diff --git a/changelog/14523.improvement.rst b/changelog/14523.improvement.rst
new file mode 100644
index 00000000000..f809c6803a6
--- /dev/null
+++ b/changelog/14523.improvement.rst
@@ -0,0 +1,4 @@
+Large assertion comparison diffs are now built lazily and capped to the
+truncation budget, so a huge diff is no longer formatted in full just to
+be truncated. As a result the truncation footer no longer
+reports the exact number of hidden lines.
diff --git a/doc/en/example/reportingdemo.rst b/doc/en/example/reportingdemo.rst
index 1eef1322927..65e99a14a1f 100644
--- a/doc/en/example/reportingdemo.rst
+++ b/doc/en/example/reportingdemo.rst
@@ -148,7 +148,7 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     E           1
     E           1...
     E
-    E         ...Full output truncated (7 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     failure_demo.py:62: AssertionError
     _________________ TestSpecialisedExplanations.test_eq_list _________________
diff --git a/doc/en/how-to/output.rst b/doc/en/how-to/output.rst
index db36a5a7206..752d0206526 100644
--- a/doc/en/how-to/output.rst
+++ b/doc/en/how-to/output.rst
@@ -172,7 +172,7 @@ Now we can increase pytest's verbosity:
     E               'banana',
     E               'apple',...
     E
-    E         ...Full output truncated (7 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     test_verbosity_example.py:8: AssertionError
     ____________________________ test_numbers_fail _____________________________
@@ -190,7 +190,7 @@ Now we can increase pytest's verbosity:
     E         {'10': 10, '20': 20, '30': 30, '40': 40}
     E         ...
     E
-    E         ...Full output truncated (16 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     test_verbosity_example.py:14: AssertionError
     ___________________________ test_long_text_fail ____________________________
diff --git a/src/_pytest/assertion/__init__.py b/src/_pytest/assertion/__init__.py
index e33f8b29609..82d412e7750 100644
--- a/src/_pytest/assertion/__init__.py
+++ b/src/_pytest/assertion/__init__.py
@@ -12,6 +12,8 @@
 from _pytest.assertion import rewrite
 from _pytest.assertion import truncate
 from _pytest.assertion import util
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.assertion.rewrite import assertstate_key
 from _pytest.config import Config
 from _pytest.config import hookimpl
@@ -181,13 +183,21 @@ def callbinrepr(op, left: object, right: object) -> str | None:
             config=item.config, op=op, left=left, right=right
         )
         for new_expl in hook_result:
+            # Plugin-supplied lists are truncated here; the built-in impl
+            # already truncates as it streams, so re-applying truncation
+            # to its output is a near no-op (the body fits the budget,
+            # only the footer line is re-emitted with the same wording).
+            # ``materialize_with_truncation`` can return ``[]`` when the
+            # input was a truthy-but-empty iterable, so re-check after
+            # materialising.
             if new_expl:
-                new_expl = truncate.truncate_if_required(new_expl, item)
-                new_expl = [line.replace("\n", "\\n") for line in new_expl]
-                res = "\n~".join(new_expl)
-                if item.config.getvalue("assertmode") == "rewrite":
-                    res = res.replace("%", "%%")
-                return res
+                new_expl = truncate.materialize_with_truncation(new_expl, item.config)
+                if new_expl:
+                    new_expl = [line.replace("\n", "\\n") for line in new_expl]
+                    res = "\n~".join(new_expl)
+                    if item.config.getvalue("assertmode") == "rewrite":
+                        res = res.replace("%", "%%")
+                    return res
         return None
 
     saved_assert_hooks = util._reprcompare, util._assertion_pass
@@ -218,19 +228,51 @@ def pytest_sessionfinish(session: Session) -> None:
 def pytest_assertrepr_compare(
     config: Config, op: str, left: Any, right: Any
 ) -> list[str] | None:
+    """Return an explanation for ``left op right``.
+
+    Internally ``util.assertrepr_compare`` is a generator; we feed it
+    through ``materialize_with_truncation`` so a huge comparison
+    short-circuits at the truncation threshold without building the
+    full diff, while still returning the ``list[str] | None`` shape
+    the hook spec advertises.
+    """
     if config.pluginmanager.has_plugin("terminalreporter"):
         highlighter = config.get_terminal_writer()._highlight
     else:
         # Keep it plaintext when not using terminalrepoterer (#14377).
         highlighter = util.dummy_highlighter
-    explanation = list(
-        util.assertrepr_compare(
-            op=op,
-            left=left,
-            right=right,
-            verbose=config.get_verbosity(Config.VERBOSITY_ASSERTIONS),
-            highlighter=highlighter,
-            assertion_text_diff_style=util.get_assertion_text_diff_style(config),
+    # When truncation is going to clip the explanation downstream, tell the
+    # comparison helpers to cap their pformat output at the same budget so they
+    # don't spend O(N) formatting lines/chars we're about to drop. The cap is
+    # ``(max_lines, max_chars)`` per side, matching what the truncator will
+    # actually pull (the raw limit plus the footer slack — see
+    # ``truncate.TRUNCATION_FOOTER_LINES`` / ``TRUNCATION_FOOTER_CHARS``), so a
+    # side is never under-formatted.
+    #
+    # ``difflib.ndiff`` over two K-line/char pformat outputs produces at least
+    # K output lines/chars (more when the sides differ), and the truncator
+    # pulls at most that much, so a per-side budget covers the worst case. A
+    # dimension whose limit is 0 (disabled) stays ``None`` so it isn't bounded;
+    # with truncation off both stay ``None`` and the user gets the full diff.
+    should_truncate, trunc_lines, trunc_chars = truncate._get_truncation_parameters(
+        config
+    )
+    if should_truncate:
+        truncation_budget = TruncationBudget(
+            trunc_lines + truncate.TRUNCATION_FOOTER_LINES + 1
+            if trunc_lines > 0
+            else None,
+            trunc_chars + truncate.TRUNCATION_FOOTER_CHARS if trunc_chars > 0 else None,
         )
+    else:
+        truncation_budget = NO_TRUNCATION_BUDGET
+    lines = util.assertrepr_compare(
+        op=op,
+        left=left,
+        right=right,
+        verbose=config.get_verbosity(Config.VERBOSITY_ASSERTIONS),
+        highlighter=highlighter,
+        assertion_text_diff_style=util.get_assertion_text_diff_style(config),
+        truncation_budget=truncation_budget,
     )
-    return explanation or None
+    return truncate.materialize_with_truncation(lines, config) or None
diff --git a/src/_pytest/assertion/_compare_any.py b/src/_pytest/assertion/_compare_any.py
index 9e577683736..2c84ba2b0be 100644
--- a/src/_pytest/assertion/_compare_any.py
+++ b/src/_pytest/assertion/_compare_any.py
@@ -19,6 +19,8 @@
 from _pytest.assertion._guards import istext
 from _pytest.assertion._typing import _AssertionTextDiffStyle
 from _pytest.assertion._typing import _HighlightFunc
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.assertion.compare_text import _compare_eq_text
 
 
@@ -28,6 +30,7 @@ def _compare_eq_any(
     highlighter: _HighlightFunc,
     verbose: int,
     assertion_text_diff_style: _AssertionTextDiffStyle,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
     """Yield the per-line explanation for ``left == right`` (without summary).
 
