feat(slack): stream trigger responses to agent sessions

Author: TheodoreSpeaks

apps/docs/content/docs/integrations/slack.mdx

@@ -28,6 +28,18 @@ With the Slack integration in Sim, you can:
 
 In Sim, the Slack integration enables your agents to programmatically interact with Slack as part of their workflows. This allows for automation scenarios such as sending notifications with dynamic updates, managing conversational flows with editable status messages, acknowledging important messages with reactions, and maintaining clean channels by removing outdated bot messages. The integration can also be used in trigger mode to start a workflow when a message is sent to a channel.
 
+## Stream Trigger Responses
+
+Custom-bot Slack triggers can stream workflow outputs directly back into the conversation that started a run. Enable **Stream response to Slack** on a Message, App Mention, or Assistant Thread Started trigger, then select the outputs to deliver.
+
+- A selected Agent output streams as it is generated. Intermediate pre-tool turns are held back so only the final answer is appended.
+- A selected non-streaming block output is sent when that block invocation completes.
+- Loop and parallel invocations each create their own Slack response.
+- Optional thinking and tool-call updates appear as Slack tasks in a timeline or plan.
+- Slack Agent Sessions remain in processing state for the run, return to active when it finishes, and the native Slack stop button cancels active workflow executions.
+
+Automatic trigger responses require a custom bot created by the Slack setup wizard. They are not available with the shared Sim Slack app.
+
 ## AI-Generated Content
 
 Sim workflows may use AI models to generate messages and responses sent to Slack. AI-generated content may be inaccurate or contain errors. Always review automated outputs, especially for critical communications.
@@ -1992,6 +2004,11 @@ Trigger from Slack events (mentions, messages, reactions)
 | `eventType` | string | Yes | The single Slack event this trigger fires on. Add another trigger block for another event. |
 | `customBotCredential` | string | Yes | Choose a custom Slack bot you set up once and reuse across triggers. |
 | `manualBotCredential` | string | Yes | Set the custom bot credential ID directly. |
+| `streamResponse` | boolean | No | Stream selected workflow outputs into the Slack conversation that started this run. Custom bots only. |
+| `streamOutputs` | workflow-output-selector | No | Each selected block invocation creates its own Slack response. Agent outputs stream live; other outputs are sent when the block completes. |
+| `streamIncludeThinking` | boolean | No | Show agent thinking as Slack task updates while the response is generated. |
+| `streamIncludeToolCalls` | boolean | No | Show tool execution lifecycle as Slack task updates. |
+| `streamTaskDisplayMode` | string | No | Choose how Slack displays thinking and tool progress. |
 | `source` | string | No | Restrict to direct messages, public channels, or private channels. Leave empty to match any. |
 | `channelFilter` | channel-selector | No | Restrict to specific channels. Leave empty to trigger on any channel the bot has been added to. |
 | `manualChannelFilter` | string | No | Comma-separated channel IDs to restrict to. Set IDs directly here. |
@@ -2025,6 +2042,7 @@ Trigger from Slack events (mentions, messages, reactions)
 |   ↳ `tab` | string | App Home tab that was opened, including messages for Agent View |
 |   ↳ `context` | json | Current Agent View context. Normalized from context on app_context_changed/app_home_opened or app_context on message.im |
 |   ↳ `team_id` | string | Slack workspace/team ID |
+|   ↳ `user_team_id` | string | Slack workspace/team ID of the user who triggered the event. Used for Slack Connect response streaming. |
 |   ↳ `enterprise_id` | string | Slack Enterprise Grid organization ID |
 |   ↳ `event_id` | string | Unique event identifier |
 |   ↳ `reaction` | string | Emoji reaction name \(e.g., thumbsup\). Present for reaction_added/reaction_removed events |

apps/sim/app/api/webhooks/slack/custom/[credentialId]/route.ts

@@ -6,6 +6,7 @@ import { parseWebhookBody } from '@/lib/webhooks/processor'
 import { handleSlackChallenge } from '@/lib/webhooks/providers/slack'
 import {
   dispatchSlackCustomBotCredential,
+  handleSlackAgentSessionStopped,
   verifySlackCustomBotCredentialRequest,
 } from '@/lib/webhooks/slack-custom-ingress'
 import { getSlackDispatchResponse } from '@/lib/webhooks/slack-dispatch'
@@ -68,12 +69,15 @@ async function handleSlackCustomBotWebhook(
     return authError
   }
 
