI_NODE_.+|DYNO|ENV_DIR|HEROKU_TEST_RUN_.+|HOME|OLDPWD|PORT|PWD|SHLVL|STACK|TERM)='
diff --git a/spec/fixtures/ci_requirements/app.json b/spec/fixtures/ci_requirements/app.json
index 99d1d237f..59617c169 100644
--- a/spec/fixtures/ci_requirements/app.json
+++ b/spec/fixtures/ci_requirements/app.json
@@ -2,7 +2,7 @@
   "environments": {
     "test": {
       "scripts": {
-        "test": "pytest --version"
+        "test": "./bin/print-env-vars.sh && pytest --version"
       }
     }
   }
diff --git a/spec/fixtures/ci_requirements/bin/compile b/spec/fixtures/ci_requirements/bin/compile
new file mode 100755
index 000000000..fcb7055e3
--- /dev/null
+++ b/spec/fixtures/ci_requirements/bin/compile
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack, and tests that the environment is
+# configured as expected for buildpacks that run after the Python buildpack.
+
+set -euo pipefail
+
+BUILD_DIR="${1}"
+
+cd "${BUILD_DIR}"
+
+exec bin/print-env-vars.sh
diff --git a/spec/fixtures/ci_requirements/bin/detect b/spec/fixtures/ci_requirements/bin/detect
new file mode 100755
index 000000000..68cdcc4a2
--- /dev/null
+++ b/spec/fixtures/ci_requirements/bin/detect
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack.
+
+set -euo pipefail
+
+echo "Inline"
diff --git a/spec/fixtures/ci_requirements/bin/post_compile b/spec/fixtures/ci_requirements/bin/post_compile
new file mode 100644
index 000000000..15362f6b1
--- /dev/null
+++ b/spec/fixtures/ci_requirements/bin/post_compile
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+exec bin/print-env-vars.sh
diff --git a/spec/fixtures/ci_requirements/bin/print-env-vars.sh b/spec/fixtures/ci_requirements/bin/print-env-vars.sh
new file mode 100755
index 000000000..9e0bebe6b
--- /dev/null
+++ b/spec/fixtures/ci_requirements/bin/print-env-vars.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+printenv | sort | grep -vE '^(_|BUILDPACK_LOG_FILE|BUILD_DIR|CACHE_DIR|CI_NODE_.+|DYNO|ENV_DIR|HEROKU_TEST_RUN_.+|HOME|OLDPWD|PORT|PWD|SHLVL|STACK|TERM)='
diff --git a/spec/fixtures/ci_requirements/requirements-test.txt b/spec/fixtures/ci_requirements/requirements-test.txt
index e079f8a60..40543aaba 100644
--- a/spec/fixtures/ci_requirements/requirements-test.txt
+++ b/spec/fixtures/ci_requirements/requirements-test.txt
@@ -1 +1 @@
-pytest
+pytest==8.3.3
diff --git a/spec/fixtures/ci_requirements/requirements.txt b/spec/fixtures/ci_requirements/requirements.txt
index a42590beb..eec3a2223 100644
--- a/spec/fixtures/ci_requirements/requirements.txt
+++ b/spec/fixtures/ci_requirements/requirements.txt
@@ -1 +1,2 @@
-urllib3
+# This package has been picked since it has no dependencies and is small/fast to install.
+typing-extensions==4.12.2
diff --git a/spec/fixtures/hooks/bin/post_compile b/spec/fixtures/hooks/bin/post_compile
index 373d33889..4bd8ed0e8 100644
--- a/spec/fixtures/hooks/bin/post_compile
+++ b/spec/fixtures/hooks/bin/post_compile
@@ -2,5 +2,6 @@
 
 set -euo pipefail
 
-echo 'post_compile ran with env vars:'
-printenv | cut -d '=' -f 1 | sort
+echo '~ post_compile ran with env vars:'
+bin/print-env-vars.sh
+echo '~ post_compile complete'
diff --git a/spec/fixtures/hooks/bin/pre_compile b/spec/fixtures/hooks/bin/pre_compile
index 40d1a402e..9f3737bef 100644
--- a/spec/fixtures/hooks/bin/pre_compile
+++ b/spec/fixtures/hooks/bin/pre_compile
@@ -2,5 +2,6 @@
 
 set -euo pipefail
 
-echo 'pre_compile ran with env vars:'
-printenv | cut -d '=' -f 1 | sort
+echo '~ pre_compile ran with env vars:'
+bin/print-env-vars.sh
+echo '~ pre_compile complete'
diff --git a/spec/fixtures/hooks/bin/print-env-vars.sh b/spec/fixtures/hooks/bin/print-env-vars.sh
new file mode 100755
index 000000000..dbd598a54
--- /dev/null
+++ b/spec/fixtures/hooks/bin/print-env-vars.sh
@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+printenv | sort \
+  | grep -vE '^(_|BUILDPACK_LOG_FILE|DYNO|OLDPWD|REQUEST_ID|SHLVL)=' \
+  | sed --regexp-extended \
+      --expression 's#(=/tmp/build_)[^:/]+#\1<hash>#' \
+      --expression 's#^(ENV_DIR=/tmp/).*#\1...#' \
+      --expression 's#^(SOURCE_VERSION=).*#\1...#'
diff --git a/spec/fixtures/pipenv_editable/bin/compile b/spec/fixtures/pipenv_editable/bin/compile
index c506ad6b7..df17e9401 100755
--- a/spec/fixtures/pipenv_editable/bin/compile
+++ b/spec/fixtures/pipenv_editable/bin/compile
@@ -9,4 +9,4 @@ BUILD_DIR="${1}"
 
 cd "${BUILD_DIR}"
 
