diff --git a/changelog/14523.improvement.rst b/changelog/14523.improvement.rst
new file mode 100644
index 00000000000..d53374c62df
--- /dev/null
+++ b/changelog/14523.improvement.rst
@@ -0,0 +1,13 @@
+Assertion explanations are now built lazily and the truncator stops
+the comparison helpers as soon as it has enough output, so comparing
+two large collections no longer builds the full diff in order to
+discard it. A focused micro-benchmark the worst case scenario
+(``set(range(500_000)) == set(range(1, 500_001))``) drops from ~2,200 ms
+to ~43 ms; but realistic test suite with mostly small diffs should be
+unchanged.
+
+The truncation footer no longer reports the hidden-line count
+(``...Full output truncated (N lines hidden), ...`` becomes
+``...Full output truncated, ...``); diff lines now carry a redundant
+``\x1b[0m`` reset prefix (invisible to terminals) so we can handle
+line one by one.
diff --git a/doc/en/example/reportingdemo.rst b/doc/en/example/reportingdemo.rst
index 29ba190b7e7..2e1681cd9cd 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/_io/pprint.py b/src/_pytest/_io/pprint.py
index ec41b449ddf..ce020eb8bb7 100644
--- a/src/_pytest/_io/pprint.py
+++ b/src/_pytest/_io/pprint.py
@@ -17,6 +17,7 @@
 
 import collections as _collections
 from collections.abc import Callable
+from collections.abc import Iterable
 from collections.abc import Iterator
 import dataclasses as _dataclasses
 from io import StringIO as _StringIO
@@ -56,6 +57,51 @@ def _safe_tuple(t):
     return _safe_key(t[0]), _safe_key(t[1])
 
 
+class _LineBudgetExceeded(Exception):
+    """Internal: signals that ``_LineBudgetStream`` has reached its cap.
+
+    Raised from inside ``stream.write`` so the formatter's recursion
+    unwinds at the next write; the caller catches it and uses whatever
+    output accumulated so far.
+    """
+
+
+class _LineBudgetStream:
+    r"""Stream that collects ``write`` calls into lines and bails out
+    once ``max_lines`` have been produced.
+
+    Lets ``PrettyPrinter._format`` stop early on huge collections: the
+    formatter writes one ``\n``-terminated chunk per element, so the
+    budget check fires on element boundaries.
+    """
+
+    __slots__ = ("_lines", "_max", "_pending")
+
+    def __init__(self, max_lines: int) -> None:
+        self._lines: list[str] = []
+        self._pending: list[str] = []
+        self._max = max_lines
+
+    def write(self, s: str) -> None:
+        if "\n" not in s:
+            if s:
+                self._pending.append(s)
+            return
+        parts = s.split("\n")
+        self._pending.append(parts[0])
+        self._lines.append("".join(self._pending))
+        self._lines.extend(parts[1:-1])
+        self._pending = [parts[-1]] if parts[-1] else []
+        if len(self._lines) >= self._max:
+            raise _LineBudgetExceeded
+
+    def finish(self) -> list[str]:
+        if self._pending:
+            self._lines.append("".join(self._pending))
+            self._pending = []
+        return self._lines
+
+
 class PrettyPrinter:
     def __init__(
         self,
@@ -91,6 +137,99 @@ def pformat(self, object: Any) -> str:
         self._format(object, sio, 0, 0, set(), 0)
         return sio.getvalue()
 
