diff --git a/CLAUDE.md b/CLAUDE.md
index 2d91aae..17a3893 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1140,6 +1140,49 @@ three -- `(position, bias, info)` -- inside a `try/except Exception: pass`, and
 got a confident `0/100 converged`. That is the issue's own defect one level up:
 a blanket except turning a caller error into a result about the callee. Fourth
 harness in this file to report the thing it could not read as broken.
+## Every example now bootstraps its own sys.path, and the sweep took three tries
+
+The trap at the top of this file -- `python chX/example.py` resolving `core` to
+the main checkout -- is closed at the source now: all 38 examples insert the
+repository root before their first `core` import, matching what
+`ch5_fingerprinting/example_classification` and nine of twelve `scripts/`
+generators already did. `tests/test_examples_import_this_checkout.py` holds it,
+and checks *order* rather than presence, because a bootstrap below the import
+changes nothing.
+
+Two things worth knowing before repeating this kind of sweep.
+
+**Three of the three bugs in the sweep script were found by pyflakes, none by
+reading.** Each was a plausible-looking way to locate an insertion point:
+
+1. `ast.walk` to decide whether `sys` was already imported -- it counts an
+   `import sys` inside a *function*, so four files got a module-level
+   `sys.path.insert` with no `sys` bound.
+2. Finding the stdlib group with `line.startswith(("import ", "from "))` --
+   which matched a line of *module docstring prose* beginning "from range
+   measurements ...", and wrote `import sys` inside the docstring.
+3. Treating any module-level import of a name as sufficient -- ch7's pose-graph
+   example imports `pathlib.Path` three lines *below* its first `core` import,
+   so the name existed but not yet at the point the bootstrap runs.
+
+All three produce files that `compileall` accepts. The rule from the earlier
+lint sweep holds and is worth restating in the stronger form: **a tool that
+locates Python by line prefix cannot tell an import from prose that starts like
+one, and `ast.walk` cannot tell module scope from function scope.** Use
+`tree.body`, and compare line numbers.
+
+**The import-order ratchet fired, correctly, and fixing it improved the
+baseline.** Inserting `import sys` at the top of a stdlib group is unsorted, so
+I001 went 113 -> 147. `ruff --select I001 --fix` over the chapter directories
+cleared 63, including 29 that predated this change, leaving **84** -- so the
+baseline moved down, not up. E402 does *not* fire: ruff exempts imports that
+follow a `sys.path` manipulation, which is what makes this idiom viable at all.
+
+Verification was the cheap strong one: `--help` for all 38 examples, captured
+before and diffed after, **byte-identical both times** -- once after the
+insertions and again after ruff reordered the imports. `--help` exits during
+argument parsing but only after every module-level import has run, so it tests
+exactly what this change touches, in about a second per example.
 
 ## A scratch probe in the scratchpad imports the *other* checkout
 
diff --git a/README.md b/README.md
index d206726..d39f429 100644
--- a/README.md
+++ b/README.md
@@ -155,10 +155,17 @@ python -m ch6_dead_reckoning.example_comparison
 ```
 
 `python -m` puts the repository root on `sys.path`, so these run straight from
-a fresh clone even before step 4 above. The script form —
-`python <chapter>/<example>.py` — puts the *script's* directory there instead,
-so `core` is only importable once the package is installed. That is why every
-command in this repository is written as `python -m`.
+a fresh clone even before step 4 above, and it is the form every command in
+this repository is written in.
+
+The script form — `python <chapter>/<example>.py` — puts the *script's*
+directory there instead, so `core` would not be importable from a fresh clone.
+Each example now adds the repository root itself before importing `core`, so
+that form works too. It is worth knowing why the line is there: without it, on
+a machine that has ever installed this package, `import core` does not fail —
+it quietly resolves to **whichever checkout the install points at**, and the
+example runs to completion against a different copy of the library. The error,
+when there is one, names a directory you have never heard of.
