diff --git a/backend/docs/summarization.md b/backend/docs/summarization.md
index e478f07a9..ccd8c0779 100644
--- a/backend/docs/summarization.md
+++ b/backend/docs/summarization.md
@@ -95,6 +95,18 @@ summarization:
      value: 0.8  # 80% of max input tokens
    ```
 
+   The percentage resolves from the **summary model's** declared `context_window`
+   — the anchor that generates summaries: `summarization.model_name` when set,
+   otherwise the run's own model. Declare `context_window` on that models entry
+   in `config.yaml`. Third-party OpenAI-compatible models carry no built-in
+   capacity profile, so without a declared `context_window` the fraction clause
+   is dropped with a warning at agent build — any remaining absolute clauses
+   (`tokens` / `messages`) keep working. Caveat: when a separate summary model
+   is configured, its window sizes the threshold — a 64k run model paired with
+   a 128k-window summary model resolves `fraction: 0.8` to ~102k tokens and
+   auto-summarization cannot fire before the run model overflows; in that setup
+   prefer absolute `tokens` thresholds sized for the run model.
+
 **Multiple Triggers:**
 ```yaml
 trigger:
diff --git a/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
index c57be988b..7d4067f4e 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import html
 import logging
+from collections.abc import Mapping
 from dataclasses import dataclass
 from typing import Any, Protocol, override, runtime_checkable
 
@@ -18,6 +19,7 @@ from langgraph.runtime import Runtime
 
 from deerflow.agents.middlewares.dynamic_context_middleware import is_dynamic_context_reminder
 from deerflow.config.app_config import get_app_config
+from deerflow.config.summarization_config import DEFAULT_KEEP
 from deerflow.extensions.notify import notify_context_compacted
 from deerflow.models import create_chat_model
 from deerflow.utils.messages import is_real_user_message
@@ -799,6 +801,73 @@ def _build_summary_anchor(candidate_names: list[str | None], app_config: Any) ->
     return None, None
 
 
+def _anchor_profile_max_input_tokens(model: Any) -> int | None:
+    """Pre-construction mirror of the parent's ``_get_profile_limits`` validation.
+
+    Same rules the parent will apply moments later: ``model.profile`` must be a
+    ``Mapping`` carrying an ``int`` ``max_input_tokens``. Anything else counts as
+    "no usable profile".
+    """
+    profile = getattr(model, "profile", None)
+    if not isinstance(profile, Mapping):
+        return None
+    max_input_tokens = profile.get("max_input_tokens")
+    return max_input_tokens if isinstance(max_input_tokens, int) else None
+
+
+def _drop_unusable_fraction_clauses(
+    anchor_model: Any,
+    trigger: Any,
+    keep: tuple[str, int | float],
+) -> tuple[Any, tuple[str, int | float], bool]:
+    """Drop fraction clauses the anchor model cannot resolve (no usable profile).
+
+    LangChain's parent constructor raises ``ValueError`` for a fraction clause when
+    ``profile["max_input_tokens"]`` is unavailable, which on a third-party
+    OpenAI-compatible model without a declared ``context_window`` would otherwise
+    fail the whole agent build (#3103). Fraction trigger clauses are dropped
+    (absolute clauses survive), and a fraction ``keep`` falls back to the messages
+    default.
+
+    Returns ``(trigger, keep, has_usable_trigger)``; ``has_usable_trigger`` is
+    ``False`` only when trigger clauses were configured and every one of them was
+    a dropped fraction clause. A ``trigger`` that was ``None`` to begin with passes
+    through unchanged with ``has_usable_trigger=True``, preserving the long-standing
+    "enabled but never auto-triggers" configuration.