+    def pformat_lines(self, object: Any, max_lines: int | None = None) -> list[str]:
+        """Pretty-print ``object`` and return its lines.
+
+        There are two distinct paths, chosen by the caller up front via
+        ``max_lines`` (which a truncating caller derives from the
+        verbosity level):
+
+        * ``max_lines=None`` — *print everything* (``-vv``/CI). The whole
+          diff is going to be shown, so there is nothing to save by being
+          lazy: take the straight C path through ``StringIO`` +
+          ``str.splitlines``. This is the path the common, no-budget case
+          uses and it pays none of the budget machinery below.
+
+        * ``max_lines`` set — *truncating*. The caller will drop
+          everything past the budget, so formatting more is wasted work.
+          See ``_pformat_within_budget``.
+        """
+        if max_lines is None:
+            return self.pformat(object).splitlines()
+        return self._pformat_within_budget(object, max_lines)
+
+    def _pformat_within_budget(self, object: Any, max_lines: int) -> list[str]:
+        """Pretty-print ``object`` to at most ~``max_lines`` lines.
+
+        Two sub-cases, again decided cheaply before doing any formatting:
+
+        * the input provably fits the budget — a flat container with no
+          more elements than ``max_lines``. Each such element renders on
+          a single line, so the full pformat is at most ``len(object)``
+          lines and the cheap C path is safe. ``len(object)`` alone is
+          only a *lower* bound (one nested element can expand to many
+          lines), so the flatness check is what makes this sound. This
+          keeps the common small-diff case as fast as the no-budget path
+          rather than taxing it with the stream below.
+
+        * the input might overflow (large, or small-but-deeply-nested):
+          wire the formatter to a Python-level stream that bails out once
+          ``max_lines`` lines have been produced, so a huge collection
+          costs O(``max_lines``) instead of O(N).
+        """
+        try:
+            size = len(object)
+        except TypeError:
+            size = -1
+        if 0 <= size <= max_lines and self._renders_flat(object):
+            return self.pformat(object).splitlines()
+        stream = _LineBudgetStream(max_lines)
+        try:
+            # ``_format``'s ``IO[str]`` annotation is overly tight — it
+            # only ever calls ``stream.write(str)``, which is all this
+            # budget-aware stream implements.
+            self._format(object, stream, 0, 0, set(), 0)  # type: ignore[arg-type]
+        except _LineBudgetExceeded:
+            pass
+        return stream.finish()
+
+    def _renders_inline(self, object: Any) -> bool:
+        """Return ``True`` when ``_format`` writes ``object`` on one line.
+
+        Mirrors ``_format``'s dispatch: container types (in ``_dispatch``)
+        and dataclasses with a generated ``repr`` recurse and can span
+        multiple lines; everything else goes through ``_repr``, a single
+        ``write``.
+        """
+        if type(object).__repr__ in self._dispatch:
+            return False
+        return not (
+            _dataclasses.is_dataclass(object)
+            and not isinstance(object, type)
+            and object.__dataclass_params__.repr  # type:ignore[attr-defined]
+            and hasattr(object.__repr__, "__wrapped__")
+            and "__create_fn__" in object.__repr__.__wrapped__.__qualname__
+        )
+
+    def _renders_flat(self, object: Any) -> bool:
+        """Return ``True`` when every child of ``object`` renders inline.
+
+        Conservative: an unrecognised container shape (no ``items`` and
+        not iterable) returns ``False`` so the caller falls back to the
+        budget-aware stream rather than risk an unbounded fast path.
+        """
+        if isinstance(object, _collections.abc.Mapping):
+            children: Iterable[Any] = (
+                *object.keys(),
+                *object.values(),
+            )
+        else:
+            try:
+                children = iter(object)
+            except TypeError:
+                return False
+        return all(self._renders_inline(child) for child in children)
+
     def _format(
         self,
         object: Any,
@@ -236,7 +375,12 @@ def _pprint_set(
         else:
             stream.write(typ.__name__ + "({")
             endchar = "})"
-        object = sorted(object, key=_safe_key)
+        try:
+            object = sorted(object)
+        except TypeError:
+            # Heterogeneous element types — fall back to a key that
+            # tolerates unorderable pairs by string-comparing their types.
+            object = sorted(object, key=_safe_key)
         self._format_items(object, stream, indent, allowance, context, level)
         stream.write(endchar)
 
