diff --git a/apps/sim/app/api/knowledge/[id]/connectors/[connectorId]/route.ts b/apps/sim/app/api/knowledge/[id]/connectors/[connectorId]/route.ts
index 33717fdcb39..3ff1d479cd0 100644
--- a/apps/sim/app/api/knowledge/[id]/connectors/[connectorId]/route.ts
+++ b/apps/sim/app/api/knowledge/[id]/connectors/[connectorId]/route.ts
@@ -1,12 +1,6 @@
 import { AuditAction, AuditResourceType, recordAudit } from '@sim/audit'
 import { db } from '@sim/db'
-import {
-  document,
-  embedding,
-  knowledgeBase,
-  knowledgeConnector,
-  knowledgeConnectorSyncLog,
-} from '@sim/db/schema'
+import { document, embedding, knowledgeConnector, knowledgeConnectorSyncLog } from '@sim/db/schema'
 import { createLogger } from '@sim/logger'
 import { and, desc, eq, inArray, isNull, sql } from 'drizzle-orm'
 import { type NextRequest, NextResponse } from 'next/server'
@@ -17,6 +11,7 @@ import { checkSessionOrInternalAuth } from '@/lib/auth/hybrid'
 import { hasWorkspaceLiveSyncAccess } from '@/lib/billing/core/subscription'
 import { generateRequestId } from '@/lib/core/utils/request'
 import { withRouteHandler } from '@/lib/core/utils/with-route-handler'
+import { resolveCredentialTokenIdentity } from '@/lib/credentials/access'
 import { deleteDocumentStorageFiles } from '@/lib/knowledge/documents/service'
 import { cleanupUnusedTagDefinitions } from '@/lib/knowledge/tags/service'
 import { captureServerEvent } from '@/lib/posthog/server'
@@ -157,16 +152,6 @@ export const PATCH = withRouteHandler(async (request: NextRequest, context: Rout
         )
       }
 
-      const kbRows = await db
-        .select({ userId: knowledgeBase.userId })
-        .from(knowledgeBase)
-        .where(eq(knowledgeBase.id, knowledgeBaseId))
-        .limit(1)
-
-      if (kbRows.length === 0) {
-        return NextResponse.json({ error: 'Knowledge base not found' }, { status: 404 })
-      }
-
       let accessToken: string | null = null
       if (connectorConfig.auth.mode === 'apiKey') {
         if (!existing.encryptedApiKey) {
@@ -183,9 +168,32 @@ export const PATCH = withRouteHandler(async (request: NextRequest, context: Rout
             { status: 400 }
           )
         }
+        const connectorWorkspaceId = writeCheck.knowledgeBase.workspaceId
+        if (!connectorWorkspaceId) {
+          return NextResponse.json(
+            { error: 'Knowledge base is missing workspace context' },
+            { status: 409 }
+          )
+        }
+        /**
+         * Resolve the credential's own account owner, not the knowledge base owner:
+         * workspace credentials are shared, and token reads are scoped to
+         * `account.userId`.
+         */
+        const identity = await resolveCredentialTokenIdentity(
+          existing.credentialId,
+          connectorWorkspaceId
+        )
+        if (!identity) {
+          return NextResponse.json(
+            { error: 'Credential is no longer usable in this workspace. Please reconnect it.' },
+            { status: 400 }
+          )
+        }
         accessToken = await refreshAccessTokenIfNeeded(
           existing.credentialId,
-          kbRows[0].userId,
+          // Service accounts mint their own token and ignore the acting user.
+          identity.kind === 'oauth' ? identity.userId : auth.userId,
           `patch-${connectorId}`
         )
       }
diff --git a/apps/sim/app/workspace/[workspaceId]/knowledge/[id]/components/add-connector-modal/add-connector-modal.tsx b/apps/sim/app/workspace/[workspaceId]/knowledge/[id]/components/add-connector-modal/add-connector-modal.tsx
index 8738542fa6d..934e4b07011 100644
--- a/apps/sim/app/workspace/[workspaceId]/knowledge/[id]/components/add-connector-modal/add-connector-modal.tsx
+++ b/apps/sim/app/workspace/[workspaceId]/knowledge/[id]/components/add-connector-modal/add-connector-modal.tsx
@@ -92,7 +92,7 @@ export function AddConnectorModal({
   )
 
   const {
-    data: credentials = [],
+    data: rawCredentials = [],
     isLoading: credentialsLoading,
     refetch: refetchCredentials,
   } = useOAuthCredentials(connectorProviderId ?? undefined, {
@@ -100,6 +100,20 @@ export function AddConnectorModal({
     workspaceId,
   })
 
