diff --git a/packages/server/src/services/mongodb/relay.test.ts b/packages/server/src/services/mongodb/relay.test.ts
index cc932406..4baf3ac4 100644
--- a/packages/server/src/services/mongodb/relay.test.ts
+++ b/packages/server/src/services/mongodb/relay.test.ts
@@ -49,4 +49,19 @@ describe("mongodb relay", () => {
   ])("rejects %s", async (_label, invoke, message) => {
     await expect(invoke()).rejects.toThrow(message);
   });
+
+  it("treats a connection string with malformed percent-encoding as having no URI database", async () => {
+    await expect(
+      listMongoCollections({
+        credentials: {
+          connectionString: "mongodb://user:pass@localhost:27017/db%zz",
+          database: "admin",
+          type: "mongodb" as const,
+        },
+        request: { database: "other" },
+      })
+    ).rejects.toThrow(
+      'Database "other" is not allowed by this data source configuration'
+    );
+  });
 });
diff --git a/packages/server/src/services/mongodb/relay.ts b/packages/server/src/services/mongodb/relay.ts
index 73cc5d7a..611a2a8c 100644
--- a/packages/server/src/services/mongodb/relay.ts
+++ b/packages/server/src/services/mongodb/relay.ts
@@ -187,7 +187,16 @@ function extractDatabaseFromMongoUri(connectionString: string): string | null {
   }
   const url = new URL(normalizedConnectionString);
   const pathname = url.pathname ? url.pathname.replace(/^\//, "") : "";
-  const decoded = decodeURIComponent(pathname).trim();
+  // Comment: URL parsing does not validate percent-encoding, so a connection
+  // string whose database path contains malformed escapes (for example a
+  // literal "%") must be treated like an unparseable URI instead of crashing
+  // with an uncaught URIError.
+  let decoded: string;
+  try {
+    decoded = decodeURIComponent(pathname).trim();
+  } catch {
+    return null;
+  }
   if (decoded.length === 0) {
     return null;
   }