diff --git a/packages/cli-exec/src/baseline.js b/packages/cli-exec/src/baseline.js
index 71774521c..c3b29d042 100644
--- a/packages/cli-exec/src/baseline.js
+++ b/packages/cli-exec/src/baseline.js
@@ -124,18 +124,35 @@ export async function findBaselineProvider({ cwd = process.cwd(), log } = {}) {
   return null;
 }
 
+// The seed-build status poll needs a READ-capable token, but `percy exec` normally runs with the
+// project's default write-only token — the read then 403s (build reads are master/read_only-gated
+// server-side). That's a token-choice problem the user can fix, not a transient failure, so it is
+// classified here and surfaced as its own sentinel instead of the generic wait-timeout warning.
+function isAuthFailure(error) {
+  let status = error.response?.statusCode;
+  if (status === 401 || status === 403) return true;
+  return /\b(401|403|forbidden|unauthori[sz]ed)\b/i.test(error.message || '');
+}
+
 // The seed build keeps processing (renders + auto-approval) after finalize. The head build must
 // not start until it reaches a terminal state — head snapshots select their baseline as they are
 // processed, and an unapproved seed means the whole first run shows as new instead of diffing.
 // The timeout matches the pipeline latency budget (~99% of builds finish under 5 minutes) —
 // a seed of committed screenshots still renders server-side, so first runs can hold for minutes.
+// Returns the terminal state, or 'unauthorized' when the token cannot read build status at all
+// (every retry would fail identically, so polling stops on the first auth failure).
 export async function waitForSeedBuild(client, buildId, { log, timeout = 600000, interval = 5000 }) {
   let deadline = Date.now() + timeout;
   let state = 'pending';
   let polls = 0;
 
   for (;;) {
-    ({ state } = (await client.getBuild(buildId)).data.attributes);
+    try {
+      ({ state } = (await client.getBuild(buildId)).data.attributes);
+    } catch (err) {
+      if (isAuthFailure(err)) return 'unauthorized';
+      throw err;
+    }
     if (state !== 'pending' && state !== 'processing') return state;
     if (Date.now() >= deadline) return state;
     // A visible heartbeat every ~30s so a multi-minute first-run hold doesn't look like a hang.
@@ -228,6 +245,16 @@ export async function maybeSeedBaseline(percy, provider, { log, waitTimeout, wai
     if (state === 'finished') {
       log.info(`Baseline established from ${seeded}/${baselines.length} committed snapshot(s) ` +
         'and auto-approved — this run diffs against it.');
+    } else if (state === 'unauthorized') {
+      // Token-choice problem, not a transient one: build reads need a read-capable token, so
+      // with the default write-only token Percy cannot hold the run until the baseline is
+      // ready. Ask for the full access token up front — this fires BEFORE the head build's
+      // snapshots are taken, while switching tokens can still save the first run.
+      log.warn(`Uploaded ${seeded}/${baselines.length} baseline snapshot(s), but this token ` +
+        'cannot read build status, so Percy cannot wait for your baseline to finish before ' +
+        'tests start. Use your project\'s FULL ACCESS token as PERCY_TOKEN for this first run ' +
+        '(Project settings → Tokens). If snapshots in this run appear as new instead of ' +
+        'diffing, approve build #1 in the dashboard.');
     } else {
       log.warn(`Baseline build did not finish processing in time (state: ${state || 'unknown'}) — ` +
         'snapshots in this run may show as new instead of diffing against the baseline');
diff --git a/packages/cli-exec/test/baseline.test.js b/packages/cli-exec/test/baseline.test.js
index 419452c02..9616d6b22 100644
--- a/packages/cli-exec/test/baseline.test.js
+++ b/packages/cli-exec/test/baseline.test.js
@@ -5,6 +5,7 @@ import {
   findBaselineProvider,
   maybeSeedBaseline,
   uploadBaselines,
+  waitForSeedBuild,
   sanitizePath,
   sanitizeDirentName
 } from '../src/baseline.js';
@@ -146,6 +147,47 @@ describe('exec baseline seeding', () => {
       expect(log.entries.warn.join('\n')).toContain('did not finish processing in time');
     });
 
+    it('asks for the full access token when the wait 403s (write-only token)', async () => {
+      // `percy exec` normally runs with the write-only token, which cannot read build status —
+      // the poll 403s. That's a fixable token choice, so the user gets targeted guidance BEFORE
+      // their tests run instead of the generic wait-timeout warning.
+      let polls = 0;
+      let client = fakeClient();
+      client.getBuild = async () => {
+        polls += 1;
+        throw Object.assign(new Error('403 Forbidden'), { response: { statusCode: 403 } });
+      };
+      let log = fakeLog();
+      let provider = { discoverBaselines: async () => ({ baselines: BASELINES }) };
+
+      let seeded = await maybeSeedBaseline({ client, projectType: 'web' }, provider, { log });
+
+      expect(seeded).toBe(true);
+      // Auth failures never resolve on retry — the wait must stop after the first poll.
+      expect(polls).toBe(1);
+      let warned = log.entries.warn.join('\n');
+      expect(warned).toContain('FULL ACCESS token');
+      expect(warned).toContain('cannot read build status');
+      expect(warned).not.toContain('did not finish processing in time');
+    });
+
+    it('classifies auth failures from the message when no response object is attached', async () => {
+      let client = fakeClient();
+      client.getBuild = async () => { throw new Error('401 Unauthorized'); };
+
+      let state = await waitForSeedBuild(client, 'seed-build-1', { log: fakeLog() });
+
+      expect(state).toBe('unauthorized');
+    });
+
+    it('a message-less non-auth error keeps the generic degrade path', async () => {
+      let client = fakeClient();
+      client.getBuild = async () => { throw new Error(); };
+
+      await expectAsync(waitForSeedBuild(client, 'seed-build-1', { log: fakeLog() }))
+        .toBeRejected();
+    });
+
     it('abandons the seed when the API hands back a non-first build (pre-candidate API)', async () => {
       let client = fakeClient({ buildNumber: 3 });
       let log = fakeLog();