diff --git a/benchmarks/swebench/run_infer.py b/benchmarks/swebench/run_infer.py
index 33f802373..a5836351a 100644
--- a/benchmarks/swebench/run_infer.py
+++ b/benchmarks/swebench/run_infer.py
@@ -26,6 +26,7 @@
 from benchmarks.utils.console_logging import summarize_instance
 from benchmarks.utils.constants import EVAL_AGENT_SERVER_IMAGE
 from benchmarks.utils.conversation import build_event_persistence_callback
+from benchmarks.utils.cost_cap import build_cost_cap_callback
 from benchmarks.utils.critics import create_critic
 from benchmarks.utils.dataset import get_dataset
 from benchmarks.utils.evaluation import Evaluation
@@ -302,14 +303,30 @@ def evaluate_instance(
             instance_id=instance.id,
             attempt=self.current_attempt,
         )
+        callbacks: list = [persist_callback]
+
+        # Optional per-instance cost cap (defence-in-depth against runaway
+        # cost on hard instances). Constructed before the Conversation so it
+        # is part of the composed callback chain from the first event, then
+        # bound to the conversation right after construction so it can call
+        # ``conversation.pause()`` and read accumulated cost.
+        cost_cap_callback = None
+        if self.metadata.max_cost_per_instance is not None:
+            cost_cap_callback = build_cost_cap_callback(
+                max_cost_per_instance=self.metadata.max_cost_per_instance,
+                instance_id=instance.id,
+            )
+            callbacks.append(cost_cap_callback)
 
         conversation = Conversation(
             agent=agent,
             workspace=workspace,
-            callbacks=[persist_callback],
+            callbacks=callbacks,
             max_iteration_per_run=self.metadata.max_iterations,
             delete_on_close=True,
         )
+        if cost_cap_callback is not None:
+            cost_cap_callback.bind(conversation)
 
         logger.info("repo_path: %s", repo_path)
         source_repo_path = self.get_source_repo_path(instance)
@@ -441,6 +458,7 @@ def main() -> None:
         enable_condenser=enable_condenser,
         condenser_max_size=args.condenser_max_size,
         condenser_keep_first=args.condenser_keep_first,
+        max_cost_per_instance=args.max_cost_per_instance,
     )
 
     # Run orchestrator with a simple JSONL writer
diff --git a/benchmarks/utils/args_parser.py b/benchmarks/utils/args_parser.py
index 4df17499b..c97469571 100644
--- a/benchmarks/utils/args_parser.py
+++ b/benchmarks/utils/args_parser.py
@@ -129,6 +129,16 @@ def get_parser(add_llm_config: bool = True) -> argparse.ArgumentParser:
         type=int,
         help="Number of initial events to always keep when condensing",
     )
+    parser.add_argument(
+        "--max-cost-per-instance",
+        type=float,
+        default=None,
+        help=(
+            "Optional per-instance accumulated cost cap in USD. When set "
+            "(must be > 0), the conversation is paused as soon as "
+            "accumulated_cost exceeds this value. Defaults to no cap."
+        ),
+    )
     return parser
 
 
diff --git a/benchmarks/utils/cost_cap.py b/benchmarks/utils/cost_cap.py
new file mode 100644
index 000000000..455c4fde5
--- /dev/null
+++ b/benchmarks/utils/cost_cap.py
@@ -0,0 +1,121 @@
+"""Per-instance cost cap callback for benchmarks.
+
+Some evaluations have shown that a small minority of instances can consume
+disproportionately large amounts of money (e.g. for the Gemini-3.5-Flash
+SWE-bench run, 22 instances cost >$10 each and accounted for ~20% of the
+total $1900+ spend). The dominant mechanism is a combination of:
+
+* large iteration counts (some instances ran 300+ iterations);
+* the LLM-summarising condenser, which periodically rewrites the prompt
+  prefix and therefore invalidates the provider's prompt cache (cache-read
+  ratio dropped from ~45% on cheap instances to ~10% on the expensive
+  ones); and
+* high reasoning effort, which makes every uncached call more costly.
+
+This module provides a small, optional defence-in-depth: a
+:class:`Conversation` callback that pauses the conversation once the
+accumulated per-instance cost exceeds a configured threshold. It does not
+attempt to fix the root cause (which would require SDK-level changes to the
+condenser or to enforce ``Metrics.max_budget_per_task``); it simply caps
+the blast radius.
+
+When the cap is hit, the callback calls ``conversation.pause()`` which
+takes effect at the next iteration boundary, mirroring the existing
+behaviour of ``max_iteration_per_run``. Any patch produced up to that point
+is still collected and submitted.