-exec bin/test-entrypoints
+exec bin/test-entrypoints.sh
diff --git a/spec/fixtures/pipenv_editable/bin/post_compile b/spec/fixtures/pipenv_editable/bin/post_compile
index 460dc2de7..6e77d159a 100755
--- a/spec/fixtures/pipenv_editable/bin/post_compile
+++ b/spec/fixtures/pipenv_editable/bin/post_compile
@@ -2,4 +2,4 @@
 
 set -euo pipefail
 
-exec bin/test-entrypoints
+exec bin/test-entrypoints.sh
diff --git a/spec/fixtures/pipenv_editable/bin/test-entrypoints b/spec/fixtures/pipenv_editable/bin/test-entrypoints.sh
similarity index 100%
rename from spec/fixtures/pipenv_editable/bin/test-entrypoints
rename to spec/fixtures/pipenv_editable/bin/test-entrypoints.sh
diff --git a/spec/fixtures/pipenv_python_version_unspecified/Pipfile b/spec/fixtures/pipenv_python_version_unspecified/Pipfile
index 67803f45b..496fa1558 100644
--- a/spec/fixtures/pipenv_python_version_unspecified/Pipfile
+++ b/spec/fixtures/pipenv_python_version_unspecified/Pipfile
@@ -4,6 +4,6 @@ verify_ssl = true
 name = "pypi"
 
 [packages]
-urllib3 = "*"
+typing-extensions = "*"
 
 [dev-packages]
diff --git a/spec/fixtures/pipenv_python_version_unspecified/Pipfile.lock b/spec/fixtures/pipenv_python_version_unspecified/Pipfile.lock
index 4c426c2ae..e744fd450 100644
--- a/spec/fixtures/pipenv_python_version_unspecified/Pipfile.lock
+++ b/spec/fixtures/pipenv_python_version_unspecified/Pipfile.lock
@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "a95c318e4395270fbd1e0f52dd8f12185db4c3258d4e03dd80de54b7c7aad8b1"
+            "sha256": "beb76460a63ef2f29eec7b281a3c7114d442db105096d7472b4b72a7504df8fe"
         },
         "pipfile-spec": 6,
         "requires": {},
@@ -14,14 +14,14 @@
         ]
     },
     "default": {
-        "urllib3": {
+        "typing-extensions": {
             "hashes": [
-                "sha256:450b20ec296a467077128bff42b73080516e71b56ff59a60a02bef2232c4fa9d",
-                "sha256:d0570876c61ab9e520d776c38acbbb5b05a776d3f9ff98a5c8fd5162a444cf19"
+                "sha256:04e5ca0351e0f3f85c6853954072df659d0d13fac324d0072316b67d7794700d",
+                "sha256:1a7ead55c7e559dd4dee8856e3a88b41225abfe1ce8df57b7c13915fe121ffb8"
             ],
             "index": "pypi",
             "markers": "python_version >= '3.8'",
-            "version": "==2.2.1"
+            "version": "==4.12.2"
         }
     },
     "develop": {}
diff --git a/spec/fixtures/pipenv_python_version_unspecified/bin/compile b/spec/fixtures/pipenv_python_version_unspecified/bin/compile
new file mode 100755
index 000000000..db00975d6
--- /dev/null
+++ b/spec/fixtures/pipenv_python_version_unspecified/bin/compile
@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack, and tests that the environment is
+# configured as expected for buildpacks that run after the Python buildpack.
+
+set -euo pipefail
+
+printenv | sort | grep -vE '^(_|BUILDPACK_LOG_FILE|DYNO|HOME|PWD|REQUEST_ID|SHLVL|SOURCE_VERSION|STACK)='
+echo
+
+python -c 'import pprint, sys; pprint.pp(sys.path)'
+echo
+
+# TODO: Investigate why 'pipenv graph' doesn't work here.
+# TODO: Remove --disable-pip-version-check in favour of exporting PIP_DISABLE_PIP_VERSION_CHECK
+pip list --disable-pip-version-check
+echo
+
+python -c 'import typing_extensions; print(typing_extensions)'
diff --git a/spec/fixtures/pipenv_python_version_unspecified/bin/detect b/spec/fixtures/pipenv_python_version_unspecified/bin/detect
new file mode 100755
index 000000000..68cdcc4a2
--- /dev/null
+++ b/spec/fixtures/pipenv_python_version_unspecified/bin/detect
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack.
+
+set -euo pipefail
+
+echo "Inline"
diff --git a/spec/fixtures/pipenv_python_version_unspecified/setup.py b/spec/fixtures/pipenv_python_version_unspecified/setup.py
new file mode 100644
index 000000000..de90e907e
--- /dev/null
+++ b/spec/fixtures/pipenv_python_version_unspecified/setup.py
@@ -0,0 +1,9 @@
+# This file is here to confirm we don't try and create the fallback requirements
+# file containing '-e .' when using Pipenv.
+
+from setuptools import setup
+
+setup(
+  name='test',
+  install_requires=['six'],
+)
diff --git a/spec/fixtures/python_3.10_outdated/runtime.txt b/spec/fixtures/python_3.10_outdated/runtime.txt
index a5da7cc4d..fadb07024 100644
--- a/spec/fixtures/python_3.10_outdated/runtime.txt
+++ b/spec/fixtures/python_3.10_outdated/runtime.txt
@@ -1 +1 @@
-python-3.10.5
+python-3.10.0
diff --git a/spec/fixtures/python_3.8_outdated/runtime.txt b/spec/fixtures/python_3.8_outdated/runtime.txt
index d9a16ae1e..73b1cf81d 100644
--- a/spec/fixtures/python_3.8_outdated/runtime.txt
+++ b/spec/fixtures/python_3.8_outdated/runtime.txt
@@ -1 +1 @@
-python-3.8.12
+python-3.8.0
diff --git a/spec/fixtures/python_3.9_outdated/runtime.txt b/spec/fixtures/python_3.9_outdated/runtime.txt
index 540296197..f72c5111f 100644
--- a/spec/fixtures/python_3.9_outdated/runtime.txt
+++ b/spec/fixtures/python_3.9_outdated/runtime.txt
@@ -1 +1 @@
-python-3.9.12
+python-3.9.0
diff --git a/spec/fixtures/requirements_basic/bin/compile b/spec/fixtures/requirements_basic/bin/compile
new file mode 100755
index 000000000..68d243e7b
--- /dev/null
+++ b/spec/fixtures/requirements_basic/bin/compile
@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack, and tests that the environment is
+# configured as expected for buildpacks that run after the Python buildpack.
+
+set -euo pipefail
+
+printenv | sort | grep -vE '^(_|BUILDPACK_LOG_FILE|DYNO|HOME|PWD|REQUEST_ID|SHLVL|SOURCE_VERSION|STACK)='
+echo
+
+python -c 'import pprint, sys; pprint.pp(sys.path)'
+echo
+
+# TODO: Remove --disable-pip-version-check in favour of exporting PIP_DISABLE_PIP_VERSION_CHECK
+pip list --disable-pip-version-check
+echo
+
+python -c 'import typing_extensions; print(typing_extensions)'
diff --git a/spec/fixtures/requirements_basic/bin/detect b/spec/fixtures/requirements_basic/bin/detect
new file mode 100755
index 000000000..68cdcc4a2
--- /dev/null
+++ b/spec/fixtures/requirements_basic/bin/detect
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# This file is run by the inline buildpack.
+
+set -euo pipefail
+
+echo "Inline"
diff --git a/spec/fixtures/requirements_basic/requirements.txt b/spec/fixtures/requirements_basic/requirements.txt
new file mode 100644
index 000000000..eec3a2223
--- /dev/null
+++ b/spec/fixtures/requirements_basic/requirements.txt
@@ -0,0 +1,2 @@
+# This package has been picked since it has no dependencies and is small/fast to install.
+typing-extensions==4.12.2
diff --git a/spec/fixtures/requirements_editable/bin/compile b/spec/fixtures/requirements_editable/bin/compile
index c506ad6b7..df17e9401 100755
--- a/spec/fixtures/requirements_editable/bin/compile
+++ b/spec/fixtures/requirements_editable/bin/compile
@@ -9,4 +9,4 @@ BUILD_DIR="${1}"
 
