diff --git a/sdks/python/src/honcho/api_types.py b/sdks/python/src/honcho/api_types.py
index 23692f2f..32db3226 100644
--- a/sdks/python/src/honcho/api_types.py
+++ b/sdks/python/src/honcho/api_types.py
@@ -45,6 +45,7 @@ class SummaryConfiguration(BaseModel):
     enabled: bool | None = None
     messages_per_short_summary: int | None = None
     messages_per_long_summary: int | None = None
+    custom_instructions: str | None = None
 
 
 class DreamConfiguration(BaseModel):
diff --git a/src/schemas/configuration.py b/src/schemas/configuration.py
index 53a8c11a..22fd78ea 100644
--- a/src/schemas/configuration.py
+++ b/src/schemas/configuration.py
@@ -65,6 +65,15 @@ class SummaryConfiguration(BaseModel):
         ge=20,
         description="Number of messages per long summary. Must be positive, greater than or equal to 20, and greater than messages_per_short_summary.",
     )
+    custom_instructions: str | None = Field(
+        default=None,
+        description="Optional custom instructions for session summaries. Validated against DERIVER.MAX_CUSTOM_INSTRUCTIONS_TOKENS.",
+    )
+
+    @field_validator("custom_instructions")
+    @classmethod
+    def validate_custom_instructions(cls, value: str | None) -> str | None:
+        return _validate_custom_instructions_budget(value)
 
     @model_validator(mode="after")
     def validate_summary_thresholds(self) -> Self:
@@ -130,7 +139,7 @@ class WorkspaceConfiguration(BaseModel):
     )
     dream: DreamConfiguration | None = Field(
         default=None,
-        description="Configuration for dream functionality. If reasoning is disabled, dreams will also be disabled and these settings will be ignored.",
+        description="Configuration for dream functionality. If reasoning is disabled, dreams will also be disabled and this setting will be ignored.",
     )
 
 
@@ -155,6 +164,10 @@ class MessageConfiguration(BaseModel):
         default=None,
         description="Configuration for reasoning functionality.",
     )
+    summary: SummaryConfiguration | None = Field(
+        default=None,
+        description="Configuration for summary functionality.",
+    )
 
 
 class ResolvedReasoningConfiguration(BaseModel):
@@ -176,6 +189,12 @@ class ResolvedSummaryConfiguration(BaseModel):
     enabled: bool
     messages_per_short_summary: int
     messages_per_long_summary: int
+    custom_instructions: str | None = None
+
+    @field_validator("custom_instructions")
+    @classmethod
+    def validate_custom_instructions(cls, value: str | None) -> str | None:
+        return _validate_custom_instructions_budget(value)
 
 
 class ResolvedDreamConfiguration(BaseModel):
diff --git a/src/utils/config_helpers.py b/src/utils/config_helpers.py
index b4afa078..6c1ae124 100644
--- a/src/utils/config_helpers.py
+++ b/src/utils/config_helpers.py
@@ -12,18 +12,34 @@ from src.schemas import (
 
 logger = logging.getLogger(__name__)
 
+_NONE_OVERRIDE_PATHS: set[tuple[str, ...]] = {
+    ("summary", "custom_instructions"),
+}
 
-def deep_update(base: dict[str, Any], update: dict[str, Any]) -> None:
+
+def deep_update(
+    base: dict[str, Any],
+    update: dict[str, Any],
+    path: tuple[str, ...] = (),
+) -> None:
     """
     Recursive update of a dictionary.
-    Skips None values in the update dictionary.
+    Skips None values unless None explicitly clears a nullable field.
     """
     for key, value in update.items():
+        current_path = (*path, key)
+
         if value is None:
+            if current_path in _NONE_OVERRIDE_PATHS:
+                base[key] = None
             continue
 
         if isinstance(value, dict) and key in base and isinstance(base[key], dict):
-            deep_update(cast(dict[str, Any], base[key]), cast(dict[str, Any], value))
+            deep_update(
+                cast(dict[str, Any], base[key]),
+                cast(dict[str, Any], value),
+                current_path,
+            )
         else:
             base[key] = value
 