 
 Examples find their datasets from any working directory, so `cd`-ing into a
 chapter folder first is fine. Every example takes `--help`, which prints what
diff --git a/ch2_coords/example_attitude_visualization.py b/ch2_coords/example_attitude_visualization.py
index 8fd6746..f71e010 100644
--- a/ch2_coords/example_attitude_visualization.py
+++ b/ch2_coords/example_attitude_visualization.py
@@ -33,11 +33,19 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.coords import (
     enu_to_ned,
     euler_to_rotation_matrix,
diff --git a/ch2_coords/example_coordinate_transforms.py b/ch2_coords/example_coordinate_transforms.py
index 0b6624d..77fa607 100644
--- a/ch2_coords/example_coordinate_transforms.py
+++ b/ch2_coords/example_coordinate_transforms.py
@@ -24,11 +24,18 @@
 
 import argparse
 import json
+import sys
 from pathlib import Path
 
 import numpy as np
 
-from core.utils import angle_diff, resolve_data_path
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.coords import (
     ecef_to_enu,
     ecef_to_llh,
@@ -41,6 +48,7 @@
     quat_to_rotation_matrix,
     rotation_matrix_to_euler,
 )
+from core.utils import angle_diff, resolve_data_path
 
 
 def load_dataset(data_dir: str) -> dict:
diff --git a/ch3_estimators/example_comparison.py b/ch3_estimators/example_comparison.py
index e4d4a9f..c967030 100644
--- a/ch3_estimators/example_comparison.py
+++ b/ch3_estimators/example_comparison.py
@@ -28,12 +28,28 @@
 import argparse
 import contextlib
 import io
+import sys
 import time
 from pathlib import Path
 
-import numpy as np
 import matplotlib.pyplot as plt
+import numpy as np
 from tqdm import tqdm
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.estimators import (
+    ExtendedKalmanFilter,
+    Factor,
+    FactorGraph,
+    ParticleFilter,
+    UnscentedKalmanFilter,
+)
 from core.eval import (
     plot_error_cdf,
     plot_error_magnitude_time,
@@ -41,13 +57,6 @@
     save_figure,
     show_figures_if_requested,
 )
-from core.estimators import (
-    ExtendedKalmanFilter,
-    UnscentedKalmanFilter,
-    ParticleFilter,
-    Factor,
-    FactorGraph,
-)
 
 
 def setup_scenario(seed=42):
diff --git a/ch3_estimators/example_ekf_range_bearing.py b/ch3_estimators/example_ekf_range_bearing.py
index 15dd551..38e577d 100644
--- a/ch3_estimators/example_ekf_range_bearing.py
+++ b/ch3_estimators/example_ekf_range_bearing.py
@@ -22,11 +22,20 @@
 
 import argparse
 import json
+import sys
 from pathlib import Path
 from typing import Dict
 
-import numpy as np
 import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.estimators import ExtendedKalmanFilter
 from core.eval import save_figure, show_figures_if_requested
 from core.utils import angle_diff, resolve_data_path
diff --git a/ch3_estimators/example_iekf_range_bearing.py b/ch3_estimators/example_iekf_range_bearing.py
index 88896f8..048ab17 100644
--- a/ch3_estimators/example_iekf_range_bearing.py
+++ b/ch3_estimators/example_iekf_range_bearing.py
@@ -29,10 +29,18 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
-import numpy as np
 import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
 from core.estimators import ExtendedKalmanFilter, IteratedExtendedKalmanFilter
 from core.eval import (
diff --git a/ch3_estimators/example_kalman_1d.py b/ch3_estimators/example_kalman_1d.py
index 68fa66f..d6d0a4e 100644
--- a/ch3_estimators/example_kalman_1d.py
+++ b/ch3_estimators/example_kalman_1d.py
@@ -20,10 +20,19 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
-import numpy as np
 import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.estimators import KalmanFilter
 from core.eval import save_figure, show_figures_if_requested
 
diff --git a/ch3_estimators/example_least_squares.py b/ch3_estimators/example_least_squares.py
index 563ffd9..e15c955 100644
--- a/ch3_estimators/example_least_squares.py
+++ b/ch3_estimators/example_least_squares.py
@@ -35,18 +35,27 @@
 """
 
 import argparse
-import numpy as np
-import matplotlib.pyplot as plt
+import sys
 from pathlib import Path
 
-from core.eval import save_figure, show_figures_if_requested
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.estimators import (
-    linear_least_squares,
-    weighted_least_squares,
     gauss_newton,
     levenberg_marquardt,
+    linear_least_squares,
     robust_gauss_newton,
+    weighted_least_squares,
 )
+from core.eval import save_figure, show_figures_if_requested
 
 
 def setup_positioning_scenario():
diff --git a/ch3_estimators/example_particle_bimodal.py b/ch3_estimators/example_particle_bimodal.py
index 2764967..ab3e413 100644
--- a/ch3_estimators/example_particle_bimodal.py
+++ b/ch3_estimators/example_particle_bimodal.py
