diff --git a/packages/wire/CHANGELOG.md b/packages/wire/CHANGELOG.md
index 6e75b705b..422951a63 100644
--- a/packages/wire/CHANGELOG.md
+++ b/packages/wire/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Bumped `COLLAB_PROTO` to `3`: the `ui-request`/`ui-request-end` host frames and `ui-response` guest frame are part of the handshake contract. Proto-2 guests, which silently dropped host ask requests, are now rejected at hello with the protocol-mismatch error.
+
 ### Added
 
 - Added collaboration UI request/response wireframes, enabling browser guests to respond to host-side interactive prompts.
diff --git a/packages/wire/src/index.ts b/packages/wire/src/index.ts
index 752ce8919..57d549770 100644
--- a/packages/wire/src/index.ts
+++ b/packages/wire/src/index.ts
@@ -389,8 +389,12 @@ export type WireFrame = GuestFrame | HostFrame;
  *   transcript entries follow in `snapshot-chunk` frames, so multi-MB
  *   sessions are not gated on a single welcome frame fitting under the
  *   guest's first-welcome timeout.
+ * - `3`: host asks guests through `ui-request`/`ui-request-end` host frames
+ *   answered by the `ui-response` guest frame. Guests that predate the
+ *   grammar would silently drop `ui-request` (asks hang forever on the
+ *   host), so they must be rejected at hello.
  */
-export const COLLAB_PROTO = 2;
+export const COLLAB_PROTO = 3;
 
 /** Parameter key used for intent tracing (e.g. prompt explanation/reasoning) */
 export const INTENT_FIELD = "i";
diff --git a/packages/wire/test/constants.test.ts b/packages/wire/test/constants.test.ts
index 6acd1491c..282d0ca86 100644
--- a/packages/wire/test/constants.test.ts
+++ b/packages/wire/test/constants.test.ts
@@ -9,7 +9,7 @@ import {
 
 describe("collab wire constants", () => {
 	it("exports the protocol constants consumed by host, guest, and relay links", () => {
-		expect(COLLAB_PROTO).toBe(2);
+		expect(COLLAB_PROTO).toBe(3);
 		expect(COLLAB_PROMPT_MESSAGE_TYPE).toBe("collab-prompt");
 		expect(ENVELOPE_HEADER_LENGTH).toBe(4);
 		expect(ROOM_ID_BYTES).toBe(16);
diff --git a/scripts/install-tests/run-ci.sh b/scripts/install-tests/run-ci.sh
index 64ae5c18f..247904412 100755
--- a/scripts/install-tests/run-ci.sh
+++ b/scripts/install-tests/run-ci.sh
@@ -170,7 +170,7 @@ mkdir -p "$TARBALL_APP_DIR"
       exit 1
    }
    wire_proto="$(bun -e 'import { COLLAB_PROTO } from "@oh-my-pi/pi-wire"; process.stdout.write(String(COLLAB_PROTO));')"
-   [ "$wire_proto" = "2" ] || {
+   [ "$wire_proto" = "3" ] || {
       echo "Unexpected @oh-my-pi/pi-wire COLLAB_PROTO: $wire_proto"
       exit 1
    }