@@ -113,6 +129,7 @@ def get_configuration(
             "enabled": settings.SUMMARY.ENABLED,
             "messages_per_short_summary": settings.SUMMARY.MESSAGES_PER_SHORT_SUMMARY,
             "messages_per_long_summary": settings.SUMMARY.MESSAGES_PER_LONG_SUMMARY,
+            "custom_instructions": None,
         },
         "dream": {"enabled": settings.DREAM.ENABLED},
     }
@@ -130,7 +147,7 @@ def get_configuration(
         deep_update(
             config_dict,
             normalize_configuration_dict(
-                message_configuration.model_dump(exclude_none=True)
+                message_configuration.model_dump(exclude_unset=True)
             ),
         )
 
diff --git a/src/utils/summarizer.py b/src/utils/summarizer.py
index 2abdb0f9..780065ee 100644
--- a/src/utils/summarizer.py
+++ b/src/utils/summarizer.py
@@ -2,7 +2,7 @@ import asyncio
 import logging
 import time
 from enum import Enum
-from functools import cache
+from functools import cache, lru_cache
 from inspect import cleandoc as c
 from typing import TypedDict
 
@@ -15,6 +15,8 @@ from src.cache.client import cache as cache_client
 from src.config import ConfiguredModelSettings, settings
 from src.crud.session import session_cache_key
 from src.dependencies import tracked_db
+# TODO: move _custom_instructions_section to shared utility
+from src.deriver.prompts import _custom_instructions_section
 from src.exceptions import ResourceNotFoundException
 from src.llm import HonchoLLMCallResponse, honcho_llm_call
 from src.llm.types import LLMTelemetryContext
@@ -58,6 +60,7 @@ class Summary(TypedDict):
 
 
 def to_schema_summary(s: Summary) -> schemas.Summary:
+    """Convert a Summary TypedDict to a Pydantic Summary schema object."""
     return schemas.Summary(
         content=s["content"],
         message_id=s["message_id"],
@@ -82,6 +85,7 @@ __all__ = [
 
 
 def _get_summary_model_config() -> ConfiguredModelSettings:
+    """Return the configured model settings for summary generation."""
     return settings.SUMMARY.MODEL_CONFIG
 
 
@@ -102,8 +106,10 @@ def short_summary_prompt(
     formatted_messages: str,
     output_words: int,
     previous_summary_text: str,
+    custom_instructions: str | None = None,
 ) -> str:
     """Generate the short summary prompt."""
+    custom_instructions_section = _custom_instructions_section(custom_instructions)
     return c(f"""
 You are a system that summarizes parts of a conversation to create a concise and accurate summary. Focus on capturing:
 
@@ -118,6 +124,7 @@ Provide a concise, factual summary that captures the essence of the conversation
 
 Return only the summary without any explanation or meta-commentary.
 
+{custom_instructions_section}
 <previous_summary>
 {previous_summary_text}
 </previous_summary>
@@ -134,8 +141,10 @@ def long_summary_prompt(
     formatted_messages: str,
     output_words: int,
     previous_summary_text: str,
+    custom_instructions: str | None = None,
 ) -> str:
     """Generate the long summary prompt."""
+    custom_instructions_section = _custom_instructions_section(custom_instructions)
     return c(f"""
 You are a system that creates thorough, comprehensive summaries of conversations. Focus on capturing:
 
@@ -152,6 +161,7 @@ Provide a thorough and detailed summary that captures the essence of the convers
 
 Return only the summary without any explanation or meta-commentary.
 