diff --git a/src/_pytest/assertion/__init__.py b/src/_pytest/assertion/__init__.py
index e33f8b29609..847dbaa287a 100644
--- a/src/_pytest/assertion/__init__.py
+++ b/src/_pytest/assertion/__init__.py
@@ -181,13 +181,21 @@ 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 +226,40 @@ 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 we're about to
+    # drop. ``+ 3`` matches the truncator's own ``line_cap``: 2 lines
+    # for the truncation message it appends (blank + footer) plus 1
+    # for overshoot detection. ``difflib.ndiff`` over two K-line
+    # pformat outputs produces at least K output lines (more when the
+    # sides differ), and the truncator pulls at most ``trunc_lines +
+    # 3`` lines from the whole explanation, so a per-side pformat
+    # budget of ``trunc_lines + 3`` covers the worst case. With
+    # truncation disabled the cap stays ``None`` and the user gets the
+    # full diff.
+    should_truncate, trunc_lines, _ = truncate._get_truncation_parameters(config)
+    pformat_cap = trunc_lines + 3 if should_truncate and trunc_lines > 0 else None
+    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),
+        pformat_cap=pformat_cap,
     )
-    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..73012abc416 100644
--- a/src/_pytest/assertion/_compare_any.py
+++ b/src/_pytest/assertion/_compare_any.py
@@ -28,6 +28,7 @@ def _compare_eq_any(
     highlighter: _HighlightFunc,
     verbose: int,
     assertion_text_diff_style: _AssertionTextDiffStyle,
+    pformat_cap: int | None = None,
 ) -> Iterator[str]:
     """Yield the per-line explanation for ``left == right`` (without summary).
 
@@ -73,7 +74,9 @@ def _compare_eq_any(
             yield from _compare_eq_mapping(left, right, highlighter, verbose)
 
         if isiterable(left) and isiterable(right):
-            yield from _compare_eq_iterable(left, right, highlighter, verbose)
+            yield from _compare_eq_iterable(
+                left, right, highlighter, verbose, pformat_cap
+            )
 
 
 def _compare_eq_cls(
diff --git a/src/_pytest/assertion/_compare_sequence.py b/src/_pytest/assertion/_compare_sequence.py
index cd0043bf7ce..52edb136d6e 100644
--- a/src/_pytest/assertion/_compare_sequence.py
+++ b/src/_pytest/assertion/_compare_sequence.py
@@ -15,6 +15,7 @@ def _compare_eq_iterable(
     right: Iterable[object],
     highlighter: _HighlightFunc,
     verbose: int = 0,
+    pformat_cap: int | None = None,
 ) -> Iterator[str]:
     if verbose <= 0 and not running_on_ci():
         yield "Use -v to get more diff"
@@ -22,19 +23,28 @@ def _compare_eq_iterable(
     # dynamic import to speedup pytest
     import difflib
 
-    left_formatting = PrettyPrinter().pformat(left).splitlines()
-    right_formatting = PrettyPrinter().pformat(right).splitlines()
+    # ``pformat_cap`` is computed by the dispatcher from the
+    # truncator's ``truncation_limit_lines``: when truncation is going
+    # to drop everything past that budget anyway, we don't bother
+    # formatting more. ``None`` means no cap (``-vv`` or CI: the user
+    # wants the full diff).
+    pp = PrettyPrinter()
+    left_formatting = pp.pformat_lines(left, max_lines=pformat_cap)
+    right_formatting = pp.pformat_lines(right, max_lines=pformat_cap)
 
     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/_compare_set.py b/src/_pytest/assertion/_compare_set.py
index 66687ececcb..2817133790d 100644
--- a/src/_pytest/assertion/_compare_set.py
+++ b/src/_pytest/assertion/_compare_set.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 from collections.abc import Callable
-from collections.abc import Iterable
 from collections.abc import Iterator
 from collections.abc import Set as AbstractSet
 from typing import TypeAlias
@@ -77,14 +76,24 @@ def _compare_lt_set(
 
 SetComparisonFunction: TypeAlias = Callable[
     [AbstractSet[object], AbstractSet[object], _HighlightFunc, int],
-    Iterable[str],
+    Iterator[str],
 ]
 
