diff --git a/.coderabbit.yaml b/.coderabbit.yaml
index 4597dc1..ec0e324 100644
--- a/.coderabbit.yaml
+++ b/.coderabbit.yaml
@@ -253,9 +253,10 @@ reviews:
         instructions: >-
           PASS if the PR is a release preparation (title matches
           `chore: prepare the * release`), or touches only .github/, benchmarks/,
-          planning/, test files, or developer tooling configuration that never
-          reaches the published package (dot-files in the repository root such as
-          .coderabbit.yaml, .pre-commit-config.yaml, .gitignore).
+          planning/, CONTRIBUTING.md, test files, or developer tooling
+          configuration that never reaches the published package (dot-files in
+          the repository root such as .coderabbit.yaml, .pre-commit-config.yaml,
+          .gitignore).
           Otherwise FAIL unless CHANGELOG.md gains at
           least one bullet under the `## [Unreleased]` heading describing the
           user-visible effect of the change.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 30c4fe7..53d1b29 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -137,7 +137,7 @@ deterministic suite only — the hypothesis suites are excluded, because a test
 that reaches a branch only sometimes makes both mutmut's test-selection mapping
 and the score itself irreproducible.
 
-Baseline: **526 of 549 mutants killed (95.8%)**. Every survivor is an equivalent
+Baseline: **614 of 637 mutants killed (96.4%)**. Every survivor is an equivalent
 mutant, in one of five classes:
 
 | Class | Why it cannot be killed |
diff --git a/pyproject.toml b/pyproject.toml
index 4e7ddec..2472ee1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -245,16 +245,20 @@ do_not_mutate_patterns = ["cast\\("]
 # mutmut runs pytest in-process and copies pyproject.toml into `mutants/`, so
 # the `-n auto` default would fork the collector away from it.
 pytest_add_cli_args = ["-n", "0", "-p", "no:cacheprovider"]
-# Excluded from the mutation suite, for three different reasons:
+# Excluded from the mutation suite, for four different reasons:
 #   - the wall-clock example runner and the extras integration tests cost
 #     minutes and exercise third-party code, not the two modules under mutation;
 #   - the hypothesis suites are randomised, and mutmut maps tests to functions
 #     from a single stats run — a test that reaches a branch only sometimes
 #     makes both that mapping and the score irreproducible. The score is a
 #     tracked number, so it is measured against the deterministic suite: a
-#     survivor then means a missing example-based test, every time.
+#     survivor then means a missing example-based test, every time;
+#   - the README link checks read repository files that mutmut does not copy
+#     into `mutants/`, so collection fails there; they import no library code
+#     and could not kill a mutant anyway.
 pytest_add_cli_args_test_selection = [
   "tests",
+  "--ignore=tests/test_readme.py",
   "--ignore=tests/test_examples.py",
   "--ignore=tests/test_aiohttp.py",
   "--ignore=tests/test_fastapi.py",
diff --git a/tests/test_coordination.py b/tests/test_coordination.py
index fbc7d4e..dc7ae9f 100644
--- a/tests/test_coordination.py
+++ b/tests/test_coordination.py
@@ -8,6 +8,7 @@
 
 import asyncio
 import gc
+import re
 import threading
 import weakref
 from dataclasses import replace
@@ -1496,8 +1497,19 @@ def test__breaker__backoff_with_shared_storage__rejected(fake_clock: FakeClock)
     failed-round count crosses the wire, so a multiplier set here would be read,
     validated and then quietly ignored — the exact trap the option exists to
     remove.
+
+    The whole message is asserted because it is the whole remedy: a caller who
+    hits this has to learn which two ways out exist, and half a sentence would
+    leave them guessing.
     """
-    with pytest.raises(ValueError, match='wait_duration_backoff_multiplier'):
+    refusal = (
+        'wait_duration_backoff_multiplier has no effect on a breaker with shared '
+        'storage: reopening is decided by the backend from wait_duration_in_open, '
+        'and no failed-round count is shared. Leave it at 1.0, or drop the storage '
+        'to run this breaker locally. Got 2.0.'
+    )
+
+    with pytest.raises(ValueError, match=rf'\A{re.escape(refusal)}\Z'):
         CircuitBreaker(
             name='payments',
             config=Config(wait_duration_backoff_multiplier=2.0),
diff --git a/tests/test_engine.py b/tests/test_engine.py
index d176702..f85dc97 100644
--- a/tests/test_engine.py
+++ b/tests/test_engine.py
@@ -482,6 +482,30 @@ def test__half_open__every_probe_unreachable__reopens(
     assert engine.state is State.OPEN
 
 
+def test__half_open__ordinary_exception__stays_a_probe_verdict(
+    config: Config, fake_clock: FakeClock
+) -> None:
+    """Configuring unreachable exceptions must not turn every failure into one.
+
+    An inconclusive probe hands its slot back and ends no round, so a breaker
+    that mistook an ordinary failure for one would sit in HALF_OPEN while the
+    dependency kept failing.
+    """
+
+    def boom() -> None:
+        raise ValueError('boom')
+
+    engine = _unreachable_engine(config, fake_clock)
+    _trip_to_open(engine)
+    fake_clock.advance(5.0)
+
+    with pytest.raises(ValueError, match='boom'):
+        engine.call_sync(boom)
+    engine.call_sync(lambda: 'ok')
+
+    assert engine.state is State.OPEN
+
+
 def test__unreachable_not_configured__probe_reopens_as_before(
     config: Config, fake_clock: FakeClock
 ) -> None:
@@ -539,7 +563,7 @@ def test__unreachable_exceptions__not_a_tuple__rejected_at_construction(
     config: Config, fake_clock: FakeClock
 ) -> None:
     """``isinstance`` refuses a list of types, so refuse it here with a clear message."""
-    with pytest.raises(TypeError, match='must be a tuple'):
+    with pytest.raises(TypeError, match=r'\Aunreachable_exceptions must be a tuple, got: list\Z'):
         Engine(
             name='test',
             config=config,
diff --git a/tests/test_state_machine.py b/tests/test_state_machine.py
index a4c3778..9c26a8b 100644
--- a/tests/test_state_machine.py
+++ b/tests/test_state_machine.py
@@ -723,6 +723,10 @@ def test__half_open__probe_round_passes__backoff_resets(
     _trip_to_open(machine, 2)
     assert machine.retry_after() == pytest.approx(5.0)
 
+    # Counting again from one, not resuming from three: 5 * 2, never 5 * 2**3.
+    _fail_probe_round(machine, fake_clock, wait=5.0)
+    assert machine.retry_after() == pytest.approx(10.0)
+
 
 def test__open__grown_wait__probe_not_admitted_before_it_elapses(
     config: Config, fake_clock: FakeClock