diff --git a/apps/sim/lib/workflows/application/read-workflow-version.ts b/apps/sim/lib/workflows/application/read-workflow-version.ts
index 4cab173c1a2..5e9c47a8ecf 100644
--- a/apps/sim/lib/workflows/application/read-workflow-version.ts
+++ b/apps/sim/lib/workflows/application/read-workflow-version.ts
@@ -5,6 +5,7 @@ import { defineAuthorizedWorkflowUseCase } from '@/lib/workflows/application/aut
 import { resolveActiveWorkflowApplicationContext } from '@/lib/workflows/application/context'
 import { workflowOperations } from '@/lib/workflows/application/operations'
 import { assertedWorkflowWorkspaceId } from '@/lib/workflows/application/principal-scope'
+import { projectLegacySlackV2Auth } from '@/lib/workflows/compatibility/slack-v2-auth'
 import { sanitizeWorkflowForSharing } from '@/lib/workflows/credentials/credential-extractor'
 import { getWorkflowDeploymentVersion } from '@/lib/workflows/persistence/utils'
 import type { WorkflowState } from '@/stores/workflows/workflow/types'
@@ -68,7 +69,10 @@ export const readWorkflowVersion = defineAuthorizedWorkflowUseCase({
     if (!isWorkflowState(state)) {
       throw new Error('Deployment version contains invalid workflow state')
     }
-    const presentedState = input.includeCredentialValues ? state : sanitizeVersionState(state)
+    const compatibleState = { ...state, blocks: projectLegacySlackV2Auth(state.blocks ?? {}) }
+    const presentedState = input.includeCredentialValues
+      ? compatibleState
+      : sanitizeVersionState(compatibleState)
     logger.info('Read workflow version', {
       workspaceId: context.workspaceId,
       workflowId: context.workflowId,
diff --git a/apps/sim/lib/workflows/application/workflow-crud.test.ts b/apps/sim/lib/workflows/application/workflow-crud.test.ts
index b2004215b6e..cc7c29cae9a 100644
--- a/apps/sim/lib/workflows/application/workflow-crud.test.ts
+++ b/apps/sim/lib/workflows/application/workflow-crud.test.ts
@@ -106,6 +106,7 @@ import { listWorkflowVersions } from '@/lib/workflows/application/list-workflow-
 import { readWorkflow } from '@/lib/workflows/application/read-workflow'
 import { readWorkflowVersion } from '@/lib/workflows/application/read-workflow-version'
 import { updateWorkflow } from '@/lib/workflows/application/update-workflow'
+import { createHistoricalSlackV2Block } from '@/lib/workflows/compatibility/slack-v2-auth.fixtures'
 
 const WORKSPACE_ID = 'workspace-1'
 const WORKFLOW_ID = 'workflow-1'
@@ -466,4 +467,31 @@ describe('authorized workflow CRUD and version reads', () => {
     ).resolves.toMatchObject({ version: { id: 'version-1', version: 1 } })
     expect(mocks.resolveWorkflowContext).toHaveBeenCalledBefore(mocks.readVersion)
   })
+
+  it('presents historical Slack v2 auth canonically without mutating the stored version', async () => {
+    const historicalSlack = createHistoricalSlackV2Block('slack')
+    const state = {
+      blocks: { slack: historicalSlack },
+      edges: [],
+      loops: {},
+      parallels: {},
+    }
+    mocks.readVersion.mockResolvedValue({
+      id: 'version-legacy-slack',
+      version: 1,
+      state,
+    })
+
+    const result = await readWorkflowVersion.execute({
+      principal: personalPrincipal,
+      input: { workflowId: WORKFLOW_ID, version: 1, includeCredentialValues: true },
+    })
+
+    expect(result.version.state.blocks.slack.subBlocks.credential.value).toBe(
+      'credential-custom-bot'
+    )
+    expect(result.version.state.blocks.slack.subBlocks).not.toHaveProperty('authMethod')
+    expect(historicalSlack.subBlocks.authMethod.value).toBe('bot_token')
+    expect(historicalSlack.subBlocks.credential.value).toBe('dormant-oauth')
+  })
 })
diff --git a/apps/sim/lib/workflows/compatibility/slack-v2-auth.fixtures.ts b/apps/sim/lib/workflows/compatibility/slack-v2-auth.fixtures.ts
new file mode 100644
index 00000000000..d69a8a785ea
--- /dev/null
+++ b/apps/sim/lib/workflows/compatibility/slack-v2-auth.fixtures.ts
@@ -0,0 +1,41 @@