+{custom_instructions_section}
 <previous_summary>
 {previous_summary_text}
 </previous_summary>
@@ -164,46 +174,65 @@ Hard limit: {output_words} words maximum. If needed, drop lower-priority detail
 """)
 
 
-@cache
-def estimate_short_summary_prompt_tokens() -> int:
-    """Estimate tokens for the short summary prompt (without messages/previous_summary)."""
+@lru_cache(maxsize=128)
+def estimate_short_summary_prompt_tokens(
+    custom_instructions: str | None = None,
+) -> int:
+    """Estimate tokens for the short summary prompt, optionally including custom instructions."""
     try:
         return estimate_tokens(
             short_summary_prompt(
                 formatted_messages="",
                 output_words=0,
                 previous_summary_text="",
+                custom_instructions=custom_instructions,
             )
         )
     except Exception:
-        # Return a rough estimate if estimation fails
         return 200
 
 
-@cache
-def estimate_long_summary_prompt_tokens() -> int:
-    """Estimate tokens for the long summary prompt (without messages/previous_summary)."""
+@lru_cache(maxsize=128)
+def estimate_long_summary_prompt_tokens(
+    custom_instructions: str | None = None,
+) -> int:
+    """Estimate tokens for the long summary prompt, optionally including custom instructions."""
     try:
         return estimate_tokens(
             long_summary_prompt(
                 formatted_messages="",
                 output_words=0,
                 previous_summary_text="",
+                custom_instructions=custom_instructions,
             )
         )
     except Exception:
-        # Return a rough estimate if estimation fails
         return 200
 
 
+
 @conditional_observe(name="Create Short Summary")
 async def create_short_summary(
     formatted_messages: str,
     input_tokens: int,
     previous_summary: str | None = None,
+    custom_instructions: str | None = None,
     *,
     workspace_name: str | None = None,
 ) -> HonchoLLMCallResponse[str]:
+    """
+    Generate a short summary via an LLM call.
+
+    Args:
+        formatted_messages: Pre-formatted conversation messages.
+        input_tokens: Token count of the input (messages + previous summary).
+        previous_summary: Previous summary text for continuity, if any.
+        custom_instructions: Optional custom instructions from configuration.
+        workspace_name: Workspace name for telemetry attribution.
+
+    Returns:
+        The LLM response containing the short summary text and token counts.
+    """
     # input_tokens indicates how many tokens the message list + previous summary take up
     # we want to optimize short summaries to be smaller than the actual content being summarized
     # so we ask the agent to produce a word count roughly equal to either the input, or the max
@@ -217,7 +246,10 @@ async def create_short_summary(
         previous_summary_text = "There is no previous summary -- the messages are the beginning of the conversation."
 
     prompt = short_summary_prompt(
-        formatted_messages, output_words, previous_summary_text
+        formatted_messages,
+        output_words,
+        previous_summary_text,
+        custom_instructions=custom_instructions,
     )
 
     # Mint a root span id.
@@ -242,9 +274,22 @@ async def create_short_summary(
 async def create_long_summary(
     formatted_messages: str,
     previous_summary: str | None = None,
+    custom_instructions: str | None = None,
     *,
     workspace_name: str | None = None,
 ) -> HonchoLLMCallResponse[str]:
+    """
+    Generate a comprehensive long summary via an LLM call.
+
+    Args:
+        formatted_messages: Pre-formatted conversation messages.
+        previous_summary: Previous summary text for continuity, if any.
+        custom_instructions: Optional custom instructions from configuration.
+        workspace_name: Workspace name for telemetry attribution.
+
+    Returns:
+        The LLM response containing the long summary text and token counts.
+    """
     # the word/token ratio is roughly 4:3 so we multiply by 0.75.
     # LLMs *seem* to respond better to getting asked for a word count but should workshop this.
     output_words = int(settings.SUMMARY.MAX_TOKENS_LONG * 0.75)
@@ -255,7 +300,10 @@ async def create_long_summary(
         previous_summary_text = "There is no previous summary -- the messages are the beginning of the conversation."
 
     prompt = long_summary_prompt(
-        formatted_messages, output_words, previous_summary_text
+        formatted_messages,
+        output_words,
+        previous_summary_text,
+        custom_instructions=custom_instructions,
     )
 
     # Mint a root span id.
@@ -452,6 +500,13 @@ async def _create_and_save_summary(
         previous_summary_tokens = latest_summary["token_count"] if latest_summary else 0
         input_tokens = messages_tokens + previous_summary_tokens
 
