diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index da712d6e2a..62936ad45d 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -380,8 +380,8 @@ def array_summary(data, n_max, n_edge, linewidth, precision):
                 # or "years") cannot be converted to a date using
                 # `num2date`, so gracefully fall back to printing
                 # values as numbers.
-                if not self.units.is_long_time_interval():
-                    # Otherwise ... replace all with strings.
+                try:
+                    # Replace all with strings.
                     if ma.is_masked(data):
                         mask = data.mask
                     else:
@@ -391,6 +391,10 @@ def array_summary(data, n_max, n_edge, linewidth, precision):
                     # Masked datapoints do not survive num2date.
                     if mask is not None:
                         data = np.ma.masked_array(data, mask)
+                except (ValueError, TypeError):
+                    # Fall back to numeric values if num2date fails
+                    # (e.g., for long time intervals like months/years)
+                    pass
 
             if ma.is_masked(data):
                 # Masks are not handled by np.array2string, whereas
diff --git a/lib/iris/experimental/raster.py b/lib/iris/experimental/raster.py
index 0b5057136c..7c387d58c9 100644
--- a/lib/iris/experimental/raster.py
+++ b/lib/iris/experimental/raster.py
@@ -208,3 +208,4 @@ def export_geotiff(cube, fname):
     x_min = np.min(x_bounds)
     y_max = np.max(coord_y.bounds)
     _gdal_write_array(x_min, x_step, y_max, y_step, coord_system, data, fname, "GTiff")
+    
\ No newline at end of file
diff --git a/lib/iris/tests/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py b/lib/iris/tests/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
index 216c3383c8..11f0242786 100644
--- a/lib/iris/tests/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
+++ b/lib/iris/tests/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
@@ -13,6 +13,12 @@
 import pytest
 
 import iris.analysis._interpolation
+# Suppress deprecation warnings for experimental.regrid, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.regrid.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.regrid import (
     regrid_area_weighted_rectilinear_src_and_grid as regrid_area_weighted,
 )
diff --git a/lib/iris/tests/graphics/__init__.py b/lib/iris/tests/graphics/__init__.py
index 2c9fc0b345..04c25131fb 100644
--- a/lib/iris/tests/graphics/__init__.py
+++ b/lib/iris/tests/graphics/__init__.py
@@ -285,7 +285,7 @@ def skip_plot(fn: Callable) -> Callable:
     ...     pass
 
     """
-    skip = pytest.mark.skipIf(
+    skip = pytest.mark.skipif(
         condition=not MPL_AVAILABLE,
         reason="Graphics tests require the matplotlib library.",
     )
diff --git a/lib/iris/tests/integration/experimental/regrid/test_regrid_conservative_via_esmpy.py b/lib/iris/tests/integration/experimental/regrid/test_regrid_conservative_via_esmpy.py
index e6d209c691..aa3483b10c 100644
--- a/lib/iris/tests/integration/experimental/regrid/test_regrid_conservative_via_esmpy.py
+++ b/lib/iris/tests/integration/experimental/regrid/test_regrid_conservative_via_esmpy.py
@@ -10,6 +10,7 @@
 import numpy as np
 import pytest
 
+from iris._deprecation import IrisDeprecation
 from iris.tests import _shared_utils
 
 # Import ESMF if installed, else fail quietly + disable all the tests.
@@ -24,6 +25,13 @@
 import iris
 import iris.analysis
 import iris.analysis.cartography as i_cartog
+
+# The following functions are deprecated but testing their behavior is the purpose
+# of this module.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*regrid_conservative_via_esmpy.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.regrid_conservative import regrid_conservative_via_esmpy
 import iris.tests.stock as istk
 