 cd "${BUILD_DIR}"
 
-exec bin/test-entrypoints
+exec bin/test-entrypoints.sh
diff --git a/spec/fixtures/requirements_editable/bin/post_compile b/spec/fixtures/requirements_editable/bin/post_compile
index 460dc2de7..6e77d159a 100755
--- a/spec/fixtures/requirements_editable/bin/post_compile
+++ b/spec/fixtures/requirements_editable/bin/post_compile
@@ -2,4 +2,4 @@
 
 set -euo pipefail
 
-exec bin/test-entrypoints
+exec bin/test-entrypoints.sh
diff --git a/spec/fixtures/requirements_editable/bin/test-entrypoints b/spec/fixtures/requirements_editable/bin/test-entrypoints.sh
similarity index 100%
rename from spec/fixtures/requirements_editable/bin/test-entrypoints
rename to spec/fixtures/requirements_editable/bin/test-entrypoints.sh
diff --git a/spec/fixtures/requirements_git/requirements.txt b/spec/fixtures/requirements_git/requirements.txt
deleted file mode 100644
index 81e9dbfe0..000000000
--- a/spec/fixtures/requirements_git/requirements.txt
+++ /dev/null
@@ -1,2 +0,0 @@
-# This relies upon the VCS binaries from the stack image.
-git+https://github.com/certifi/python-certifi
diff --git a/spec/fixtures/requirements_invalid/requirements.txt b/spec/fixtures/requirements_invalid/requirements.txt
new file mode 100644
index 000000000..db42b7ee6
--- /dev/null
+++ b/spec/fixtures/requirements_invalid/requirements.txt
@@ -0,0 +1 @@
+an-invalid-requirement!
diff --git a/spec/hatchet/ci_spec.rb b/spec/hatchet/ci_spec.rb
index eb0b2c5db..ad8053abf 100644
--- a/spec/hatchet/ci_spec.rb
+++ b/spec/hatchet/ci_spec.rb
@@ -3,46 +3,163 @@
 require_relative '../spec_helper'
 