+import type { BlockState, SubBlockState } from '@sim/workflow-types/workflow'
+
+function subBlock(
+  id: string,
+  type: SubBlockState['type'],
+  value: SubBlockState['value']
+): SubBlockState {
+  return { id, type, value }
+}
+
+/** Relevant persisted fields produced by the slack_v2 schema introduced in f4d47ed. */
+export function createHistoricalSlackV2Block(id = 'slack-1'): BlockState {
+  return {
+    id,
+    type: 'slack_v2',
+    name: 'Slack',
+    position: { x: 0, y: 0 },
+    enabled: true,
+    triggerMode: false,
+    subBlocks: {
+      operation: subBlock('operation', 'dropdown', 'send'),
+      authMethod: subBlock('authMethod', 'dropdown', 'bot_token'),
+      credential: subBlock('credential', 'oauth-input', 'dormant-oauth'),
+      manualCredential: subBlock('manualCredential', 'short-input', null),
+      customBotCredential: subBlock('customBotCredential', 'oauth-input', 'credential-custom-bot'),
+      manualCustomBotCredential: subBlock('manualCustomBotCredential', 'short-input', null),
+      destinationType: subBlock('destinationType', 'dropdown', 'channel'),
+      channel: subBlock('channel', 'channel-selector', 'C123456789'),
+      text: subBlock('text', 'long-input', 'Hello'),
+      messageFormat: subBlock('messageFormat', 'dropdown', 'text'),
+    },
+    data: {
+      canonicalModes: {
+        oauthCredential: 'basic',
+        botCredential: 'basic',
+        channel: 'basic',
+      },
+    },
+    outputs: {},
+  }
+}
diff --git a/apps/sim/lib/workflows/compatibility/slack-v2-auth.test.ts b/apps/sim/lib/workflows/compatibility/slack-v2-auth.test.ts
new file mode 100644
index 00000000000..bf97872fcc6
--- /dev/null
+++ b/apps/sim/lib/workflows/compatibility/slack-v2-auth.test.ts
@@ -0,0 +1,120 @@
+/**
+ * @vitest-environment node
+ */
+
+import { omit } from '@sim/utils/object'
+import type { BlockState, WorkflowState } from '@sim/workflow-types/workflow'
+import { afterAll, describe, expect, it, vi } from 'vitest'
+
+vi.unmock('@/blocks/registry')
+
+import { generateWorkflowDiffSummary } from '@/lib/workflows/comparison/compare'
+import { projectLegacySlackV2Auth } from '@/lib/workflows/compatibility/slack-v2-auth'
+import { createHistoricalSlackV2Block } from '@/lib/workflows/compatibility/slack-v2-auth.fixtures'
+import { buildSelectorContextFromBlock } from '@/lib/workflows/subblocks/context'
+import * as blocksBarrel from '@/blocks'
+import { getBlock as getRealBlock } from '@/blocks/registry'
+import { extractBlockParams } from '@/serializer'
+
+const getBlockSpy = vi.spyOn(blocksBarrel, 'getBlock').mockImplementation(getRealBlock)
+
+afterAll(() => {
+  getBlockSpy.mockRestore()
+})
+
+function workflowWith(block: BlockState): WorkflowState {
+  return { blocks: { [block.id]: block }, edges: [], loops: {}, parallels: {} }
+}
+
+describe('projectLegacySlackV2Auth', () => {
+  it('makes the historical custom-bot action behave like its current equivalent', () => {
+    const historical = createHistoricalSlackV2Block()
+    const original = structuredClone(historical)
+    const equivalentCurrent = structuredClone(historical)
+    equivalentCurrent.subBlocks = omit(equivalentCurrent.subBlocks, [
+      'authMethod',
+      'customBotCredential',
+      'manualCustomBotCredential',
+    ])
+    equivalentCurrent.subBlocks.credential.value = 'credential-custom-bot'
+    const blocks = projectLegacySlackV2Auth({ [historical.id]: historical })
+    const projected = blocks[historical.id]
+
+    expect(historical).toEqual(original)
+    expect(projected.subBlocks).not.toHaveProperty('authMethod')
+    expect(projected.subBlocks).not.toHaveProperty('customBotCredential')
+    expect(projected.subBlocks.credential.value).toBe('credential-custom-bot')
+    expect(projected.data?.canonicalModes).toMatchObject({
+      oauthCredential: 'basic',
+      botCredential: 'basic',
+    })
+
+    const selectorContext = buildSelectorContextFromBlock(projected.type, projected.subBlocks, {