diff --git a/lib/iris/tests/integration/experimental/test_regrid_ProjectedUnstructured.py b/lib/iris/tests/integration/experimental/test_regrid_ProjectedUnstructured.py
index 85f10a0c87..5e5ddae90b 100644
--- a/lib/iris/tests/integration/experimental/test_regrid_ProjectedUnstructured.py
+++ b/lib/iris/tests/integration/experimental/test_regrid_ProjectedUnstructured.py
@@ -12,6 +12,12 @@
 import iris
 import iris.aux_factory
 from iris.coord_systems import GeogCS
+# Suppress deprecation warnings for experimental.regrid, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.regrid.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.regrid import (
     ProjectedUnstructuredLinear,
     ProjectedUnstructuredNearest,
diff --git a/lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py b/lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py
index faf57d0e65..78c8f5fb39 100644
--- a/lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py
+++ b/lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py
@@ -213,7 +213,7 @@ def chunkspecs(points: _Chunkspec, bounds: _Chunkspec) -> Tuple[_Chunks, _Chunks
 
 
 class Test_rechunk:
-    class TestAuxFact(AuxCoordFactory):
+    class AuxFactImpl(AuxCoordFactory):
         """A minimal AuxCoordFactory that enables us to test the re-chunking logic."""
 
         def __init__(self, nx, ny, nz):
@@ -267,7 +267,7 @@ def test_rechunk(self, nz, deptypes):
         #   (10, 10, 10) = 1,000: ok
         #   (10, 10, 100) = 10,000 --> (5, 10, 100) = 5000 --> rechunk, dividing X by 2
         #   (10, 10, 1000) = 100,000 --> (1, 5, 1000) --> rechunk both X and Y
-        aux_co = self.TestAuxFact(nx, ny, nz)
+        aux_co = self.AuxFactImpl(nx, ny, nz)
 
         if deptypes != "all_lazy":
             # Touch all dependencies to realise
@@ -299,7 +299,7 @@ def test_rechunk(self, nz, deptypes):
         }[nz]
         assert result_pts_bds_chunks == expected_pts_bds_chunks
 
-    class MultiDimTestFactory(TestAuxFact):
+    class MultiDimTestFactory(AuxFactImpl):
         """An extended test factory with an added multidimensional term."""
 
         # Use fixed test dimensions, for simplicity.
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index d91c7e81c0..e71ec1e499 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -69,7 +69,12 @@ def test_masked_no_mask(self):
     def test_matrix(self):
         # Subclasses of np.ndarray should be coerced back to np.ndarray.
         # (Except for np.ma.MaskedArray.)
-        data = np.matrix([[1, 2, 3], [4, 5, 6]])
+        # Create a custom ndarray subclass to replace deprecated np.matrix.
+        class NdArraySubclass(np.ndarray):
+            pass
+        
+        data_base = np.array([[1, 2, 3], [4, 5, 6]])
+        data = data_base.view(NdArraySubclass)
         cube = Cube(data)
         assert type(cube.data) is np.ndarray
         _shared_utils.assert_array_equal(cube.data, data)
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 2471b00316..b52fa0527c 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -332,7 +332,7 @@ def test_with_lazy_mask_array__not_masked(self):
         assert dm.has_lazy_data()
         result = dm.data
         assert not dm.has_lazy_data()
-        assert isinstance(result, np.core.ndarray)
+        assert isinstance(result, np.ndarray)
         assert dm.dtype == self.dtype
         assert result.fill_value == self.fill_value
         _shared_utils.assert_array_equal(result, self.real_array)
@@ -543,11 +543,15 @@ def test_coerce_to_ndarray(self):
         shape = (2, 3)
         size = np.prod(shape)
         real_array = np.arange(size).reshape(shape)
-        matrix = np.matrix(real_array)
+        # Create a custom ndarray subclass to replace deprecated np.matrix.
+        class NdArraySubclass(np.ndarray):
+            pass
+        
+        matrix = real_array.view(NdArraySubclass)
         dm = DataManager(real_array)
         dm.data = matrix
-        assert isinstance(dm._real_array, np.core.ndarray)
-        assert isinstance(dm.data, np.core.ndarray)
+        assert isinstance(dm._real_array, np.ndarray)
+        assert isinstance(dm.data, np.ndarray)
         _shared_utils.assert_array_equal(dm.data, real_array)
 
     def test_real_masked_constant_to_array(self):
diff --git a/lib/iris/tests/unit/experimental/raster/test_export_geotiff.py b/lib/iris/tests/unit/experimental/raster/test_export_geotiff.py
index e1c76f50de..1b950580e6 100644
--- a/lib/iris/tests/unit/experimental/raster/test_export_geotiff.py
+++ b/lib/iris/tests/unit/experimental/raster/test_export_geotiff.py
@@ -8,6 +8,12 @@
 