 RSpec.describe 'Heroku CI' do
-  it 'installs both normal and test dependencies and uses cache on subsequent runs' do
-    Hatchet::Runner.new('spec/fixtures/ci_requirements', allow_failure: true).run_ci do |test_run|
-      expect(test_run.output).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
-        -----> Python app detected
-        -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
-               To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
-        -----> Installing python-#{DEFAULT_PYTHON_VERSION}
-        -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
-        -----> Installing SQLite3
-        -----> Installing requirements with pip
-               .*
-               Successfully installed urllib3-.*
-        -----> Installing test dependencies...
-               .*
-               Successfully installed .* pytest-.*
-        -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
-        -----> No test-setup command provided. Skipping.
-        -----> Running test command `pytest --version`...
-        pytest .*
-        -----> test command `pytest --version` completed successfully
-      REGEX
-
-      test_run.run_again
-
-      expect(test_run.output).to match(Regexp.new(<<~REGEX))
-        -----> Python app detected
-        -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
-               To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
-        -----> No change in requirements detected, installing from cache
-        -----> Using cached install of python-#{DEFAULT_PYTHON_VERSION}
-        -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
-        -----> Installing SQLite3
-        -----> Installing requirements with pip
-        -----> Installing test dependencies...
-        -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
-        -----> No test-setup command provided. Skipping.
-        -----> Running test command `pytest --version`...
-        pytest .*
-        -----> test command `pytest --version` completed successfully
-      REGEX
+  let(:buildpacks) { [:default, 'heroku-community/inline'] }
+
+  context 'when using pip' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/ci_requirements', buildpacks:) }
+
+    it 'installs both normal and test dependencies and uses cache on subsequent runs' do
+      app.run_ci do |test_run|
+        expect(test_run.output).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
+          -----> Python app detected
+          -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
+                 To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          -----> Installing python-#{DEFAULT_PYTHON_VERSION}
+          -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          -----> Installing SQLite3
+          -----> Installing requirements with pip
+                 .*
+                 Successfully installed typing-extensions-4.12.2
+          -----> Installing test dependencies...
+                 .*
+                 Successfully installed .* pytest-8.3.3
+          -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
+          -----> Running post-compile hook
+          CI=true
+          CPLUS_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          C_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          DISABLE_COLLECTSTATIC=1
+          INSTALL_TEST=1
+          LANG=en_US.UTF-8
+          LC_ALL=C.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/app/.heroku/vendor/bin::/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/
+          PIP_NO_PYTHON_VERSION_WARNING=1
+          PKG_CONFIG_PATH=/app/.heroku/vendor/lib/pkg-config:/app/.heroku/python/lib/pkg-config:
+          PYTHONUNBUFFERED=1
+          -----> Inline app detected
+          LANG=en_US.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/
+          PYTHONHASHSEED=random
+          PYTHONHOME=/app/.heroku/python
+          PYTHONPATH=/app
+          PYTHONUNBUFFERED=true
+          -----> No test-setup command provided. Skipping.
+          -----> Running test command `./bin/print-env-vars.sh && pytest --version`...
+          CI=true
+          DYNO_RAM=2560
+          FORWARDED_ALLOW_IPS=\\*
+          GUNICORN_CMD_ARGS=--access-logfile -
+          LANG=en_US.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/:/app/.sprettur/bin/
+          PYTHONHASHSEED=random
+          PYTHONHOME=/app/.heroku/python
+          PYTHONPATH=/app
+          PYTHONUNBUFFERED=true
+          WEB_CONCURRENCY=5
+          pytest 8.3.3
+          -----> test command `./bin/print-env-vars.sh && pytest --version` completed successfully
+        REGEX
+
+        test_run.run_again
+
+        expect(test_run.output).to include(<<~OUTPUT)
+          -----> Python app detected
+          -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
+                 To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          -----> No change in requirements detected, installing from cache
+          -----> Using cached install of python-#{DEFAULT_PYTHON_VERSION}
+          -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          -----> Installing SQLite3
+          -----> Installing requirements with pip
+          -----> Installing test dependencies...
+          -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
+          -----> Running post-compile hook
+        OUTPUT
+      end
+    end
+  end
+
+  context 'when using Pipenv' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/ci_pipenv', buildpacks:) }
+
+    it 'installs both normal and test dependencies and uses cache on subsequent runs' do
+      app.run_ci do |test_run|
+        expect(test_run.output).to match(Regexp.new(<<~REGEX))
+          -----> Python app detected
+          -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
+                 To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          -----> Installing python-#{DEFAULT_PYTHON_VERSION}
+          -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          -----> Installing test dependencies
+                 Installing dependencies from Pipfile.lock \\(.+\\)...
+                 Installing dependencies from Pipfile.lock \\(.+\\)...
+          -----> Installing SQLite3
+          -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
+          -----> Running post-compile hook
+          CI=true
+          CPLUS_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          C_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          DISABLE_COLLECTSTATIC=1
+          INSTALL_TEST=1
+          LANG=en_US.UTF-8
+          LC_ALL=C.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/app/.heroku/vendor/bin::/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/
+          PIP_NO_PYTHON_VERSION_WARNING=1
+          PKG_CONFIG_PATH=/app/.heroku/vendor/lib/pkg-config:/app/.heroku/python/lib/pkg-config:
+          PYTHONUNBUFFERED=1
+          SKIP_PIP_INSTALL=1
+          -----> Inline app detected
+          LANG=en_US.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/
+          PYTHONHASHSEED=random
+          PYTHONHOME=/app/.heroku/python
+          PYTHONPATH=/app
+          PYTHONUNBUFFERED=true
+          -----> No test-setup command provided. Skipping.
+          -----> Running test command `./bin/print-env-vars.sh && pytest --version`...
+          CI=true
+          DYNO_RAM=2560
+          FORWARDED_ALLOW_IPS=\\*
+          GUNICORN_CMD_ARGS=--access-logfile -
+          LANG=en_US.UTF-8
+          LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          PATH=/app/.heroku/python/bin:/usr/local/bin:/usr/bin:/bin:/app/.sprettur/bin/:/app/.sprettur/bin/
+          PYTHONHASHSEED=random
+          PYTHONHOME=/app/.heroku/python
+          PYTHONPATH=/app
+          PYTHONUNBUFFERED=true
+          WEB_CONCURRENCY=5
+          pytest 8.3.3
+          -----> test command `./bin/print-env-vars.sh && pytest --version` completed successfully
+        REGEX
+
+        test_run.run_again
+
+        expect(test_run.output).to match(Regexp.new(<<~REGEX))
+          -----> Python app detected
+          -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
+                 To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          -----> Using cached install of python-#{DEFAULT_PYTHON_VERSION}
+          -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          -----> Installing test dependencies
+                 Installing dependencies from Pipfile.lock \\(.+\\)...
+                 Installing dependencies from Pipfile.lock \\(.+\\)...
+          -----> Installing SQLite3
+          -----> Skipping Django collectstatic since the env var DISABLE_COLLECTSTATIC is set.
+          -----> Running post-compile hook
+        REGEX
+      end
     end
   end
 end
diff --git a/spec/hatchet/django_spec.rb b/spec/hatchet/django_spec.rb
index 865d5b833..bd7feb1a9 100644
--- a/spec/hatchet/django_spec.rb
+++ b/spec/hatchet/django_spec.rb
@@ -97,10 +97,19 @@
           expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
             remote: -----> \\$ python manage.py collectstatic --noinput
             remote:        Traceback \\(most recent call last\\):
-            remote:        .*
+            remote:        .+
             remote:        ModuleNotFoundError: No module named 'gettingstarted'
             remote: 
             remote:  !     Error while running '\\$ python manage.py collectstatic --noinput'.
+            remote:        See traceback above for details.
+            remote: 
+            remote:        You may need to update application code to resolve this error.
+            remote:        Or, you can disable collectstatic for this application:
+            remote: 
+            remote:           \\$ heroku config:set DISABLE_COLLECTSTATIC=1
+            remote: 
+            remote:        https://devcenter.heroku.com/articles/django-assets
+            remote:  !     Push rejected, failed to compile Python app.
           REGEX
         end
       end
diff --git a/spec/hatchet/hooks_spec.rb b/spec/hatchet/hooks_spec.rb
index dd114a76b..6d66adf54 100644
--- a/spec/hatchet/hooks_spec.rb
+++ b/spec/hatchet/hooks_spec.rb
@@ -3,49 +3,62 @@
 require_relative '../spec_helper'
 