+      selectorKey: 'slack.channels',
+      dependsOn: ['credential'],
+      canonicalModes: projected.data?.canonicalModes,
+    })
+    expect(selectorContext.oauthCredential).toBe('credential-custom-bot')
+
+    const params = extractBlockParams(projected)
+    expect(params).toMatchObject({
+      oauthCredential: 'credential-custom-bot',
+      channel: 'C123456789',
+    })
+    expect(params).not.toHaveProperty('botCredential')
+
+    expect(
+      generateWorkflowDiffSummary(workflowWith(equivalentCurrent), workflowWith(projected))
+        .hasChanges
+    ).toBe(false)
+  })
+
+  it('honors the historical custom-bot and OAuth modes', () => {
+    const historical = createHistoricalSlackV2Block()
+    historical.data!.canonicalModes!.botCredential = 'advanced'
+    historical.subBlocks.manualCustomBotCredential.value = 'credential-manual-bot'
+
+    const projected = projectLegacySlackV2Auth({ [historical.id]: historical })[historical.id]
+
+    expect(projected.subBlocks.credential.value).toBeNull()
+    expect(projected.subBlocks.manualCredential.value).toBe('credential-manual-bot')
+    expect(projected.data?.canonicalModes?.oauthCredential).toBe('advanced')
+
+    const historicalOauth = createHistoricalSlackV2Block()
+    historicalOauth.subBlocks.authMethod.value = 'oauth'
+    const projectedOauth = projectLegacySlackV2Auth({ slack: historicalOauth }).slack
+    expect(projectedOauth.subBlocks.credential.value).toBe('dormant-oauth')
+  })
+
+  it('leaves current, trigger, and unidentifiable states untouched', () => {
+    const cases = [
+      createHistoricalSlackV2Block(),
+      createHistoricalSlackV2Block(),
+      createHistoricalSlackV2Block(),
+    ]
+    cases[0].subBlocks = omit(cases[0].subBlocks, ['authMethod'])
+    cases[1].triggerMode = true
+    cases[2].subBlocks.authMethod.value = null
+
+    for (const block of cases) {
+      const blocks = { [block.id]: block }
+      expect(projectLegacySlackV2Auth(blocks)).toBe(blocks)
+    }
+  })
+
+  it('does not substitute a dormant OAuth account for a missing historical bot credential', () => {
+    const historical = createHistoricalSlackV2Block()
+    historical.subBlocks.customBotCredential.value = null
+
+    const projected = projectLegacySlackV2Auth({ [historical.id]: historical })[historical.id]
+
+    expect(projected.subBlocks.credential.value).toBeNull()
+    expect(
+      buildSelectorContextFromBlock(projected.type, projected.subBlocks, {
+        selectorKey: 'slack.channels',
+        dependsOn: ['credential'],
+        canonicalModes: projected.data?.canonicalModes,
+      }).oauthCredential
+    ).toBeUndefined()
+  })
+})
diff --git a/apps/sim/lib/workflows/compatibility/slack-v2-auth.ts b/apps/sim/lib/workflows/compatibility/slack-v2-auth.ts
new file mode 100644
index 00000000000..ba092e31aa2
--- /dev/null
+++ b/apps/sim/lib/workflows/compatibility/slack-v2-auth.ts
@@ -0,0 +1,98 @@
+import { omit } from '@sim/utils/object'
+import type { BlockState, SubBlockState } from '@sim/workflow-types/workflow'
+import { isNonEmptyValue } from '@/lib/workflows/subblocks/visibility'
+
+type CanonicalMode = 'basic' | 'advanced'
+
+function resolveLegacyMode(
+  override: CanonicalMode | undefined,
+  basicValue: unknown,
+  advancedValue: unknown
+): CanonicalMode {
+  if (override === 'basic' || override === 'advanced') return override
+  return !isNonEmptyValue(basicValue) && isNonEmptyValue(advancedValue) ? 'advanced' : 'basic'
+}
+
+function withValue(
+  subBlock: SubBlockState | undefined,
+  id: string,
+  type: SubBlockState['type'],
+  value: SubBlockState['value']
+): SubBlockState {
+  return { ...(subBlock ?? { id, type }), value }
+}
+
+/**
+ * Projects the preview-era slack_v2 action auth shape into the merged credential picker added in
+ * 5be35b5. The returned view is safe for current readers but is never marked for persistence, so
+ * frozen deployment snapshots and normalized workflow rows remain unchanged.
