fix(sdk): pass through summarization factory prompt knobs (#3559)

Closes #2622

---

Adds keyword-only controls for summarization prompt, trim limit, token
counter, and compact-tool system prompt while preserving model-aware
trigger/keep defaults. Brings to parity with class init.

_Opened collaboratively by Mason Daugherty and open-swe._

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Mason Daugherty <61371264+mdrxy@users.noreply.github.com>
Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/deepagents/deepagents/middleware/summarization.py

@@ -1158,6 +1158,10 @@ This is the name external callers should import and reference.
 def create_summarization_middleware(
     model: BaseChatModel,
     backend: BACKEND_TYPES,
+    *,
+    summary_prompt: str = DEFAULT_SUMMARY_PROMPT,
+    trim_tokens_to_summarize: int | None = None,
+    token_counter: TokenCounter = count_tokens_approximately,
 ) -> _DeepAgentsSummarizationMiddleware:
     """Create a Deep Agents `SummarizationMiddleware` with model-aware defaults.
 
@@ -1199,6 +1203,9 @@ def create_summarization_middleware(
 
             Use `resolve_model()` first if needed for model strings.
         backend: Backend instance or factory for persisting conversation history.
+        summary_prompt: Prompt template for generating summaries.
+        trim_tokens_to_summarize: Max tokens to include when generating summary.
+        token_counter: Function to count tokens in messages.
 
     Returns:
         Configured `SummarizationMiddleware` instance.
@@ -1218,7 +1225,9 @@ def create_summarization_middleware(
         backend=backend,
         trigger=defaults["trigger"],
         keep=defaults["keep"],
-        trim_tokens_to_summarize=None,
+        token_counter=token_counter,
+        summary_prompt=summary_prompt,
+        trim_tokens_to_summarize=trim_tokens_to_summarize,
         truncate_args_settings=defaults["truncate_args_settings"],
     )
 
@@ -1226,6 +1235,8 @@ def create_summarization_middleware(
 def create_summarization_tool_middleware(
     model: str | BaseChatModel,
     backend: BACKEND_TYPES,
+    *,
+    system_prompt: str | None = SUMMARIZATION_SYSTEM_PROMPT,
 ) -> SummarizationToolMiddleware:
     """Create a `SummarizationToolMiddleware` with model-aware defaults.
 
@@ -1257,6 +1268,8 @@ def create_summarization_tool_middleware(
         model: Chat model instance, or a model string
             (e.g. `"anthropic:claude-sonnet-4-6"`).
         backend: Backend instance or factory for persisting conversation history.
+        system_prompt: System-prompt fragment nudging the model to call
+            `compact_conversation`. Pass `None` to skip appending the nudge.
 
     Returns:
         Configured `SummarizationToolMiddleware` instance.
@@ -1307,7 +1320,7 @@ def create_summarization_tool_middleware(
     if isinstance(model, str):
         model = resolve_model(model)
     summarization = create_summarization_middleware(model, backend)
-    return SummarizationToolMiddleware(summarization)
+    return SummarizationToolMiddleware(summarization, system_prompt=system_prompt)
 
 
 class SummarizationToolMiddleware(AgentMiddleware):

libs/deepagents/tests/unit_tests/middleware/test_compact_tool.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from inspect import Parameter, signature
 from typing import Any
 from unittest.mock import MagicMock, NonCallableMagicMock, patch
 
@@ -640,6 +641,22 @@ def test_create_summarization_tool_middleware_returns_instance() -> None:
     assert mw.tools[0].name == "compact_conversation"
 
 
+def test_create_summarization_tool_middleware_accepts_system_prompt() -> None:
+    """Factory passes explicit `system_prompt` through to the tool middleware."""
+    model = GenericFakeChatModel(messages=iter([AIMessage(content="ok")]))
+    model.profile = {"max_input_tokens": 120_000}
+    mw = create_summarization_tool_middleware(model, MagicMock(), system_prompt="custom nudge")
+
+    assert mw.system_prompt == "custom nudge"
+
+
+def test_create_summarization_tool_middleware_system_prompt_is_keyword_only() -> None:
+    """Requires the optional tool nudge to be passed by name."""
+    params = signature(create_summarization_tool_middleware).parameters
+
+    assert params["system_prompt"].kind is Parameter.KEYWORD_ONLY
+
+
 # --- system_prompt override / suppression --------------------------------
 
 

libs/deepagents/tests/unit_tests/middleware/test_summarization_factory.py

@@ -1,10 +1,12 @@
 """Unit tests for the summarization middleware factory."""
 
+from collections.abc import Iterable
+from inspect import Parameter, signature
 from typing import Any, cast
 from unittest.mock import MagicMock
 
 import pytest
-from langchain_core.messages import AIMessage
+from langchain_core.messages import AIMessage, MessageLikeRepresentation
 
 from deepagents.middleware.summarization import create_summarization_middleware
 from tests.unit_tests.chat_model import GenericFakeChatModel
@@ -42,6 +44,35 @@ def test_factory_uses_fallback_defaults_without_profile() -> None:
     assert middleware._truncate_args_keep == ("messages", 20)
 
 
+def test_factory_surfaces_summarization_knobs() -> None:
+    """Passes explicit summarization settings through to the middleware."""
+    model = _make_model(with_profile_limit=120_000)
+
+    def token_counter(messages: Iterable[MessageLikeRepresentation]) -> int:
+        return len(list(messages))
+
+    middleware = create_summarization_middleware(
+        model,
+        cast("Any", MagicMock()),
+        summary_prompt="custom summary prompt: {messages}",
+        trim_tokens_to_summarize=123,
+        token_counter=token_counter,
+    )
+
+    assert middleware._lc_helper.summary_prompt == "custom summary prompt: {messages}"
+    assert middleware._lc_helper.trim_tokens_to_summarize == 123
+    assert middleware._lc_helper.token_counter is token_counter
+
+
+def test_factory_summarization_knobs_are_keyword_only() -> None:
+    """Requires optional factory controls to be passed by name."""
+    params = signature(create_summarization_middleware).parameters
+
+    assert params["summary_prompt"].kind is Parameter.KEYWORD_ONLY
+    assert params["trim_tokens_to_summarize"].kind is Parameter.KEYWORD_ONLY
+    assert params["token_counter"].kind is Parameter.KEYWORD_ONLY
+
+
 def test_factory_rejects_string_model() -> None:
     """Raises `TypeError` when called with a string model name."""
     with pytest.raises(TypeError, match="BaseChatModel"):