diff --git a/ossai/handlers.py b/ossai/handlers.py
index 06a94ac..f24e0c4 100644
--- a/ossai/handlers.py
+++ b/ossai/handlers.py
@@ -22,6 +22,7 @@
     get_since_timeframe_presets,
 )
 
+_custom_prompt_cache = {}
 
 def handler_feedback(body):
     """
@@ -102,7 +103,6 @@ async def handler_tldr_extended_slash_command(
     client: WebClient, ack, payload, say, user_id: str
 ):
     await ack()
-    text = payload.get("text", None)
     channel_name = payload["channel_name"]
     channel_id = payload["channel_id"]
     dm_channel_id = None
@@ -110,14 +110,12 @@ async def handler_tldr_extended_slash_command(
     dm_channel_id = await get_direct_message_channel_id(client, user_id)
     await say(channel=dm_channel_id, text="...")
 
-    if text:
-        return await say("ERROR: custom prompt support coming soon!")
-
     history = await get_channel_history(client, channel_id)
     history.reverse()
     user = await get_user_context(client, user_id)
     title = f"*Summary of #{channel_name}* (last {len(history)} messages)\n"
-    summarizer = Summarizer()
+    custom_prompt = payload.get("text", None)
+    summarizer = Summarizer(custom_prompt=custom_prompt)
     summary, run_id = summarizer.summarize_slack_messages(
         client,
         history,
@@ -126,7 +124,7 @@ async def handler_tldr_extended_slash_command(
         user=user,
     )
     text, blocks = get_text_and_blocks_for_say(
-        title=title, run_id=run_id, messages=summary
+        title=title, run_id=run_id, messages=summary, custom_prompt=custom_prompt
     )
     return await say(channel=dm_channel_id, text=text, blocks=blocks)
 
@@ -136,9 +134,6 @@ async def handler_topics_slash_command(
     client: WebClient, ack, payload, say, user_id: str
 ):
     await ack()
-    text = payload.get("text", None)
-    if text:
-        return await say("ERROR: custom prompt support coming soon!")
     channel_id = payload["channel_id"]
     dm_channel_id = await get_direct_message_channel_id(client, user_id)
     await say(channel=dm_channel_id, text="...")
@@ -149,6 +144,11 @@ async def handler_topics_slash_command(
     messages = get_parsed_messages(client, history, with_names=False)
     user = await get_user_context(client, user_id)
     is_private, channel_name = get_is_private_and_channel_name(client, channel_id)
+    custom_prompt = payload.get("text", None)
+    if custom_prompt:
+        # todo: add support for custom prompts to /tldr
+        await say(channel=dm_channel_id, text="Sorry, this command doesn't support custom prompts yet so I'm processing your request without it.")
+
     topic_overview, run_id = await analyze_topics_of_history(
         channel_name, messages, user=user, is_private=is_private
     )
@@ -164,11 +164,10 @@ async def handler_tldr_since_slash_command(client: WebClient, ack, payload, say)
     await ack()
     title = "Choose your summary timeframe."
     dm_channel_id = await get_direct_message_channel_id(client, payload["user_id"])
-    text = payload.get("text", None)
-    if text:
-        return await say("ERROR: custom prompt support coming soon!")
+    
+    custom_prompt = payload.get("text", None)
 
-    client.chat_postEphemeral(
+    result = client.chat_postEphemeral(
         channel=payload["channel_id"],
         user=payload["user_id"],
         text=title,
@@ -184,17 +183,23 @@ async def handler_tldr_since_slash_command(client: WebClient, ack, payload, say)
                             "text": "Select a date",
                             "emoji": True,
                         },
-                        "action_id": "summarize_since",
+                        "action_id": f"summarize_since",
                     },
                 ],
             }
         ],
     )
 
+    # get `custom_prompt` into handler_action_summarize_since_date()
+    key = f"{result['message_ts']}__{payload['user_id']}"
+    _custom_prompt_cache[key] = custom_prompt
+    logger.debug(f"Storing `custom_prompt` at {key}: {custom_prompt}")
+
     await say(
         channel=dm_channel_id,
         text=f'In #{payload["channel_name"]}, choose a date or timeframe to get your summary',
     )
+    return
 
 
 @catch_errors_dm_user
@@ -227,7 +232,11 @@ async def handler_action_summarize_since_date(client: WebClient, ack, body):
     history = await get_channel_history(client, channel_id, since=since_datetime)
     history.reverse()
     user = await get_user_context(client, user_id)
-    summarizer = Summarizer()
+    custom_prompt = None
+    if 'container' in body and 'message_ts' in body['container']:
+        key = f"{body['container']['message_ts']}__{user_id}"
+        custom_prompt = _custom_prompt_cache.get(key, None)
+    summarizer = Summarizer(custom_prompt=custom_prompt)
     summary, run_id = summarizer.summarize_slack_messages(
         client, history, channel_id, feature_name=feature_name, user=user
     )
@@ -235,6 +244,7 @@ async def handler_action_summarize_since_date(client: WebClient, ack, body):
         title=f'*Summary of #{channel_name}* since {since_datetime.strftime("%A %b %-d, %Y")} ({len(history)} messages)\n',
         run_id=run_id,
         messages=summary,
+        custom_prompt=custom_prompt,
     )
     # todo: somehow add date/preset choice to langsmith metadata
     #   feature_name: str -> feature: str || Tuple[str, List(Tuple[str, str])]
@@ -245,17 +255,26 @@ async def handler_action_summarize_since_date(client: WebClient, ack, body):
 async def handler_sandbox_slash_command(
     client: WebClient, ack, payload, say, user_id: str
 ):
-    text = payload.get("text", None)
-    if text:
-        return await say("ERROR: custom prompt support coming soon!")
     logger.debug(f"Handling /sandbox command")
     await ack()
-    run_id = str(uuid.uuid4())
-    run_id = None
-    text = """-- Better error handling coming soon! Useful summary of content goes here -- (no run id)"""
-    lines = text.strip().split("\n")
+    channel_id = payload["channel_id"]
+    custom_prompt = payload.get("text", None)
+    summarizer = Summarizer(custom_prompt=custom_prompt)
+    summary, run_id = summarizer.summarize_slack_messages(
+        client,
+        [
+            {"text": "bacon", "user": user_id},
+            {"text": "eggs", "user": user_id},
+            {"text": "spam", "user": user_id},
+            {"text": "orange juice", "user": user_id},
+            {"text": "coffee", "user": user_id},
+        ],
+        channel_id=channel_id,
+        feature_name="sandbox",
+        user=user_id,
+    )
     title = "This is a test of the /sandbox command."
     text, blocks = get_text_and_blocks_for_say(
-        title=title, run_id=run_id, messages=lines
+        title=title, run_id=run_id, messages=summary, custom_prompt=custom_prompt
     )
     return await say(text=text, blocks=blocks)
diff --git a/ossai/summarizer.py b/ossai/summarizer.py
index 030ce16..4d9af10 100644
--- a/ossai/summarizer.py
+++ b/ossai/summarizer.py
@@ -20,10 +20,12 @@
 
 
 class Summarizer:
-    def __init__(self):
+    def __init__(self, custom_prompt: str | None = None):
+        # todo: apply pydantic model
         self.config = get_llm_config()
         self.model = ChatOpenAI(model=self.config["chat_model"], temperature=self.config["temperature"])
         self.parser = StrOutputParser()
+        self.custom_prompt = custom_prompt
 
     def summarize(
         self,
@@ -62,18 +64,21 @@ def summarize(
         So, The assistant needs to speak in {language}.
         """
 
-        human_msg = """\
+        base_human_msg = """\
         Please summarize the following chat log to a flat markdown formatted bullet list.
         Do not write a line by line summary. Instead, summarize the overall conversation.
         Do not include greeting/salutation/polite expressions in summary.
         Make the summary easy to read while maintaining a conversational tone and retaining meaning.
         Write in conversational English.
+        {custom_instructions}
 
         {text}
         """
 
