Preserve Slack participant identity in embedded thread context

[dcramer]

Problem

When Junior embeds prior Slack thread context, participant identity is not guaranteed to survive into the model-facing transcript. The bot can therefore treat prior messages as generic users and fail to know who reported something, who is asking now, or which person a follow-up refers to.

Observed concern: in multi-person Slack threads, Junior seems to never know people by username or display name when using prior thread context.

Current behavior to investigate

• ConversationMessage.author can store userId, userName, and fullName.
• Prior thread context rendering prefers fullName, then userName, then falls back to the role label.
• Backfilled and persisted prior messages appear to rely on whatever author fields the Chat SDK message already has.
• The current requester has a Slack profile lookup fallback, but older/prior thread participants are not clearly hydrated before the embedded transcript is built.
• When names are missing, the rendered context can collapse distinct humans into generic user attribution instead of at least preserving a stable Slack user id.

Desired contract

Prior thread context should always preserve a stable person identity for every human-authored message.

• Prefer Slack display name or real name when available.
• Include username when available.
• Always include or fall back to Slack user id so distinct people do not collapse into user.
• Hydrate missing names for prior participants through the existing Slack user lookup path, without refetching full Slack thread history on every turn.
• Preserve the distinction between the original reporter, later participants, and the current requester.

Suggested acceptance criteria

• A thread with Alice posting an earlier message and David later asking Junior to act embeds Alice as Alice or U_ALICE, not as generic user.
• The current requester remains separately represented in the per-turn requester context.
• Stored conversation state remains the canonical source for prompt context, consistent with the Slack context sourcing contract.
• Add deterministic coverage for missing userName and fullName fields in prior thread messages.
• Add or update an integration/eval scenario that proves reporter/requester attribution survives a multi-user Slack thread.

References

Relevant areas:

• packages/junior/src/chat/services/conversation-memory.ts
• packages/junior/src/chat/runtime/turn-preparation.ts
• packages/junior/src/chat/runtime/reply-executor.ts
• packages/junior/src/chat/slack/user.ts
• specs/slack-agent-delivery-spec.md