+    """
+    clauses = list(trigger) if isinstance(trigger, list) else ([] if trigger is None else [trigger])
+    has_fraction_trigger = any(isinstance(clause, tuple) and clause[0] == "fraction" for clause in clauses)
+    keep_is_fraction = isinstance(keep, tuple) and keep[0] == "fraction"
+    if not (has_fraction_trigger or keep_is_fraction):
+        return trigger, keep, True
+    if _anchor_profile_max_input_tokens(anchor_model) is not None:
+        return trigger, keep, True
+
+    kept = [clause for clause in clauses if not (isinstance(clause, tuple) and clause[0] == "fraction")]
+    dropped = [clause for clause in clauses if isinstance(clause, tuple) and clause[0] == "fraction"]
+    if dropped:
+        logger.warning(
+            "Dropped summarization fraction trigger clause(s) %s: the summary model exposes no context window to resolve them against. Declare `context_window` on the model in config.yaml, or use absolute token/message thresholds.",
+            dropped,
+        )
+    new_keep = keep
+    if keep_is_fraction:
+        # The shared constant keeps this fallback identical to SummarizationConfig's
+        # documented default keep.
+        new_keep = DEFAULT_KEEP
+        logger.warning(
+            "Summarization keep %s is unusable without a model context window; falling back to %s. Declare `context_window` on the model in config.yaml to use fraction retention.",
+            keep,
+            new_keep,
+        )
+    if not kept:
+        # No trigger clause survived, but only treat that as "nothing usable" when
+        # clauses were configured at all: a trigger of None keeps constructing the
+        # never-firing middleware, exactly as it does outside this degradation path.
+        return None, new_keep, not clauses
+    return (kept if isinstance(trigger, list) else kept[0]), new_keep, True
+
+
 def create_summarization_middleware(
     *,
     app_config: Any | None = None,
@@ -858,10 +927,32 @@ def create_summarization_middleware(
         logger.warning("Summarization is enabled but no summary model could be constructed; compaction is unavailable for this build")
         return None
 
+    # LangChain's SummarizationMiddleware raises ValueError at construction when a
+    # fraction clause is configured but the anchor exposes no usable profile
+    # (``profile["max_input_tokens"]``) — the default for any third-party
+    # OpenAI-compatible model whose ``context_window`` was not declared in
+    # config.yaml (#3103: `trigger: fraction` used to fail the whole agent build).
+    # Degrade instead: drop the unusable fraction clauses (absolute ones survive)
+    # and fall the keep policy back to its messages default. When every configured
+    # trigger clause is dropped, construction continues with ``trigger=None`` —
+    # the never-firing shape — so manual compaction (``/compact``, which runs with
+    # ``force=True`` and never consults trigger clauses) keeps working for a
+    # profile-less model instead of reporting "compaction is disabled". The factory
+    # attaches a profile from a declared ``context_window``, so this path is
+    # reached only when the model's capacity is genuinely unknown.
+    trigger, keep_tuple, has_usable_trigger = _drop_unusable_fraction_clauses(anchor_model, trigger, keep or config.keep.to_tuple())
+    if not has_usable_trigger:
+        logger.warning(
+            "Every configured summarization trigger is fraction-based but anchor model %r "
+            "exposes no context window (no `context_window` on the model in config.yaml, no provider profile); "
+            "auto-compaction will not fire for this build. Declare `context_window` on the model to enable fraction "
+            "triggers. Manual compaction (/compact) remains available.",
+            anchor_name,
+        )
     kwargs: dict[str, Any] = {
         "model": anchor_model,
         "trigger": trigger,
-        "keep": keep or config.keep.to_tuple(),
+        "keep": keep_tuple,
     }
     if config.trim_tokens_to_summarize is not None:
         kwargs["trim_tokens_to_summarize"] = config.trim_tokens_to_summarize
diff --git a/backend/packages/harness/deerflow/config/model_config.py b/backend/packages/harness/deerflow/config/model_config.py
index 0d212bdd1..36ba5ed90 100644
--- a/backend/packages/harness/deerflow/config/model_config.py
+++ b/backend/packages/harness/deerflow/config/model_config.py
@@ -37,9 +37,11 @@ class ModelConfig(BaseModel):
         gt=0,
         description=(
             "Positive total context window size in tokens (prompt + completion). Used to compute the real-time "
-            "context usage percentage displayed in the chat UI. Distinct from `max_tokens`, which is the "
-            "per-call output cap passed to the provider. Leave unset if unknown; the UI will hide the "
-            "percentage."
+            "context usage percentage displayed in the chat UI, and attached to the model's langchain profile "
+            "(`max_input_tokens`) so fraction-based summarization triggers can resolve their thresholds for "
+            "third-party OpenAI-compatible models that carry no built-in profile. Distinct from `max_tokens`, "
+            "which is the per-call output cap passed to the provider. Leave unset if unknown; the UI will hide "
+            "the percentage and fraction summarization clauses will degrade with a warning."
         ),
     )
     stream_chunk_timeout: float | None = Field(
diff --git a/backend/packages/harness/deerflow/config/summarization_config.py b/backend/packages/harness/deerflow/config/summarization_config.py
index ac130ee29..965cd2ac1 100644
--- a/backend/packages/harness/deerflow/config/summarization_config.py
+++ b/backend/packages/harness/deerflow/config/summarization_config.py
@@ -1,11 +1,16 @@
 """Configuration for conversation summarization."""
 
+import math
 from typing import Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 ContextSizeType = Literal["fraction", "tokens", "messages"]
 DEFAULT_SKILL_FILE_READ_TOOL_NAMES: tuple[str, ...] = ("read_file", "read", "view", "cat")
+#: Documented default retention policy after summarization. Shared with the
+#: summarization middleware's fraction-keep degradation fallback so the two
+#: cannot drift apart.
+DEFAULT_KEEP: tuple[ContextSizeType, int] = ("messages", 20)
 
 
 class ContextSize(BaseModel):
@@ -14,6 +19,34 @@ class ContextSize(BaseModel):
     type: ContextSizeType = Field(description="Type of context size specification")
     value: int | float = Field(description="Value for the context size specification")
 
+    @model_validator(mode="after")
+    def _validate_value_range(self) -> "ContextSize":
+        """Reject value ranges that would silently produce a dead threshold.
+
+        A fraction written percent-style (``value: 80`` instead of ``0.8``) resolves
+        to ``int(max_input_tokens * 80)`` — a threshold the context can never reach,
+        so the trigger silently never fires. Non-finite floats (YAML ``.nan`` /
+        ``.inf`` pass pydantic's float parsing) are dead thresholds the same way
+        (``count >= nan`` is always False), and ``nan <= 0`` is False so the
+        positivity check alone would not catch them. Failing at config load turns
+        these foot-guns into actionable errors, consistent with how fraction
+        clauses degrade (loudly) elsewhere. Absolute ``tokens`` values must simply
+        be positive to describe a usable threshold, while ``messages`` values must
+        additionally be whole numbers: langchain slices the message list with them
+        (``messages[-keep:]``), and a float index raises ``TypeError: list indices
+        must be integers or slices, not float`` mid-compaction.
+        """
+        if not math.isfinite(self.value):
+            raise ValueError(f"ContextSize value must be finite (got {self.value!r})")
+        if self.type == "fraction":
+            if not 0 < self.value <= 1:
+                raise ValueError(f"fraction ContextSize value must be in (0, 1] (got {self.value!r}) — write 0.8 for 80%, not 80")
+        elif self.type == "messages" and not isinstance(self.value, int):
+            raise ValueError(f"messages ContextSize value must be a whole number of messages (got {self.value!r}) — it slices the message list, so a float index would raise TypeError at compaction time")
+        elif self.value <= 0:
+            raise ValueError(f"{self.type} ContextSize value must be positive (got {self.value!r})")
+        return self
+
     def to_tuple(self) -> tuple[ContextSizeType, int | float]:
         """Convert to tuple format expected by SummarizationMiddleware."""