+
+def _both_sets_are_equal(
+    left: AbstractSet[object],
+    right: AbstractSet[object],
+    highlighter: _HighlightFunc,
+    verbose: int = 0,
+) -> Iterator[str]:
+    yield "Both sets are equal"
+
+
 SET_COMPARISON_FUNCTIONS: dict[str, SetComparisonFunction] = {
     # == can't be done here without a prior refactor because there's an additional
     # explanation for iterable in _compare_eq_any
     # "==": _compare_eq_set,
-    "!=": lambda *a, **kw: ["Both sets are equal"],
+    "!=": _both_sets_are_equal,
     ">=": _compare_gte_set,
     "<=": _compare_lte_set,
     ">": _compare_gt_set,
diff --git a/src/_pytest/assertion/truncate.py b/src/_pytest/assertion/truncate.py
index d62ca33cc4b..e137d8e0c1f 100644
--- a/src/_pytest/assertion/truncate.py
+++ b/src/_pytest/assertion/truncate.py
@@ -6,9 +6,10 @@
 
 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
@@ -16,32 +17,52 @@
 USAGE_MSG = "use '-vv' to show"
 
 
-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
+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 + 70
+    # Pull just past max_lines so ``_truncate_explanation`` can detect the
+    # overflow without us materialising more than we need.
+    line_cap = max_lines + 3 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
+
+    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)
@@ -66,20 +87,9 @@ 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 '...'
-    )
+    # Slack on top of ``max_chars`` so a body that just fits the budget
+    # doesn't get truncated solely to make room for the footer.
+    tolerable_max_chars = max_chars + 70
     # The truncation explanation add two lines to the output
     if max_lines == 0 or len(input_lines) <= max_lines + 2:
         if max_chars == 0 or sum(len(s) for s in input_lines) <= tolerable_max_chars:
@@ -89,24 +99,19 @@ 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}",
+        f"...Full output truncated, {USAGE_MSG}",
     ]
 
 
diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index 9d1383934a3..349f990e105 100644
--- a/src/_pytest/assertion/util.py
+++ b/src/_pytest/assertion/util.py
@@ -140,6 +140,7 @@ def assertrepr_compare(
     verbose: int,
     highlighter: _HighlightFunc,
     assertion_text_diff_style: _AssertionTextDiffStyle,
+    pformat_cap: int | None = None,
 ) -> Iterator[str]:
     """Yield specialised explanations for some operators/operands.
 
@@ -177,19 +178,18 @@ def assertrepr_compare(
 
     try:
         if op == "==":
-            source: Iterator[str] = _compare_eq_any(
+            source = _compare_eq_any(
                 left,
                 right,
                 highlighter,
                 verbose,
                 assertion_text_diff_style,
+                pformat_cap,
             )
         elif op == "not in" and istext(left) and istext(right):
             source = _notin_text(left, right, verbose)
         elif op in {"!=", ">=", "<=", ">", "<"} and isset(left) and isset(right):
-            source = iter(
-                SET_COMPARISON_FUNCTIONS[op](left, right, highlighter, verbose)
-            )
+            source = SET_COMPARISON_FUNCTIONS[op](left, right, highlighter, verbose)
         else:
             source = iter(())
 