@@ -42,6 +45,7 @@ def _compare_eq_any(
             highlighter,
             verbose,
             assertion_text_diff_style,
+            truncation_budget,
         )
     else:
         from _pytest.python_api import ApproxBase
@@ -70,10 +74,14 @@ def _compare_eq_any(
         elif isset(left) and isset(right):
             yield from _compare_eq_set(left, right, highlighter, verbose)
         elif ismapping(left) and ismapping(right):
-            yield from _compare_eq_mapping(left, right, highlighter, verbose)
+            yield from _compare_eq_mapping(
+                left, right, highlighter, verbose, truncation_budget
+            )
 
         if isiterable(left) and isiterable(right):
-            yield from _compare_eq_iterable(left, right, highlighter, verbose)
+            yield from _compare_eq_iterable(
+                left, right, highlighter, verbose, truncation_budget
+            )
 
 
 def _compare_eq_cls(
diff --git a/src/_pytest/assertion/_compare_mapping.py b/src/_pytest/assertion/_compare_mapping.py
index 4edb47026c6..7c53adab312 100644
--- a/src/_pytest/assertion/_compare_mapping.py
+++ b/src/_pytest/assertion/_compare_mapping.py
@@ -1,11 +1,16 @@
 from __future__ import annotations
 
+from collections.abc import Collection
 from collections.abc import Iterator
 from collections.abc import Mapping
+import heapq
 import pprint
 
+from _pytest._io.pprint import _safe_key
 from _pytest._io.saferepr import saferepr
 from _pytest.assertion._typing import _HighlightFunc
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 
 
 def _compare_eq_mapping(
@@ -13,7 +18,9 @@ def _compare_eq_mapping(
     right: Mapping[object, object],
     highlighter: _HighlightFunc,
     verbose: int = 0,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
+    max_lines = truncation_budget.max_lines
     set_left = set(left)
     set_right = set(right)
     common = set_left.intersection(set_right)
@@ -36,13 +43,35 @@ def _compare_eq_mapping(
     len_extra_left = len(extra_left)
     if len_extra_left:
         yield f"Left contains {len_extra_left} more item{'' if len_extra_left == 1 else 's'}:"
-        yield from highlighter(
-            pprint.pformat({k: left[k] for k in extra_left})
-        ).splitlines()
+        yield from _format_extra_items(left, extra_left, highlighter, max_lines)
     extra_right = set_right - set_left
     len_extra_right = len(extra_right)
     if len_extra_right:
         yield f"Right contains {len_extra_right} more item{'' if len_extra_right == 1 else 's'}:"
+        yield from _format_extra_items(right, extra_right, highlighter, max_lines)
+
+
+def _format_extra_items(
+    mapping: Mapping[object, object],
+    keys: Collection[object],
+    highlighter: _HighlightFunc,
+    max_lines: int | None,
+) -> Iterator[str]:
+    """Render the "X contains N more items" subdict.
+
+    Small (or untruncated, ``max_lines is None``) output keeps the compact,
+    key-sorted ``pprint`` block. When there are more extra keys than the
+    truncation budget, ``pprint.pformat`` would format the whole subdict
+    just to have all but the first few lines dropped, so instead emit only
+    the smallest ``max_lines`` keys, one per line — deterministic via the
+    same safe sort ``pprint`` uses, char-bounded via ``saferepr``. (This
+    differs from the ``pprint`` block, but only in the truncated tail; the
+    smallest keys shown are the same ones ``pprint`` would have led with.)
+    """
+    if max_lines is None or len(keys) <= max_lines:
         yield from highlighter(
-            pprint.pformat({k: right[k] for k in extra_right})
+            pprint.pformat({k: mapping[k] for k in keys})
         ).splitlines()
+        return
+    for k in heapq.nsmallest(max_lines, keys, key=_safe_key):
+        yield highlighter(saferepr({k: mapping[k]}))
diff --git a/src/_pytest/assertion/_compare_sequence.py b/src/_pytest/assertion/_compare_sequence.py
index cd0043bf7ce..6c8fa5558ba 100644
--- a/src/_pytest/assertion/_compare_sequence.py
+++ b/src/_pytest/assertion/_compare_sequence.py
@@ -7,6 +7,8 @@
 from _pytest._io.pprint import PrettyPrinter
 from _pytest._io.saferepr import saferepr
 from _pytest.assertion._typing import _HighlightFunc
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.compat import running_on_ci
 
 
@@ -15,6 +17,7 @@ def _compare_eq_iterable(
     right: Iterable[object],
     highlighter: _HighlightFunc,
     verbose: int = 0,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
     if verbose <= 0 and not running_on_ci():
         yield "Use -v to get more diff"
@@ -22,19 +25,30 @@ def _compare_eq_iterable(
     # dynamic import to speedup pytest
     import difflib
 
-    left_formatting = PrettyPrinter().pformat(left).splitlines()
-    right_formatting = PrettyPrinter().pformat(right).splitlines()
+    # ``truncation_budget`` is ``(max_lines, max_chars)``, computed by the
+    # dispatcher from the truncator's ``truncation_limit_lines`` /
+    # ``truncation_limit_chars``: when truncation is going to drop
+    # everything past those budgets anyway, we don't bother formatting
+    # more. ``(None, None)`` means no cap (``-vv`` or CI: the user wants
+    # the full diff).
+    pp = PrettyPrinter()
+    max_lines, max_chars = truncation_budget
+    left_formatting = pp.pformat_lines(left, max_lines=max_lines, max_chars=max_chars)
+    right_formatting = pp.pformat_lines(right, max_lines=max_lines, max_chars=max_chars)
 
     yield ""
     yield "Full diff:"
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
-    yield from highlighter(
-        "\n".join(
-            line.rstrip() for line in difflib.ndiff(right_formatting, left_formatting)
-        ),
-        lexer="diff",
-    ).splitlines()
+    #
+    # Yield each ndiff line through the highlighter individually so the
+    # streaming truncator can stop pulling from ``difflib.ndiff`` as
+    # soon as its budget is full. The diff lexer is line-oriented, so
+    # per-line highlighting is equivalent — it just adds a redundant
+    # ``\x1b[0m`` reset at the start of each line (invisible to the
+    # terminal).
+    for line in difflib.ndiff(right_formatting, left_formatting):
+        yield highlighter(line.rstrip(), lexer="diff")
 
 
 def _compare_eq_sequence(
diff --git a/src/_pytest/assertion/_typing.py b/src/_pytest/assertion/_typing.py
index f032f34e0bc..b61c760dcfb 100644
--- a/src/_pytest/assertion/_typing.py
+++ b/src/_pytest/assertion/_typing.py
@@ -1,12 +1,31 @@
 from __future__ import annotations
 
 from typing import Literal
+from typing import NamedTuple
 from typing import Protocol
 