 RSpec.describe 'Compile hooks' do
-  context 'when an app has bin/pre_compile and bin/post_compile scripts' do
+  # TODO: Run this on Heroku-22 too, once it has also migrated to the new build infrastructure.
+  # (Currently the test fails on the old infrastructure due to subtle differences in system PATH elements.)
+  context 'when an app has bin/pre_compile and bin/post_compile scripts', stacks: %w[heroku-20 heroku-24] do
     let(:app) { Hatchet::Runner.new('spec/fixtures/hooks', config: { 'SOME_APP_CONFIG_VAR' => '1' }) }
 
     it 'runs the hooks with the correct environment' do
-      expected_env_vars = %w[
-        _
-        BUILD_DIR
-        BUILDPACK_LOG_FILE
-        CACHE_DIR
-        C_INCLUDE_PATH
-        CPLUS_INCLUDE_PATH
-        DYNO
-        ENV_DIR
-        HOME
-        LANG
-        LD_LIBRARY_PATH
-        LIBRARY_PATH
-        OLDPWD
-        PATH
-        PIP_NO_PYTHON_VERSION_WARNING
-        PKG_CONFIG_PATH
-        PWD
-        PYTHONUNBUFFERED
-        REQUEST_ID
-        SHLVL
-        SOME_APP_CONFIG_VAR
-        SOURCE_VERSION
-        STACK
-      ]
-
       app.deploy do |app|
-        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
+        output = clean_output(app.output)
+
+        expect(output).to include(<<~OUTPUT)
           remote: -----> Python app detected
           remote: -----> Running pre-compile hook
-          remote: pre_compile ran with env vars:
-          remote: #{expected_env_vars.join("\nremote: ")}
-          remote: -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
-          remote: .*
+          remote: ~ pre_compile ran with env vars:
+          remote: BUILD_DIR=/tmp/build_<hash>
+          remote: CACHE_DIR=/tmp/codon/tmp/cache
+          remote: C_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          remote: CPLUS_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          remote: ENV_DIR=/tmp/...
+          remote: HOME=/app
+          remote: LANG=en_US.UTF-8
+          remote: LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: PATH=/app/.heroku/python/bin:/app/.heroku/vendor/bin::/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+          remote: PIP_NO_PYTHON_VERSION_WARNING=1
+          remote: PKG_CONFIG_PATH=/app/.heroku/vendor/lib/pkg-config:/app/.heroku/python/lib/pkg-config:
+          remote: PWD=/tmp/build_<hash>
+          remote: PYTHONUNBUFFERED=1
+          remote: SOME_APP_CONFIG_VAR=1
+          remote: SOURCE_VERSION=...
+          remote: STACK=#{app.stack}
+          remote: ~ pre_compile complete
+        OUTPUT
+
+        expect(output).to include(<<~OUTPUT)
           remote: -----> Installing requirements with pip
           remote: -----> Running post-compile hook
-          remote: post_compile ran with env vars:
-          remote: #{expected_env_vars.join("\nremote: ")}
-        REGEX
+          remote: ~ post_compile ran with env vars:
+          remote: BUILD_DIR=/tmp/build_<hash>
+          remote: CACHE_DIR=/tmp/codon/tmp/cache
+          remote: C_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          remote: CPLUS_INCLUDE_PATH=/app/.heroku/vendor/include:/app/.heroku/python/include:
+          remote: ENV_DIR=/tmp/...
+          remote: HOME=/app
+          remote: LANG=en_US.UTF-8
+          remote: LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: PATH=/app/.heroku/python/bin:/app/.heroku/vendor/bin::/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+          remote: PIP_NO_PYTHON_VERSION_WARNING=1
+          remote: PKG_CONFIG_PATH=/app/.heroku/vendor/lib/pkg-config:/app/.heroku/python/lib/pkg-config:
+          remote: PWD=/tmp/build_<hash>
+          remote: PYTHONUNBUFFERED=1
+          remote: SOME_APP_CONFIG_VAR=1
+          remote: SOURCE_VERSION=...
+          remote: STACK=#{app.stack}
+          remote: ~ post_compile complete
+        OUTPUT
       end
     end
   end
diff --git a/spec/hatchet/package_manager_spec.rb b/spec/hatchet/package_manager_spec.rb
index 10cff59b4..d247e1deb 100644
--- a/spec/hatchet/package_manager_spec.rb
+++ b/spec/hatchet/package_manager_spec.rb
@@ -35,6 +35,7 @@
           remote:  !     https://devcenter.heroku.com/articles/getting-started-with-python
           remote:  !     https://devcenter.heroku.com/articles/python-support
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
diff --git a/spec/hatchet/pip_spec.rb b/spec/hatchet/pip_spec.rb
index 704a65c4b..5351975e0 100644
--- a/spec/hatchet/pip_spec.rb
+++ b/spec/hatchet/pip_spec.rb
@@ -12,12 +12,15 @@
 end
 