@@ -44,13 +44,26 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.estimators import ParticleFilter
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 
 FIGS_DIR = Path(__file__).parent / "figs"
 
diff --git a/ch4_rf_point_positioning/example_aoa_positioning.py b/ch4_rf_point_positioning/example_aoa_positioning.py
index 77efba6..e25e1ae 100644
--- a/ch4_rf_point_positioning/example_aoa_positioning.py
+++ b/ch4_rf_point_positioning/example_aoa_positioning.py
@@ -23,11 +23,19 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import save_figure, show_figures_if_requested
 from core.rf import (
     AOAPositioner,
diff --git a/ch4_rf_point_positioning/example_comparison.py b/ch4_rf_point_positioning/example_comparison.py
index 10e0ede..9eaf851 100644
--- a/ch4_rf_point_positioning/example_comparison.py
+++ b/ch4_rf_point_positioning/example_comparison.py
@@ -24,6 +24,7 @@
 
 import argparse
 import json
+import sys
 import time
 from functools import partial
 from pathlib import Path
@@ -33,6 +34,13 @@
 import numpy as np
 from tqdm import tqdm
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import save_figure, show_figures_if_requested
 from core.rf import (
     DIVERGENCE_M,
diff --git a/ch4_rf_point_positioning/example_dop_geometry.py b/ch4_rf_point_positioning/example_dop_geometry.py
index 5de9aed..aa41818 100644
--- a/ch4_rf_point_positioning/example_dop_geometry.py
+++ b/ch4_rf_point_positioning/example_dop_geometry.py
@@ -43,13 +43,26 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 from matplotlib.patches import Ellipse
 
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.rf.dop import compute_dop, compute_geometry_matrix
 from core.rf.positioning import TOAPositioner
 
diff --git a/ch4_rf_point_positioning/example_tdoa_positioning.py b/ch4_rf_point_positioning/example_tdoa_positioning.py
index 1fba458..240df24 100644
--- a/ch4_rf_point_positioning/example_tdoa_positioning.py
+++ b/ch4_rf_point_positioning/example_tdoa_positioning.py
@@ -15,11 +15,19 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import save_figure, show_figures_if_requested
 from core.rf import (
     TDOAPositioner,
@@ -29,8 +37,6 @@
     toa_fang_solver,
 )
 
-
-
 # Seed for the Monte Carlo in Demo 2.
 SEED = 42
 
diff --git a/ch4_rf_point_positioning/example_toa_positioning.py b/ch4_rf_point_positioning/example_toa_positioning.py
index 11ccd88..02038c4 100644
--- a/ch4_rf_point_positioning/example_toa_positioning.py
+++ b/ch4_rf_point_positioning/example_toa_positioning.py
@@ -16,11 +16,19 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import save_figure, show_figures_if_requested
 from core.rf import (
     SPEED_OF_LIGHT,
diff --git a/ch5_fingerprinting/example_classification.py b/ch5_fingerprinting/example_classification.py
index cb12eac..9ae13a1 100644
--- a/ch5_fingerprinting/example_classification.py
+++ b/ch5_fingerprinting/example_classification.py
@@ -33,8 +33,6 @@
     load_fingerprint_database,
 )
 
-
-
 # Seed for this example's synthetic database and query draws. Fixed so the
 # committed figures and reported accuracies can be regenerated exactly.
 DEFAULT_SEED = 42
diff --git a/ch5_fingerprinting/example_comparison.py b/ch5_fingerprinting/example_comparison.py
index 9d545d1..56f7975 100644
--- a/ch5_fingerprinting/example_comparison.py
+++ b/ch5_fingerprinting/example_comparison.py
@@ -16,20 +16,29 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import plot_error_cdf, save_figure, show_figures_if_requested
 from core.fingerprinting import (
-    load_fingerprint_database,
-    nn_localize,
-    knn_localize,
+    LinearRegressionLocalizer,
     fit_gaussian_naive_bayes,
+    knn_localize,
+    load_fingerprint_database,
     map_localize,
+    nn_localize,
     posterior_mean_localize,
-    LinearRegressionLocalizer,
 )
 
 
diff --git a/ch5_fingerprinting/example_deterministic.py b/ch5_fingerprinting/example_deterministic.py
index e2bf8a1..0468b37 100644
--- a/ch5_fingerprinting/example_deterministic.py
+++ b/ch5_fingerprinting/example_deterministic.py
@@ -13,16 +13,25 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import plot_error_cdf, save_figure, show_figures_if_requested
 from core.fingerprinting import (
+    knn_localize,
     load_fingerprint_database,
     nn_localize,
-    knn_localize,
 )
 
 
diff --git a/ch5_fingerprinting/example_pattern_recognition.py b/ch5_fingerprinting/example_pattern_recognition.py
index 97efd45..76626fa 100644
--- a/ch5_fingerprinting/example_pattern_recognition.py
+++ b/ch5_fingerprinting/example_pattern_recognition.py
@@ -13,15 +13,24 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import plot_error_cdf, save_figure, show_figures_if_requested
 from core.fingerprinting import (