+  /**
+   * The credential list also returns the provider's service accounts, but
+   * `ConnectorAuthConfig` has no service-account mode: the sync engine resolves
+   * connector tokens through `refreshAccessTokenIfNeeded`, which passes no scopes
+   * and drops the `cloudId`/`domain`/`authStyle` a service account resolves with.
+   * Offering them here would surface credentials no connector can authenticate
+   * with, so — like a workflow picker that has not opted in via
+   * `allowServiceAccounts` — list OAuth accounts only.
+   */
+  const credentials = useMemo(
+    () => rawCredentials.filter((cred) => cred.type !== 'service_account'),
+    [rawCredentials]
+  )
+
   useCredentialRefreshTriggers(refetchCredentials, connectorProviderId ?? '', workspaceId)
 
   const effectiveCredentialId =
diff --git a/apps/sim/lib/credentials/access.test.ts b/apps/sim/lib/credentials/access.test.ts
index b5ae09c1418..1fc40f6e4ed 100644
--- a/apps/sim/lib/credentials/access.test.ts
+++ b/apps/sim/lib/credentials/access.test.ts
@@ -1,13 +1,15 @@
-import { credential, credentialMember } from '@sim/db/schema'
+import { account, credential, credentialMember } from '@sim/db/schema'
 import { queueTableRows, resetDbChainMock } from '@sim/testing'
 import { afterAll, beforeEach, describe, expect, it, vi } from 'vitest'
 
-const { mockCheckWorkspaceAccess } = vi.hoisted(() => ({
+const { mockCheckWorkspaceAccess, mockGetUserEntityPermissions } = vi.hoisted(() => ({
   mockCheckWorkspaceAccess: vi.fn(),
+  mockGetUserEntityPermissions: vi.fn(),
 }))
 
 vi.mock('@/lib/workspaces/permissions/utils', () => ({
   checkWorkspaceAccess: mockCheckWorkspaceAccess,
+  getUserEntityPermissions: mockGetUserEntityPermissions,
   resolveWorkspaceAccess: vi.fn(async (workspaceId: string, userId: string, provided?: any) =>
     provided && provided.workspace?.id === workspaceId
       ? provided
@@ -15,7 +17,7 @@ vi.mock('@/lib/workspaces/permissions/utils', () => ({
   ),
 }))
 
-import { getCredentialActorContext } from '@/lib/credentials/access'
+import { getCredentialActorContext, resolveCredentialTokenIdentity } from '@/lib/credentials/access'
 
 afterAll(resetDbChainMock)
 