 RSpec.describe 'Pip support' do
-  context 'when requirements.txt is unchanged since the last build' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/python_version_unspecified') }
+  # TODO: Run this on Heroku-22 too, once it has also migrated to the new build infrastructure.
+  # (Currently the test fails on the old infrastructure due to subtle differences in system PATH elements.)
+  context 'when requirements.txt is unchanged since the last build', stacks: %w[heroku-20 heroku-24] do
+    let(:buildpacks) { [:default, 'heroku-community/inline'] }
+    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_basic', buildpacks:) }
 
     it 're-uses packages from the cache' do
       app.deploy do |app|
-        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
+        expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
           remote: -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
           remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
@@ -25,12 +28,37 @@
           remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
           remote: -----> Installing SQLite3
           remote: -----> Installing requirements with pip
-          remote:        Collecting urllib3 \\(from -r requirements.txt \\(line 1\\)\\)
-          remote:          Downloading urllib3-.*
-          remote:        Downloading urllib3-.*
-          remote:        Installing collected packages: urllib3
-          remote:        Successfully installed urllib3-.*
-        REGEX
+          remote:        Collecting typing-extensions==4.12.2 (from -r requirements.txt (line 2))
+          remote:          Downloading typing_extensions-4.12.2-py3-none-any.whl.metadata (3.0 kB)
+          remote:        Downloading typing_extensions-4.12.2-py3-none-any.whl (37 kB)
+          remote:        Installing collected packages: typing-extensions
+          remote:        Successfully installed typing-extensions-4.12.2
+          remote: -----> Inline app detected
+          remote: LANG=en_US.UTF-8
+          remote: LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: PATH=/app/.heroku/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+          remote: PYTHONHASHSEED=random
+          remote: PYTHONHOME=/app/.heroku/python
+          remote: PYTHONPATH=/app
+          remote: PYTHONUNBUFFERED=true
+          remote: 
+          remote: ['',
+          remote:  '/app',
+          remote:  '/app/.heroku/python/lib/python312.zip',
+          remote:  '/app/.heroku/python/lib/python3.12',
+          remote:  '/app/.heroku/python/lib/python3.12/lib-dynload',
+          remote:  '/app/.heroku/python/lib/python3.12/site-packages']
+          remote: 
+          remote: Package           Version
+          remote: ----------------- -------
+          remote: pip               #{PIP_VERSION}
+          remote: setuptools        #{SETUPTOOLS_VERSION}
+          remote: typing_extensions 4.12.2
+          remote: wheel             #{WHEEL_VERSION}
+          remote: 
+          remote: <module 'typing_extensions' from '/app/.heroku/python/lib/python3.12/site-packages/typing_extensions.py'>
+        OUTPUT
         app.commit!
         app.push!
         expect(clean_output(app.output)).to include(<<~OUTPUT)
@@ -42,21 +70,21 @@
           remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
           remote: -----> Installing SQLite3
           remote: -----> Installing requirements with pip
-          remote: -----> Discovering process types
+          remote: -----> Inline app detected
         OUTPUT
       end
     end
   end
 
   context 'when requirements.txt has changed since the last build' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/python_version_unspecified') }
+    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_basic') }
 
     it 'clears the cache before installing the packages again' do
       app.deploy do |app|
         File.write('requirements.txt', 'six', mode: 'a')
         app.commit!
         app.push!
-        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
+        expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
           remote: -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
           remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
@@ -65,32 +93,50 @@
           remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
           remote: -----> Installing SQLite3
           remote: -----> Installing requirements with pip
-          remote:        Collecting urllib3 \\(from -r requirements.txt \\(line 1\\)\\)
-          remote:          Downloading urllib3-.*
-          remote:        Collecting six \\(from -r requirements.txt \\(line 2\\)\\)
-          remote:          Downloading six-.*
-          remote:        Downloading urllib3-.*
-          remote:        Downloading six-.*
-          remote:        Installing collected packages: urllib3, six
-          remote:        Successfully installed six-.* urllib3-.*
-        REGEX
+          remote:        Collecting typing-extensions==4.12.2 (from -r requirements.txt (line 2))
+          remote:          Downloading typing_extensions-4.12.2-py3-none-any.whl.metadata (3.0 kB)
+          remote:        Collecting six (from -r requirements.txt (line 3))
+          remote:          Downloading six-1.16.0-py2.py3-none-any.whl.metadata (1.8 kB)
+          remote:        Downloading typing_extensions-4.12.2-py3-none-any.whl (37 kB)
+          remote:        Downloading six-1.16.0-py2.py3-none-any.whl (11 kB)
+          remote:        Installing collected packages: typing-extensions, six
+          remote:        Successfully installed six-1.16.0 typing-extensions-4.12.2
+        OUTPUT
       end
     end
   end
 
-  context 'when requirements.txt contains popular compiled packages' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_compiled') }
+  context 'when the package manager has changed from Pipenv to pip since the last build' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_python_version_unspecified') }
 
-    include_examples 'installs successfully using pip'
+    # TODO: Fix this case so the cache is actually cleared.
+    it 'clears the cache before installing with pip' do
+      app.deploy do |app|
+        FileUtils.rm(['Pipfile', 'Pipfile.lock'])
+        FileUtils.cp(FIXTURE_DIR.join('requirements_basic/requirements.txt'), '.')
+        app.commit!
+        app.push!
+        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
+          remote: -----> Python app detected
+          remote: -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
+          remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          remote: -----> Using cached install of python-#{DEFAULT_PYTHON_VERSION}
+          remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          remote: -----> Installing SQLite3
+          remote: -----> Installing requirements with pip
+          remote: -----> Discovering process types
+        REGEX
+      end
+    end
   end
 
-  context 'when requirements.txt contains Git requirements URLs' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_git') }
+  context 'when requirements.txt contains popular compiled packages' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_compiled') }
 
     include_examples 'installs successfully using pip'
   end
 
-  context 'when requirements.txt contains editable requirements' do
+  context 'when requirements.txt contains editable requirements (both VCS and local package)' do
     let(:buildpacks) { [:default, 'heroku-community/inline'] }
     let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_editable', buildpacks:) }
 
