diff --git a/00.md b/00.md
index d7184a73..2be98266 100644
--- a/00.md
+++ b/00.md
@@ -56,11 +56,22 @@ These specifications use elliptic curve public key cryptography over the field a
 
 ### Types
 
-Values in these specification are one of:
+Values in this specification are one of:
 
 - Scalars: integers modulo the curve order `n`
 - Curve points: elements of the elliptic curve group (including the identity point at infinity)
 - Byte sequences
+- `int` / `u64`: unsigned 64-bit integer in the range: `0..2^64-1`.
+
+#### Unsigned 64-bit integers in JSON fields
+
+For JSON fields, `int` / `u64` values may be encoded as a JSON integer number or a base-10 decimal string.
+
+Implementations **MUST** accept both forms.
+
+Values greater than `9007199254740991 (2^53-1)` **MUST** be represented as decimal strings.
+
+This preserves integer precision in environments where JSON numbers use IEEE-754.
 
 ### Byte Operations And Encodings
 
diff --git a/01.md b/01.md
index 5a85bd2d..5ce8e3d1 100644
--- a/01.md
+++ b/01.md
@@ -67,7 +67,7 @@ Response `GetKeysResponse` of `Bob`:
       "input_fee_ppk": <int|null>,
       "final_expiry": <unix_timestamp_int|null>
       "keys": {
-        <amount_int>: <public_key_str>,
+        <amount_str>: <public_key_str>,
         ...
       }
     }
diff --git a/18.md b/18.md
index 34571360..1967afb6 100644
--- a/18.md
+++ b/18.md
@@ -33,7 +33,7 @@ A Payment Request is defined as follows
 Here, the fields are
 
 - `i`: Payment id to be included in the payment payload
-- `a`: The amount of the requested payment
+- `a`: The amount of the requested payment as an [unsigned int][00]
 - `u`: The unit of the requested payment (MUST be set if `a` is set)
 - `s`: Whether the payment request is for single use
 - `m`: A set of mints from which the payment is requested
diff --git a/24.md b/24.md
index 05822628..2417b907 100644
--- a/24.md
+++ b/24.md
@@ -23,7 +23,7 @@ HTTP servers may respond with the HTTP status code 402 with a `X-Cashu` header c
 }
 ```
 
-- `a`: The amount required in the specified unit
+- `a`: The amount required in the specified unit as an [unsigned int][00]
 - `u`: The currency unit (e.g., "sat", "usd", "api")
 - `m`: Array of mint URLs that the server accepts tokens from
 - `nut10`: The required [NUT-10][10] locking condition
@@ -40,6 +40,7 @@ The token MUST be from one of the mints listed in the `mints` array, and MUST be
 
 If the server receives tokens from a mint that is not in the `mints` array, an incorrect `unit`, an insufficient amount of tokens, or with insufficient locking conditions, it should respond with a HTTP `400` status code.
 
+[00]: 00.md
 [10]: 10.md
 [12]: 12.md
 [18]: 18.md