diff --git a/src/jsonrpc.ts b/src/jsonrpc.ts
index bf03afe2..3eedf42f 100644
--- a/src/jsonrpc.ts
+++ b/src/jsonrpc.ts
@@ -1792,6 +1792,16 @@ export class RequestError extends Error {
     );
   }
 
+  static unsupportedProtocolVersion(
+    requested: number,
+    supported: readonly number[],
+  ): RequestError {
+    return new RequestError(-32022, "Unsupported protocol version", {
+      requested,
+      supported: [...supported],
+    });
+  }
+
   /**
    * Converts this error to a JSON-RPC result object.
    */
diff --git a/src/protocol-router.test.ts b/src/protocol-router.test.ts
index 26e00a85..80744ebd 100644
--- a/src/protocol-router.test.ts
+++ b/src/protocol-router.test.ts
@@ -203,8 +203,9 @@ describe("AgentProtocolRouter", () => {
     expect(response).toMatchObject({
       id: "initialize-id",
       error: {
-        code: -32600,
-        data: expect.stringContaining("supports ACP protocol version 2"),
+        code: -32022,
+        message: "Unsupported protocol version",
+        data: { requested: 1, supported: [2] },
       },
     });
     expect(v2.connectionCount).toBe(0);
@@ -231,8 +232,9 @@ describe("AgentProtocolRouter", () => {
           jsonrpc: "2.0",
           id: "initialize-id",
           error: {
-            code: -32600,
-            data: expect.stringContaining("supports ACP protocol version 2"),
+            code: -32022,
+            message: "Unsupported protocol version",
+            data: { requested: 1, supported: [2] },
           },
         },
       ],
diff --git a/src/protocol-router.ts b/src/protocol-router.ts
index 65fd5363..0a1af39b 100644
--- a/src/protocol-router.ts
+++ b/src/protocol-router.ts
@@ -41,10 +41,10 @@ const MAX_PROTOCOL_VERSION = 0xffff;
  * including the experimental draft ACP v2 API.
  *
  * The router consumes the first wire item, which must be an `initialize`
- * request or a single-entry batch containing one. It selects the highest
- * configured protocol version that does not exceed the client's requested
- * version. Only the initialize params and optional batch framing are
- * normalized; every later wire item is forwarded unchanged.
+ * request or a single-entry batch containing one. Endpoints exposing v2 select
+ * the highest configured version at or below the requested version. V1-only
+ * endpoints retain the legacy fallback to v1. Only initialize params and
+ * optional batch framing are normalized. Later wire items are forwarded unchanged.
  *
  * @experimental
  */
@@ -201,15 +201,16 @@ export class AgentProtocolRouter implements AgentConnector {
       return;
     }
 
-    const selected = this.highestCompatible(requested);
+    const selected = this.selectProtocol(requested);
     if (!selected) {
       await rejectInitialize(
         reader,
         stream.writable,
         message.id,
-        RequestError.invalidRequest(
-          `unsupported ACP protocol version ${requested}; this endpoint supports ${this.supportedDescription()}`,
-        ),
+        RequestError.unsupportedProtocolVersion(requested, [
+          ...(this.v1 ? [PROTOCOL_V1] : []),
+          ...(this.v2 ? [PROTOCOL_V2] : []),
+        ]),
         batchedInitialize,
       );
       lifecycle.finish();
@@ -266,7 +267,10 @@ export class AgentProtocolRouter implements AgentConnector {
     }
   }
 
-  private highestCompatible(requested: number): AgentProtocol | undefined {
+  private selectProtocol(requested: number): AgentProtocol | undefined {
+    if (this.v1 && !this.v2) {
+      return PROTOCOL_V1;
+    }
     if (this.v2 && requested >= PROTOCOL_V2) {
       return PROTOCOL_V2;
     }
@@ -275,19 +279,6 @@ export class AgentProtocolRouter implements AgentConnector {
     }
     return undefined;
   }
-
-  private supportedDescription(): string {
-    if (this.v1 && this.v2) {
-      return "ACP protocol versions 1 and 2";
-    }
-    if (this.v1) {
-      return "ACP protocol version 1";
-    }
-    if (this.v2) {
-      return "ACP protocol version 2";
-    }
-    return "no ACP protocol versions";
-  }
 }
 
 function shouldCloseWithoutResponse(message: WireMessage): boolean {
diff --git a/src/v2/acp.ts b/src/v2/acp.ts
index b358496b..f06a89ea 100644
--- a/src/v2/acp.ts
+++ b/src/v2/acp.ts
@@ -408,19 +408,25 @@ function assertV2BatchMethods(
 }
 
 function parseV2InitializeRequest(params: unknown): schema.InitializeRequest {
+  const protocolVersion = received<schema.ProtocolVersion>(
+    validate.zProtocolVersion,
+  ).parse(
+    typeof params === "object" && params !== null && !Array.isArray(params)
+      ? (params as Record<string, unknown>)["protocolVersion"]
+      : undefined,
+  );
+  if (protocolVersion < schema.PROTOCOL_VERSION) {
+    throw RequestError.unsupportedProtocolVersion(protocolVersion, [
+      schema.PROTOCOL_VERSION,
+    ]);
+  }
   const request = received<schema.InitializeRequest>(
     validate.zInitializeRequest,
   ).parse(params);
-  if (request.protocolVersion !== schema.PROTOCOL_VERSION) {
-    throw RequestError.invalidParams(
-      {
-        expectedProtocolVersion: schema.PROTOCOL_VERSION,
-        receivedProtocolVersion: request.protocolVersion,
-      },
-      `The v2 API only supports protocol version ${schema.PROTOCOL_VERSION}`,
-    );
-  }
-  return structuredClone(request);
+  return structuredClone({
+    ...request,
+    protocolVersion: schema.PROTOCOL_VERSION,
+  });
 }
 
 function normalizeOutgoingV2InitializeRequest(params: unknown): {