@@ -120,7 +166,7 @@
         REGEX
 
         # Test rewritten paths work at runtime.
-        expect(app.run('bin/test-entrypoints')).to include(<<~OUTPUT)
+        expect(app.run('bin/test-entrypoints.sh')).to include(<<~OUTPUT)
           easy-install.pth:/app/.heroku/src/gunicorn
           easy-install.pth:/app/packages/local_package_setup_py
           __editable___local_package_pyproject_toml_0_0_1_finder.py:/app/packages/local_package_pyproject_toml/local_package_pyproject_toml'}
@@ -166,8 +212,21 @@
 
     it 'installs packages from setup.py' do
       app.deploy do |app|
-        expect(app.output).to include('Running setup.py develop for test')
-        expect(app.output).to include('Successfully installed six')
+        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
+          remote: -----> Python app detected
+          remote: -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
+          remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          remote: -----> Installing python-#{DEFAULT_PYTHON_VERSION}
+          remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          remote: -----> Installing SQLite3
+          remote: -----> Installing requirements with pip
+          remote:        Obtaining file:///tmp/build_.* \\(from -r requirements.txt \\(line 1\\)\\)
+          remote:          Preparing metadata \\(setup.py\\): started
+          remote:          Preparing metadata \\(setup.py\\): finished with status 'done'
+          remote:        .+
+          remote:        Installing collected packages: six, test
+          remote:          Running setup.py develop for test
+        REGEX
       end
     end
   end
@@ -186,6 +245,20 @@
     end
   end
 
+  context 'when requirements.txt contains an invalid requirement' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_invalid', allow_failure: true) }
+
+    it 'aborts the build and displays the pip error' do
+      app.deploy do |app|
+        expect(clean_output(app.output)).to include(<<~OUTPUT)
+          remote: -----> Installing requirements with pip
+          remote:        ERROR: Invalid requirement: 'an-invalid-requirement!' (from line 1 of requirements.txt)
+          remote:  !     Push rejected, failed to compile Python app.
+        OUTPUT
+      end
+    end
+  end
+
   context 'when requirements.txt contains GDAL but the GDAL C++ library is missing' do
     let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_gdal', allow_failure: true) }
 
@@ -195,6 +268,7 @@
           remote:  !     Hello! Package installation failed since the GDAL library was not found.
           remote:  !     For GDAL, GEOS and PROJ support, use the Geo buildpack alongside the Python buildpack:
           remote:  !     https://github.com/heroku/heroku-geo-buildpack
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
diff --git a/spec/hatchet/pipenv_spec.rb b/spec/hatchet/pipenv_spec.rb
index ff5b5ba1c..6c0e2fa97 100644
--- a/spec/hatchet/pipenv_spec.rb
+++ b/spec/hatchet/pipenv_spec.rb
@@ -30,6 +30,7 @@
         remote:  !     For a list of the supported Python versions, see:
         remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
         remote:  !     
+        remote:  !     Push rejected, failed to compile Python app.
       OUTPUT
     end
   end
@@ -63,10 +64,14 @@
   end
 
   context 'with a Pipfile.lock but no Python version specified' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_python_version_unspecified') }
+    let(:buildpacks) { [:default, 'heroku-community/inline'] }
+    let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_python_version_unspecified', buildpacks:) }
 
-    it 'builds with the default Python version' do
+    # TODO: Run this on Heroku-22 too, once it has also migrated to the new build infrastructure.
+    # (Currently the test fails on the old infrastructure due to subtle differences in system PATH elements.)
+    it 'builds with the default Python version', stacks: %w[heroku-20 heroku-24] do
       app.deploy do |app|
+        # TODO: We should not be leaking the Pipenv installation into the app environment.
         expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
           remote: -----> Python app detected
           remote: -----> No Python version was specified. Using the buildpack default: python-#{DEFAULT_PYTHON_VERSION}
@@ -76,6 +81,37 @@
           remote: -----> Installing dependencies with Pipenv #{PIPENV_VERSION}
           remote:        Installing dependencies from Pipfile.lock \\(.+\\)...
           remote: -----> Installing SQLite3
+          remote: -----> Inline app detected
+          remote: LANG=en_US.UTF-8
+          remote: LD_LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: LIBRARY_PATH=/app/.heroku/vendor/lib:/app/.heroku/python/lib:
+          remote: PATH=/app/.heroku/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+          remote: PYTHONHASHSEED=random
+          remote: PYTHONHOME=/app/.heroku/python
+          remote: PYTHONPATH=/app
+          remote: PYTHONUNBUFFERED=true
+          remote: 
+          remote: \\['',
+          remote:  '/app',
+          remote:  '/app/.heroku/python/lib/python312.zip',
+          remote:  '/app/.heroku/python/lib/python3.12',
+          remote:  '/app/.heroku/python/lib/python3.12/lib-dynload',
+          remote:  '/app/.heroku/python/lib/python3.12/site-packages'\\]
+          remote: 
+          remote: Package           Version
+          remote: ----------------- ---------
+          remote: certifi           .+
+          remote: distlib           .+
+          remote: filelock          .+
+          remote: pip               #{PIP_VERSION}
+          remote: pipenv            #{PIPENV_VERSION}
+          remote: platformdirs      .+
+          remote: setuptools        #{SETUPTOOLS_VERSION}
+          remote: typing_extensions 4.12.2
+          remote: virtualenv        .+
+          remote: wheel             #{WHEEL_VERSION}
+          remote: 
+          remote: \\<module 'typing_extensions' from '/app/.heroku/python/lib/python3.12/site-packages/typing_extensions.py'\\>
         REGEX
       end
     end
@@ -101,6 +137,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
@@ -126,6 +163,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
@@ -207,7 +245,7 @@
     let(:allow_failure) { false }
     let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_python_full_version', allow_failure:) }
 
-    it 'builds with the outdated Python version specified' do
+    it 'builds with the outdated Python version specified and displays a deprecation warning' do
       app.deploy do |app|
         expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
           remote: -----> Python app detected
@@ -241,6 +279,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
@@ -260,6 +299,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
@@ -305,6 +345,32 @@
     end
   end
 