 
 _AssertionTextDiffStyle = Literal["ndiff", "block"]
 
 
+class TruncationBudget(NamedTuple):
+    """Per-side budget for capping diff formatting before truncation.
+
+    ``max_lines`` / ``max_chars`` bound how much of each operand is
+    formatted when the explanation is going to be truncated anyway. A
+    ``None`` dimension is left unbounded (``-vv`` / CI: the full diff is
+    wanted).
+    """
+
+    max_lines: int | None = None
+    max_chars: int | None = None
+
+
+# Module-level singleton for "no cap" (the full diff is formatted), used as a
+# default argument so we do not build a fresh instance on every call (B008).
+NO_TRUNCATION_BUDGET = TruncationBudget()
+
+
 class _HighlightFunc(Protocol):  # noqa: PYI046
     def __call__(self, source: str, lexer: Literal["diff", "python"] = "python") -> str:
         """Apply highlighting to the given source."""
diff --git a/src/_pytest/assertion/compare_text.py b/src/_pytest/assertion/compare_text.py
index 31096444ba6..e5dd8a0c7c3 100644
--- a/src/_pytest/assertion/compare_text.py
+++ b/src/_pytest/assertion/compare_text.py
@@ -5,6 +5,8 @@
 from _pytest._io.saferepr import saferepr
 from _pytest.assertion._typing import _AssertionTextDiffStyle
 from _pytest.assertion._typing import _HighlightFunc
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.assertion.highlight import dummy_highlighter
 from _pytest.compat import assert_never
 
@@ -15,12 +17,13 @@ def _compare_eq_text(
     highlighter: _HighlightFunc,
     verbose: int,
     assertion_text_diff_style: _AssertionTextDiffStyle,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
     match assertion_text_diff_style:
         case "block":
             yield from _diff_text_block(left, right)
         case "ndiff":
-            yield from _diff_text(left, right, highlighter, verbose)
+            yield from _diff_text(left, right, highlighter, verbose, truncation_budget)
         case unreachable:
             assert_never(unreachable)
 
@@ -39,12 +42,27 @@ def _format_text_block_lines(text: str) -> Iterator[str]:
 
 
 def _diff_text(
-    left: str, right: str, highlighter: _HighlightFunc, verbose: int = 0
+    left: str,
+    right: str,
+    highlighter: _HighlightFunc,
+    verbose: int = 0,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
     """Yield the explanation for the diff between text.
 
     Unless --verbose is used this will skip leading and trailing
     characters which are identical to keep the diff minimal.
+
+    ``truncation_budget`` is ``(max_lines, max_chars)`` from the truncator:
+    when truncation will clip the diff anyway, the inputs to ``ndiff``
+    are capped to that budget first. ``ndiff`` is not lazy — its
+    intraline ``_fancy_replace`` refinement runs in full before the
+    first line is yielded — so an enormous comparison can't be
+    short-circuited by streaming; the only lever is feeding it less.
+    The resulting diff reflects a *local* alignment of the bounded
+    prefix rather than the global one, so the truncated head may differ
+    from truncating the full diff (use ``-vv`` for the exact diff); for
+    the common case of two mostly-similar texts the head is identical.
     """
     from difflib import ndiff
 
@@ -75,23 +93,38 @@ def _diff_text(
         left = repr(str(left))
         right = repr(str(right))
         yield "Strings contain only whitespace, escaping them using repr()"
+    # Cap the inputs to ndiff to the truncation budget: a char slice
+    # first (bounds a few huge lines, whose intraline diff is O(len^2)),
+    # then a line slice (bounds many lines). ``None`` leaves a dimension
+    # unbounded (``-vv``/CI: the full diff is wanted).
+    max_lines, max_chars = truncation_budget
+    if max_chars is not None:
+        left = left[:max_chars]
+        right = right[:max_chars]
+    left_lines = left.splitlines(keepends)
+    right_lines = right.splitlines(keepends)
+    if max_lines is not None:
+        left_lines = left_lines[:max_lines]
+        right_lines = right_lines[:max_lines]
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
     yield from highlighter(
-        "\n".join(
-            line.strip("\n")
-            for line in ndiff(right.splitlines(keepends), left.splitlines(keepends))
-        ),
+        "\n".join(line.strip("\n") for line in ndiff(right_lines, left_lines)),
         lexer="diff",
     ).splitlines()
 
 
-def _notin_text(term: str, text: str, verbose: int = 0) -> Iterator[str]:
+def _notin_text(
+    term: str,
+    text: str,
+    verbose: int = 0,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
+) -> Iterator[str]:
     index = text.find(term)
     head = text[:index]
     tail = text[index + len(term) :]
     correct_text = head + tail
-    diff = _diff_text(text, correct_text, dummy_highlighter, verbose)
+    diff = _diff_text(text, correct_text, dummy_highlighter, verbose, truncation_budget)
     yield f"{saferepr(term, maxsize=42)} is contained here:"
     for line in diff:
         if line.startswith("Skipping"):
diff --git a/src/_pytest/assertion/truncate.py b/src/_pytest/assertion/truncate.py
index d62ca33cc4b..9f12f915b50 100644
--- a/src/_pytest/assertion/truncate.py
+++ b/src/_pytest/assertion/truncate.py
@@ -6,42 +6,75 @@
 
 from __future__ import annotations
 
+from collections.abc import Iterable
+
 from _pytest.compat import running_on_ci
 from _pytest.config import Config
-from _pytest.nodes import Item
 
