Chunking to Embedding Contract

March 6, 2026 · View on GitHub

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Use this page when retrieval fails because the chunk schema is not aligned with the embedding ingestion contract.
If the retriever expects fields that were never embedded, or chunks omit IDs/offsets/anchors, then citations drift and ΔS rises.


Open these first


Core acceptance

  • Every chunk has chunk_id, section_id, source_url, offsets, tokens.
  • Embedding index was built from the same schema as retrieval contract.
  • ΔS(question, retrieved) ≤ 0.45 across 3 paraphrases.
  • Coverage ≥ 0.70 to the target section.

Typical breakpoints and the right fix

  • Missing fields in ingestion (e.g., no section_id)
    → Enforce data-contracts.md.

  • Different schema for ingest vs retrieve
    → Corpus ingested raw text, retriever expects chunk JSON → rebuild with schema.

  • Offsets not tracked
    → Cannot map back to original document → enforce offsets at ingest.

  • Tokenizer drift
    → Chunk IDs differ between preprocessing runs → use chunking-checklist.md.


Fix in 60 seconds

  1. Check ingestion schema
    Compare the fields stored in the index with the fields expected in retrieval.

  2. Align contracts
    Define chunk = {chunk_id, section_id, source_url, offsets, tokens, text}.
    Enforce that this exact object is used both in ingestion and retrieval.

  3. Rebuild index if misaligned
    If fields differ, re-ingest corpus with enforced schema.


Copy-paste schema

{
  "chunk_id": "uuid-v4",
  "section_id": "doc-23-sec-7",
  "source_url": "https://example.com/doc23",
  "offsets": [120, 320],
  "tokens": 512,
  "text": "...."
}

Target: retriever always returns this schema, LLM consumes directly.


Common gotchas

  • Only text embedded, no IDs → cannot trace back → citations drift.
  • Chunk boundaries not logged → hallucinations reappear.
  • JSON schema updated mid-deploy → index mismatch.

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