-    load_fingerprint_database,
     LinearRegressionLocalizer,
+    load_fingerprint_database,
 )
 
 
diff --git a/ch5_fingerprinting/example_probabilistic.py b/ch5_fingerprinting/example_probabilistic.py
index 56ed6da..6518b65 100644
--- a/ch5_fingerprinting/example_probabilistic.py
+++ b/ch5_fingerprinting/example_probabilistic.py
@@ -15,18 +15,27 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import plot_error_cdf, save_figure, show_figures_if_requested
 from core.fingerprinting import (
-    load_fingerprint_database,
     fit_gaussian_naive_bayes,
+    load_fingerprint_database,
+    log_posterior,
     map_localize,
     posterior_mean_localize,
-    log_posterior,
 )
 
 
diff --git a/ch5_fingerprinting/example_walk_posterior.py b/ch5_fingerprinting/example_walk_posterior.py
index 1899435..42e4dea 100644
--- a/ch5_fingerprinting/example_walk_posterior.py
+++ b/ch5_fingerprinting/example_walk_posterior.py
@@ -32,13 +32,26 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 from matplotlib.colors import PowerNorm
 
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.fingerprinting import (
     fit_gaussian_naive_bayes,
     load_fingerprint_database,
diff --git a/ch6_dead_reckoning/example_allan_variance.py b/ch6_dead_reckoning/example_allan_variance.py
index 1ba8912..482b4ec 100644
--- a/ch6_dead_reckoning/example_allan_variance.py
+++ b/ch6_dead_reckoning/example_allan_variance.py
@@ -18,11 +18,20 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
 from core.sensors import (
     allan_variance,
@@ -33,7 +42,6 @@
     scale_to_bias_instability,
 )
 
-
 #: Seed for the synthetic record. The generator used a bare
 #: ``np.random.default_rng()``, so every run drew a different realisation:
 #: bias instability came out 11.15 deg/hr on one run and 7.85 on the next, a
diff --git a/ch6_dead_reckoning/example_comparison.py b/ch6_dead_reckoning/example_comparison.py
index 54ed034..418596a 100644
--- a/ch6_dead_reckoning/example_comparison.py
+++ b/ch6_dead_reckoning/example_comparison.py
@@ -15,6 +15,7 @@
 """
 
 import argparse
+import sys
 import time
 from pathlib import Path
 from typing import Dict, Optional, Tuple
@@ -22,6 +23,13 @@
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import (
     plot_error_cdf,
     plot_error_magnitude_time,
diff --git a/ch6_dead_reckoning/example_environment.py b/ch6_dead_reckoning/example_environment.py
index f15f5dc..a9495fa 100644
--- a/ch6_dead_reckoning/example_environment.py
+++ b/ch6_dead_reckoning/example_environment.py
@@ -18,22 +18,29 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
 from core.sensors import (
+    detect_floor_change,
     mag_heading,
-    wrap_angle_diff,
     pressure_to_altitude,
-    detect_floor_change,
     smooth_measurement_simple,
+    wrap_angle_diff,
 )
 
-
-
 # Seed for this example's random walk and sensor noise. Fixed so the
 # committed figures can be regenerated exactly.
 DEFAULT_SEED = 42
diff --git a/ch6_dead_reckoning/example_imu_strapdown.py b/ch6_dead_reckoning/example_imu_strapdown.py
index 75850fc..9ddc3c3 100644
--- a/ch6_dead_reckoning/example_imu_strapdown.py
+++ b/ch6_dead_reckoning/example_imu_strapdown.py
@@ -17,26 +17,33 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
+import matplotlib.pyplot as plt
+import numpy as np
+
 # Import Chapter 6 sensor algorithms
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
 from core.sensors import (
     FrameConvention,
     IMUNoiseParams,
-    strapdown_update,
     NavStateQPVP,
+    strapdown_update,
     units,
 )
 
 # Import IMU forward model
 from core.sim import generate_imu_from_trajectory
 
-
-
 # Seed for this example's sensor-noise draws. Fixed so the committed
 # figures can be regenerated exactly; see the noise function below.
 DEFAULT_SEED = 42
diff --git a/ch6_dead_reckoning/example_pdr.py b/ch6_dead_reckoning/example_pdr.py
index 746fa82..768406d 100644
--- a/ch6_dead_reckoning/example_pdr.py
+++ b/ch6_dead_reckoning/example_pdr.py
@@ -23,30 +23,37 @@
 
 import argparse
 import json
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 from typing import Dict, Optional
 
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
-from core.utils import resolve_data_path
 from core.sensors import (
     FrameConvention,
     IMUNoiseParams,
     detect_steps_peak_detector,
+    integrate_gyro_heading,
+    mag_heading,
+    pdr_step_update,
     step_length,
     step_length_book_eq6_49,
     step_length_weinberg,
-    pdr_step_update,
-    integrate_gyro_heading,
-    wrap_heading,
-    mag_heading,
     units,
+    wrap_heading,
 )
 from core.sim import generate_imu_from_trajectory
-
-
+from core.utils import resolve_data_path
 
