diff --git a/format/spec.md b/format/spec.md
index e6f9e0425b01..c6d12b1747c5 100644
--- a/format/spec.md
+++ b/format/spec.md
@@ -1881,8 +1881,8 @@ The binary single-value serialization can be used to store the lower and upper b
 | **`timestamptz_ns`** | **`JSON string`**                       | `"2017-11-16T22:31:08.123456789+00:00"`    | Stores ISO-8601 standard timestamp with nanosecond precision; must include a zone offset and it must be '+00:00' |
 | **`string`**       | **`JSON string`**                         | `"iceberg"`                                | |
 | **`uuid`**         | **`JSON string`**                         | `"f79c3e09-677c-4bbd-a479-3f349cb785e7"`   | Stores the lowercase uuid string |
-| **`fixed(L)`**     | **`JSON string`**                         | `"000102ff"`                               | Stored as a hexadecimal string |
-| **`binary`**       | **`JSON string`**                         | `"000102ff"`                               | Stored as a hexadecimal string |
+| **`fixed(L)`**     | **`JSON string`**                         | `"000102FF"`                               | Stored as a hexadecimal string; writers should emit uppercase, readers must accept either case |
+| **`binary`**       | **`JSON string`**                         | `"000102FF"`                               | Stored as a hexadecimal string; writers should emit uppercase, readers must accept either case |
 | **`struct`**       | **`JSON object by field ID`**             | `{"1": 1, "2": "bar"}`                     | Stores struct fields using the field ID as the JSON field name; field values are stored using this JSON single-value format |
 | **`list`**         | **`JSON array of values`**                | `[1, 2, 3]`                                | Stores a JSON array of values that are serialized using this JSON single-value format |
 | **`map`**          | **`JSON object of key and value arrays`** | `{ "keys": ["a", "b"], "values": [1, 2] }` | Stores arrays of keys and values; individual keys and values are serialized using this JSON single-value format |
diff --git a/open-api/rest-catalog-open-api.py b/open-api/rest-catalog-open-api.py
index ad3eed00439b..8b900d8db0da 100644
--- a/open-api/rest-catalog-open-api.py
+++ b/open-api/rest-catalog-open-api.py
@@ -1203,7 +1203,7 @@ class TimestampTzNanoTypeValue(RootModel[str]):
 class FixedTypeValue(RootModel[str]):
     root: str = Field(
         ...,
-        description='Fixed length type values are stored and serialized as an uppercase hexadecimal string preserving the fixed length',
+        description='Fixed length type values are stored and serialized as a hexadecimal string preserving the fixed length. Writers should emit uppercase hexadecimal characters; readers must accept either case',
         examples=['78797A'],
     )
 
@@ -1211,7 +1211,7 @@ class FixedTypeValue(RootModel[str]):
 class BinaryTypeValue(RootModel[str]):
     root: str = Field(
         ...,
-        description='Binary type values are stored and serialized as an uppercase hexadecimal string',
+        description='Binary type values are stored and serialized as a hexadecimal string. Writers should emit uppercase hexadecimal characters; readers must accept either case',
         examples=['78797A'],
     )
 
diff --git a/open-api/rest-catalog-open-api.yaml b/open-api/rest-catalog-open-api.yaml
index 2064e670a46c..712edae8ee5d 100644
--- a/open-api/rest-catalog-open-api.yaml
+++ b/open-api/rest-catalog-open-api.yaml
@@ -5463,14 +5463,17 @@ components:
     FixedTypeValue:
       type: string
       description:
-        "Fixed length type values are stored and serialized as an uppercase hexadecimal string
-        preserving the fixed length"
+        "Fixed length type values are stored and serialized as a hexadecimal string
+        preserving the fixed length. Writers should emit uppercase hexadecimal
+        characters; readers must accept either case"
       example: "78797A"
 
     BinaryTypeValue:
       type: string
       description:
-        "Binary type values are stored and serialized as an uppercase hexadecimal string"
+        "Binary type values are stored and serialized as a hexadecimal string.
+        Writers should emit uppercase hexadecimal characters; readers must accept
+        either case"
       example: "78797A"
 
     CountMap: