diff --git a/packages/shared/src/oauthScope.test.ts b/packages/shared/src/oauthScope.test.ts
index 0aa4ef595a8..f5cc247a24a 100644
--- a/packages/shared/src/oauthScope.test.ts
+++ b/packages/shared/src/oauthScope.test.ts
@@ -1,6 +1,14 @@
 import { describe, expect, it } from "vite-plus/test";
+import * as Schema from "effect/Schema";
 
-import { encodeOAuthScope, parseAllowedOAuthScope, parseOAuthScope } from "./oauthScope.ts";
+import {
+  encodeOAuthScope,
+  OAuthScopeEncodingError,
+  parseAllowedOAuthScope,
+  parseOAuthScope,
+} from "./oauthScope.ts";
+
+const isOAuthScopeEncodingError = Schema.is(OAuthScopeEncodingError);
 
 describe("OAuth scopes", () => {
   it("parses an RFC 6749 space-delimited scope set without duplicating permissions", () => {
@@ -32,4 +40,22 @@ describe("OAuth scopes", () => {
       }),
     ).toBeNull();
   });
+
+  it("reports invalid encoding input structurally", () => {
+    expect.assertions(5);
+
+    try {
+      encodeOAuthScope(["access:read", "invalid scope", "access:read"]);
+    } catch (error) {
+      expect(error).toBeInstanceOf(OAuthScopeEncodingError);
+      if (!isOAuthScopeEncodingError(error)) return;
+
+      expect(error.scopes).toEqual(["access:read", "invalid scope", "access:read"]);
+      expect(error.invalidScopes).toEqual(["invalid scope"]);
+      expect(error.duplicateScopes).toEqual(["access:read"]);
+      expect(error.message).toBe(
+        "OAuth scopes must be non-empty, syntactically valid, and unique.",
+      );
+    }
+  });
 });
diff --git a/packages/shared/src/oauthScope.ts b/packages/shared/src/oauthScope.ts
index 47c6dd7051b..4f427440660 100644
--- a/packages/shared/src/oauthScope.ts
+++ b/packages/shared/src/oauthScope.ts
@@ -1,5 +1,20 @@
+import * as Schema from "effect/Schema";
+
 const OAUTH_SCOPE_TOKEN = /^[\u0021\u0023-\u005b\u005d-\u007e]+$/u;
 
+export class OAuthScopeEncodingError extends Schema.TaggedErrorClass<OAuthScopeEncodingError>()(
+  "OAuthScopeEncodingError",
+  {
+    scopes: Schema.Array(Schema.String),
+    invalidScopes: Schema.Array(Schema.String),
+    duplicateScopes: Schema.Array(Schema.String),
+  },
+) {
+  override get message(): string {
+    return "OAuth scopes must be non-empty, syntactically valid, and unique.";
+  }
+}
+
 /**
  * Decodes an RFC 6749 `scope` value as a set while preserving its first-seen
  * order for canonical responses and logs.
@@ -18,12 +33,22 @@ export function parseOAuthScope(value: string): ReadonlyArray<string> | null {
 }
 
 export function encodeOAuthScope(scopes: ReadonlyArray<string>): string {
-  const encoded = scopes.join(" ");
-  const parsed = parseOAuthScope(encoded);
-  if (parsed === null || parsed.length !== scopes.length) {
-    throw new Error("OAuth scopes must be non-empty, valid, and unique.");
+  const invalidScopes = scopes.filter((scope) => !OAUTH_SCOPE_TOKEN.test(scope));
+  const seen = new Set<string>();
+  const duplicateScopes = new Set<string>();
+  for (const scope of scopes) {
+    if (seen.has(scope)) duplicateScopes.add(scope);
+    seen.add(scope);
+  }
+
+  if (scopes.length === 0 || invalidScopes.length > 0 || duplicateScopes.size > 0) {
+    throw new OAuthScopeEncodingError({
+      scopes,
+      invalidScopes,
+      duplicateScopes: [...duplicateScopes],
+    });
   }
-  return encoded;
+  return scopes.join(" ");
 }
 
 export function oauthScopeSetEquals(value: string, expectedScopes: ReadonlyArray<string>): boolean {