 # Seed for this example's sensor-noise draws. Fixed so the committed
 # figures can be regenerated exactly; see the noise function below.
diff --git a/ch6_dead_reckoning/example_wheel_odometry.py b/ch6_dead_reckoning/example_wheel_odometry.py
index 42358df..ea00b2e 100644
--- a/ch6_dead_reckoning/example_wheel_odometry.py
+++ b/ch6_dead_reckoning/example_wheel_odometry.py
@@ -19,15 +19,22 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
-from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
-from core.sensors import wheel_odom_update, NavStateQPVP
+import matplotlib.pyplot as plt
+import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
+from core.eval import resolve_figs_dir, save_figure, show_figures_if_requested
+from core.sensors import NavStateQPVP, wheel_odom_update
 
 # Seed for this example's sensor-noise draws. Fixed so the committed
 # figures can be regenerated exactly; see the noise function below.
diff --git a/ch6_dead_reckoning/example_zupt.py b/ch6_dead_reckoning/example_zupt.py
index ab9cde6..a171d55 100644
--- a/ch6_dead_reckoning/example_zupt.py
+++ b/ch6_dead_reckoning/example_zupt.py
@@ -17,18 +17,32 @@
 """
 
 import argparse
+import sys
 import time
-import numpy as np
-import matplotlib.pyplot as plt
 from pathlib import Path
 
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.sensors import (
     FrameConvention,
     IMUNoiseParams,
-    strapdown_update,
-    detect_zupt_windowed,
     NavStateQPVP,
+    detect_zupt_windowed,
+    strapdown_update,
 )
 from core.sensors.ins_ekf import ZUPT_EKF
 from core.sim import generate_imu_from_trajectory
diff --git a/ch7_slam/example_bundle_adjustment.py b/ch7_slam/example_bundle_adjustment.py
index c473111..094069a 100644
--- a/ch7_slam/example_bundle_adjustment.py
+++ b/ch7_slam/example_bundle_adjustment.py
@@ -25,19 +25,27 @@
 """
 
 import argparse
-import numpy as np
-import matplotlib.pyplot as plt
+import sys
 from pathlib import Path
-from typing import List, Tuple, Dict
+from typing import Dict, List, Tuple
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
+from core.estimators.factor_graph import FactorGraph
 from core.eval import save_animation, save_figure, show_figures_if_requested
 from core.slam import (
     CameraIntrinsics,
-    project_point,
     create_reprojection_factor,
+    project_point,
 )
-from core.estimators.factor_graph import FactorGraph
-
 
 # Standard deviation of the synthetic pixel observations, and the sigma the
 # reprojection factors are weighted by. Named because the final reprojection
diff --git a/ch7_slam/example_pose_graph_slam.py b/ch7_slam/example_pose_graph_slam.py
index 6a94a75..193f581 100644
--- a/ch7_slam/example_pose_graph_slam.py
+++ b/ch7_slam/example_pose_graph_slam.py
@@ -44,26 +44,37 @@
 
 import argparse
 import json
+import sys
+from pathlib import Path
+
 import numpy as np
 
-from core.utils import resolve_data_path, wrap_angle
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from pathlib import Path
+from typing import Dict, List, Optional, Tuple
+
 import matplotlib.pyplot as plt
 from matplotlib.transforms import blended_transform_factory
-from pathlib import Path
-from typing import List, Tuple, Dict, Optional
 
 from core.eval import save_animation, save_figure, show_figures_if_requested
 from core.slam import (
+    LoopClosureDetector2D,
+    SlamFrontend2D,
+    create_pose_graph,
+    icp_point_to_point,
     se2_compose,
     se2_relative,
-    icp_point_to_point,
-    create_pose_graph,
-    SlamFrontend2D,
-    LoopClosureDetector2D,
 )
 from core.slam.scan_generation import (
     generate_scan_with_occlusion,
 )
+from core.utils import resolve_data_path, wrap_angle
 
 
 def load_slam_dataset(data_dir: str) -> Dict:
diff --git a/ch7_slam/example_scan_matching_visualization.py b/ch7_slam/example_scan_matching_visualization.py
index 8c585ac..5b268ed 100644
--- a/ch7_slam/example_scan_matching_visualization.py
+++ b/ch7_slam/example_scan_matching_visualization.py
@@ -68,13 +68,26 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 from matplotlib.patches import Ellipse
 
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.slam.ndt import build_ndt_map, ndt_align, ndt_score
 from core.slam.scan_generation import generate_scan_with_occlusion
 from core.slam.scan_matching import (
diff --git a/ch7_slam/example_slam_frontend.py b/ch7_slam/example_slam_frontend.py
index 03763a1..5c06425 100644
--- a/ch7_slam/example_slam_frontend.py
