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Message flow

This page traces a message from Slack through Ark and back.

Direct message flow

User sends DM to bot
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Socket Mode receives WebSocket message
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Gateway.handle_events_api() extracts inner event
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Gateway.handle_message() validates:
  - Not from the bot itself
  - Channel type is "im" (DM)
  - Subtype is nil or "file_share"
  - Text is non-empty or has files
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Add :eyes: reaction (spawned fiber)
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Download Slack files if attached (max 5, 10 MB each)
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Gateway.respond():
  1. Look up user attributes (name, timezone, title) — cached
  2. Call agent.invoke() with:
     - Input text
     - Session ID (thread_ts, dots replaced with dashes)
     - User attributes + current_datetime
     - Input files (sent as CODE_INTERPRETER use case)
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Bedrock::Agent builds signed POST request to:
  /agents/{agentId}/agentAliases/{aliasId}/sessions/{sessionId}/text
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Parse streaming response (binary event stream):
  - "chunk" events → accumulate text + collect citations
  - "files" events → collect output files
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Publish analytics event to Firehose (spawned fiber)
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Format response:
  - If response contains markdown tables → Block Kit blocks
  - Otherwise → convert markdown to mrkdwn
  - Append source citations if present
  - Split long messages at paragraph boundaries (40k limit)
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Post response to Slack channel/thread
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Upload any output files returned by the agent

Channel mention flow

The mention flow is similar with two differences:

  1. The <@BOT_ID> mention is stripped from the text before forwarding
  2. The response is always posted in a thread (using the message timestamp)

Session management

Bedrock sessions are mapped to Slack threads:

Slack context Session ID
DM — first message Message timestamp (e.g., 1710412200-123456)
DM — thread reply Thread timestamp
Channel — first mention Message timestamp
Channel — thread reply Thread timestamp

The dot in Slack timestamps (e.g., 1710412200.123456) is replaced with a dash for Bedrock compatibility.

User context

Each message includes user attributes in the Bedrock session state:

Attribute Source
current_datetime UTC ISO 8601 timestamp
user_name Slack user's real name
user_timezone Slack user's timezone
user_title Slack user's profile title

User info is cached per user ID for the lifetime of the process.

File handling

Input files (Slack to Bedrock)

  • Max 5 files per message
  • Max 10 MB per file
  • MIME type resolved from Slack metadata, with extension-based fallback
  • Sent to Bedrock as CODE_INTERPRETER use case with base64-encoded content
  • Supported types: CSV, Excel, JSON, YAML, Word, HTML, Markdown, TXT, PDF, PNG

Output files (Bedrock to Slack)

  • Files returned by the agent (e.g., code interpreter output) are uploaded to the same Slack thread
  • Uses files.uploadV2 (get upload URL, upload content, complete)