+"""
+
+from __future__ import annotations
+
+from typing import Callable
+
+from openhands.sdk import Event, get_logger
+
+
+logger = get_logger(__name__)
+
+ConversationCallback = Callable[[Event], None]
+
+
+class CostCapCallback:
+    """Callback that pauses a conversation once accumulated cost exceeds a cap.
+
+    Use :meth:`bind` to attach the callback to the conversation after the
+    conversation has been constructed; the callback can be passed to the
+    :class:`Conversation` constructor *before* binding so that the
+    callback is part of the composed callback chain from the very first
+    event.
+    """
+
+    def __init__(self, max_cost_per_instance: float, instance_id: str) -> None:
+        """
+        Args:
+            max_cost_per_instance: Maximum allowed accumulated USD cost for
+                this instance. Must be strictly positive.
+            instance_id: Identifier used only for log messages.
+
+        Raises:
+            ValueError: If ``max_cost_per_instance`` is not strictly positive.
+        """
+        if max_cost_per_instance <= 0:
+            raise ValueError(
+                f"max_cost_per_instance must be > 0, got {max_cost_per_instance}"
+            )
+        self.max_cost_per_instance = max_cost_per_instance
+        self.instance_id = instance_id
+        self._conversation = None  # type: ignore[assignment]
+        self._triggered = False
+
+    def bind(self, conversation: object) -> None:
+        """Attach the conversation whose cost will be monitored."""
+        self._conversation = conversation
+
+    def __call__(self, event: Event) -> None:  # noqa: ARG002 - event is unused
+        if self._triggered or self._conversation is None:
+            return
+        try:
+            cost = self._conversation.conversation_stats.get_combined_metrics().accumulated_cost
+        except Exception as exc:
+            # Metrics access should never block the run.
+            logger.debug(
+                "cost_cap: failed to read accumulated_cost for %s: %s",
+                self.instance_id,
+                exc,
+            )
+            return
+
+        if cost >= self.max_cost_per_instance:
+            self._triggered = True
+            logger.warning(
+                "cost_cap: instance %s exceeded per-instance budget "
+                "(accumulated_cost=$%.4f >= cap=$%.4f); pausing conversation.",
+                self.instance_id,
+                cost,
+                self.max_cost_per_instance,
+            )
+            try:
+                self._conversation.pause()
+            except Exception as exc:
+                # Defensive: if pause itself fails (e.g. remote conversation
+                # in an odd state), don't take the whole instance down.
+                logger.warning(
+                    "cost_cap: pause() raised for %s: %s",
+                    self.instance_id,
+                    exc,
+                )
+
+
+def build_cost_cap_callback(
+    max_cost_per_instance: float, instance_id: str
+) -> CostCapCallback:
+    """Convenience wrapper. See :class:`CostCapCallback`.
+
+    Returns:
+        An unbound :class:`CostCapCallback`. The caller must call
+        ``bind(conversation)`` once the conversation has been created so
+        the callback knows which conversation to read cost from and pause.
+    """
+    return CostCapCallback(
+        max_cost_per_instance=max_cost_per_instance,
+        instance_id=instance_id,
+    )
diff --git a/benchmarks/utils/models.py b/benchmarks/utils/models.py
index 9dd471383..8190b2dd8 100644
--- a/benchmarks/utils/models.py
+++ b/benchmarks/utils/models.py
@@ -92,6 +92,18 @@ class EvalMetadata(BaseModel):
         ge=0,
         description="Number of initial events to always keep when condensing",
     )
+    max_cost_per_instance: float | None = Field(
+        default=None,
+        gt=0,
+        description=(
+            "Optional per-instance accumulated cost cap in USD. When set, "
+            "the conversation is paused as soon as accumulated_cost exceeds "
+            "this value, mirroring the behaviour of max_iterations. None "
+            "(the default) disables the cap. This is a defence-in-depth "
+            "measure against runaway-cost instances and does not affect "
+            "behaviour when the cap is not reached."
+        ),
+    )
     lmnr: LaminarEvalMetadata | None = Field(
         default=None,
         description="Laminar evaluation metadata",
diff --git a/tests/test_cost_cap.py b/tests/test_cost_cap.py
new file mode 100644
index 000000000..36f2423e5
--- /dev/null
+++ b/tests/test_cost_cap.py
@@ -0,0 +1,143 @@
+"""Tests for the per-instance cost cap callback."""