 import numpy as np
 import pytest
+# Suppress deprecation warnings for experimental.raster, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.raster.*:iris._deprecation.IrisDeprecation"
+)
+
 
 try:
     from osgeo import gdal
diff --git a/lib/iris/tests/unit/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py b/lib/iris/tests/unit/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
index 630781d2ef..2dfdf05d5a 100644
--- a/lib/iris/tests/unit/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
+++ b/lib/iris/tests/unit/experimental/regrid/test_regrid_area_weighted_rectilinear_src_and_grid.py
@@ -14,6 +14,12 @@
 from iris.coord_systems import GeogCS
 from iris.coords import DimCoord
 from iris.cube import Cube
+# Suppress deprecation warnings for experimental.regrid, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.regrid.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.regrid import (
     regrid_area_weighted_rectilinear_src_and_grid as regrid,
 )
diff --git a/lib/iris/tests/unit/experimental/regrid/test_regrid_weighted_curvilinear_to_rectilinear.py b/lib/iris/tests/unit/experimental/regrid/test_regrid_weighted_curvilinear_to_rectilinear.py
index bb5d5a274d..3dbb772aba 100644
--- a/lib/iris/tests/unit/experimental/regrid/test_regrid_weighted_curvilinear_to_rectilinear.py
+++ b/lib/iris/tests/unit/experimental/regrid/test_regrid_weighted_curvilinear_to_rectilinear.py
@@ -18,6 +18,13 @@
 import iris.coords
 from iris.coords import AuxCoord, DimCoord
 import iris.cube
+
+# Suppress deprecation warnings for experimental.regrid, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.regrid.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.regrid import (
     regrid_weighted_curvilinear_to_rectilinear as regrid,
 )
diff --git a/lib/iris/tests/unit/experimental/ugrid/test_ParseUgridOnLoad.py b/lib/iris/tests/unit/experimental/ugrid/test_ParseUgridOnLoad.py
index 62961157d8..775f563028 100644
--- a/lib/iris/tests/unit/experimental/ugrid/test_ParseUgridOnLoad.py
+++ b/lib/iris/tests/unit/experimental/ugrid/test_ParseUgridOnLoad.py
@@ -11,6 +11,12 @@
 import pytest
 
 from iris._deprecation import IrisDeprecation
+# Suppress deprecation warnings for experimental.ugrid, which is the subject
+# of these tests.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:.*experimental.ugrid.*:iris._deprecation.IrisDeprecation"
+)
+
 from iris.experimental.ugrid import PARSE_UGRID_ON_LOAD, ParseUGridOnLoad
 
 
diff --git a/lib/iris/tests/unit/fileformats/ff/test_FF2PP.py b/lib/iris/tests/unit/fileformats/ff/test_FF2PP.py
index e1cd1f5912..6ac1b7816d 100644
--- a/lib/iris/tests/unit/fileformats/ff/test_FF2PP.py
+++ b/lib/iris/tests/unit/fileformats/ff/test_FF2PP.py
@@ -6,6 +6,7 @@
 
 import collections
 import contextlib
+from unittest.mock import patch
 
 import numpy as np
 import pytest
@@ -72,7 +73,7 @@ def mock_for_extract_field(self, fields, x=None, y=None):
         the "make_pp_field" call.
 
         """
-        with self.mocker.patch("iris.fileformats._ff.FFHeader"):
+        with patch("iris.fileformats._ff.FFHeader"):
             ff2pp = ff.FF2PP("mock")
         ff2pp._ff_header.lookup_table = [0, 0, len(fields)]
         # Fake level constants, with shape specifying just one model-level.
@@ -83,13 +84,13 @@ def mock_for_extract_field(self, fields, x=None, y=None):
 