+ */
+export function projectLegacySlackV2Auth(
+  blocks: Record<string, BlockState>
+): Record<string, BlockState> {
+  let projectedBlocks: Record<string, BlockState> | undefined
+
+  for (const [blockId, block] of Object.entries(blocks)) {
+    if (block.type !== 'slack_v2' || block.triggerMode) continue
+
+    const authMethod = block.subBlocks.authMethod?.value
+    if (authMethod !== 'oauth' && authMethod !== 'bot_token') continue
+
+    const canonicalModes = block.data?.canonicalModes ?? {}
+    const oauthMode = resolveLegacyMode(
+      canonicalModes.oauthCredential,
+      block.subBlocks.credential?.value,
+      block.subBlocks.manualCredential?.value
+    )
+    const botMode = resolveLegacyMode(
+      canonicalModes.botCredential,
+      block.subBlocks.customBotCredential?.value,
+      block.subBlocks.manualCustomBotCredential?.value
+    )
+    const activeMode = authMethod === 'bot_token' ? botMode : oauthMode
+    const activeValue =
+      authMethod === 'bot_token'
+        ? activeMode === 'advanced'
+          ? block.subBlocks.manualCustomBotCredential?.value
+          : block.subBlocks.customBotCredential?.value
+        : activeMode === 'advanced'
+          ? block.subBlocks.manualCredential?.value
+          : block.subBlocks.credential?.value
+    const credentialValue = isNonEmptyValue(activeValue) ? (activeValue ?? null) : null
+    const currentSubBlocks = omit(block.subBlocks, [
+      'authMethod',
+      'customBotCredential',
+      'manualCustomBotCredential',
+    ])
+
+    projectedBlocks ??= { ...blocks }
+    projectedBlocks[blockId] = {
+      ...block,
+      subBlocks: {
+        ...currentSubBlocks,
+        credential: withValue(
+          block.subBlocks.credential,
+          'credential',
+          'oauth-input',
+          activeMode === 'basic' ? credentialValue : null
+        ),
+        manualCredential: withValue(
+          block.subBlocks.manualCredential,
+          'manualCredential',
+          'short-input',
+          activeMode === 'advanced' ? credentialValue : null
+        ),
+      },
+      data: {
+        ...block.data,
+        canonicalModes: {
+          ...canonicalModes,
+          oauthCredential: activeMode,
+          botCredential: 'basic',
+        },
+      },
+    }
+  }
+
+  return projectedBlocks ?? blocks
+}
diff --git a/apps/sim/lib/workflows/persistence/utils.test.ts b/apps/sim/lib/workflows/persistence/utils.test.ts
index b4b50ab88a0..e2866667b34 100644
--- a/apps/sim/lib/workflows/persistence/utils.test.ts
+++ b/apps/sim/lib/workflows/persistence/utils.test.ts
@@ -23,6 +23,7 @@ import {
   schemaMock,
 } from '@sim/testing'
 import { afterAll, beforeEach, describe, expect, it, vi } from 'vitest'
+import { createHistoricalSlackV2Block } from '@/lib/workflows/compatibility/slack-v2-auth.fixtures'
 import type {
   BlockState as AppBlockState,
   WorkflowState as AppWorkflowState,
@@ -347,6 +348,26 @@ describe('Database Helpers', () => {
     })
   })
 
+  describe('materializeDeploymentState', () => {
+    it('projects historical Slack v2 auth without changing the frozen snapshot', async () => {
+      const historicalSlack = createHistoricalSlackV2Block('slack')
+      const frozenState = createWorkflowState({
+        blocks: { slack: historicalSlack },
