diff --git a/frontend/src/a2a.js b/frontend/src/a2a.js
index 0251692..498212a 100644
--- a/frontend/src/a2a.js
+++ b/frontend/src/a2a.js
@@ -3,17 +3,10 @@ export const CONFIG_URL = import.meta.env.DEV ? "/config.json" : "./config.json"
 export async function requestJson(url, init = {}) {
   const response = await fetch(url, { credentials: "same-origin", ...init });
   const text = await response.text();
-  let data = null;
-  try {
-    data = text ? JSON.parse(text) : null;
-  } catch {
-    data = { message: text };
-  }
+  const data = text ? JSON.parse(text) : null;
   if (!response.ok) {
-    const detail = data && (data.detail || data.message || data.error);
-    const err = new Error(detail || `request failed: ${response.status}`);
-    err.status = response.status;
-    throw err;
+    const detail = data && (data.detail || data.message);
+    throw new Error(detail || `request failed: ${response.status}`);
   }
   return data;
 }
@@ -27,6 +20,10 @@ export async function loadSession(config) {
 }
 
 export function signInUrl(config) {
+  // Always prefer authorizeUrl. The dashboard's login cookie is host-locked to
+  // the dashboard's own origin, so sending the browser straight to loginUrl
+  // signs the user in *there* and bounces back here still signed out.
+  // authorizeUrl hands this origin a code it trades for its own session.
   const target = config.auth?.authorizeUrl || config.auth?.loginUrl;
   if (!target) return null;
   const next = encodeURIComponent(
@@ -35,32 +32,54 @@ export function signInUrl(config) {
   return `${target}${target.includes("?") ? "&" : "?"}next=${next}`;
 }
 
-export async function callSkill(config, skillName, args) {
-  return requestJson(`${config.endpoints.invoke}/${encodeURIComponent(skillName)}`, {
-    method: "POST",
-    headers: { "content-type": "application/json" },
-    body: JSON.stringify({ arguments: args }),
-  });
+export async function requireSession(config) {
+  const session = await loadSession(config);
+  if (!session?.authenticated) {
+    const target = signInUrl(config);
+    if (target) window.location.assign(target);
+    throw new Error("sign in required");
+  }
+  return session;
 }
 
-export async function fileToBrowserDocument(file, maxBytes = 128000) {
-  if (file.size > maxBytes) {
-    throw new Error(`${file.name} is too large. Upload files up to ${Math.round(maxBytes / 1000)} KB.`);
+export async function callSkill(config, skillName, args) {
+  if (config.auth?.invokeRequiresSession) {
+    await requireSession(config);
   }
-  const mediaType = file.type || (file.name.endsWith(".json") ? "application/json" : file.name.endsWith(".csv") ? "text/csv" : "text/plain");
-  if (!["text/csv", "text/plain", "application/json"].includes(mediaType)) {
-    throw new Error(`${file.name} must be CSV, JSON, or plain text.`);
-  }
-  const buffer = await file.arrayBuffer();
-  let binary = "";
-  const bytes = new Uint8Array(buffer);
-  const chunkSize = 0x8000;
-  for (let i = 0; i < bytes.length; i += chunkSize) {
-    binary += String.fromCharCode(...bytes.subarray(i, i + chunkSize));
-  }
-  return {
-    filename: file.name,
-    media_type: mediaType,
-    data_base64: btoa(binary),
-  };
+  return requestJson(
+    `${config.endpoints.invoke}/${encodeURIComponent(skillName)}`,
+    {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ arguments: args }),
+    },
+  );
+}
+
+export function sampleValue(schema) {
+  if (!schema || typeof schema !== "object") return null;
+  if ("default" in schema) return schema.default;
+  if ("const" in schema) return schema.const;
+  if (Array.isArray(schema.enum) && schema.enum.length) return schema.enum[0];
+  const type = Array.isArray(schema.type)
+    ? schema.type.find((item) => item !== "null") || schema.type[0]
+    : schema.type;
+  if (type === "array") return [];
+  if (type === "boolean") return false;
+  if (type === "integer" || type === "number") return 0;
+  if (type === "string") return "";
+  const props = schema.properties || {};
+  if (type === "object" || Object.keys(props).length) {
+    const required = Array.isArray(schema.required) ? schema.required : Object.keys(props);
+    return Object.fromEntries(
+      required.map((key) => [key, sampleValue(props[key])]),
+    );
+  }
+  return null;
+}
+
+export function sampleArgs(skill) {
+  const schema = skill?.input_schema || skill?.inputSchema || {};
+  const value = sampleValue(schema);
+  return value && typeof value === "object" && !Array.isArray(value) ? value : {};
 }