         return (self.type, self.value)
@@ -41,7 +74,7 @@ class SummarizationConfig(BaseModel):
         "{'type': 'fraction', 'value': 0.8} triggers at 80% of model's max input tokens",
     )
     keep: ContextSize = Field(
-        default_factory=lambda: ContextSize(type="messages", value=20),
+        default_factory=lambda: ContextSize(type=DEFAULT_KEEP[0], value=DEFAULT_KEEP[1]),
         description="Context retention policy after summarization. Specifies how much history to preserve. "
         "Examples: {'type': 'messages', 'value': 20} keeps 20 messages, "
         "{'type': 'tokens', 'value': 3000} keeps 3000 tokens, "
diff --git a/backend/packages/harness/deerflow/models/factory.py b/backend/packages/harness/deerflow/models/factory.py
index 21f541a3d..8ebadd3ad 100644
--- a/backend/packages/harness/deerflow/models/factory.py
+++ b/backend/packages/harness/deerflow/models/factory.py
@@ -308,10 +308,29 @@ def create_chat_model(name: str | None = None, thinking_enabled: bool = False, *
         if "stream_usage" in getattr(model_class, "model_fields", {}):
             model_settings_from_config["stream_usage"] = True
 
+    # Translate the declared context window into the langchain profile so
+    # profile-dependent features (e.g. SummarizationMiddleware fraction triggers,
+    # which resolve thresholds from profile["max_input_tokens"]) work for
+    # third-party OpenAI-compatible models whose SDK ships no profile of its
+    # own (#3103). ``profile`` is a metadata-only BaseChatModel field
+    # (exclude=True) and never reaches the provider request payload. An
+    # explicit profile from a caller or model_overrides is never clobbered.
+    translate_context_window = bool(model_config.context_window) and "profile" not in kwargs and "profile" not in model_settings_from_config
+
     _warn_unknown_model_settings(model_class, name, model_settings_from_config)
 
     model_instance = model_class(**kwargs, **model_settings_from_config)
 
+    if translate_context_window:
+        # Applied *after* construction and merged into the provider's inferred
+        # profile: passing ``profile`` to the constructor would REPLACE the whole
+        # inferred metadata (tool_calling, structured_output, io capabilities,
+        # output limits) with just this one key. The declared window wins on
+        # max_input_tokens itself — the operator set it precisely because the
+        # inferred (or absent) value doesn't match their gateway.
+        inferred_profile = getattr(model_instance, "profile", None)
+        model_instance.profile = {**(inferred_profile or {}), "max_input_tokens": model_config.context_window}
+
     if attach_tracing:
         callbacks = build_tracing_callbacks()
         if callbacks:
diff --git a/backend/tests/test_model_factory.py b/backend/tests/test_model_factory.py
index 20675464d..35952f6f7 100644
--- a/backend/tests/test_model_factory.py
+++ b/backend/tests/test_model_factory.py
@@ -155,6 +155,85 @@ def test_context_window_never_reaches_the_provider_client(monkeypatch):
     assert "context_window" not in FakeChatModel.captured_kwargs
 
 
+def test_context_window_attaches_langchain_profile(monkeypatch):
+    """The declared context window is translated into the langchain ``profile``
+    so profile-dependent features (e.g. SummarizationMiddleware fraction triggers,
+    which resolve thresholds from ``profile["max_input_tokens"]``) work for
+    third-party OpenAI-compatible models whose SDK ships no profile of its own
+    (#3103: `trigger: fraction` used to crash the whole agent build)."""
+    model = _make_model("windowed")
+    model.context_window = 200_000
+    cfg = _make_app_config([model])
+    _patch_factory(monkeypatch, cfg)
+
+    FakeChatModel.captured_kwargs = {}
+    created = factory_module.create_chat_model(name="windowed")
+
+    assert "profile" not in FakeChatModel.captured_kwargs
+    assert created.profile == {"max_input_tokens": 200_000}
+
+
+def test_context_window_merges_into_inferred_profile(monkeypatch):
+    """A provider-inferred profile must survive the context_window translation:
+    passing ``profile`` to the constructor would REPLACE the whole inferred
+    metadata (tool_calling, structured_output, output limits) with the single
+    key, changing LangChain feature selection. The declared window wins on
+    ``max_input_tokens`` itself — the operator declared it because the inferred
+    value doesn't match their gateway."""