@@ -88,3 +90,74 @@ describe('getCredentialActorContext', () => {
     expect(ctx.isAdmin).toBe(false)
   })
 })
+
+describe('resolveCredentialTokenIdentity', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    resetDbChainMock()
+  })
+
+  it('resolves the account owner even when it is not the caller', async () => {
+    queueTableRows(credential, [{ workspaceId: 'ws', type: 'oauth', accountId: 'acct1' }])
+    queueTableRows(account, [{ userId: 'authorizer' }])
+    mockGetUserEntityPermissions.mockResolvedValue('write')
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toEqual({
+      kind: 'oauth',
+      userId: 'authorizer',
+    })
+  })
+
+  it('rejects a credential belonging to another workspace', async () => {
+    queueTableRows(credential, [{ workspaceId: 'other-ws', type: 'oauth', accountId: 'acct1' }])
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toBeNull()
+  })
+
+  it('reports service accounts as needing no user id', async () => {
+    queueTableRows(credential, [{ workspaceId: 'ws', type: 'service_account', accountId: null }])
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toEqual({
+      kind: 'service_account',
+    })
+  })
+
+  it('rejects a service account from another workspace', async () => {
+    queueTableRows(credential, [
+      { workspaceId: 'other-ws', type: 'service_account', accountId: null },
+    ])
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toBeNull()
+  })
+
+  it('rejects a credential type that is neither oauth nor a service account', async () => {
+    queueTableRows(credential, [{ workspaceId: 'ws', type: 'env_personal', accountId: null }])
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toBeNull()
+  })
+
+  it('rejects when the owner no longer has workspace access', async () => {
+    queueTableRows(credential, [{ workspaceId: 'ws', type: 'oauth', accountId: 'acct1' }])
+    queueTableRows(account, [{ userId: 'departed' }])
+    mockGetUserEntityPermissions.mockResolvedValue(null)
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toBeNull()
+  })
+
+  it('falls back to a legacy raw account id when no credential row exists', async () => {
+    queueTableRows(account, [{ userId: 'legacy-owner' }])
+    mockGetUserEntityPermissions.mockResolvedValue('admin')
+
+    await expect(resolveCredentialTokenIdentity('acct-legacy', 'ws')).resolves.toEqual({
+      kind: 'oauth',
+      userId: 'legacy-owner',
+    })
+  })
+
+  it('returns null when the account row is missing', async () => {
+    queueTableRows(credential, [{ workspaceId: 'ws', type: 'oauth', accountId: 'acct1' }])
+
+    await expect(resolveCredentialTokenIdentity('c1', 'ws')).resolves.toBeNull()
+    expect(mockGetUserEntityPermissions).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/sim/lib/credentials/access.ts b/apps/sim/lib/credentials/access.ts
index 005a47fe65d..185d6206db6 100644
--- a/apps/sim/lib/credentials/access.ts
+++ b/apps/sim/lib/credentials/access.ts
@@ -1,8 +1,12 @@
 import { db } from '@sim/db'
-import { credential, credentialMember, credentialTypeEnum } from '@sim/db/schema'
+import { account, credential, credentialMember, credentialTypeEnum } from '@sim/db/schema'
 import { and, eq, inArray } from 'drizzle-orm'
 import type { DbOrTx } from '@/lib/db/types'
-import { resolveWorkspaceAccess, type WorkspaceAccess } from '@/lib/workspaces/permissions/utils'
+import {
+  getUserEntityPermissions,
+  resolveWorkspaceAccess,
+  type WorkspaceAccess,
+} from '@/lib/workspaces/permissions/utils'
 
 type ActiveCredentialMember = typeof credentialMember.$inferSelect
 type CredentialRecord = typeof credential.$inferSelect
@@ -19,6 +23,66 @@ export const SHARED_CREDENTIAL_TYPES = credentialTypeEnum.enumValues.filter(
   (type) => type !== 'env_personal'
 )
 
