Summary

Enable progressive message rendering (block streaming) for Mattermost by editing a single message in-place as the model generates text, instead of sending multiple separate messages.

How it works

1. First block → send a new message, track its post id
2. Subsequent blocks → accumulate text and PATCH /api/v4/posts/{id}/patch the existing message
3. Final reply → edit the message with the complete text
4. Fallback → if edit fails, gracefully falls back to sending new messages

This is the same pattern used by the Telegram and Discord plugins.

User experience

Before: User asks question → 10s silence → wall of text appears

After: User asks question → text starts appearing after ~1-2s and writes progressively

Files changed

• extensions/mattermost/src/channel.ts — declare blockStreaming: true in capabilities
• extensions/mattermost/src/mattermost/monitor.ts — implement block streaming logic in deliver function
• extensions/mattermost/src/mattermost/client.ts — patchMattermostPost() (from feat(mattermost): add edit and delete message actions #25295)

Dependencies

Builds on #25295 (edit/delete message actions) which adds patchMattermostPost().

Testing

• Tested patchMattermostPost against live Mattermost 10.x instance
• Block streaming config already exists in schema (blockStreaming, blockStreamingCoalesce)
• Graceful fallback to new messages if edit fails

Configuration

Block streaming is enabled by default. Users can disable it per-account:

channels:
  mattermost:
    blockStreaming: false  # disable edit-in-place
    blockStreamingCoalesce:
      minChars: 1500
      idleMs: 1000

Greptile Summary

This PR adds block streaming (progressive message rendering) to Mattermost by editing a single message in-place as text generates, matching the pattern used by Telegram and Discord plugins.

Key changes:

• Enabled blockStreaming: true capability in channel configuration
• Added edit/delete message actions to ChannelMessageActionAdapter
• Implemented block streaming logic in deliver function with state tracking (blockStreamMessageId, blockStreamAccumulatedText)
• Graceful fallback to new messages if edit operations fail

How it works:

1. First block → sends new message, tracks post ID
2. Subsequent blocks → accumulates text and patches existing message via PATCH /api/v4/posts/{id}/patch
3. Final reply → edits message with complete text
4. Error handling → falls back to sending new messages on edit failures

The implementation is clean and follows established patterns in the codebase. Block streaming defaults to enabled but can be disabled per-account via config.

Confidence Score: 4/5

• This PR is safe to merge with minimal risk - implements a well-tested pattern from other channels
• The block streaming implementation follows the established pattern from Discord/Telegram, includes proper error handling with graceful fallbacks, and has clear state management. The only minor consideration is the accumulation reset on fallback, which is intentional design. Dependencies are properly added from PR feat(mattermost): add edit and delete message actions #25295.
• No files require special attention

_{Last reviewed commit: 4a062d5}

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