+    inferred = {
+        "tool_calling": True,
+        "structured_output": True,
+        "max_output_tokens": 16_384,
+        "max_input_tokens": 999_999,
+    }
+
+    class _InferredProfileChatModel(FakeChatModel):
+        def __init__(self, **kwargs):
+            super().__init__(**kwargs)
+            self.profile = dict(inferred)
+
+    model = _make_model("windowed")
+    model.context_window = 200_000
+    cfg = _make_app_config([model])
+    _patch_factory(monkeypatch, cfg, model_class=_InferredProfileChatModel)
+
+    created = factory_module.create_chat_model(name="windowed")
+
+    assert created.profile == {
+        "tool_calling": True,
+        "structured_output": True,
+        "max_output_tokens": 16_384,
+        "max_input_tokens": 200_000,
+    }
+
+
+def test_unset_context_window_leaves_profile_unset(monkeypatch):
+    """No declared window -> no invented profile; fraction triggers degrade
+    with a warning instead of silently assuming a capacity."""
+    cfg = _make_app_config([_make_model("opaque")])
+    _patch_factory(monkeypatch, cfg)
+
+    FakeChatModel.captured_kwargs = {}
+    created = factory_module.create_chat_model(name="opaque")
+
+    assert "profile" not in FakeChatModel.captured_kwargs
+    assert created.profile is None
+
+
+def test_context_window_does_not_clobber_explicit_profile(monkeypatch):
+    """A caller-supplied profile wins over the context_window translation."""
+    model = _make_model("windowed")
+    model.context_window = 200_000
+    cfg = _make_app_config([model])
+    _patch_factory(monkeypatch, cfg)
+
+    FakeChatModel.captured_kwargs = {}
+    created = factory_module.create_chat_model(name="windowed", profile={"max_input_tokens": 100})
+
+    assert FakeChatModel.captured_kwargs.get("profile") == {"max_input_tokens": 100}
+    assert created.profile == {"max_input_tokens": 100}
+
+
 def test_appends_all_tracing_callbacks(monkeypatch):
     cfg = _make_app_config([_make_model("alpha")])
     _patch_factory(monkeypatch, cfg)
diff --git a/backend/tests/test_summarization_middleware.py b/backend/tests/test_summarization_middleware.py
index eb0306da4..856f39634 100644
--- a/backend/tests/test_summarization_middleware.py
+++ b/backend/tests/test_summarization_middleware.py
@@ -10,13 +10,17 @@ from langchain_core.language_models import BaseChatModel
 from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage, SystemMessage, ToolMessage
 from langchain_core.outputs import ChatGeneration, ChatResult
 from langgraph.constants import TAG_NOSTREAM
+from pydantic import ValidationError
 
 from deerflow.agents.memory.summarization_hook import memory_flush_hook
 from deerflow.agents.middlewares.dynamic_context_middleware import _DYNAMIC_CONTEXT_REMINDER_KEY, DynamicContextMiddleware, is_dynamic_context_reminder
 from deerflow.agents.middlewares.summarization_middleware import DeerFlowSummarizationMiddleware, SummarizationEvent, SummaryGenerationError, create_summarization_middleware
 from deerflow.agents.thread_state import ThreadState
+from deerflow.config.app_config import AppConfig
 from deerflow.config.memory_config import MemoryConfig
-from deerflow.config.summarization_config import SummarizationConfig
+from deerflow.config.model_config import ModelConfig
+from deerflow.config.sandbox_config import SandboxConfig
+from deerflow.config.summarization_config import ContextSize, SummarizationConfig
 
 
 def _messages() -> list:
@@ -1077,11 +1081,11 @@ def test_before_summarization_hook_not_fired_when_summary_fails(monkeypatch: pyt
     assert captured == []
 
 
-def _factory_app_config(model_names, *, summary_model_name=None):
+def _factory_app_config(model_names, *, summary_model_name=None, summarization_kwargs=None):
     """AppConfig-shaped stub for the factory: summarization enabled + ordered models."""
     models = [SimpleNamespace(name=name) for name in model_names]
     return SimpleNamespace(
-        summarization=SummarizationConfig(enabled=True, model_name=summary_model_name),
+        summarization=SummarizationConfig(enabled=True, model_name=summary_model_name, **(summarization_kwargs or {})),
         memory=MemoryConfig(enabled=False),
         models=models,
         get_model_config=lambda name: next((model for model in models if model.name == name), None),
@@ -1141,6 +1145,184 @@ def test_factory_configured_constructor_failure_falls_back_to_run_model(monkeypa
     assert result.summary_text == "from-run-model"
 
 
+def _profileless_anchor_stub() -> MagicMock:
+    """Anchor stub whose ``.profile`` is unusable (not a Mapping), like any
+    third-party OpenAI-compatible client constructed without a profile."""
+    model = MagicMock()
+    model.with_config.return_value = model
+    model.invoke.return_value = SimpleNamespace(text="summary")
+    model.ainvoke = AsyncMock(return_value=SimpleNamespace(text="summary"))
+    return model
+
+
+def test_factory_fraction_only_trigger_degrades_to_manual_compaction_only(monkeypatch, caplog: pytest.LogCaptureFixture) -> None:
+    """#3103 mechanism (b): a fraction trigger whose anchor exposes no usable profile
+    must not raise out of ``create_summarization_middleware`` (which used to fail the
+    whole agent build). With no absolute clause to keep, the middleware still
+    constructs as never-firing so manual compaction (/compact, force=True, never
+    consults trigger clauses) keeps working; the warning names the config fix."""
+    fake_model = _profileless_anchor_stub()
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+    cfg = _factory_app_config(("models0",), summarization_kwargs={"trigger": ContextSize(type="fraction", value=0.8)})
+
+    with caplog.at_level("WARNING", logger="deerflow.agents.middlewares.summarization_middleware"):
+        middleware = create_summarization_middleware(app_config=cfg, run_model_name="models0", keep=("messages", 2))
+
+    assert middleware is not None  # degraded, not raised — and not disabled either
+    assert "context_window" in caplog.text  # the warning names the fix
+    assert "Manual compaction" in caplog.text  # and says manual /compact still works
+    result = middleware.compact_state({"messages": _messages()}, _runtime(), force=True)
+    assert result is not None  # forced compaction never consults trigger clauses
+    assert result.summary_text == "summary"
+
+
+def test_factory_drops_only_fraction_clauses_and_keeps_absolute_ones(monkeypatch, caplog: pytest.LogCaptureFixture) -> None:
+    """Mixed [fraction, messages] triggers degrade to the messages clause alone:
+    construction succeeds and message-count compaction still fires."""
+    fake_model = _profileless_anchor_stub()
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+    cfg = _factory_app_config(
+        ("models0",),
+        summarization_kwargs={"trigger": [ContextSize(type="fraction", value=0.8), ContextSize(type="messages", value=3)]},
+    )
+
+    with caplog.at_level("WARNING", logger="deerflow.agents.middlewares.summarization_middleware"):
+        middleware = create_summarization_middleware(app_config=cfg, run_model_name="models0", keep=("messages", 2))
+
+    assert middleware is not None  # the absolute clause kept the middleware alive
+    assert "context_window" in caplog.text
+    result = middleware.compact_state({"messages": _messages()}, _runtime(), force=True)
+    assert result is not None
+    assert result.summary_text == "summary"
+
+
+def test_factory_keep_fraction_falls_back_to_messages_default(monkeypatch, caplog: pytest.LogCaptureFixture) -> None:
+    """A fraction ``keep`` against a profile-less anchor falls back to the
+    messages default instead of failing construction."""