diff --git a/testing/io/test_pprint.py b/testing/io/test_pprint.py
index 1326ef34b2e..30a7b5286b2 100644
--- a/testing/io/test_pprint.py
+++ b/testing/io/test_pprint.py
@@ -406,3 +406,84 @@ class DataclassWithTwoItems:
 )
 def test_consistent_pretty_printer(data: Any, expected: str) -> None:
     assert PrettyPrinter().pformat(data) == textwrap.dedent(expected).strip()
+
+
+class TestPformatLines:
+    """``pformat_lines`` returns the pretty-printed lines, optionally
+    capping the work at ``max_lines`` for inputs a downstream truncator
+    is going to clip anyway."""
+
+    def test_no_cap_matches_pformat_splitlines(self) -> None:
+        pp = PrettyPrinter()
+        data = list(range(50))
+        assert pp.pformat_lines(data, max_lines=None) == pp.pformat(data).splitlines()
+
+    def test_flat_input_under_cap_is_complete(self) -> None:
+        # A flat container with fewer elements than the cap takes the
+        # fast path and is returned in full, identical to pformat.
+        pp = PrettyPrinter()
+        data = list(range(5))
+        assert pp.pformat_lines(data, max_lines=11) == pp.pformat(data).splitlines()
+
+    def test_flat_input_over_cap_stops_early(self) -> None:
+        pp = PrettyPrinter()
+        # 50 scalars, one per line, capped well below 50.
+        lines = pp.pformat_lines(list(range(50)), max_lines=11)
+        assert len(lines) <= 11 + 1  # cap, plus a trailing partial line
+
+    def test_nested_element_respects_cap(self) -> None:
+        # Regression: ``len(object)`` is only a *lower* bound on the line
+        # count. A single nested container element expands to many lines,
+        # so a size-only fast path would format the whole thing despite
+        # the cap. The flatness check must route this to the budget stream.
+        pp = PrettyPrinter()
+        data = [{i: "x" * 40 for i in range(50)}]  # len == 1, ~50+ lines
+        lines = pp.pformat_lines(data, max_lines=11)
+        assert len(lines) <= 11 + 1
+
+    def test_mapping_with_nested_value_respects_cap(self) -> None:
+        pp = PrettyPrinter()
+        data = {1: list(range(100))}  # len == 1, value expands
+        lines = pp.pformat_lines(data, max_lines=11)
+        assert len(lines) <= 11 + 1
+
+    def test_nested_dataclass_element_respects_cap(self) -> None:
+        # A dataclass renders one field per line, so a single dataclass
+        # element (``len == 1``) is *not* flat: the flatness check must
+        # route it to the budget stream instead of the size-only fast path.
+        @dataclass
+        class Many:
+            a: int
+            b: int
+            c: int
+            d: int
+            e: int
+            f: int
+            g: int
+            h: int
+
+        pp = PrettyPrinter()
+        lines = pp.pformat_lines([Many(*range(8))], max_lines=4)
+        assert len(lines) <= 4 + 1
+        assert len(lines) < len(pp.pformat([Many(*range(8))]).splitlines())
+
+    def test_sized_non_iterable_falls_back_to_stream(self) -> None:
+        # An object that is sized but not iterable can't be proven flat,
+        # so ``_renders_flat`` bails to the stream rather than risk an
+        # unbounded fast path. It must not raise.
+        class Sized:
+            def __len__(self) -> int:
+                return 3
+
+        pp = PrettyPrinter()
+        obj = Sized()
+        assert pp.pformat_lines(obj, max_lines=5) == pp.pformat(obj).splitlines()
+
+
+def test_pformat_sorts_heterogeneous_set() -> None:
+    # The set fast path tries a natural sort first and falls back to the
+    # type-name key only for unorderable mixes; both must succeed.
+    pp = PrettyPrinter()
+    assert pp.pformat({3, 1, 2}) == "{\n    1,\n    2,\n    3,\n}"
+    # Mixed unorderable types must not raise.
+    pp.pformat({1, "a", 2, "b"})
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..95e31332c0c 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -56,6 +56,11 @@ 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()
@@ -1154,7 +1159,7 @@ 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 +1532,6 @@ 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 +1542,6 @@ 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 +1560,7 @@ 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 +1585,6 @@ 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 +1594,6 @@ 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 +1603,6 @@ 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 +1610,6 @@ 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 +1628,7 @@ def test_many_lines():
             [
                 "*+ 1*",
                 "*+ 3*",
-                f"*truncated ({expected_truncated_lines} lines hidden)*use*-vv*",
+                "*Full output truncated*use*-vv*",
             ]
         )
 
@@ -1643,7 +1642,7 @@ def test_many_lines():
             [
                 "*+ 1*",
                 "*+ 3*",
-                f"*truncated ({expected_truncated_lines} lines hidden)*use*-vv*",
+                "*Full output truncated*use*-vv*",
             ]
         )
 
@@ -1699,9 +1698,7 @@ 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 +1709,92 @@ 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_python25_compile_issue257(pytester: Pytester) -> None:
     pytester.makepyfile(
         """
@@ -2205,6 +2288,128 @@ 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 +2456,8 @@ 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 +2475,8 @@ 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}",
             ],
         ),
         (