diff --git a/src/domain.rs b/src/domain.rs
index fd5b743..aaebdeb 100644
--- a/src/domain.rs
+++ b/src/domain.rs
@@ -22,6 +22,15 @@ pub type VersionClock = BTreeMap<DeviceId, u64>;
 /// Version of the signed folder roster and manifest model.
 pub const FOLDER_PROTOCOL_VERSION: u16 = 2;
 
+/// Encode data used in signed payloads and Merkle hashes.
+///
+/// Appa's protocol types use declaration-ordered structs and `BTreeMap` for
+/// every map in these payloads. Changing either representation changes the
+/// bytes, so it requires a matching protocol version bump.
+pub(crate) fn canonical_json_bytes<T: Serialize>(value: &T) -> anyhow::Result<Vec<u8>> {
+    Ok(serde_json::to_vec(value)?)
+}
+
 #[derive(Clone, Copy, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum FolderMode {
diff --git a/src/domain/merkle.rs b/src/domain/merkle.rs
index dd9b1c4..594553f 100644
--- a/src/domain/merkle.rs
+++ b/src/domain/merkle.rs
@@ -1,6 +1,6 @@
 use std::collections::BTreeMap;
 
-use super::{Entry, Manifest, RelativePath};
+use super::{Entry, Manifest, RelativePath, canonical_json_bytes};
 
 const MAX_LEAF_ENTRIES: usize = 64;
 const BLAKE3_HEX_LENGTH: usize = 64;
@@ -93,7 +93,7 @@ fn build_index_node(
     let node = if entries.len() <= MAX_LEAF_ENTRIES || prefix.len() == BLAKE3_HEX_LENGTH {
         ManifestMerkleNode::Leaf {
             prefix: prefix.to_owned(),
-            hash: blake3::hash(&serde_json::to_vec(&entries)?)
+            hash: blake3::hash(&canonical_json_bytes(&entries)?)
                 .to_hex()
                 .to_string(),
             entries,
@@ -116,7 +116,7 @@ fn build_index_node(
         }
         ManifestMerkleNode::Branch {
             prefix: prefix.to_owned(),
-            hash: blake3::hash(&serde_json::to_vec(&children)?)
+            hash: blake3::hash(&canonical_json_bytes(&children)?)
                 .to_hex()
                 .to_string(),
             children,
diff --git a/src/domain/roster.rs b/src/domain/roster.rs
index 2468cfd..76f2e36 100644
--- a/src/domain/roster.rs
+++ b/src/domain/roster.rs
@@ -4,7 +4,7 @@ use anyhow::Context;
 use iroh::{EndpointId, SecretKey, Signature};
 use serde::{Deserialize, Serialize};
 
-use super::{DeviceId, FOLDER_PROTOCOL_VERSION, FolderId};
+use super::{DeviceId, FOLDER_PROTOCOL_VERSION, FolderId, canonical_json_bytes};
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub enum MemberRole {
@@ -127,7 +127,7 @@ impl FolderRoster {
     }
 
     fn signing_bytes(&self) -> anyhow::Result<Vec<u8>> {
-        Ok(serde_json::to_vec(&UnsignedFolderRoster::from(self))?)
+        canonical_json_bytes(&UnsignedFolderRoster::from(self))
     }
 
     fn invalidate_signature(&mut self) {
diff --git a/src/protocol.rs b/src/protocol.rs
index 132a853..8c792b0 100644
--- a/src/protocol.rs
+++ b/src/protocol.rs
@@ -1,6 +1,6 @@
 use anyhow::Context;
 
-use crate::domain::{DeviceId, FolderId, FolderRoster, ManifestMerkleNode};
+use crate::domain::{DeviceId, FolderId, FolderRoster, ManifestMerkleNode, canonical_json_bytes};
 #[cfg(test)]
 use crate::domain::{Manifest, manifest_root_hash};
 use base64::{Engine, engine::general_purpose::URL_SAFE_NO_PAD};
@@ -67,9 +67,36 @@ impl Invite {
     }
 
     fn signing_bytes(&self) -> anyhow::Result<Vec<u8>> {
-        let mut unsigned = self.clone();
-        unsigned.signature = None;
-        Ok(serde_json::to_vec(&unsigned)?)
+        canonical_json_bytes(&UnsignedInvite::from(self))
+    }
+}
+
+#[derive(Serialize)]
+struct UnsignedInvite<'a> {
+    protocol_version: u16,
+    folder_id: FolderId,
+    folder_name: &'a str,
+    inviter_device_id: &'a str,
+    inviter_endpoint: &'a EndpointAddr,
+    capability: &'a str,
+    roster: &'a FolderRoster,
+    expires_at: OffsetDateTime,
+    signature: Option<Signature>,
+}
+
+impl<'a> From<&'a Invite> for UnsignedInvite<'a> {
+    fn from(invite: &'a Invite) -> Self {
+        Self {
+            protocol_version: invite.protocol_version,
+            folder_id: invite.folder_id,
+            folder_name: &invite.folder_name,
+            inviter_device_id: &invite.inviter_device_id,
+            inviter_endpoint: &invite.inviter_endpoint,
+            capability: &invite.capability,
+            roster: &invite.roster,
+            expires_at: invite.expires_at,
+            signature: None,
+        }
     }
 }
 
@@ -175,6 +202,15 @@ mod tests {
             signature: None,
         };
         invite.sign(&identity)?;
+
+        let mut legacy_unsigned_invite = invite.clone();
+        legacy_unsigned_invite.signature = None;
+        assert_eq!(
+            invite.signing_bytes()?,
+            serde_json::to_vec(&legacy_unsigned_invite)?
+        );
+        assert!(decode_invite(&encode_invite(&invite)?).is_ok());
+
         invite.folder_name = "altered".to_owned();
 
         assert!(decode_invite(&encode_invite(&invite)?).is_err());