 
 DEFAULT_MAX_LINES = 8
 DEFAULT_MAX_CHARS = DEFAULT_MAX_LINES * 80
 USAGE_MSG = "use '-vv' to show"
+TRUNCATION_MSG = f"...Full output truncated, {USAGE_MSG}"
+
+# Truncating appends a footer to the kept body: ``...`` on the last kept line,
+# a blank separator line, then ``TRUNCATION_MSG``. ``TRUNCATION_FOOTER_LINES``
+# / ``TRUNCATION_FOOTER_CHARS`` are that footer's cost. We let a body exceed
+# the raw budget by this much before truncating, so a body that *nearly* fits
+# is not cut just to make room for the footer. Both are derived from the
+# footer itself so they cannot drift out of date (the char slack was once a
+# hand-rounded 70 that also covered a now-removed "(N lines hidden)" count).
+TRUNCATION_FOOTER_LINES = 2  # blank separator + message line
+TRUNCATION_FOOTER_CHARS = len("...") + len(TRUNCATION_MSG)
+
+
+def materialize_with_truncation(lines: Iterable[str], config: Config) -> list[str]:
+    """Materialise a streaming explanation, applying truncation lazily.
+
+    Pulls from ``lines`` only until the truncation threshold is reached;
+    once exceeded, the rest of the iterator is dropped without being
+    consumed. This lets a huge comparison short-circuit instead of
+    building (and immediately discarding) megabytes of explanation text.
+    """
+    should_truncate, max_lines, max_chars = _get_truncation_parameters(config)
+    if not should_truncate:
+        return list(lines)
+
+    tolerable_max_chars = max_chars + TRUNCATION_FOOTER_CHARS
+    # Pull one line past the point where ``_truncate_explanation`` keeps the
+    # body whole (``max_lines + TRUNCATION_FOOTER_LINES``) so it can detect the
+    # overflow, without us materialising more than we need.
+    line_cap = max_lines + TRUNCATION_FOOTER_LINES + 1 if max_lines > 0 else None
+    buffered: list[str] = []
+    char_count = 0
+    for line in lines:
+        buffered.append(line)
+        char_count += len(line)
+        if line_cap is not None and len(buffered) >= line_cap:
+            break
+        if max_chars > 0 and char_count > tolerable_max_chars:
+            break
+    else:
+        # Iterator exhausted within limits — nothing to truncate.
+        return buffered
 
-
-def truncate_if_required(explanation: list[str], item: Item) -> list[str]:
-    """Truncate this assertion explanation if the given test item is eligible."""
-    should_truncate, max_lines, max_chars = _get_truncation_parameters(item)
-    if should_truncate:
-        return _truncate_explanation(
-            explanation,
-            max_lines=max_lines,
-            max_chars=max_chars,
-        )
-    return explanation
+    return _truncate_explanation(buffered, max_lines=max_lines, max_chars=max_chars)
 
 
-def _get_truncation_parameters(item: Item) -> tuple[bool, int, int]:
-    """Return the truncation parameters related to the given item, as (should truncate, max lines, max chars)."""
+def _get_truncation_parameters(config: Config) -> tuple[bool, int, int]:
+    """Return the truncation parameters from the given config, as (should truncate, max lines, max chars)."""
     # We do not need to truncate if one of conditions is met:
     # 1. Verbosity level is 2 or more;
     # 2. Test is being run in CI environment;
     # 3. Both truncation_limit_lines and truncation_limit_chars
     #    .ini parameters are set to 0 explicitly.
-    max_lines = item.config.getini("truncation_limit_lines")
+    max_lines = config.getini("truncation_limit_lines")
     max_lines = int(max_lines if max_lines is not None else DEFAULT_MAX_LINES)
 
-    max_chars = item.config.getini("truncation_limit_chars")
+    max_chars = config.getini("truncation_limit_chars")
     max_chars = int(max_chars if max_chars is not None else DEFAULT_MAX_CHARS)
 
-    verbose = item.config.get_verbosity(Config.VERBOSITY_ASSERTIONS)
+    verbose = config.get_verbosity(Config.VERBOSITY_ASSERTIONS)
 
     should_truncate = verbose < 2 and not running_on_ci()
     should_truncate = should_truncate and (max_lines > 0 or max_chars > 0)
@@ -66,22 +99,10 @@ def _truncate_explanation(
     When this function is launched we know max_lines > 0 or max_chars > 0
     because _get_truncation_parameters was called first.
     """
-    # The length of the truncation explanation depends on the number of lines
-    # removed but is at least 68 characters:
-    # The real value is
-    # 64 (for the base message:
-    # '...\n...Full output truncated (1 line hidden), use '-vv' to show")'
-    # )
-    # + 1 (for plural)
-    # + int(math.log10(len(input_lines) - max_lines)) (number of hidden line, at least 1)
-    # + 3 for the '...' added to the truncated line
-    # But if there's more than 100 lines it's very likely that we're going to
-    # truncate, so we don't need the exact value using log10.
-    tolerable_max_chars = (
-        max_chars + 70  # 64 + 1 (for plural) + 2 (for '99') + 3 for '...'
-    )
-    # The truncation explanation add two lines to the output
-    if max_lines == 0 or len(input_lines) <= max_lines + 2:
+    # ``max_chars`` bounds the body only; the footer slack is added on top
+    # (see ``TRUNCATION_FOOTER_CHARS``).
+    tolerable_max_chars = max_chars + TRUNCATION_FOOTER_CHARS
+    if max_lines == 0 or len(input_lines) <= max_lines + TRUNCATION_FOOTER_LINES:
         if max_chars == 0 or sum(len(s) for s in input_lines) <= tolerable_max_chars:
             return input_lines
         truncated_explanation = input_lines
@@ -89,24 +110,19 @@ def _truncate_explanation(
         # Truncate first to max_lines, and then truncate to max_chars if necessary
         truncated_explanation = input_lines[:max_lines]
     # We reevaluate the need to truncate chars following removal of some lines
-    need_to_truncate_char = (
+    if (
         max_chars > 0
         and sum(len(e) for e in truncated_explanation) > tolerable_max_chars
-    )
-    if need_to_truncate_char:
+    ):
         truncated_explanation = _truncate_by_char_count(
             truncated_explanation, max_chars
         )
     # Something was truncated, adding '...' at the end to show that
     truncated_explanation[-1] += "..."
-    truncated_line_count = (
-        len(input_lines) - len(truncated_explanation) + int(need_to_truncate_char)
-    )
     return [
         *truncated_explanation,
         "",
-        f"...Full output truncated ({truncated_line_count} line"
-        f"{'' if truncated_line_count == 1 else 's'} hidden), {USAGE_MSG}",
+        TRUNCATION_MSG,
     ]
 
 
diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index 5e5ef543c13..3898a6f79a8 100644
--- a/src/_pytest/assertion/util.py
+++ b/src/_pytest/assertion/util.py
@@ -19,6 +19,8 @@
 from _pytest.assertion._guards import istext
 from _pytest.assertion._typing import _AssertionTextDiffStyle
 from _pytest.assertion._typing import _HighlightFunc
+from _pytest.assertion._typing import NO_TRUNCATION_BUDGET
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.assertion.compare_text import _notin_text
 from _pytest.assertion.highlight import dummy_highlighter as dummy_highlighter
 from _pytest.config import Config
@@ -140,6 +142,7 @@ def assertrepr_compare(
     verbose: int,
     highlighter: _HighlightFunc,
     assertion_text_diff_style: _AssertionTextDiffStyle,
+    truncation_budget: TruncationBudget = NO_TRUNCATION_BUDGET,
 ) -> Iterator[str]:
     """Yield specialised explanations for some operators/operands.
 
