fix(search): prevent accidental agent index creation

[ehayes2000]

Agent-session parent writes could silently create a physical agent_sessions index when the alias was missing, inferring the relation as text and breaking subsequent child writes. Require an existing alias for normal parent and bulk writes so missing setup fails instead of creating the wrong index.

Index creation remains manual using the existing canonical declaration, which already defines the correct join mapping. Explicit backfill index overrides remain supported. Remove the dedicated one-off migration script and document its manual atomic repair instead, including deletion of the old search projection and no historical backfill. Deployment workflows and generic provisioning helpers are unchanged.

Validation: 138 OpenSearch-client Rust tests and one doctest; 35 existing helper tests; just check. The alias guard and explicit override behavior are covered by HTTP request tests. The unchanged guard was also previously exercised against local OpenSearch, where a missing alias was rejected without creating an index.

Production repair has not been executed by this PR.

---

Note

Medium Risk
Changes live indexing behavior for agent sessions: missing alias now fails writes instead of silently creating a bad index, so rollout depends on correct provisioning; backfill override behavior is preserved and covered by tests.

Overview
Agent-session OpenSearch writes now refuse to auto-create a bare agent_sessions index. Parent index and bulk reconcile calls set require_alias=true when no index_override is set, so a missing alias fails loudly instead of inferring a wrong mapping (e.g. join as text). Backfills that pass an explicit physical index still set require_alias=false.

Tests capture real HTTP requests and assert alias vs override query parameters on both write paths.

Docs state that indices/aliases must be provisioned manually before deploying search-processing, add a one-time operator runbook to atomically remove a mistaken bare agent_sessions index and point the alias at agent_sessions_v1 (future writes only, no reindex), and clarify that unmapped fields under dynamic: false stay in _source but are not searchable.

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📒 Files selected for processing (4)

• crates/opensearch_client/src/upsert/agent_session.rs
• crates/opensearch_client/src/upsert/agent_session/test.rs
• infra/stacks/opensearch/helpers/README.md
• services/search_processing_service/README.md

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added a migration tool to repair agent-session indexing for future writes.
  • Supports dry-run previews, safe atomic alias cutovers, validation, and repeatable execution.
  • Preserves existing data by avoiding reindexing or backfilling; applying the migration removes the old bare index and its historical data.
  • Added documentation covering usage, safety checks, limitations, and post-migration verification.

Walkthrough

Adds migrate_agent_sessions.ts to repair agent-session indexing for future writes. The script validates the canonical mapping, checks alias state, creates agent_sessions_v1 when needed, waits for cluster health, and performs an atomic alias cutover. It supports dry-run and idempotent execution. New tests cover successful migrations, invalid mappings, failures, and cutover behavior. The README documents usage, data effects, failure cases, and verification.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to d914e

Normal writes now fail safely when the required alias is absent instead of creating an incorrectly mapped index. No actionable current-head risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the conventional commit format fix(search): and is 52 characters long. It accurately describes the prevention of accidental agent index creation.
Description check	✅ Passed	The description clearly explains the alias guard, supported backfill overrides, documentation changes, tests, validation, and deployment impact. It directly matches the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
In `@infra/stacks/opensearch/helpers/scripts/migrate_agent_sessions.ts`:
- Around line 48-54: Update aliasState to inspect the alias metadata after
validating the single target: return the repair state when
response.body[index].aliases[alias].is_write_index is explicitly false, while
treating an omitted value as ready. Add a fixture covering is_write_index: false
and assert that migration uses the existing add action with is_write_index:
true.

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• infra/stacks/opensearch/helpers/README.md
• infra/stacks/opensearch/helpers/scripts/migrate_agent_sessions.test.ts
• infra/stacks/opensearch/helpers/scripts/migrate_agent_sessions.ts

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