+
+from __future__ import annotations
+
+import pytest
+
+from benchmarks.utils.cost_cap import CostCapCallback, build_cost_cap_callback
+
+
+class _FakeMetrics:
+    def __init__(self, cost: float) -> None:
+        self.accumulated_cost = cost
+
+
+class _FakeStats:
+    def __init__(self, cost: float) -> None:
+        self._cost = cost
+
+    def get_combined_metrics(self) -> _FakeMetrics:
+        return _FakeMetrics(self._cost)
+
+
+class _FakeConversation:
+    """Minimal stand-in for ``BaseConversation`` for unit testing.
+
+    Exposes only the surface the cost-cap callback touches:
+    ``conversation_stats`` and ``pause()``.
+    """
+
+    def __init__(self, cost: float) -> None:
+        self._cost = cost
+        self.paused = False
+        self.pause_call_count = 0
+
+    @property
+    def conversation_stats(self) -> _FakeStats:
+        return _FakeStats(self._cost)
+
+    def set_cost(self, cost: float) -> None:
+        self._cost = cost
+
+    def pause(self) -> None:
+        self.paused = True
+        self.pause_call_count += 1
+
+
+def test_rejects_non_positive_cap():
+    with pytest.raises(ValueError):
+        build_cost_cap_callback(max_cost_per_instance=0.0, instance_id="x")
+    with pytest.raises(ValueError):
+        build_cost_cap_callback(max_cost_per_instance=-1.0, instance_id="x")
+
+
+def test_no_pause_below_cap():
+    convo = _FakeConversation(cost=2.5)
+    cb = build_cost_cap_callback(max_cost_per_instance=10.0, instance_id="inst")
+    cb.bind(convo)
+    cb(event=object())  # event is unused
+    assert convo.paused is False
+
+
+def test_pauses_when_cap_reached():
+    convo = _FakeConversation(cost=10.0)
+    cb = build_cost_cap_callback(max_cost_per_instance=10.0, instance_id="inst")
+    cb.bind(convo)
+    cb(event=object())
+    assert convo.paused is True
+    assert convo.pause_call_count == 1
+
+
+def test_pauses_when_cap_exceeded():
+    convo = _FakeConversation(cost=0.0)
+    cb = build_cost_cap_callback(max_cost_per_instance=5.0, instance_id="inst")
+    cb.bind(convo)
+    cb(event=object())
+    assert convo.paused is False
+
+    convo.set_cost(7.5)
+    cb(event=object())
+    assert convo.paused is True
+
+
+def test_idempotent_once_triggered():
+    """Once paused, the callback must not call pause() again on subsequent events."""
+    convo = _FakeConversation(cost=20.0)
+    cb = build_cost_cap_callback(max_cost_per_instance=5.0, instance_id="inst")
+    cb.bind(convo)
+    for _ in range(5):
+        cb(event=object())
+    assert convo.pause_call_count == 1
+
+
+def test_no_op_when_not_bound():
+    """Calling the callback before bind() should be a safe no-op."""
+    cb = build_cost_cap_callback(max_cost_per_instance=1.0, instance_id="inst")
+    # Should not raise.
+    cb(event=object())
+    # Still works after binding.
+    convo = _FakeConversation(cost=2.0)
+    cb.bind(convo)
+    cb(event=object())
+    assert convo.paused is True
+
+
+def test_metrics_failure_does_not_crash():
+    """If reading metrics raises, the callback must swallow the error."""
+
+    class _BrokenConversation(_FakeConversation):
+        @property
+        def conversation_stats(self):
+            raise RuntimeError("metrics unavailable")
+
+    convo = _BrokenConversation(cost=0.0)
+    cb = build_cost_cap_callback(max_cost_per_instance=1.0, instance_id="inst")
+    cb.bind(convo)
+    cb(event=object())  # must not raise
+    assert convo.paused is False
+
+
+def test_pause_failure_does_not_crash():
+    """If pause() raises, the callback must swallow the error and stay triggered."""
+
+    class _BrokenPauseConversation(_FakeConversation):
+        def pause(self) -> None:
+            self.pause_call_count += 1
+            raise RuntimeError("cannot pause")
+
+    convo = _BrokenPauseConversation(cost=10.0)
+    cb = build_cost_cap_callback(max_cost_per_instance=1.0, instance_id="inst")
+    cb.bind(convo)
+    cb(event=object())  # must not raise
+    # Once it tried to pause once, it stays triggered and won't try again.
+    cb(event=object())
+    assert convo.pause_call_count == 1
+
+
+def test_callback_class_directly():
+    """CostCapCallback can be constructed and used directly."""
+    convo = _FakeConversation(cost=15.0)
+    cb = CostCapCallback(max_cost_per_instance=10.0, instance_id="direct")
+    cb.bind(convo)
+    cb(event=object())
+    assert convo.paused is True