+    fake_model = _profileless_anchor_stub()
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+    cfg = _factory_app_config(
+        ("models0",),
+        summarization_kwargs={"trigger": ContextSize(type="messages", value=3), "keep": ContextSize(type="fraction", value=0.3)},
+    )
+
+    with caplog.at_level("WARNING", logger="deerflow.agents.middlewares.summarization_middleware"):
+        middleware = create_summarization_middleware(app_config=cfg, run_model_name="models0")
+
+    assert middleware is not None
+    assert middleware.keep == ("messages", 20)  # SummarizationConfig's documented default
+
+
+def test_factory_null_trigger_with_fraction_keep_still_constructs(monkeypatch, caplog: pytest.LogCaptureFixture) -> None:
+    """``trigger: null`` + fraction ``keep``: the long-standing "enabled but never
+    auto-triggers" setup must keep constructing — with the keep degraded to the
+    messages default — rather than disabling compaction. On main this exact config
+    crashes the agent build (fraction keep needs a profile)."""
+    fake_model = _profileless_anchor_stub()
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+    cfg = _factory_app_config(("models0",), summarization_kwargs={"keep": ContextSize(type="fraction", value=0.3)})
+
+    with caplog.at_level("WARNING", logger="deerflow.agents.middlewares.summarization_middleware"):
+        middleware = create_summarization_middleware(app_config=cfg, run_model_name="models0")
+
+    assert middleware is not None  # never-firing but constructed, same as any trigger: null setup
+    assert middleware.keep == ("messages", 20)
+    assert "context_window" in caplog.text
+
+
+def test_factory_fraction_trigger_survives_when_anchor_has_profile(monkeypatch) -> None:
+    """With a usable profile on the anchor (the factory attaches one from
+    ``context_window``), the fraction clause is kept as configured — construction
+    succeeding is itself the regression pin (#3103: it used to raise)."""
+    model = _StaticChatModel(profile={"max_input_tokens": 65536})
+    assert model.profile == {"max_input_tokens": 65536}
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: model)
+    cfg = _factory_app_config(("models0",), summarization_kwargs={"trigger": ContextSize(type="fraction", value=0.8)})
+
+    middleware = create_summarization_middleware(app_config=cfg, run_model_name="models0", keep=("messages", 2))
+
+    assert middleware is not None
+    result = middleware.compact_state({"messages": _messages()}, _runtime(), force=True)
+    assert result is not None
+    assert result.summary_text == "ok"
+
+
+def test_context_size_rejects_percent_style_fraction_value() -> None:
+    """A fraction written percent-style (80 instead of 0.8) would resolve to
+    int(capacity * 80) — a threshold the context can never reach, so the trigger
+    silently never fires. Config load is the failure point, not a dead trigger."""
+    with pytest.raises(ValidationError, match="fraction ContextSize value must be in"):
+        ContextSize(type="fraction", value=80)
+
+
+def test_context_size_rejects_non_positive_absolute_values() -> None:
+    with pytest.raises(ValidationError, match="tokens ContextSize value must be positive"):
+        ContextSize(type="tokens", value=0)
+    with pytest.raises(ValidationError, match="messages ContextSize value must be positive"):
+        ContextSize(type="messages", value=-5)
+
+
+def test_context_size_rejects_non_finite_values() -> None:
+    """YAML .nan / .inf pass pydantic's float parsing but never describe a usable
+    threshold (``count >= nan`` is always False, and ``nan <= 0`` is False so the
+    positivity check alone would not catch them) — they must fail at config load."""
+    with pytest.raises(ValidationError, match="ContextSize value must be finite"):
+        ContextSize(type="tokens", value=float("nan"))
+    with pytest.raises(ValidationError, match="ContextSize value must be finite"):
+        ContextSize(type="fraction", value=float("inf"))
+
+
+def test_context_size_rejects_fractional_message_counts() -> None:
+    """``messages`` values slice the message list at compaction time
+    (``messages[-keep:]``) — a float raises ``TypeError: list indices must be
+    integers or slices, not float`` mid-compaction, so config load must reject
+    it first. Even an integral float (``20.0``) is a float index to a slice."""