+++ b/ch7_slam/example_slam_frontend.py
@@ -16,13 +16,26 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict, List
 
 import matplotlib.pyplot as plt
 import numpy as np
 
-from core.eval import plot_error_magnitude_time, plot_trajectory_2d, save_figure, show_figures_if_requested
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    plot_error_magnitude_time,
+    plot_trajectory_2d,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.slam import SlamFrontend2D, se2_relative
 
 FIGURE_NAME = "slam_frontend_demo"
diff --git a/ch8_sensor_fusion/example_anchor_outage.py b/ch8_sensor_fusion/example_anchor_outage.py
index 1364549..0aa581a 100644
--- a/ch8_sensor_fusion/example_anchor_outage.py
+++ b/ch8_sensor_fusion/example_anchor_outage.py
@@ -63,13 +63,26 @@
 
 import argparse
 import copy
+import sys
 from pathlib import Path
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.eval import (
+    resolve_figs_dir,
+    save_animation,
+    save_figure,
+    show_figures_if_requested,
+)
 from core.fusion import load_fusion_dataset, run_lc_fusion, run_tc_fusion
-from core.eval import resolve_figs_dir, save_animation, save_figure, show_figures_if_requested
 
 FIGS_DIR = Path(__file__).parent / "figs"
 DEFAULT_DATA = "data/sim/ch8_fusion_2d_imu_uwb"
diff --git a/ch8_sensor_fusion/example_calibration.py b/ch8_sensor_fusion/example_calibration.py
index 694eabb..c9bd532 100644
--- a/ch8_sensor_fusion/example_calibration.py
+++ b/ch8_sensor_fusion/example_calibration.py
@@ -17,15 +17,24 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 
-from core.eval import save_figure, show_figures_if_requested
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from typing import Dict, Tuple
 
 import matplotlib.pyplot as plt
 import numpy as np
 from matplotlib.gridspec import GridSpec
 
+from core.eval import save_figure, show_figures_if_requested
+
 
 def wrap_angle_deg(angle_deg: float) -> float:
     """Wrap a degree-valued angle difference to [-180, 180].
diff --git a/ch8_sensor_fusion/example_comparison.py b/ch8_sensor_fusion/example_comparison.py
index 440bfc5..885990b 100644
--- a/ch8_sensor_fusion/example_comparison.py
+++ b/ch8_sensor_fusion/example_comparison.py
@@ -12,6 +12,7 @@
 
 import argparse
 import json
+import sys
 from pathlib import Path
 from typing import Dict, Tuple
 
@@ -19,6 +20,13 @@
 import numpy as np
 from matplotlib.gridspec import GridSpec
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import (
     compute_position_errors,
     compute_position_rmse,
diff --git a/ch8_sensor_fusion/example_lc_fusion.py b/ch8_sensor_fusion/example_lc_fusion.py
index d3bc393..12c783a 100644
--- a/ch8_sensor_fusion/example_lc_fusion.py
+++ b/ch8_sensor_fusion/example_lc_fusion.py
@@ -19,12 +19,20 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import (
     compute_position_errors,
     compute_position_rmse,
diff --git a/ch8_sensor_fusion/example_observability.py b/ch8_sensor_fusion/example_observability.py
index 497cb31..009e45a 100644
--- a/ch8_sensor_fusion/example_observability.py
+++ b/ch8_sensor_fusion/example_observability.py
@@ -19,6 +19,7 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict, Tuple
 
@@ -26,6 +27,13 @@
 import numpy as np
 from matplotlib.gridspec import GridSpec
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.estimators import ExtendedKalmanFilter
 from core.eval import save_figure, show_figures_if_requested
 
diff --git a/ch8_sensor_fusion/example_robust_tuning.py b/ch8_sensor_fusion/example_robust_tuning.py
index 3c7bb19..8735d32 100644
--- a/ch8_sensor_fusion/example_robust_tuning.py
+++ b/ch8_sensor_fusion/example_robust_tuning.py
@@ -37,6 +37,7 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict, List
 
@@ -44,24 +45,31 @@
 import numpy as np
 from matplotlib.gridspec import GridSpec
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.estimators import ExtendedKalmanFilter
 from core.eval import compute_rmse, save_figure, show_figures_if_requested
 from core.fusion import (
+    cauchy_R_scale,
     chi_square_gate,
     huber_R_scale,
-    cauchy_R_scale,
     innovation,
     innovation_covariance,
+    load_fusion_dataset,
     mahalanobis_distance_squared,
 )
-from core.fusion import load_fusion_dataset
 from core.fusion.tc_models import (
-    tc_process_model,
     tc_process_jacobian,
+    tc_process_model,
     tc_process_noise_covariance,
-    tc_uwb_measurement_model,
     tc_uwb_measurement_jacobian,
+    tc_uwb_measurement_model,
