diff --git a/src/anthropic/resources/beta/messages/batches.py b/src/anthropic/resources/beta/messages/batches.py
index 5c2ad54da..0d3864b9b 100644
--- a/src/anthropic/resources/beta/messages/batches.py
+++ b/src/anthropic/resources/beta/messages/batches.py
@@ -366,7 +366,9 @@ def results(
 
         Each line in the file is a JSON object containing the result of a single request
         in the Message Batch. Results are not guaranteed to be in the same order as
-        requests. Use the `custom_id` field to match results to requests.
+        requests. Use the `custom_id` field to match results to requests. Each
+        `custom_id` must be unique within the batch and be at most 64 characters
+        long.
 
         Learn more about the Message Batches API in our
         [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
@@ -755,7 +757,9 @@ async def results(
 
         Each line in the file is a JSON object containing the result of a single request
         in the Message Batch. Results are not guaranteed to be in the same order as
-        requests. Use the `custom_id` field to match results to requests.
+        requests. Use the `custom_id` field to match results to requests. Each
+        `custom_id` must be unique within the batch and be at most 64 characters
+        long.
 
         Learn more about the Message Batches API in our
         [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
diff --git a/src/anthropic/resources/messages/batches.py b/src/anthropic/resources/messages/batches.py
index 213337119..e6bc5ac9a 100644
--- a/src/anthropic/resources/messages/batches.py
+++ b/src/anthropic/resources/messages/batches.py
@@ -291,7 +291,9 @@ def results(
 
         Each line in the file is a JSON object containing the result of a single request
         in the Message Batch. Results are not guaranteed to be in the same order as
-        requests. Use the `custom_id` field to match results to requests.
+        requests. Use the `custom_id` field to match results to requests. Each
+        `custom_id` must be unique within the batch and be at most 64 characters
+        long.
 
         Learn more about the Message Batches API in our
         [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
@@ -594,7 +596,9 @@ async def results(
 
         Each line in the file is a JSON object containing the result of a single request
         in the Message Batch. Results are not guaranteed to be in the same order as
-        requests. Use the `custom_id` field to match results to requests.
+        requests. Use the `custom_id` field to match results to requests. Each
+        `custom_id` must be unique within the batch and be at most 64 characters
+        long.
 
         Learn more about the Message Batches API in our
         [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
diff --git a/src/anthropic/types/beta/messages/batch_create_params.py b/src/anthropic/types/beta/messages/batch_create_params.py
index 9fcd90fb1..e52e85706 100644
--- a/src/anthropic/types/beta/messages/batch_create_params.py
+++ b/src/anthropic/types/beta/messages/batch_create_params.py
@@ -31,7 +31,8 @@ class Request(TypedDict, total=False):
     Useful for matching results to requests, as results may be given out of request
     order.
 
-    Must be unique for each request within the Message Batch.
+    Must be unique for each request within the Message Batch and be at most 64
+    characters long.
     """
 
     params: Required[MessageCreateParamsNonStreaming]
diff --git a/src/anthropic/types/messages/batch_create_params.py b/src/anthropic/types/messages/batch_create_params.py
index e60c74f13..8790a4066 100644
--- a/src/anthropic/types/messages/batch_create_params.py
+++ b/src/anthropic/types/messages/batch_create_params.py
@@ -25,7 +25,8 @@ class Request(TypedDict, total=False):
     Useful for matching results to requests, as results may be given out of request
     order.
 
-    Must be unique for each request within the Message Batch.
+    Must be unique for each request within the Message Batch and be at most 64
+    characters long.
     """
 
     params: Required[MessageCreateParamsNonStreaming]