+    with pytest.raises(ValidationError, match="messages ContextSize value must be a whole number"):
+        ContextSize(type="messages", value=1.5)
+    with pytest.raises(ValidationError, match="messages ContextSize value must be a whole number"):
+        ContextSize(type="messages", value=20.0)
+
+
+def test_context_size_accepts_boundary_values() -> None:
+    assert ContextSize(type="fraction", value=1).to_tuple() == ("fraction", 1)
+    assert ContextSize(type="fraction", value=0.8).to_tuple() == ("fraction", 0.8)
+    assert ContextSize(type="messages", value=20).to_tuple() == ("messages", 20)
+
+
+def test_factory_wiring_context_window_to_fraction_trigger_end_to_end() -> None:
+    """Pins the two halves of the fix together without monkeypatching
+    ``create_chat_model``: a ``context_window``-declared model gets a profile from
+    the real model factory, the fraction clause survives
+    ``_drop_unusable_fraction_clauses``, and the middleware constructs with the
+    trigger intact. The middleware-side tests stub the factory and the
+    factory-side tests stop at captured kwargs — this is the automated pin of the
+    shipped contract (the manual E2E in the PR body was the only wiring proof)."""
+    model = ModelConfig(
+        name="gw-64k",
+        display_name="gw-64k",
+        description=None,
+        use="langchain_openai:ChatOpenAI",
+        model="some-64k-model",
+        base_url="https://third-party-gateway.example.com/v1",
+        api_key="sk-test",
+        supports_thinking=False,
+        supports_reasoning_effort=False,
+        supports_vision=False,
+        context_window=65_536,
+    )
+    cfg = AppConfig(
+        models=[model],
+        sandbox=SandboxConfig(use="deerflow.sandbox.local:LocalSandboxProvider"),
+        summarization=SummarizationConfig(enabled=True, trigger=ContextSize(type="fraction", value=0.8)),
+        memory=MemoryConfig(enabled=False),
+    )
+
+    middleware = create_summarization_middleware(app_config=cfg, run_model_name="gw-64k")
+
+    assert middleware is not None  # on main this raises: no profile without the factory translation
+    assert middleware.model.profile == {"max_input_tokens": 65_536}
+
+
 class _RaisingTextResponse:
     """A provider response whose ``.text`` accessor fails — a realistic malformed result."""
 
diff --git a/config.example.yaml b/config.example.yaml
index e2c098750..168d06c05 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -102,8 +102,12 @@ max_recursion_limit: 1000
 # Two token fields look similar but mean different things:
 # - `max_tokens` is the per-call OUTPUT cap passed to the provider.
 # - `context_window` is a positive integer for the total context capacity
-#   (prompt + completion) and drives the real-time "% context used" indicator
-#   in the chat UI.
+#   (prompt + completion). It drives the real-time "% context used" indicator
+#   in the chat UI and feeds the model's langchain profile, which fraction-based
+#   summarization triggers (`summarization.trigger: [{type: fraction, ...}]`)
+#   resolve their thresholds from. Third-party OpenAI-compatible models carry no
+#   built-in profile, so without `context_window` a fraction trigger degrades
+#   (dropped with a warning) instead of crashing the agent build.
 # Leave `context_window` unset if the provider limit is unknown; the percentage
 # will not render. Verify configured values against the provider's model docs.
 #
@@ -1760,7 +1764,14 @@ summarization:
     # Uncomment to also trigger when message count reaches 50
     # - type: messages
     #   value: 50
-    # Uncomment to trigger when 80% of model's max input tokens is reached
+    # Uncomment to trigger when 80% of model's max input tokens is reached.
+    # The percentage resolves from the SUMMARY model's declared `context_window`
+    # (summarization.model_name when set, else the run's own model): declare it
+    # on that models entry — third-party OpenAI-compatible models carry no
+    # built-in profile. Without it the fraction clause is dropped with a warning
+    # and any remaining absolute clauses keep working. If a separate summary
+    # model with a larger window is configured, prefer absolute `tokens`
+    # thresholds sized for the run model instead.
     # - type: fraction
     #   value: 0.8