fix(slack): remove dead call to __create_holmes_callback in SlackSinkPreview

[saintetix]

Problem

When using the SlackSinkPreview sink, all alerts were falling back to the legacy Slack output format with the following error:

2026-03-11 10:01:38.616 ERROR    Failed to render slack preview template, defaulting to legacy slack output
Traceback (most recent call last):
  File "/app/src/robusta/integrations/slack/sender.py", line 669, in send_finding_to_slack
    return self.__send_finding_to_slack_preview(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/src/robusta/integrations/slack/sender.py", line 810, in __send_finding_to_slack_preview
    blocks.append(self.__create_holmes_callback(finding))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'SlackSender' object has no attribute '_SlackSender__create_holmes_callback'

Root Cause

The __create_holmes_callback method was removed in #1826 as part of the Holmes Slack format refactor, which replaced the callback button approach with the get_holmes_block text-based prompt. However, the call to __create_holmes_callback inside __send_finding_to_slack_preview was not removed, leaving a dangling reference that raised an AttributeError at runtime.

Fix

Remove the dead call to self.__create_holmes_callback(finding) and its surrounding condition in __send_finding_to_slack_preview.

Testing

• Deploy with SlackSinkPreview configured and verify alerts render correctly without falling back to legacy output.

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 131af460-03c5-4d1b-a827-71781cf0e9f2

📥 Commits

Reviewing files that changed from the base of the PR and between 6bb5943 and 84cc3f7.

📒 Files selected for processing (1)

• src/robusta/integrations/slack/sender.py

💤 Files with no reviewable changes (1)

• src/robusta/integrations/slack/sender.py

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Walkthrough

Removed the conditional that appended the Holmes "ask" callback/button block to Slack preview messages; all other preview construction (headers, links, source/description blocks) is unchanged.

Changes

Cohort / File(s)	Summary
Holmes callback removal src/robusta/integrations/slack/sender.py	Removed the conditional block (3 lines) that appended the Holmes "ask" callback/button to Slack preview blocks when HOLMES and HOLMES_ASK_SLACK_BUTTON are enabled; no signature or other behavior changes.

Sequence Diagram(s)

(omitted)

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• [ROB-1946] Bugfix - Automatic slack Investigation fix #1899: Edits Holmes guidance text returned by get_holmes_block in the same Slack sender module.
• [ROB-1286] Updated holmes slack format #1826: Also modifies handling of the Holmes "ask" callback/block in the Slack sender.

Suggested reviewers

• RoiGlinik
• moshemorad

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: removing a dead call to __create_holmes_callback from SlackSinkPreview, which matches the actual code modification.
Description check	✅ Passed	The description is directly related to the changeset, providing clear context about the problem, root cause, fix, and testing approach for the removed Holmes callback code.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

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