diff --git a/apps/server/src/wsServer.ts b/apps/server/src/wsServer.ts
index 2e6ac51b7..f893eef82 100644
--- a/apps/server/src/wsServer.ts
+++ b/apps/server/src/wsServer.ts
@@ -20,7 +20,9 @@ import {
   PROVIDER_SEND_TURN_MAX_IMAGE_BYTES,
   ProjectId,
   ThreadId,
+  type WsError,
   WS_CHANNELS,
+  WS_ERROR_CODES,
   WS_METHODS,
   WebSocketRequest,
   type WsResponse as WsResponseMessage,
@@ -231,6 +233,40 @@ class RouteRequestError extends Schema.TaggedErrorClass<RouteRequestError>()("Ro
   message: Schema.String,
 }) {}
 
+function buildWsErrorForClient(cause: Cause.Cause<unknown>): WsError {
+  const defect = Cause.squash(cause);
+  const rawMessage = defect instanceof Error ? defect.message : Cause.pretty(cause);
+  const firstLine = rawMessage.split("\n")[0]?.trim() ?? rawMessage;
+  const message = firstLine.replace(/^Error:\s*/i, "");
+
+  const dirNotExistPrefix = "Project directory does not exist: ";
+  const pathNotDirPrefix = "Project path is not a directory: ";
+  if (message.startsWith(dirNotExistPrefix)) {
+    return {
+      code: WS_ERROR_CODES.projectDirNotExist,
+      message: "Project directory does not exist",
+      path: message.slice(dirNotExistPrefix.length).trim() || undefined,
+    };
+  }
+  if (message.startsWith(pathNotDirPrefix)) {
+    return {
+      code: WS_ERROR_CODES.projectPathNotDirectory,
+      message: "Project path is not a directory",
+      path: message.slice(pathNotDirPrefix.length).trim() || undefined,
+    };
+  }
+  if (Schema.is(RouteRequestError)(defect)) {
+    return {
+      code: WS_ERROR_CODES.routeRequest,
+      message,
+    };
+  }
+  return {
+    code: WS_ERROR_CODES.unknown,
+    message,
+  };
+}
+
 export const createServer = Effect.fn(function* (): Effect.fn.Return<
   http.Server,
   ServerLifecycleError,
@@ -903,7 +939,10 @@ export const createServer = Effect.fn(function* (): Effect.fn.Return<
     if (messageText === null) {
       return yield* sendWsResponse({
         id: "unknown",
-        error: { message: "Invalid request format: Failed to read message" },
+        error: {
+          code: WS_ERROR_CODES.invalidRequest,
+          message: "Invalid request format: Failed to read message",
+        },
       });
     }
 
@@ -911,7 +950,10 @@ export const createServer = Effect.fn(function* (): Effect.fn.Return<
     if (Result.isFailure(request)) {
       return yield* sendWsResponse({
         id: "unknown",
-        error: { message: `Invalid request format: ${formatSchemaError(request.failure)}` },
+        error: {
+          code: WS_ERROR_CODES.invalidRequest,
+          message: `Invalid request format: ${formatSchemaError(request.failure)}`,
+        },
       });
     }
 
@@ -919,7 +961,7 @@ export const createServer = Effect.fn(function* (): Effect.fn.Return<
     if (Exit.isFailure(result)) {
       return yield* sendWsResponse({
         id: request.success.id,
-        error: { message: Cause.pretty(result.cause) },
+        error: buildWsErrorForClient(result.cause),
       });
     }
 
diff --git a/apps/web/src/components/Sidebar.tsx b/apps/web/src/components/Sidebar.tsx
index 1b43eb4c1..5b9ba903e 100644
--- a/apps/web/src/components/Sidebar.tsx
+++ b/apps/web/src/components/Sidebar.tsx
@@ -1215,7 +1215,7 @@ export default function Sidebar() {
           </div>
 
           {shouldShowProjectPathEntry && (
-            <div className="mb-2 px-1">
+            <div className="mb-2 min-w-0 px-1 overflow-hidden">
               {isElectron && (
                 <button
                   type="button"
@@ -1227,7 +1227,7 @@ export default function Sidebar() {
                   {isPickingFolder ? "Picking folder..." : "Browse for folder"}
                 </button>
               )}
-              <div className="flex gap-1.5">
+              <div className="flex min-w-0 gap-1.5">
                 <input
                   ref={addProjectInputRef}
                   className={`min-w-0 flex-1 rounded-md border bg-secondary px-2 py-1 font-mono text-xs text-foreground placeholder:text-muted-foreground/40 focus:outline-none ${
@@ -1260,7 +1260,7 @@ export default function Sidebar() {
                 </button>
               </div>
               {addProjectError && (
-                <p className="mt-1 px-0.5 text-[11px] leading-tight text-red-400">
+                <p className="mt-1 min-w-0 break-words px-0.5 text-[11px] leading-tight text-red-400">
                   {addProjectError}
                 </p>
               )}
diff --git a/packages/contracts/src/ws.ts b/packages/contracts/src/ws.ts
index ebb76138b..c9c7ff8c2 100644
--- a/packages/contracts/src/ws.ts
+++ b/packages/contracts/src/ws.ts
@@ -147,14 +147,26 @@ export const WebSocketRequest = Schema.Struct({
 });
 export type WebSocketRequest = typeof WebSocketRequest.Type;
 
+export const WS_ERROR_CODES = {
+  invalidRequest: "INVALID_REQUEST",
+  projectDirNotExist: "PROJECT_DIR_NOT_EXIST",
+  projectPathNotDirectory: "PROJECT_PATH_NOT_DIRECTORY",
+  routeRequest: "ROUTE_REQUEST_ERROR",
+  unknown: "UNKNOWN",
+} as const;
+export type WsErrorCode = (typeof WS_ERROR_CODES)[keyof typeof WS_ERROR_CODES];
+
+export const WsError = Schema.Struct({
+  code: Schema.String,
+  message: Schema.String,
+  path: Schema.optional(Schema.String),
+});
+export type WsError = typeof WsError.Type;
+
 export const WebSocketResponse = Schema.Struct({
   id: TrimmedNonEmptyString,
   result: Schema.optional(Schema.Unknown),
-  error: Schema.optional(
-    Schema.Struct({
-      message: Schema.String,
-    }),
-  ),
+  error: Schema.optional(WsError),
 });
 export type WebSocketResponse = typeof WebSocketResponse.Type;