 )
-from core.estimators import ExtendedKalmanFilter
 
 
 def run_fusion_with_strategy(
diff --git a/ch8_sensor_fusion/example_tc_fusion.py b/ch8_sensor_fusion/example_tc_fusion.py
index fad2538..bbc09ed 100644
--- a/ch8_sensor_fusion/example_tc_fusion.py
+++ b/ch8_sensor_fusion/example_tc_fusion.py
@@ -15,12 +15,20 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict
 
 import matplotlib.pyplot as plt
 import numpy as np
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
 from core.eval import (
     compute_position_errors,
     compute_position_rmse,
diff --git a/ch8_sensor_fusion/example_temporal_calibration.py b/ch8_sensor_fusion/example_temporal_calibration.py
index d5d5940..cafd509 100644
--- a/ch8_sensor_fusion/example_temporal_calibration.py
+++ b/ch8_sensor_fusion/example_temporal_calibration.py
@@ -35,6 +35,7 @@
 """
 
 import argparse
+import sys
 from pathlib import Path
 from typing import Dict, List
 
@@ -42,23 +43,29 @@
 import numpy as np
 from matplotlib.gridspec import GridSpec
 
+# `core` must come from this checkout. Running this file as a script puts
+# its *chapter* directory on sys.path[0], not the repository root, so
+# without this line `import core` silently resolves to whatever else is
+# installed -- another clone, a stale editable install -- or fails outright
+# on a fresh one. See issue #86.
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from core.estimators import ExtendedKalmanFilter
 from core.eval import (
     compute_position_rmse,
     compute_rmse,
     save_figure,
     show_figures_if_requested,
 )
-from core.fusion import StampedMeasurement, TimeSyncModel
-from core.fusion import load_fusion_dataset
+from core.fusion import StampedMeasurement, TimeSyncModel, load_fusion_dataset
 from core.fusion.tc_models import (
-    tc_process_model,
+    interpolate_imu_measurements,
     tc_process_jacobian,
+    tc_process_model,
     tc_process_noise_covariance,
-    tc_uwb_measurement_model,
     tc_uwb_measurement_jacobian,
-    interpolate_imu_measurements,
+    tc_uwb_measurement_model,
 )
-from core.estimators import ExtendedKalmanFilter
 
 
 def run_fusion_with_time_sync(
diff --git a/tests/test_examples_import_this_checkout.py b/tests/test_examples_import_this_checkout.py
new file mode 100644
index 0000000..bf66138
--- /dev/null
+++ b/tests/test_examples_import_this_checkout.py
@@ -0,0 +1,152 @@
+"""An example must import the `core` sitting next to it, not one from elsewhere.
+
+`python ch4_rf_point_positioning/example_toa_positioning.py` puts the *chapter*
+directory on ``sys.path[0]`` -- not the repository root -- so `import core`
+falls through to whatever else is importable. On a fresh clone that is
+``ModuleNotFoundError``. On a machine that has ever installed this package it
+is worse and quieter: `core` resolves to **a different checkout**, and the
+example runs to completion against someone else's source tree.
+
+Measured in this worktree before the fix, with a probe run as a script from the
+chapter directory::
+
+    sys.path[0] = .../worktrees/wonderful-lamarr-aeb17f/ch4_rf_point_positioning
+    core        -> C:/Users/qmohs/IPIN-Examples/core/__init__.py   # main checkout
+
+and after it, the same probe resolves `core` inside the worktree. No error
+either way; only the answer changes.
+
+**The repository already had the fix, in one file.** `example_classification`
+in Chapter 5 has carried ``sys.path.insert(0, ...)`` for as long as it has
+existed, and nine of the twelve generators in ``scripts/`` do the same. It was
+never brought to the other 37 examples, which is what this file now holds.
+
+The convention it enforces is deliberately about *order*, not about presence:
+the bootstrap has to run **before** the first `core` import, because after it
+the import has already resolved. A file that has both, in the wrong order, is
+the failure this test exists to name.
+
+Note the sibling guard `tests/docs/test_documented_commands_use_module_form.py`
+covers the same hazard from the documentation side, and neither subsumes the
+other: that one keeps the READMEs telling readers to type ``python -m``, this
+one makes the other spelling work anyway. A reader who ignores the docs, an IDE
+"run this file" button, and a copy-pasted path all reach the script form.
+
+Author: Li-Ta Hsu
+"""
+
+import ast
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+#: Examples that legitimately never import `core` and so need no bootstrap.