@@ -183,9 +186,10 @@ def assertrepr_compare(
                 highlighter,
                 verbose,
                 assertion_text_diff_style,
+                truncation_budget,
             )
         elif op == "not in" and istext(left) and istext(right):
-            source = _notin_text(left, right, verbose)
+            source = _notin_text(left, right, verbose, truncation_budget)
         elif op in {"!=", ">=", "<=", ">", "<"} and isset(left) and isset(right):
             source = SET_COMPARISON_FUNCTIONS[op](left, right, highlighter, verbose)
         else:
diff --git a/testing/python/approx.py b/testing/python/approx.py
index 88d46cbb755..c5ca03fe823 100644
--- a/testing/python/approx.py
+++ b/testing/python/approx.py
@@ -313,7 +313,7 @@ def test_error_messages_with_different_verbosity(self, assert_approx_raises_rege
                 rf"^  \(0,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}e-{SOME_INT}$",
                 rf"^  \(1,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}e-{SOME_INT}\.\.\.$",
                 "^  $",
-                rf"^  ...Full output truncated \({SOME_INT} lines hidden\), use '-vv' to show$",
+                r"^  ...Full output truncated, use '-vv' to show$",
             ],
             verbosity_level=0,
         )
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index 492834ba9de..426da2b8dec 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -17,7 +17,9 @@
 from _pytest.assertion import truncate
 from _pytest.assertion import util
 from _pytest.assertion._compare_any import _compare_eq_cls
+from _pytest.assertion._typing import TruncationBudget
 from _pytest.assertion.compare_text import _compare_eq_text
+from _pytest.assertion.compare_text import _notin_text
 from _pytest.config import Config as _Config
 from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pytester import Pytester
@@ -56,6 +58,11 @@ def get_verbosity(self, verbosity_type: str | None = None) -> int:
         def getini(self, name: str) -> str:
             if name == util.ASSERTION_TEXT_DIFF_STYLE_INI:
                 return assertion_text_diff_style
+            # Disable truncation so ``callop``-style tests can compare
+            # against the full explanation. Dedicated truncation tests
+            # use their own config in :class:`TestTruncateMaterialize`.
+            if name in ("truncation_limit_lines", "truncation_limit_chars"):
+                return "0"
             raise KeyError(f"Not mocked out: {name}")
 
     return Config()
@@ -491,6 +498,60 @@ def test_text_diff_ndiff_style(self) -> None:
             "+ spam",
         ]
 
+    def test_text_diff_budget_caps_ndiff_input(self) -> None:
+        # A large text diff fed a truncation budget caps the inputs to
+        # ndiff, so the result is bounded instead of growing with N.
+        left = "\n".join(f"left {i}" for i in range(1000))
+        right = "\n".join(f"right {i}" for i in range(1000))
+        ndiff_style = util.ASSERTION_TEXT_DIFF_STYLE_NDIFF
+        capped = list(
+            _compare_eq_text(
+                left,
+                right,
+                util.dummy_highlighter,
+                1,
+                ndiff_style,
+                TruncationBudget(11, 710),
+            )
+        )
+        full = list(
+            _compare_eq_text(
+                left, right, util.dummy_highlighter, 1, ndiff_style, TruncationBudget()
+            )
+        )
+        assert len(capped) < 80
+        assert len(full) > 1500
+        # a few huge lines: the char budget bounds each emitted line.
+        capped_chars = list(
+            _compare_eq_text(
+                "x" * 100_000,
+                "y" * 100_000,
+                util.dummy_highlighter,
+                1,
+                ndiff_style,
+                TruncationBudget(11, 710),
+            )
+        )
+        assert all(len(line) < 1000 for line in capped_chars)
+
+    def test_notin_text_budget_caps_ndiff_input(self) -> None:
+        # ``assert term not in huge_text`` runs the same ndiff machinery as
+        # ``==`` and must be capped the same way: without a budget it scans
+        # the whole text (O(N)); with one the inputs to ndiff are sliced
+        # first, so the work is bounded regardless of how big the haystack
+        # is. The needle sits in the middle so an uncapped diff would have
+        # to walk past ~N identical chars to reach it.
+        needle = "NEEDLE"
+        text = "a" * 100_000 + needle + "a" * 100_000
+        capped = list(_notin_text(needle, text, 1, TruncationBudget(11, 710)))
+        full = list(_notin_text(needle, text, 1, TruncationBudget()))
+        # Capped: a handful of short lines, each within the char budget.
+        assert len(capped) < 80
+        assert all(len(line) < 1000 for line in capped)
+        # Uncapped: the diff balloons with the input — proving the cap is
+        # what bounds it, not some unrelated early-out.
+        assert sum(len(line) for line in full) > 100_000
+
     def test_text_skipping(self) -> None:
         lines = callequal("a" * 50 + "spam", "a" * 50 + "eggs")
         assert lines is not None
@@ -902,6 +963,40 @@ def test_dict_different_items(self) -> None:
             "  }",
         ]
 