+    # Extract custom_instructions from the summarizer's own configuration.
+    # This is separate from reasoning custom_instructions — workspace
+    # operators may want summaries in a different style than deriver output.
+    custom_instructions: str | None = None
+    if configuration.summary and configuration.summary.custom_instructions is not None:
+        custom_instructions = configuration.summary.custom_instructions
+
     (
         new_summary,
         is_fallback,
@@ -466,6 +521,7 @@ async def _create_and_save_summary(
         last_message_id=last_message_id,
         last_message_content_preview=last_message_content_preview,
         message_count=message_count,
+        custom_instructions=custom_instructions,
         workspace_name=workspace_name,
     )
 
@@ -473,9 +529,13 @@ async def _create_and_save_summary(
     # save-summary path and the telemetry emit below can use it
     # without basedpyright tripping on a possibly-unbound name.
     if summary_type == SummaryType.SHORT:
-        prompt_tokens = estimate_short_summary_prompt_tokens()
+        prompt_tokens = estimate_short_summary_prompt_tokens(
+            custom_instructions
+        )
     else:
-        prompt_tokens = estimate_long_summary_prompt_tokens()
+        prompt_tokens = estimate_long_summary_prompt_tokens(
+            custom_instructions
+        )
 
     # Step 3: Save to database with new transaction
     if not is_fallback:
@@ -561,6 +621,7 @@ async def _create_summary(
     last_message_id: int,
     last_message_content_preview: str,
     message_count: int,
+    custom_instructions: str | None = None,
     *,
     workspace_name: str | None = None,
 ) -> tuple[Summary, bool, int, int]:
@@ -576,6 +637,8 @@ async def _create_summary(
         last_message_id: ID of the last message
         last_message_content_preview: Preview of last message content for fallback
         message_count: Number of messages for fallback
+        custom_instructions: Optional workspace-level custom instructions for prompt
+        workspace_name: Optional workspace name for telemetry
 
     Returns:
         A tuple of (Summary, is_fallback, llm_input_tokens, llm_output_tokens)
@@ -594,12 +657,14 @@ async def _create_summary(
                 formatted_messages,
                 input_tokens,
                 previous_summary_text,
+                custom_instructions=custom_instructions,
                 workspace_name=workspace_name,
             )
         else:
             response = await create_long_summary(
                 formatted_messages,
                 previous_summary_text,
+                custom_instructions=custom_instructions,
                 workspace_name=workspace_name,
             )
 
diff --git a/tests/test_config_helpers.py b/tests/test_config_helpers.py
new file mode 100644
index 00000000..79231a1a
--- /dev/null
+++ b/tests/test_config_helpers.py
@@ -0,0 +1,51 @@
+from types import SimpleNamespace
+from typing import Any
+
+from src.schemas import MessageConfiguration, SummaryConfiguration
+from src.utils.config_helpers import deep_update, get_configuration
+
+
+def _configured_node(configuration: dict[str, Any]) -> Any:
+    return SimpleNamespace(configuration=configuration)
+
+
+class TestDeepUpdate:
+    def test_summary_custom_instructions_none_clears_inherited_value(self) -> None:
+        base = {
+            "summary": {
+                "enabled": True,
+                "custom_instructions": "Write summaries in German.",
+            }
+        }
+
+        deep_update(
+            base,
+            {"summary": {"enabled": None, "custom_instructions": None}},
+        )
+
+        assert base["summary"]["enabled"] is True
+        assert base["summary"]["custom_instructions"] is None
+
+
+class TestGetConfiguration:
+    def test_session_can_clear_workspace_summary_custom_instructions(self) -> None:
+        workspace = _configured_node(
+            {"summary": {"custom_instructions": "Write summaries in German."}}
+        )
+        session = _configured_node({"summary": {"custom_instructions": None}})
+
+        configuration = get_configuration(None, session, workspace)
+
+        assert configuration.summary.custom_instructions is None
+
+    def test_message_can_clear_session_summary_custom_instructions(self) -> None:
+        session = _configured_node(
+            {"summary": {"custom_instructions": "Write summaries in German."}}
+        )
+        message_configuration = MessageConfiguration(
+            summary=SummaryConfiguration(custom_instructions=None)
+        )
+
+        configuration = get_configuration(message_configuration, session)
+
+        assert configuration.summary.custom_instructions is None