         open_func = "builtins.open"
         with (
-            self.mocker.patch(
+            patch(
                 "iris.fileformats._ff._parse_binary_stream", return_value=[0]
             ),
-            self.mocker.patch(open_func),
-            self.mocker.patch("struct.unpack_from", return_value=[4]),
-            self.mocker.patch("iris.fileformats.pp.make_pp_field", side_effect=fields),
-            self.mocker.patch(
+            patch(open_func),
+            patch("struct.unpack_from", return_value=[4]),
+            patch("iris.fileformats.pp.make_pp_field", side_effect=fields),
+            patch(
                 "iris.fileformats._ff.FF2PP._payload", return_value=(0, 0)
             ),
         ):
@@ -217,7 +218,7 @@ def _setup(self, mocker):
         field.boundary_packing = None
 
     def _test(self, mock_field, expected_depth, expected_dtype, word_depth=None):
-        with self.mocker.patch("iris.fileformats._ff.FFHeader", return_value=None):
+        with patch("iris.fileformats._ff.FFHeader", return_value=None):
             kwargs = {}
             if word_depth is not None:
                 kwargs["word_depth"] = word_depth
diff --git a/plan.md b/plan.md
new file mode 100644
index 0000000000..e815212e2d
--- /dev/null
+++ b/plan.md
@@ -0,0 +1,136 @@
+# Agentic Plan for Delivery
+
+## Goal
+Enable pytest to fail on problem warnings (particularly `DeprecationWarning`),
+making it easy to detect newly-introduced issues. Achieve this by:
+1. Classifying all warnings currently in the log
+2. Fixing addressable warnings in Iris source/test code
+3. Asserting expected warnings inside tests (not leaking)
+4. Suppressing unavoidable external warnings
+5. Promoting DeprecationWarning (and related) to errors in pyproject.toml
+
+---
+
+## Warning Inventory (from pytest_warnings.log, Jun 2026)
+
+### A. IrisDeprecation — Iris-owned, still used in tests (MUST FIX)
+- `GraphicsTestMixin`, `IrisTest`, `GraphicsTest`, `PPTest` classes deprecated (tests/__init__.py)
+- `iris.experimental.regrid` / `regrid_conservative` deprecated since v3.2 (used in test_regrid_* tests)
+- `iris.experimental.raster` deprecated since v3.2
+- `iris.experimental.ugrid` deprecated → `iris.mesh`
+- `iris.fileformats.abf` / `iris.fileformats.dot` deprecated
+- `env_bin_path` deprecated
+- `iris.fileformats.netcdf.saver` legacy attribute mode deprecated since v3.8
+- `iris.analysis.maths.intersection_of_cubes` deprecated
+- `iris.coord_systems.RotatedMercator` deprecated → `ObliqueMercator`
+- Various `regrid_weighted_curvilinear_to_rectilinear`, `ProjectedUnstructuredNearest/Linear` deprecated
+
+### B. DeprecationWarning — Iris test code (MUST FIX)
+- `np.core.ndarray` → `np.ndarray` in test_DataManager.py (lines 335, 549, 550)
+- Replace deprecated `np.matrix` coercion tests with a custom `np.ndarray` subclass instance (created via `.view(Subclass)`), preserving the original intent: verify ndarray-subclass inputs are coerced back to plain `np.ndarray`
+- `Unit.is_long_time_interval()` path is deprecated in cf-units; align Iris with cf-units PR #279 by removing this pre-check and using `num2date` with try-except fallback in coords.py
+
+### C. Pytest infrastructure warnings (MUST FIX)
+- `PytestUnknownMarkWarning`: `pytest.mark.skipIf` (capital I) → `pytest.mark.skipif` in tests/graphics/__init__.py:288
+- `PytestCollectionWarning`: `TestAuxFact` class with `__init__` in test_AuxCoordFactory.py:216
+- `PytestMockWarning`: mocker.patch used as context manager in test_FF2PP.py (lines 75, 86, 89, 90, 91, 92, 220)
+
+### D. IrisUserWarning / Iris domain warnings — intentionally triggered (ASSERT IN TESTS)
+These are raised by Iris logic under test; tests should explicitly assert/capture them:
+- `IrisUserWarning`: collapsing spatial coord without weighting, CRS mismatch, concatenate warnings