+    def test_dict_extra_items_bounded_under_budget(self) -> None:
+        # Many extra keys + a truncation budget: the subdict is not
+        # pretty-printed in full; only the smallest ``max_lines`` keys are
+        # emitted, one per line (deterministic, char-bounded).
+        from _pytest.assertion._compare_mapping import _compare_eq_mapping
+
+        out = list(
+            _compare_eq_mapping(
+                {i: i for i in range(1000)},
+                {},
+                util.dummy_highlighter,
+                0,
+                TruncationBudget(5, 350),
+            )
+        )
+        assert out[0] == "Left contains 1000 more items:"
+        body = out[1:]
+        assert body == [f"{{{i}: {i}}}" for i in range(5)]  # smallest 5, sorted
+
+    def test_dict_extra_items_small_keeps_pformat_block(self) -> None:
+        # Under the budget, the compact key-sorted pprint block is unchanged.
+        from _pytest.assertion._compare_mapping import _compare_eq_mapping
+
+        out = list(
+            _compare_eq_mapping(
+                {"b": 2, "a": 1},
+                {},
+                util.dummy_highlighter,
+                0,
+                TruncationBudget(5, 350),
+            )
+        )
+        assert out == ["Left contains 2 more items:", "{'a': 1, 'b': 2}"]
+
     def test_mapping_different_items(self) -> None:
         class SimpleMapping(Mapping[str, int]):
             def __init__(self, values: dict[str, int]) -> None:
@@ -1154,7 +1249,7 @@ def test_recursive_dataclasses(self, pytester: Pytester) -> None:
                 "E         Drill down into differing attribute g:",
                 "E           g: S(a=10, b='ten') != S(a=20, b='xxx')...",
                 "E         ",
-                "E         ...Full output truncated (51 lines hidden), use '-vv' to show",
+                "E         ...Full output truncated, use '-vv' to show",
             ],
             consecutive=True,
         )
@@ -1527,7 +1622,6 @@ def test_truncates_at_8_lines_when_given_list_of_empty_strings(self) -> None:
         assert result != expl
         assert len(result) == 8 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "42 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1538,7 +1632,6 @@ def test_truncates_at_8_lines_when_first_8_lines_are_LT_max_chars(self) -> None:
         assert result != expl
         assert len(result) == 8 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert f"{total_lines - 8} lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1557,7 +1650,7 @@ def test_truncates_full_line_because_of_max_chars(self) -> None:
             "a" * 10,
             "...",
             "",
-            "...Full output truncated (1 line hidden), use '-vv' to show",
+            "...Full output truncated, use '-vv' to show",
         ]
 
     def test_truncates_edgecase_when_truncation_message_makes_the_result_longer_for_chars(
@@ -1582,7 +1675,6 @@ def test_truncates_at_8_lines_when_first_8_lines_are_EQ_max_chars(self) -> None:
         assert result != expl
         assert len(result) == 16 - 8 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "8 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1592,7 +1684,6 @@ def test_truncates_at_4_lines_when_first_4_lines_are_GT_max_chars(self) -> None:
         assert result != expl
         assert len(result) == 4 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "7 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1602,7 +1693,6 @@ def test_truncates_at_1_line_when_first_line_is_GT_max_chars(self) -> None:
         assert result != expl
         assert len(result) == 1 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "1000 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1610,7 +1700,6 @@ def test_full_output_truncated(self, monkeypatch, pytester: Pytester) -> None:
         """Test against full runpytest() output."""
         line_count = 7
         line_len = 100
-        expected_truncated_lines = 2
         pytester.makepyfile(
             rf"""
             def test_many_lines():
@@ -1629,7 +1718,7 @@ def test_many_lines():
             [
                 "*+ 1*",
                 "*+ 3*",
-                f"*truncated ({expected_truncated_lines} lines hidden)*use*-vv*",
+                "*Full output truncated*use*-vv*",
             ]
         )
 
@@ -1643,7 +1732,7 @@ def test_many_lines():
             [
                 "*+ 1*",
                 "*+ 3*",
-                f"*truncated ({expected_truncated_lines} lines hidden)*use*-vv*",
+                "*Full output truncated*use*-vv*",
             ]
         )
 
@@ -1658,7 +1747,7 @@ def test_many_lines():
             (4, None, 0),
             (0, None, 0),
             (None, 8, 6),
-            (None, 9, 0),
+            (None, 33, 0),
             (None, 0, 0),
             (0, 0, 0),
             (0, 1000, 0),
@@ -1685,7 +1774,7 @@ def test():
 
         # This test produces 6 lines of diff output or 79 characters
         # So the effect should be when threshold is < 4 lines (considering 2 additional lines for explanation)
-        # Or < 9 characters (considering 70 additional characters for explanation)
+        # Or < 33 characters (considering the ~46-char footer slack, see truncate.TRUNCATION_FOOTER_CHARS)
 
         monkeypatch.delenv("CI", raising=False)
 
@@ -1699,9 +1788,7 @@ def test():
         result = pytester.runpytest()
 
         if expected_lines_hidden != 0:
-            result.stdout.fnmatch_lines(
-                [f"*truncated ({expected_lines_hidden} lines hidden)*"]
-            )
+            result.stdout.fnmatch_lines(["*Full output truncated*"])
         else:
             result.stdout.no_fnmatch_line("*truncated*")
             result.stdout.fnmatch_lines(
@@ -1712,6 +1799,229 @@ def test():
             )
 
