# X Retrieval Systems > Use this skill when analyzing how the system narrows down 500 million daily tweets to a candidate pool of ~1,500. This is the "top-of-funnel" engineering that determines what is even eligible to be ranked. - Skill: `tomevault-io/x-retrieval-systems` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/x-retrieval-systems` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/x-retrieval-systems/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/x-retrieval-systems --- # X Retrieval Systems Expertise in X's multi-stage retrieval architecture, including Earlybird search indexing and Phoenix-based ANN similarity search. ## Context Retrieval at X is split between **In-Network** (content from people you follow) and **Out-of-Network** (discovery). In-Network retrieval relies on **Earlybird** (Lucene-based search), while Out-of-Network retrieval uses **Phoenix** (Two-Tower embeddings) and **ANN** (Approximate Nearest Neighbor) algorithms like HNSW. - [Earlybird Search Index](./references/earlybird-search-index.md) - [ANN & Embedding Retrieval](./references/ann-retrieval.md) ## What it does * **Decodes In-Network Sourcing:** Explains how Earlybird shards the index into Realtime, Protected, and Archive clusters. * **Explains Discovery Logic:** Details how Two-Tower models enable "semantic" search for content you don't follow. * **Analyzes Latency:** Breaks down the single-writer/multi-reader concurrency model that allows for sub-second global retrieval. ## Example Trigger Prompts * "/find-candidates how Earlybird shards real-time index" * "/find-candidates retrieving 1,500 candidates from 500M tweets" * "Role of HNSW in embedding-based discovery" * "In-Network (Thunder) vs Out-of-Network (Phoenix) retrieval" * "Trace 'Discovery' request: User Embedding → Candidate Source" --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/elemontcapital) — claim your Tome and manage your conversions.