+      })
+
+      const materialized = await dbHelpers.materializeDeploymentState(
+        mockWorkflowId,
+        { id: 'legacy-slack-version', state: frozenState },
+        'test-workspace-id'
+      )
+
+      expect(materialized.blocks.slack.subBlocks.credential.value).toBe('credential-custom-bot')
+      expect(materialized.blocks.slack.subBlocks).not.toHaveProperty('authMethod')
+      expect(historicalSlack.subBlocks.authMethod.value).toBe('bot_token')
+      expect(historicalSlack.subBlocks.credential.value).toBe('dormant-oauth')
+    })
+  })
+
   describe('loadWorkflowFromNormalizedTables', () => {
     it('should successfully load workflow data from normalized tables', async () => {
       queueLoadFixtures({
diff --git a/apps/sim/lib/workflows/persistence/utils.ts b/apps/sim/lib/workflows/persistence/utils.ts
index 4e21627e27a..d570d939e3f 100644
--- a/apps/sim/lib/workflows/persistence/utils.ts
+++ b/apps/sim/lib/workflows/persistence/utils.ts
@@ -26,6 +26,7 @@ import type { InferSelectModel } from 'drizzle-orm'
 import { and, desc, eq, inArray, lt, sql } from 'drizzle-orm'
 import { LRUCache } from 'lru-cache'
 import { releaseWebhookPathClaims } from '@/lib/webhooks/path-claims'
+import { projectLegacySlackV2Auth } from '@/lib/workflows/compatibility/slack-v2-auth'
 import { remapConditionBlockIds, remapConditionEdgeHandle } from '@/lib/workflows/condition-ids'
 import { isDynamicHandleSubblock } from '@/lib/workflows/dynamic-handle-topology'
 import {
@@ -206,6 +207,7 @@ export async function materializeDeploymentState(
     workspaceId,
     executor
   )
+  const compatibleBlocks = projectLegacySlackV2Auth(migratedBlocks)
   /*
    * Read straight out of the version's jsonb blob, so unlike every path that
    * goes through `loadWorkflowFromNormalizedTables` these handles were never
@@ -228,7 +230,7 @@ export async function materializeDeploymentState(
    */
   const errorSourceBlockIds = collectErrorSourceBlockIds(edges)
   const blocks: DeployedWorkflowData['blocks'] = {}
-  for (const [blockId, block] of Object.entries(migratedBlocks)) {
+  for (const [blockId, block] of Object.entries(compatibleBlocks)) {
     blocks[blockId] =
       block.errorEnabled || !errorSourceBlockIds.has(blockId)
         ? block
@@ -556,11 +558,12 @@ export async function loadWorkflowFromNormalizedTables(
   const raw = await loadWorkflowFromNormalizedTablesRaw(workflowId, externalTx)
   if (!raw) return null
 
-  const { blocks: finalBlocks, migrated } = await applyBlockMigrations(
+  const { blocks: migratedBlocks, migrated } = await applyBlockMigrations(
     raw.blocks,
     raw.workspaceId,
     externalTx ?? db
   )
+  const finalBlocks = projectLegacySlackV2Auth(migratedBlocks)
 
   if (migrated) {
     // Deliberate fire-and-forget persistence on the global pool: it must not
@@ -568,7 +571,7 @@ export async function loadWorkflowFromNormalizedTables(
     // it escapes the transaction context instead of tripping the wire.
     runOutsideTransactionContext(() => {
       Promise.resolve().then(() =>
-        persistMigratedBlocks(workflowId, raw.blocks, finalBlocks, raw.blockUpdatedAtById)
+        persistMigratedBlocks(workflowId, raw.blocks, migratedBlocks, raw.blockUpdatedAtById)
       )
     })
   }