 
+class TestMaterializeWithTruncation:
+    """Tests for ``truncate.materialize_with_truncation``.
+
+    Assertions check *behaviour* — that truncation kicks in / doesn't,
+    that the original lines are preserved, that the iterator's contract
+    is honoured — and never the literal footer wording. That way the
+    tests survive any future change to the truncation message format.
+    """
+
+    @staticmethod
+    def _config_with_limits(verbose: int = 0):
+        # Minimal stand-in for ``Config`` that ``materialize_with_truncation``
+        # uses through ``_get_truncation_parameters``.
+        class C:
+            def getini(self, name: str) -> object:
+                return None  # use defaults (8 lines / 640 chars)
+
+            def get_verbosity(self, _verbosity_type: str | None = None) -> int:
+                return verbose
+
+        return C()
+
+    def test_iterator_within_limits_returns_all_lines(self) -> None:
+        lines = iter(["one", "two", "three"])
+        result = truncate.materialize_with_truncation(lines, self._config_with_limits())
+        assert result == ["one", "two", "three"]
+
+    def test_iterator_exceeding_limits_is_truncated(self) -> None:
+        lines = (f"line {i}" for i in range(1000))
+        result = truncate.materialize_with_truncation(lines, self._config_with_limits())
+        # Bounded length — we kept the truncation footer plus at most a few
+        # lines past the cap; we never collect the full 1000-line stream.
+        assert len(result) < 20
+        # The first lines we kept are the first lines of the input.
+        assert result[0] == "line 0"
+        # Some truncation marker is present (wording deliberately not asserted).
+        assert any("truncated" in line for line in result)
+
+    def test_sized_input_returns_same_shape_as_iterator_input(self) -> None:
+        # When the input is already a sized container, the function still
+        # returns the truncated form; behaviour is the same as for an
+        # iterator over the same content.
+        content = [f"line {i}" for i in range(50)]
+        sized = truncate.materialize_with_truncation(
+            content, self._config_with_limits()
+        )
+        unsized = truncate.materialize_with_truncation(
+            iter(content), self._config_with_limits()
+        )
+        assert sized[0] == unsized[0] == "line 0"
+        assert any("truncated" in line for line in sized)
+        assert any("truncated" in line for line in unsized)
+
+    def test_truncation_disabled_returns_full_input(self) -> None:
+        # verbose >= 2 disables truncation; the iterator is fully drained.
+        lines = (f"line {i}" for i in range(50))
+        result = truncate.materialize_with_truncation(
+            lines, self._config_with_limits(verbose=2)
+        )
+        assert result == [f"line {i}" for i in range(50)]
+        assert not any("truncated" in line for line in result)
+
+    def test_first_lines_are_preserved_verbatim(self) -> None:
+        lines = (f"line {i}" for i in range(200))
+        result = truncate.materialize_with_truncation(lines, self._config_with_limits())
+        # The first kept lines should match the start of the input exactly
+        # (modulo the "..." appended to the last surviving line by the
+        # truncator, which we strip before comparing).
+        kept = [line.rstrip(".") for line in result if "truncated" not in line]
+        for i, line in enumerate(kept):
+            if line == "":
+                # Blank line separating content from the footer.
+                continue
+            assert line.startswith(f"line {i}")
+
+    def test_idempotent_on_already_truncated_list(self) -> None:
+        # The dispatcher applies ``materialize_with_truncation`` after the
+        # built-in hook impl already truncated. Re-applying it must not
+        # corrupt the footer count or chop further lines.
+        once = truncate.materialize_with_truncation(
+            (f"line {i}" for i in range(200)), self._config_with_limits()
+        )
+        twice = truncate.materialize_with_truncation(once, self._config_with_limits())
+        assert twice == once
+
+    def test_does_not_over_consume_the_stream(self) -> None:
+        # Regression guard for laziness: an eager implementation that drains
+        # the whole iterator before truncating would still produce bounded
+        # output (so the other tests pass), but it would pull every line.
+        # This generator trips if drained past a small bound, so an eager
+        # impl fails deterministically — no wall-clock timing involved.
+        pulled = 0
+
+        def tripwire() -> Iterator[str]:
+            nonlocal pulled
+            for i in range(10_000):
+                pulled += 1
+                yield f"line {i}"
+            raise AssertionError(
+                "materialize_with_truncation drained the whole stream — "
+                "the explanation iterator is no longer consumed lazily"
+            )
+
+        result = truncate.materialize_with_truncation(
+            tripwire(), self._config_with_limits()
+        )
+        assert any("truncated" in line for line in result)
+        # A handful of lines past the 8-line cap, never the full stream.
+        assert pulled < 20
+
+    def test_pull_count_is_independent_of_input_size(self) -> None:
+        # Scaling invariance: the number of lines pulled to truncate must
+        # not grow with the input. Constant pulls ⇒ O(1) work regardless of
+        # how huge the comparison is. Catches an O(N) materialisation
+        # regression deterministically.
+        def count_pulls(n: int) -> int:
+            pulled = 0
+
+            def gen() -> Iterator[str]:
+                nonlocal pulled
+                for i in range(n):
+                    pulled += 1
+                    yield f"line {i}"
+
+            truncate.materialize_with_truncation(gen(), self._config_with_limits())
+            return pulled
+
+        assert count_pulls(100) == count_pulls(100_000)
+
+    def _explain_capped(self, left: object, right: object) -> list[str]:
+        # Drive the comparison exactly as the dispatcher does: derive the
+        # pformat budget from the truncation limits, run the lazy
+        # ``assertrepr_compare`` generator, then materialise with truncation.
+        config = self._config_with_limits()
+        should, lines_lim, chars_lim = truncate._get_truncation_parameters(config)
+        cap = (
+            TruncationBudget(
+                lines_lim + 3 if lines_lim > 0 else None,
+                chars_lim + 70 if chars_lim > 0 else None,
+            )
+            if should
+            else TruncationBudget()
+        )
+        src = util.assertrepr_compare(
+            op="==",
+            left=left,
+            right=right,
+            verbose=1,
+            highlighter=util.dummy_highlighter,
+            assertion_text_diff_style=util.ASSERTION_TEXT_DIFF_STYLE_NDIFF,
+            truncation_budget=cap,
+        )
+        return truncate.materialize_with_truncation(src, config)
+
+    @pytest.mark.parametrize("shape", ["list", "tuple", "dict", "set"])
+    def test_formatting_work_is_bounded_for_a_10_line_display(self, shape: str) -> None:
+        # The real cost guard, exercised on every element-bearing
+        # comparison branch (sequence/iterable, mapping, set): a huge
+        # comparison that still displays only ~10 truncated lines must not
+        # *format* the whole input to get there. Wall-clock would catch an
+        # O(N) regression but flakes in CI, so we count the element
+        # ``repr`` calls instead — a deterministic, machine-independent
+        # proxy for the CPU work. With the pformat cap (sequence/mapping)
+        # or stream laziness (set) in place this stays flat as the input
+        # grows; lose either and it becomes O(N) (~200k reprs for N=100k)
+        # while the output is still 10 lines — which a line-count check
+        # misses. (The ``str``/ndiff branch has no element objects to
+        # count; it is guarded by ``test_text_diff_budget_caps_ndiff_input``.)
+        class Tracked:
+            reprs = 0
+
+            def __init__(self, v: int) -> None:
+                self.v = v
+
+            def __repr__(self) -> str:
+                Tracked.reprs += 1
+                return f"T({self.v})"
+
+            def __eq__(self, o: object) -> bool:
+                return isinstance(o, Tracked) and self.v == o.v
+
+            def __hash__(self) -> int:
+                return hash(self.v)
+
+        def make(n: int) -> tuple[object, object]:
+            # Build two near-identical containers of ``n`` elements that
+            # differ in exactly one spot, so the explanation is a real
+            # (truncated) diff rather than an empty one.
+            if shape in ("list", "tuple"):
+                seq_left = [Tracked(i) for i in range(n)]
+                seq_right = [Tracked(i) for i in range(n)]
+                seq_right[0] = Tracked(-1)
+                if shape == "tuple":
+                    return tuple(seq_left), tuple(seq_right)
+                return seq_left, seq_right
+            if shape == "dict":
+                map_left = {i: Tracked(i) for i in range(n)}
+                map_right = {i: Tracked(i) for i in range(n)}
+                map_right[0] = Tracked(-1)
+                return map_left, map_right
+            # set
+            set_left = {Tracked(i) for i in range(n)}
+            set_right = {Tracked(i) for i in range(n)}
+            set_right.discard(Tracked(0))
+            set_right.add(Tracked(-1))
+            return set_left, set_right
+
+        def work(n: int) -> tuple[int, int]:
+            left, right = make(n)
+            Tracked.reprs = 0  # count only the formatting, not the construction
+            out = self._explain_capped(left, right)
+            assert any("truncated" in line for line in out)
+            return len(out), Tracked.reprs
+
+        lines_small, reprs_small = work(1_000)
+        lines_big, reprs_big = work(100_000)
+        # Same small display either way ...
+        assert lines_small == lines_big
+        # ... for the same (bounded, input-independent) amount of work.
+        assert reprs_small == reprs_big
+        assert reprs_big < 200  # nowhere near the 100_000-element input
+
+
 def test_python25_compile_issue257(pytester: Pytester) -> None:
     pytester.makepyfile(
         """
@@ -2205,6 +2515,128 @@ def raise_exit(obj):
         callequal(1, 1)
 