-  const dispatchResults = await dispatchSlackCustomBotCredential({
-    credentialId,
-    body,
-    request,
-    requestId,
-    receivedAt,
-  })
+  const [, dispatchResults] = await Promise.all([
+    handleSlackAgentSessionStopped(credentialId, body),
+    dispatchSlackCustomBotCredential({
+      credentialId,
+      body,
+      request,
+      requestId,
+      receivedAt,
+    }),
+  ])
   return getSlackDispatchResponse(dispatchResults)
 }

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/index.ts

@@ -38,4 +38,5 @@ export { Text } from './text'
 export { TimeInput } from './time-input'
 export { ToolInput } from './tool-input'
 export { VariablesInput } from './variables-input'
+export { WorkflowOutputSelector } from './workflow-output-selector'
 export { WorkflowSelectorInput } from './workflow-selector'

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/workflow-output-selector/index.ts

@@ -0,0 +1 @@
+export { WorkflowOutputSelector } from './workflow-output-selector'

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/workflow-output-selector/workflow-output-selector.tsx

@@ -0,0 +1,41 @@
+import { OutputSelect } from '@/app/workspace/[workspaceId]/w/[workflowId]/components/chat/components/output-select/output-select'
+import { useSubBlockValue } from '@/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/hooks/use-sub-block-value'
+import { useWorkflowRegistry } from '@/stores/workflows/registry/store'
+
+const EMPTY_OUTPUTS: string[] = []
+
+interface WorkflowOutputSelectorProps {
+  blockId: string
+  subBlockId: string
+  isPreview?: boolean
+  previewValue?: string[] | null
+  disabled?: boolean
+  placeholder?: string
+}
+
+export function WorkflowOutputSelector({
+  blockId,
+  subBlockId,
+  isPreview = false,
+  previewValue,
+  disabled = false,
+  placeholder,
+}: WorkflowOutputSelectorProps) {
+  const workflowId = useWorkflowRegistry((state) => state.activeWorkflowId)
+  const [storedValue, setStoredValue] = useSubBlockValue<string[]>(blockId, subBlockId)
+  const selectedOutputs = isPreview
+    ? (previewValue ?? EMPTY_OUTPUTS)
+    : (storedValue ?? EMPTY_OUTPUTS)
+
+  return (
+    <OutputSelect
+      workflowId={workflowId}
+      selectedOutputs={selectedOutputs}
+      onOutputSelect={setStoredValue}
+      disabled={disabled || isPreview}
+      placeholder={placeholder}
+      size='md'
+      className='w-full'
+    />
+  )
+}

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/sub-block.tsx

@@ -47,6 +47,7 @@ import {
   TimeInput,
   ToolInput,
   VariablesInput,
+  WorkflowOutputSelector,
   WorkflowSelectorInput,
 } from '@/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components'
 import { MODAL_REGISTRY } from '@/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/modal-registry'
@@ -1097,6 +1098,18 @@ function SubBlockComponent({
           />
         )
 
+      case 'workflow-output-selector':
+        return (
+          <WorkflowOutputSelector
+            blockId={blockId}
+            subBlockId={config.id}
+            isPreview={isPreview}
+            previewValue={previewValue as string[] | null | undefined}
+            disabled={isDisabled}
+            placeholder={config.placeholder}
+          />
+        )
+
       case 'mcp-server-selector':
         return (
           <McpServerSelector

apps/sim/background/webhook-execution.ts

@@ -66,6 +66,8 @@ import {
   createWebhookExecutionPrincipal,
 } from '@/lib/webhooks/execution-principal'
 import { getProviderHandler } from '@/lib/webhooks/providers'
+import { SlackExecutionStreamController } from '@/lib/webhooks/slack-execution-stream'
+import { readSlackStreamResponseConfig } from '@/lib/webhooks/slack-stream-config'
 import {
   executeWorkflowCore,
   wasExecutionFinalizedByCore,
@@ -1054,25 +1056,72 @@ async function executeWebhookJobInternal(
       })
     }
 