+  context 'when the package manager has changed from pip to Pipenv since the last build' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/python_version_unspecified') }
+
+    # TODO: Fix this case so the cache is actually cleared.
+    it 'clears the cache before installing with Pipenv' do
+      app.deploy do |app|
+        FileUtils.rm('requirements.txt')
+        FileUtils.cp(FIXTURE_DIR.join('pipenv_python_version_unspecified/Pipfile'), '.')
+        FileUtils.cp(FIXTURE_DIR.join('pipenv_python_version_unspecified/Pipfile.lock'), '.')
+        app.commit!
+        app.push!
+        expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
+          remote: -----> Python app detected
+          remote: -----> No Python version was specified. Using the same version as the last build: python-#{DEFAULT_PYTHON_VERSION}
+          remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+          remote: -----> Using cached install of python-#{DEFAULT_PYTHON_VERSION}
+          remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
+          remote: -----> Installing dependencies with Pipenv #{PIPENV_VERSION}
+          remote:        Installing dependencies from Pipfile.lock \\(.+\\)...
+          remote: -----> Installing SQLite3
+          remote: -----> Discovering process types
+        REGEX
+      end
+    end
+  end
+
   context 'when there is both a Pipfile.lock and a requirements.txt' do
     let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_and_requirements_txt') }
 
@@ -338,6 +404,7 @@
           remote:        Your Pipfile.lock \\(.+\\) is out of date.  Expected: \\(.+\\).
           remote:        .+
           remote:        ERROR:: Aborting deploy
+          remote:  !     Push rejected, failed to compile Python app.
         REGEX
       end
     end
@@ -373,7 +440,7 @@
         REGEX
 
         # Test rewritten paths work at runtime.
-        expect(app.run('bin/test-entrypoints')).to include(<<~OUTPUT)
+        expect(app.run('bin/test-entrypoints.sh')).to include(<<~OUTPUT)
           easy-install.pth:/app/.heroku/src/gunicorn
           easy-install.pth:/app/packages/local_package_setup_py
           __editable___local_package_pyproject_toml_0_0_1_finder.py:/app/packages/local_package_pyproject_toml/local_package_pyproject_toml'}
diff --git a/spec/hatchet/python_update_warning_spec.rb b/spec/hatchet/python_update_warning_spec.rb
index 6c7f66d62..df909ee73 100644
--- a/spec/hatchet/python_update_warning_spec.rb
+++ b/spec/hatchet/python_update_warning_spec.rb
@@ -30,13 +30,17 @@
         remote:  !     For a list of the supported Python versions, see:
         remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
         remote:  !     
+        remote:  !     Push rejected, failed to compile Python app.
       OUTPUT
     end
   end
 end
 
+# NOTE: We use the oldest patch releases (ie the '.0' releases) since we also want to test against
+# the oldest Python versions available to users. This is particularly important given that older
+# patch releases will bundle older pip, and the buildpack uses that pip during bootstrapping.
 RSpec.describe 'Python update warnings' do
-  context 'with a runtime.txt containing python-3.8.12' do
+  context 'with a runtime.txt containing python-3.8.0' do
     let(:allow_failure) { false }
     let(:app) { Hatchet::Runner.new('spec/fixtures/python_3.8_outdated', allow_failure:) }
 
@@ -60,7 +64,7 @@
             remote:  !     A Python security update is available! Upgrade as soon as possible to: python-#{LATEST_PYTHON_3_8}
             remote:  !     See: https://devcenter.heroku.com/articles/python-runtimes
             remote:  !     
-            remote: -----> Installing python-3.8.12
+            remote: -----> Installing python-3.8.0
           REGEX
         end
       end
@@ -70,31 +74,31 @@
       let(:allow_failure) { true }
 
       # We only support Python 3.8 on Heroku-20 and older.
-      include_examples 'aborts the build without showing an update warning', '3.8.12'
+      include_examples 'aborts the build without showing an update warning', '3.8.0'
     end
   end
 
-  context 'with a runtime.txt containing python-3.9.12' do
+  context 'with a runtime.txt containing python-3.9.0' do
     let(:allow_failure) { false }
     let(:app) { Hatchet::Runner.new('spec/fixtures/python_3.9_outdated', allow_failure:) }
 
     context 'when using Heroku-22 or older', stacks: %w[heroku-20 heroku-22] do
-      include_examples 'warns there is a Python update available', '3.9.12', LATEST_PYTHON_3_9
+      include_examples 'warns there is a Python update available', '3.9.0', LATEST_PYTHON_3_9
     end
 
     context 'when using Heroku-24', stacks: %w[heroku-24] do
       let(:allow_failure) { true }
 
       # We only support Python 3.9 on Heroku-22 and older.
-      include_examples 'aborts the build without showing an update warning', '3.9.12'
+      include_examples 'aborts the build without showing an update warning', '3.9.0'
     end
   end
 
-  context 'with a runtime.txt containing python-3.10.5' do
+  context 'with a runtime.txt containing python-3.10.0' do
     let(:allow_failure) { false }
     let(:app) { Hatchet::Runner.new('spec/fixtures/python_3.10_outdated', allow_failure:) }
 
-    include_examples 'warns there is a Python update available', '3.10.5', LATEST_PYTHON_3_10
+    include_examples 'warns there is a Python update available', '3.10.0', LATEST_PYTHON_3_10
   end
 
   context 'with a runtime.txt containing python-3.11.0' do
diff --git a/spec/hatchet/python_version_spec.rb b/spec/hatchet/python_version_spec.rb
index f4d96c376..a324089ba 100644
--- a/spec/hatchet/python_version_spec.rb
+++ b/spec/hatchet/python_version_spec.rb
@@ -31,6 +31,7 @@
         remote:  !     For a list of the supported Python versions, see:
         remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
         remote:  !     
+        remote:  !     Push rejected, failed to compile Python app.
       OUTPUT
     end
   end
@@ -102,6 +103,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
@@ -127,6 +129,7 @@
           remote:  !     For a list of the supported Python versions, see:
           remote:  !     https://devcenter.heroku.com/articles/python-support#supported-runtimes
           remote:  !     
+          remote:  !     Push rejected, failed to compile Python app.
         OUTPUT
       end
     end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 6d72bc8ee..93e3b2bdb 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -7,6 +7,8 @@
 require 'rspec/retry'
 require 'hatchet'
 
+FIXTURE_DIR = Pathname.new(__FILE__).parent.join('fixtures')
+
 LATEST_PYTHON_3_8 = '3.8.20'
 LATEST_PYTHON_3_9 = '3.9.20'
 LATEST_PYTHON_3_10 = '3.10.15'