 
+def test_plugin_hook_returning_none_is_skipped(pytester: Pytester) -> None:
+    """A ``pytest_assertrepr_compare`` impl returning ``None`` is skipped
+    so the next impl (or the built-in) can produce the explanation.
+    Covers the ``if not new_expl: continue`` branch in ``callbinrepr``.
+    """
+    pytester.makeconftest(
+        """
+        def pytest_assertrepr_compare(op, left, right):
+            # Always defer to the next plugin / the built-in.
+            return None
+        """
+    )
+    pytester.makepyfile(
+        """
+        def test_diff():
+            assert {1, 2} == {1, 3}
+        """
+    )
+    result = pytester.runpytest()
+    # The built-in set-comparison explanation still reaches the user
+    # (so the None-returning hook did not swallow it).
+    result.stdout.fnmatch_lines(
+        ["*Extra items in the left set:*", "*Extra items in the right set:*"]
+    )
+
+
+def test_plugin_hook_returning_empty_iterator_is_skipped(pytester: Pytester) -> None:
+    """A plugin returning a truthy but ultimately empty iterable is
+    skipped after materialisation. Covers the second
+    ``if not new_expl: continue`` branch in ``callbinrepr``.
+    """
+    pytester.makeconftest(
+        """
+        def pytest_assertrepr_compare(op, left, right):
+            # An iterator object is truthy, so it slips past the first
+            # falsy check; once materialised through truncation it is
+            # empty and the dispatcher must move on.
+            return iter([])
+        """
+    )
+    pytester.makepyfile(
+        """
+        def test_diff():
+            assert {1, 2} == {1, 3}
+        """
+    )
+    result = pytester.runpytest()
+    # The built-in set-comparison explanation still reaches the user.
+    result.stdout.fnmatch_lines(
+        ["*Extra items in the left set:*", "*Extra items in the right set:*"]
+    )
+
+
+def test_callbinrepr_falls_through_when_all_hooks_return_none(
+    pytester: Pytester,
+) -> None:
+    """When every ``pytest_assertrepr_compare`` impl returns ``None``
+    (no specialised explanation applies, e.g. ``assert 1 == 2``), the
+    dispatcher exhausts ``hook_result``, exits the loop, and returns
+    ``None``. Covers the ``continue → loop exit`` branch on the first
+    ``if not new_expl: continue`` line.
+    """
+    pytester.makepyfile(
+        """
+        def test_trivial():
+            assert 1 == 2
+        """
+    )
+    result = pytester.runpytest()
+    # Just the plain ``assert 1 == 2`` rewrite, with no specialised
+    # comparator explanation appended (because the dispatcher fell
+    # through to ``return None``).
+    result.stdout.fnmatch_lines(["*assert 1 == 2*"])
+    result.assert_outcomes(failed=1)
+
+
+def test_callbinrepr_plain_assert_mode(pytester: Pytester) -> None:
+    """In ``--assert=plain`` mode ``callbinrepr`` skips the ``%`` escape.
+    Covers the false branch of ``if item.config.getvalue("assertmode")
+    == "rewrite"``.
+    """
+    pytester.makepyfile(
+        """
+        def test_diff():
+            assert {1, 2} == {1, 3}
+        """
+    )
+    result = pytester.runpytest("--assert=plain")
+    # In plain mode the comparator still runs via ``callbinrepr`` (it
+    # is the rewrite escaping that's skipped), so the explanation is
+    # still produced.
+    result.stdout.fnmatch_lines(
+        ["*Extra items in the left set:*", "*Extra items in the right set:*"]
+    )
+
+
+def test_exception_before_first_yield_emits_summary_and_notice(monkeypatch) -> None:
+    """When the comparator raises *before* any explanation line has been
+    yielded, ``assertrepr_compare`` should still produce the summary so
+    the reader sees what was being compared, then append the failure
+    notice. Covers the ``summary_yielded is False`` branch of the
+    exception handler.
+    """
+    from _pytest.assertion import _compare_any
+
+    def raise_value_error(obj):
+        raise ValueError("synthetic repr failure")
+
+    # ``istext`` is called inside ``_compare_eq_any`` before the first
+    # yield, so this triggers the failure path on the very first
+    # ``next()`` call from ``assertrepr_compare``.
+    monkeypatch.setattr(_compare_any, "istext", raise_value_error)
+
+    expl = callequal(1, 1)
+    assert expl is not None
+    # Summary line still produced.
+    assert expl[0] == "1 == 1"
+    # The failure notice survives in the output; wording deliberately not
+    # asserted, only the underlying error's signature.
+    assert any("ValueError" in line or "synthetic" in line for line in expl)
+
+
 def test_assertion_location_with_coverage(pytester: Pytester) -> None:
     """This used to report the wrong location when run with coverage (#5754)."""
     p = pytester.makepyfile(
@@ -2251,8 +2683,8 @@ def test():
             """,
             [
                 "{bold}{red}E         At index 1 diff: {reset}{number}1{hl-reset}{endline} != {reset}{number}2*",
-                "{bold}{red}E         {light-red}-     2,{hl-reset}{endline}{reset}",
-                "{bold}{red}E         {light-green}+     1,{hl-reset}{endline}{reset}",
+                "{bold}{red}E         {reset}{light-red}-     2,{hl-reset}{endline}{reset}",
+                "{bold}{red}E         {reset}{light-green}+     1,{hl-reset}{endline}{reset}",
             ],
         ),
         (
@@ -2270,8 +2702,8 @@ def test():
                 "{bold}{red}E         Right contains 1 more item:{reset}",
                 "{bold}{red}E         {reset}{{{str}'{hl-reset}{str}number-is-0{hl-reset}{str}'{hl-reset}: {number}0*",
                 "{bold}{red}E         {reset}{light-gray} {hl-reset} {{{endline}{reset}",
-                "{bold}{red}E         {light-gray} {hl-reset}     'number-is-1': 1,{endline}{reset}",
-                "{bold}{red}E         {light-green}+     'number-is-5': 5,{hl-reset}{endline}{reset}",
+                "{bold}{red}E         {reset}{light-gray} {hl-reset}     'number-is-1': 1,{endline}{reset}",
+                "{bold}{red}E         {reset}{light-green}+     'number-is-5': 5,{hl-reset}{endline}{reset}",
             ],
         ),
         (