diff --git a/common/client_types.proto b/common/client_types.proto
index e758464..a1ae8c5 100644
--- a/common/client_types.proto
+++ b/common/client_types.proto
@@ -206,6 +206,7 @@ enum MfaMethod {
   OIDC = 2;
   BIOMETRIC = 3;
   MOBILE_APPROVE = 4;
+  FIDO2 = 5;
 }
 
 // Multi-step MFA (added for 2.2)
@@ -295,7 +296,13 @@ message ClientMfaStepStartResponse {
   // callbacks can be matched against the attempt that is actually current.
   string step_attempt_id = 1;
   // Biometric or mobile-approve challenge, when the method needs one.
+  // [2.2] For FIDO2 this holds the challenge the key has to sign.
   optional string challenge = 2;
+  // [2.2] For FIDO2: the credentials registered for this user, base64url as
+  // webauthn-rs serializes them. The client offers the whole list to the key,
+  // which answers for the one it holds, and names it in the finish request so
+  // later attempts can be narrowed to that credential.
+  repeated string credential_ids = 3;
 }
 
 message ClientMfaStartRequest {
@@ -330,17 +337,28 @@ message ClientMfaStartResponse {
   // [2.2] Per-step rejections, sparse - only failing steps appear. Non-empty
   // means the plan was refused and no session was created.
   repeated MfaStepRejection rejections = 3;
+  // [2.2] For FIDO2 on the legacy fused path: the credentials registered for
+  // this user, alongside the challenge above.
+  repeated string credential_ids = 4;
 }
 
 message ClientMfaFinishRequest {
   string token = 1;
+  // [2.2] For FIDO2 this holds the hash of RP ID.
   optional string code = 2;
+  // [2.2] For FIDO2 this holds the signature.
   optional string auth_pub_key = 3;
   // [2.2] The attempt id minted by StepStart for the step being submitted. It binds
   // this proof to a specific attempt, so a stale or duplicate proof cannot advance
   // the step twice. Optional so pre-2.2 clients that omit it still parse; a None
   // value keeps the legacy single-step path working.
   optional string step_attempt_id = 4;
+  // [2.2] FIDO2
+  optional bytes auth_data = 5;
+  // [2.2] For FIDO2: which credential actually signed, base64url. Picked by the
+  // key out of the list it was offered, so Core knows which of the user's
+  // security keys is in use.
+  optional string credential_id = 6;
 }
 
 message ClientMfaFinishResponse {