diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c6132d6..ac63cb4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -35,6 +35,12 @@ jobs:
         run: uvx --from check-jsonschema==0.38.2 check-jsonschema --builtin-schema vendor.dependabot .github/dependabot.yml
       - name: Run complete mocked test suite
         run: uv run pytest
+      - name: Check configured mypy target and API compatibility (Python 3.8)
+        if: matrix.python == '3.8'
+        run: uv run --extra dev python checks/mypy_compatibility.py -v
+      - name: Check configured mypy target and API compatibility (Python 3.13)
+        if: matrix.python == '3.8'
+        run: uv run --isolated --python 3.13 --extra dev python checks/mypy_compatibility.py -v
       - name: Build wheel and source distribution
         run: uv build
       - name: Require both package artifacts
diff --git a/agent-notes/repair/task6-installer-mypy-range.md b/agent-notes/repair/task6-installer-mypy-range.md
new file mode 100644
index 0000000..931068d
--- /dev/null
+++ b/agent-notes/repair/task6-installer-mypy-range.md
@@ -0,0 +1,88 @@
+# Goal
+
+Preserve configured Python 3.8 target/API compatibility in original same-org
+installer_builder PR #4 without combining its dependency change with the
+separate unresolved full-package typing characterization in draft #5.
+
+# State
+
+Local-only repair/task6-installer-mypy-range starts directly at verified actual
+merged master 11ec654ac80ef0bd7423156e76a22aa1d8e3a453. It changes the optional
+mypy range from >=0.991,<1.0 to >=0.991,<1.15, copies the exact authored
+three-case checks/mypy_compatibility.py probe, and invokes it on tool Python
+3.8 and 3.13 in the existing Windows Python 3.8 CI leg. Current default
+tests/build, support >=3.8.6, target 3.8, strict flags, action pins, permissions
+and every existing step remain.
+
+# Decisions and why
+
+- Original #4 at 62d473dd04ad154d3b4aade2b8723dc76ce73909 proposes <3.0.
+  Actual mypy 2.4.0 on tool Python 3.13 replaces the configured 3.8 target after
+  warning and accepts the 3.9-only str.removeprefix API. The positive startswith
+  control and negative untyped-definition control distinguish target behavior
+  from an unloaded configuration. The same three assertions are copied intact.
+- The first local successor wired only the 3.8 tool runtime, which would select
+  an older compatible mypy even with the original <3.0 proposal. That missed the
+  demonstrated modern-tool regression. Add a separately named 3.13 invocation
+  of the identical probe with actual --extra dev resolution, not a pinned tool.
+  Its uv run --isolated --python 3.13 uses a fresh project environment rather
+  than changing the existing 3.8 .venv. No step-level environment override
+  persists. The following build keeps the existing setup-uv runtime selection.
+  Separate steps preserve ordinary failure propagation for both invocations.
+- The installed uv run help supports --isolated, --python and --extra. The
+  official CLI reference documents fresh isolated project environments:
+  https://docs.astral.sh/uv/reference/cli/#uv-run
+- Official mypy 1.15 release notes drop tool runtime 3.8. Mypy 1.14.1 declares
+  both tool and target minimum 3.8. The <1.15 cap permits that proven endpoint
+  without changing the package floor or substituting a newer checker target.
+- This explicit target/API/config probe checks generated snippets through the
+  public mypy.api.run with the actual project config. It does not check package
+  source or establish whole-package typing correctness. It is outside ordinary
+  pytest collection, so default mocked tests do not import an optional tool.
+- Draft #5 remains open/unmerged and retains its full configured package gate
+  on its own branch. Its historical 115 diagnostics are annotation/import/stub
+  debt plus separately classified helper issues; they are not suppressed here.
+  Current default has no full package gate to remove. This independent change
+  does not claim that the separate full-package baseline is green or landed.
+
+# Evidence and limits
+
+Historical focused execution before the true rebases, with isolated dependencies
+and --no-project, exercised only this identical authored check file:
+
+- Old <1.0, mypy 0.991 / Python 3.8.20: three passed in 6.924s.
+- Original <3.0, mypy 2.4.0 / Python 3.13.15: two failed in 4.350s; target 3.8
+  warning requires 3.10 or higher and removeprefix incorrectly returns success.
+- Narrowed <1.15, mypy 1.14.1 / Python 3.8.20: three passed in 9.228s.
+- Same narrowed range / Python 3.13.15: three passed in 6.305s.
+
+An initial blanket stderr assertion failed the old control because configured
+unused override warnings apply to snippet-only checks. The positive assertion
+was narrowed to target warnings before the recorded broad-range red/narrowed
+greens. No config flag or negative API/strictness assertion was disabled.
+Those focused results are historical, not execution at this new branch head.
+Hosted execution is pending; no aggregate or full-package-green claim is made.
+No local full checker, package install, tests/build or native installer ran.
+Actionlint 1.7.12 passed with optional shellcheck/pyflakes disabled. Zizmor
+1.31.1 offline pedantic reported no findings, including after the 3.13 workflow
+correction. No local probe, source tests or full checker were rerun. SHA256 of
+the copied probe and the preserved stacked probe is identical:
+AE3B6AAB59F9B34C92501F8CE0B0DA42B39E66DB32077CFB0FD363FA08EE63A5.
+
+# Preservation
+
+Original #4 remains unchanged in backup/task6-installer-mypy-original-pr.
+archive/task6-installer-mypy-baseline-before-11ec retains old 748ddadf;
+archive/task6-installer-mypy-compatibility-before-11ec retains old e89bff68.
+The genuine rebased baseline 2f18a792 and stacked probe fd0103dc remain intact.
+The coordinator has published the rebased #5; its fresh run 39552363115 failed,
+with exact new diagnostics assessed separately. It remains unmerged.
+Canonical master 099ceb3 and all canonical dirty/untracked user work are intact.
+No remote branch, PR description, settings or model route was changed.
+
+# Next step
+
+Return complete local head/diff and static workflow audits for coordinator
+review before any publication. Any hosted probe/test/build green must describe
+that exact scope; draft #5's full-package typing characterization stays separate
+and unresolved after its fresh hosted failure. No publication, model or merge.
diff --git a/checks/mypy_compatibility.py b/checks/mypy_compatibility.py
new file mode 100644
index 0000000..b85cbc8
--- /dev/null
+++ b/checks/mypy_compatibility.py
@@ -0,0 +1,43 @@
+"""Check mypy's configured target and strictness, not package-wide typing."""
+from pathlib import Path
+import sys
+from tempfile import TemporaryDirectory
+import unittest
+
+from mypy import api
+from mypy.version import __version__
+
+
+class MypyCompatibilityTests(unittest.TestCase):
+    def check_code(self, source):
+        config = Path(__file__).resolve().parents[1] / "pyproject.toml"
+        with TemporaryDirectory() as cache:
+            return api.run([
+                "--config-file", str(config), "--no-incremental",
+                "--cache-dir", cache, "-c", source,
+            ])
+
+    def test_python38_api_is_accepted_with_configured_target(self):
+        output, errors, status = self.check_code(
+            "def has_prefix(value: str) -> bool:\n"
+            "    return value.startswith('prefix')\n")
+        self.assertEqual(status, 0, output + errors)
+        self.assertNotIn("python_version:", errors)
+
+    def test_python39_only_api_is_rejected(self):
+        output, errors, status = self.check_code(
+            "def remove_prefix(value: str) -> str:\n"
+            "    return value.removeprefix('prefix')\n")
+        self.assertEqual(status, 1, output + errors)
+        self.assertIn('has no attribute "removeprefix"', output)
+
+    def test_config_still_rejects_untyped_definitions(self):
+        output, errors, status = self.check_code(
+            "def identity(value):\n    return value\n")
+        self.assertEqual(status, 1, output + errors)
+        self.assertIn("[no-untyped-def]", output)
+
+
+if __name__ == "__main__":
+    print("mypy {} on Python {}".format(__version__, sys.version.split()[0]))
+    unittest.main()
diff --git a/pyproject.toml b/pyproject.toml
index a57357f..c4c3ac7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -31,7 +31,7 @@ dependencies = [
 
 [project.optional-dependencies]
 dev = [
-    "mypy>=0.991,<1.0",
+    "mypy>=0.991,<1.15",
 ]
 
 [dependency-groups]