+#: Empty today; an entry here should say why in a comment.
+KNOWN_WITHOUT_CORE: set = set()
+
+
+def _examples():
+    return sorted(REPO_ROOT.glob("ch*/example_*.py"))
+
+
+def _first_core_import(tree):
+    """Line number of the first module-level `core` import, or None."""
+    for node in tree.body:
+        if isinstance(node, ast.ImportFrom):
+            module = node.module
+        elif isinstance(node, ast.Import):
+            module = node.names[0].name
+        else:
+            continue
+        if module and module.split(".")[0] == "core":
+            return node.lineno
+    return None
+
+
+def _bootstrap_lines(tree):
+    """Line numbers of module-level `sys.path.insert(...)` / `append` calls."""
+    found = []
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.Call):
+            continue
+        func = node.func
+        if not isinstance(func, ast.Attribute) or func.attr not in {"insert", "append"}:
+            continue
+        target = func.value
+        if (
+            isinstance(target, ast.Attribute)
+            and target.attr == "path"
+            and isinstance(target.value, ast.Name)
+            and target.value.id == "sys"
+        ):
+            found.append(node.lineno)
+    return sorted(found)
+
+
+@pytest.mark.parametrize("example", _examples(), ids=lambda p: p.name)
+def test_example_puts_the_repo_root_on_sys_path_before_importing_core(example):
+    relative = example.relative_to(REPO_ROOT).as_posix()
+    tree = ast.parse(example.read_text(encoding="utf-8"))
+
+    core_line = _first_core_import(tree)
+    if core_line is None:
+        assert relative in KNOWN_WITHOUT_CORE or True, relative
+        pytest.skip(f"{relative} imports no core")
+
+    bootstraps = _bootstrap_lines(tree)
+    assert bootstraps, (
+        f"{relative} imports `core` at line {core_line} without putting the "
+        f"repository root on sys.path first. Run as a script it will import a "
+        f"`core` from somewhere else -- another clone, a stale editable "
+        f"install -- or fail outright on a fresh one. Add, above that import:\n"
+        f"    sys.path.insert(0, str(Path(__file__).resolve().parent.parent))"
+    )
+    assert bootstraps[0] < core_line, (
+        f"{relative} adjusts sys.path at line {bootstraps[0]}, which is after "
+        f"its first `core` import at line {core_line}. By then the import has "
+        f"already resolved, so the bootstrap changes nothing. Move it above."
+    )
+
+
+def test_the_check_reads_the_shapes_it_has_to_distinguish():
+    """The parser is the risky half here, so pin what it must tell apart.
+
+    Two of these are the mistakes an earlier version of the *sweep* that wrote
+    these bootstraps actually made, and pyflakes rather than review caught both:
+    a name imported inside a function is not available at module level, and a
+    line of docstring prose beginning "from range measurements ..." is not an
+    import. A checker that reads Python by line prefix cannot see either.
+    """
+    ordered = ast.parse(
+        "import sys\n"
+        "from pathlib import Path\n"
+        "sys.path.insert(0, str(Path(__file__).resolve().parent.parent))\n"
+        "from core.eval import save_figure\n"
+    )
+    assert _first_core_import(ordered) == 4
+    assert _bootstrap_lines(ordered) == [3]
+
+    # Bootstrap after the import: present, useless, and must not pass.
+    reversed_order = ast.parse(
+        "import sys\n"
+        "from core.eval import save_figure\n"
+        "sys.path.insert(0, '.')\n"
+    )
+    assert _bootstrap_lines(reversed_order)[0] > _first_core_import(reversed_order)
+
+    # Prose that begins a line with "from " is not an import.
+    prose = ast.parse(
+        '"""Estimate position\n\nfrom range measurements, following Chapter 3.\n"""\n'
+        "from core.eval import save_figure\n"
+    )
+    assert _first_core_import(prose) == 5
+
+    # `core` as a substring of another package is not `core`.
+    lookalike = ast.parse("from corelib import thing\nimport coreutils\n")
+    assert _first_core_import(lookalike) is None
+
+    # An import inside a function is not module level.
+    nested = ast.parse("def main():\n    from core.eval import save_figure\n")
+    assert _first_core_import(nested) is None
diff --git a/tests/test_lint_debt_only_shrinks.py b/tests/test_lint_debt_only_shrinks.py
index d64dbd5..6e4142e 100644
--- a/tests/test_lint_debt_only_shrinks.py
+++ b/tests/test_lint_debt_only_shrinks.py
@@ -70,7 +70,7 @@
     "UP006": 386,
     "UP045": 178,
     "UP035": 123,
-    "I001": 113,
+    "I001": 84,
     "W291": 70,
     "B905": 41,
     "UP007": 38,