+/**
+ * Which user a credential's token must be read as.
+ *
+ * Service-account credentials mint their own token and ignore the acting user
+ * entirely, so they carry no user id — callers pass their existing one through.
+ */
+export type CredentialTokenIdentity =
+  | { kind: 'service_account' }
+  | { kind: 'oauth'; userId: string }
+
+/**
+ * Resolves which user a credential's token must be read as, for background jobs
+ * that run without a request context (connector syncs, scheduled runs).
+ *
+ * Workspace-scoped OAuth credentials are shared, so the member who authorized one
+ * is frequently not the user driving the job. Token reads are scoped to
+ * `account.userId`, so a job passing its own user id resolves no token at all.
+ * Mirrors the ownership resolution in `authorizeCredentialUse`: the credential must
+ * belong to `workspaceId`, and its owner must still have access to that workspace.
+ *
+ * @returns the identity to read the token as, or `null` when the credential is
+ * unusable from this workspace (wrong workspace, missing account, owner lost access).
+ */
+export async function resolveCredentialTokenIdentity(
+  credentialId: string,
+  workspaceId: string
+): Promise<CredentialTokenIdentity | null> {
+  const [platformCredential] = await db
+    .select({
+      workspaceId: credential.workspaceId,
+      type: credential.type,
+      accountId: credential.accountId,
+    })
+    .from(credential)
+    .where(eq(credential.id, credentialId))
+    .limit(1)
+
+  if (platformCredential) {
+    if (platformCredential.workspaceId !== workspaceId) return null
+    if (platformCredential.type === 'service_account') return { kind: 'service_account' }
+    if (platformCredential.type !== 'oauth' || !platformCredential.accountId) return null
+  }
+
+  // Credentials predating the workspace-scoped `credential` table are raw account ids.
+  const accountId = platformCredential?.accountId ?? credentialId
+
+  const [accountRow] = await db
+    .select({ userId: account.userId })
+    .from(account)
+    .where(eq(account.id, accountId))
+    .limit(1)
+
+  if (!accountRow) return null
+
+  const ownerPerm = await getUserEntityPermissions(accountRow.userId, 'workspace', workspaceId)
+  if (ownerPerm === null) return null
+
+  return { kind: 'oauth', userId: accountRow.userId }
+}
+
 /** Whether a credential is shared at the workspace level (i.e. not a personal env var). */
 export function isSharedCredentialType(type: CredentialType): boolean {
   return type !== 'env_personal'
diff --git a/apps/sim/lib/knowledge/connectors/sync-engine.ts b/apps/sim/lib/knowledge/connectors/sync-engine.ts
index 65b0d04542f..8ca978ec829 100644
--- a/apps/sim/lib/knowledge/connectors/sync-engine.ts
+++ b/apps/sim/lib/knowledge/connectors/sync-engine.ts
@@ -17,6 +17,7 @@ import {
   type BillingAttributionSnapshot,
 } from '@/lib/billing/core/billing-attribution'
 import { getInternalApiBaseUrl } from '@/lib/core/utils/urls'
+import { resolveCredentialTokenIdentity } from '@/lib/credentials/access'
 import type { DocumentData } from '@/lib/knowledge/documents/service'
 import { hardDeleteDocuments, processDocumentsWithQueue } from '@/lib/knowledge/documents/service'
 import { StorageService } from '@/lib/uploads'
@@ -377,6 +378,10 @@ export function resolveTagMapping(
  * Resolves an access token for a connector based on its auth mode.
  * OAuth connectors refresh via the credential system; API key connectors
  * decrypt the key stored in the dedicated `encryptedApiKey` column.
+ *
+ * `userId` must be the user who owns the credential's OAuth account — not the
+ * knowledge base owner. Workspace-scoped credentials are routinely authorized by
+ * a different member, and token reads are scoped to `account.userId`.
  */
 async function resolveAccessToken(
   connector: { credentialId: string | null; encryptedApiKey: string | null },
@@ -529,7 +534,31 @@ export async function executeSync(
   let syncExitedCleanly = false
 
   try {
-    let accessToken = await resolveAccessToken(connector, connectorConfig, userId)
+    /**
+     * OAuth credentials are workspace-scoped and shared, so the member who authorized
+     * one is often not the knowledge base owner. Resolve the credential's own account
+     * owner — token reads are scoped to `account.userId`, so passing the KB owner
+     * resolves no token at all. Resolved once here rather than inside
+     * `resolveAccessToken` so per-page refreshes don't repeat the lookup.
+     */
+    let credentialUserId = userId
+    if (connectorConfig.auth.mode === 'oauth' && connector.credentialId) {
+      const identity = await resolveCredentialTokenIdentity(
+        connector.credentialId,
+        kbOwner.workspaceId
+      )
+      if (!identity) {
+        throw new Error(
+          `Credential ${connector.credentialId} is not usable from workspace ${kbOwner.workspaceId} — reconnect the credential`
+        )
+      }
+      // Service accounts mint their own token and ignore the acting user.
+      if (identity.kind === 'oauth') {
+        credentialUserId = identity.userId
+      }
+    }
+
+    let accessToken = await resolveAccessToken(connector, connectorConfig, credentialUserId)
 
     const externalDocs: ExternalDocument[] = []
     let cursor: string | undefined
@@ -605,7 +634,7 @@ export async function executeSync(
 
     for (let pageNum = 0; hasMore && pageNum < MAX_PAGES; pageNum++) {
       if (pageNum > 0 && connectorConfig.auth.mode === 'oauth') {
-        accessToken = await resolveAccessToken(connector, connectorConfig, userId)
+        accessToken = await resolveAccessToken(connector, connectorConfig, credentialUserId)
       }
 
       const page = await connectorConfig.listDocuments(
@@ -795,7 +824,7 @@ export async function executeSync(
 
       if (deferredOps.length > 0) {
         if (connectorConfig.auth.mode === 'oauth') {
-          accessToken = await resolveAccessToken(connector, connectorConfig, userId)
+          accessToken = await resolveAccessToken(connector, connectorConfig, credentialUserId)
         }
 
         const hydrated = await Promise.allSettled(