+        # todo: guard against prompt injection
+
         prompt_template = ChatPromptTemplate.from_messages(
-            [("system", system_msg), ("user", human_msg)]
+            [("system", system_msg), ("user", base_human_msg)]
         )
 
         chain = prompt_template | self.model | self.parser
@@ -87,7 +92,12 @@ def summarize(
         )
         logger.info(f"{langsmith_config=}")
         result = chain.invoke(
-            {"text": text, "language": self.config["language"]}, config=langsmith_config
+            {
+                "text": text,
+                "language": self.config["language"],
+                "custom_instructions": f"\n\nAdditionally, please follow these specific instructions for this summary:\n{self.custom_prompt}" if self.custom_prompt else "",
+            },
+            config=langsmith_config
         )
         return result, langsmith_config["run_id"]
 
diff --git a/ossai/utils.py b/ossai/utils.py
index 49a848d..86fbbe8 100644
--- a/ossai/utils.py
+++ b/ossai/utils.py
@@ -203,7 +203,7 @@ def parse_message(msg):
 
 
 def get_text_and_blocks_for_say(
-    title: str, run_id: Union[uuid.UUID, None], messages: list
+    title: str, run_id: Union[uuid.UUID, None], messages: list, custom_prompt: str = None
 ) -> tuple[str, list]:
     CHAR_LIMIT = 3000
     text = "\n".join(messages)
@@ -260,6 +260,18 @@ def get_text_and_blocks_for_say(
             }
         )
 
+    if custom_prompt:
+        blocks.append({
+            "type": "context",
+            "elements": [
+                {
+                    "type": "plain_text",
+                    "text": f"Custom Prompt: {custom_prompt}",
+                    "emoji": True
+                }
+            ]
+        })
+
     return text.split("\n")[0], blocks
 
 
diff --git a/tests/test_handlers.py b/tests/test_handlers.py
index 4cbcf0b..4d4e400 100644
--- a/tests/test_handlers.py
+++ b/tests/test_handlers.py
@@ -1,4 +1,4 @@
-from unittest.mock import AsyncMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 import uuid
 import pytest
 from slack_sdk import WebClient
@@ -380,7 +380,7 @@ async def test_handler_tldr_extended_slash_command_non_public(
 @patch("ossai.handlers.Summarizer")
 @patch("ossai.handlers.get_text_and_blocks_for_say")
 @patch("ossai.handlers.get_direct_message_channel_id")
-@patch("aiohttp.ClientSession.post", new_callable=AsyncMock)  # Change this line
+@patch("aiohttp.ClientSession.post", new_callable=AsyncMock)
 async def test_handler_action_summarize_since_date(
     mock_post,
     get_direct_message_channel_id_mock,
@@ -440,6 +440,7 @@ async def test_handler_action_summarize_since_date(
         title="*Summary of #general* since Tuesday Feb 21, 2023 (2 messages)\n",
         run_id="run_id",
         messages="summary",
+        custom_prompt=None,
     )
     client.chat_postMessage.assert_called_with(
         channel="DM123", text="text", blocks="blocks"
@@ -456,7 +457,7 @@ async def test_handler_tldr_since_slash_command_happy_path(
 ):
     # Setup
     client = AsyncMock(spec=WebClient)
-    client.chat_postEphemeral = AsyncMock()
+    client.chat_postEphemeral = MagicMock()
     say = AsyncMock()
     payload = {"user_id": "U123", "channel_id": "C123", "channel_name": "general"}
     get_since_timeframe_presets_mock.return_value = {"foo": "bar"}
@@ -646,10 +647,6 @@ async def test_handler_sandbox_slash_command_happy_path():
 
     await handler_sandbox_slash_command(client, ack, payload, say, user_id="foo123")
     say.assert_called_once()
-    assert any(
-        "Useful summary of content goes here" in str(block)
-        for block in say.call_args[1]["blocks"]
-    )
     assert any(
         "This is a test of the /sandbox command." in str(block)
         for block in say.call_args[1]["blocks"]
diff --git a/tests/test_utils.py b/tests/test_utils.py
index cf27d3d..20d833b 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -304,5 +304,8 @@ def test_get_text_and_blocks_for_say_block_size():
     assert len(blocks[-1]['elements']) == 3  # Three buttons
 
 
+# todo: test get_text_and_blocks_for_say with custom prompt
+
+
 def test_main_as_script():
     utils.main()