+    const persistedProviderConfig = isRecordLike(resolvedWebhookRecord.providerConfig)
+      ? resolvedWebhookRecord.providerConfig
+      : {}
+    const slackStreamConfig =
+      payload.provider === 'slack' || payload.provider === 'slack_app'
+        ? readSlackStreamResponseConfig(persistedProviderConfig)
+        : null
+    if (slackStreamConfig && payload.provider !== 'slack') {
+      throw new Error('Slack trigger response streaming is only supported for custom bots')
+    }
+    const slackStreamCredentialId =
+      typeof persistedProviderConfig.credentialId === 'string'
+        ? persistedProviderConfig.credentialId
+        : null
+    if (slackStreamConfig && !slackStreamCredentialId) {
+      throw new Error('Slack stream configuration is missing its custom bot credential')
+    }
+    const slackStreamController = slackStreamConfig
+      ? await SlackExecutionStreamController.create({
+          credentialId: slackStreamCredentialId!,
+          workspaceId,
+          workflowId: payload.workflowId,
+          executionId,
+          userId: actorUserId,
+          triggerInput,
+          config: slackStreamConfig,
+          loggingSession,
+          abortSignal: timeoutController.signal,
+        })
+      : null
+
     const snapshot = new ExecutionSnapshot(
       metadata,
       workflowRecord,
       triggerInput,
       workflowVariables,
-      []
+      slackStreamController?.selectedOutputs ?? []
     )
 
     workflowCoreStarted = true
-    const executionResult = await executeWorkflowCore({
-      snapshot,
-      callbacks: {},
-      loggingSession,
-      trustedInitialResolvedSecretTraceProvenance:
-        resolvedSecretTraceRegistry.exportProvenanceForValue(triggerInput),
-      includeFileBase64: false,
-      base64MaxBytes: undefined,
-      abortSignal: timeoutController.signal,
-    })
+    let executionResult: ExecutionResult
+    try {
+      executionResult = await executeWorkflowCore({
+        snapshot,
+        callbacks: slackStreamController?.callbacks ?? {},
+        loggingSession,
+        trustedInitialResolvedSecretTraceProvenance:
+          resolvedSecretTraceRegistry.exportProvenanceForValue(triggerInput),
+        includeFileBase64: false,
+        base64MaxBytes: undefined,
+        abortSignal: timeoutController.signal,
+      })
+    } catch (error) {
+      if (slackStreamController) {
+        await slackStreamController.finalize({
+          success: false,
+          output: {},
+          error: toError(error).message,
+        })
+      }
+      throw error
+    }
+    if (slackStreamController) {
+      await slackStreamController.finalize(executionResult)
+      slackStreamController.assertSucceeded()
+    }
 
     await handleExecutionResult(executionResult, {
       loggingSession,

apps/sim/blocks/blocks.test.ts

@@ -582,6 +582,7 @@ describe.concurrent('Blocks Module', () => {
         'variables-input',
         'messages-input',
         'workflow-selector',
+        'workflow-output-selector',
         'workflow-input-mapper',
         'text',
         'router-input',

apps/sim/blocks/types.ts

@@ -417,8 +417,9 @@ export interface SubBlockConfig {
    * `watchFields` is treated as a credential ID and fetched via the credentials
    * API. The subblock is hidden unless `credential.type` matches `requiredType`.
    *
-   * Only one subblock per block may use this. The serializer ignores it —
-   * the field is always serialized when it has a value.
+   * Every reactive subblock on a block must watch the same credential fields.
+   * The serializer ignores this — the field is always serialized when it has
+   * a value, so server-side validation must reject unsupported credentials.
    */
   reactiveCondition?: {
     watchFields: string[]

apps/sim/executor/execution/block-executor.ts

@@ -292,7 +292,8 @@ export class BlockExecutor {
             block,
             streamingExec,
             resolvedInputs,
-            normalizeStringArray(blockCtx.selectedOutputs)
+            normalizeStringArray(blockCtx.selectedOutputs),
+            blockLog?.executionOrder
           )
         } catch (streamError) {
           const resultRegistry = blockCtx.resolvedSecretTraceRegistry
@@ -1150,7 +1151,8 @@ export class BlockExecutor {
     block: SerializedBlock,
     streamingExec: StreamingExecution,
     resolvedInputs: Record<string, any>,
-    selectedOutputs: string[]
+    selectedOutputs: string[],
+    executionOrder?: number
   ): Promise<void> {
     const blockId = node.id
     const piiEnabled = Boolean(ctx.piiBlockOutputRedaction?.enabled)
@@ -1202,6 +1204,8 @@ export class BlockExecutor {
       onStreamPromise = ctx
         .onStream({
           ...streamingExecutionForConsumer,
+          blockId,
+          ...(executionOrder !== undefined ? { executionOrder } : {}),
           stream: processedClientStream,
           streamFormat: 'text',
           subscribe: pump.subscribe,