+- `IrisLoadWarning`, `IrisSaveWarning`, `IrisCfMissingVarWarning`, `IrisCfNonSpanningVarWarning`
+- `IrisCfLabelVarWarning`, `IrisGuessBoundsWarning`, `IrisVagueMetadataWarning`
+- `IrisDefaultingWarning`, `IrisIgnoringBoundsWarning`, `IrisPpClimModifiedWarning`
+- `IrisNimrodTranslationWarning`, `IrisGeometryExceedWarning`
+- `_WarnComboDefaultingLoad`, `_WarnComboIgnoringCfLoad`, `_WarnComboLoadIgnoring`
+
+### E. External/third-party warnings (SUPPRESS in pyproject.toml)
+- `DeprecationWarning` from `distributed/client.py` (dask large graph)
+- `DeprecationWarning` from `osgeo/gdal.py` (GDAL NumPy scalar conversion)
+- `DeprecationWarning` from `numpy.ma.extras` / `numpy._core` (NumPy internal changes)
+- `UserWarning` from `cartopy/crs.py` (Orthographic/NearsidePerspective elliptical globes)
+- `UserWarning` from `distributed/client.py` (large dask graph size warning)
+- `UserWarning` from `numpy.ma.core` (masked element to nan)
+- `RuntimeWarning` from `numpy.ma.core`/`numpy._core` (invalid value in cast — in numpy internals)
+- `RuntimeWarning` from `matplotlib/collections.py` (invalid value in sqrt)
+
+### F. RuntimeWarning — Iris/test code (ASSESS)
+- Divide by zero in test_divide.py/maths.py (intentional, test should assert)
+- `invalid value encountered in cast` in analysis/_regrid.py:818 (may need investigation)
+
+---
+
+## PR Split
+1. PR 1 covers Phases 1-3: pytest infrastructure fixes, direct deprecation fixes in Iris/tests, and migration off deprecated Iris test-framework utilities.
+2. PR 2 covers Phases 4-5: assert expected domain warnings and then tighten warning policy/filtering in pytest configuration.
+3. PR 2 depends on PR 1 merging first, to avoid large noise from unresolved deprecations and to keep warning-policy changes reviewable.
+4. Merge gate between PRs: warning count from PR 1 branch must be stable and attributable, so PR 2 only changes behavior by policy, not by hidden code migrations.
+
+## Handoff Protocol
+1. Scope lock per run: before any agent changes, explicitly choose the active target (`PR 1` or `PR 2`) and confirm no cross-PR work is allowed in that run.
+2. Plan re-read at session start: the agent must re-read this plan and state the active scope before editing.
+3. Drift check before edits: verify branch state, current warning baseline, and plan changes since last run; if any drift exists, record a delta note first.
+4. PR contract boundaries:
+   - `PR 1` only: Phases 1-3 (infrastructure/deprecation fixes + migration off deprecated test-framework utilities).
+   - `PR 2` only: Phases 4-5 (expected-warning assertions + pytest warning-policy tightening).
+5. No scope bleed: out-of-scope findings are logged as deferred follow-ups, not implemented in the current PR.
+6. Execution log continuity: keep a dated session log of completed items, deferred items, blockers, and decisions to bridge multi-day gaps.
+7. Re-entry rule after gaps: first action is always plan reload + baseline warning check, then continue implementation.
+8. Human review checklist mirrors plan phases: reviewer verifies each changed file matches the active PR scope and rejects unrelated "bonus" changes.
+
+## Phased Implementation Plan
+
+### Phase 1: Fix pytest infrastructure warnings (no functional changes, quick wins)
+1. Fix `pytest.mark.skipIf` → `pytest.mark.skipif` in `lib/iris/tests/graphics/__init__.py:288`
+2. Fix `TestAuxFact` class `__init__` in `lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py:216`
+3. Fix mocker.patch context manager usage in `lib/iris/tests/unit/fileformats/ff/test_FF2PP.py` (lines 75, 86, 89-92, 220)
+
+### Phase 2: Fix DeprecationWarnings in test code
+4. Replace `np.core.ndarray` with `np.ndarray` in `lib/iris/tests/unit/data_manager/test_DataManager.py` (lines 335, 549, 550)
+5. Replace deprecated `np.matrix` usages in `lib/iris/tests/unit/cube/test_Cube.py:72` and `lib/iris/tests/unit/data_manager/test_DataManager.py:546` with a purpose-built `np.ndarray` subclass input (for example `arr.view(Subclass)`), then assert output type is plain `np.ndarray` to preserve coercion semantics
+6. Replace deprecated `is_long_time_interval()` guard in `lib/iris/coords.py:383` with a `num2date` attempt wrapped in `try/except` so invalid long-interval units fall back to numeric formatting (matching cf-units PR #279 guidance)
+
+### Phase 3: Remove deprecated test-framework usage and selectively assert remaining deprecations
+For each group of tests that exercise deprecated APIs:
+1. Prefer code migration/removal of deprecated usage; only use `pytest.warns(IrisDeprecation)` when the deprecated API itself is the behavior under test
+   - `test_regrid_area_weighted_rectilinear_src_and_grid.py` (regrid deprecated)
+   - `test_regrid_conservative_via_esmpy.py` (regrid_conservative deprecated)
+   - `test_raster.py` / `test_export_geotiff.py` (raster deprecated)
+   - `test_regrid_ProjectedUnstructured.py` (ProjectedUnstructured deprecated)
+   - `test_regrid_weighted_curvilinear_to_rectilinear.py`
+   - `test_RotatedMercator.py`
+   - Tests using IrisTest/GraphicsTestMixin/PPTest/GraphicsTest/env_bin_path
+   - `test_intersect.py` (intersection_of_cubes)
+   - tests using netcdf legacy save mode
+   - tests using iris.experimental.ugrid
+
+### Phase 4: Assert expected Iris domain warnings in tests (Category D)
+8. Audit tests for each Iris warning class and add `pytest.warns()` assertions where warnings are expected but not currently being tested for
+   - Highest priority: tests that use `IrisUserWarning`, `IrisLoadWarning`, `IrisSaveWarning`
+   - Secondary: vague metadata, guess bounds, ignore bounds, etc.
+
+### Phase 5: Configure pyproject.toml filterwarnings
+9. Update `pyproject.toml` `[tool.pytest.ini_options]` to:
+   - Promote `DeprecationWarning` (and `PendingDeprecationWarning`) to errors: `"error::DeprecationWarning"`, `"error::PendingDeprecationWarning"`
+   - Add targeted ignores for unavoidable external warnings (Category E above)
+   - Keep `"default"` as a fallback or replace with `"error"` for Iris-specific categories
+
+---
+
+## Key Files
+- `pyproject.toml` — `[tool.pytest.ini_options].filterwarnings`
+- `lib/iris/tests/graphics/__init__.py` — skipIf typo (line 288)
+- `lib/iris/tests/unit/aux_factory/test_AuxCoordFactory.py` — TestAuxFact __init__ (line 216)
+- `lib/iris/tests/unit/fileformats/ff/test_FF2PP.py` — PytestMockWarning (lines 75, 86, 89-92, 220)
+- `lib/iris/tests/unit/data_manager/test_DataManager.py` — np.core (lines 335, 549, 550), ndarray-subclass coercion test (line 546)
+- `lib/iris/tests/unit/cube/test_Cube.py` — ndarray-subclass coercion test (line 72)
+- `lib/iris/coords.py` — cftime.is_long_time_interval (line 383)
+- All experimental regrid/raster/ugrid test files
+
+## Verification
+1. PR 1 verification: run `pytest lib/iris --tb=no -q` and confirm infrastructure/deprecation-warning reductions from Phases 1-3 are real and expected.
+2. PR 1 verification: confirm deprecated Iris test-framework utilities (IrisTest/GraphicsTestMixin/PPTest/GraphicsTest/env_bin_path) are migrated where in scope, not merely warning-wrapped.
+3. PR 2 verification: run targeted warning-sensitive suites first, then full `pytest lib/iris --tb=no -q`, and confirm Phase 4 assertions pass without leaking expected warnings.
+4. PR 2 verification: confirm warning policy in pytest config causes non-zero exit for newly introduced `DeprecationWarning`/`PendingDeprecationWarning`, with only explicitly approved external suppressions.
+5. Final verification: no legitimate regression failures introduced by warning-policy tightening.
+