Chunk ID Schema: Deterministic addressing for citations and reranking

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A practical spec to mint stable chunk_id values that survive re-ingest, reruns, and light edits. Use it to make citations auditable and to keep retrieval traces consistent across stores and seeds.

Open these first

Traceability and why-this-snippet: retrieval-traceability.md
Snippet schema and payload locks: data-contracts.md
Build the section tree: title_hierarchy.md, section_detection.md
Reindex and migrations: reindex_migration.md

Acceptance targets

chunk_id is unique within the corpus and recomputable from source plus deterministic rules.
Citation hit rate ≥ 0.90 on a gold set after re-ingest.
ΔS(question, retrieved) ≤ 0.45 and λ stays convergent across 3 paraphrases when citing by chunk_id.
Reindex migration preserves a one-to-one mapping for at least 0.95 of chunks, with redirects for the rest.

Design goals

Deterministic
The same input text and layout produce the same chunk_id, regardless of store.
Localizable
Given a chunk_id, you can open the document and jump to the exact span without embeddings.
Diff-tolerant
Small edits should not re-key the entire document. Local block changes only.
Human-inspectable
IDs are compact but interpretable during manual audits.

Canonical fields

Every chunk record must carry these fields in your payload. See the full schema in data-contracts.md.

{
  "chunk_id": "docA|r=8c1e9a02|s=1.2.3|p=002|b=007",
  "section_id": "1.2.3",
  "source_url": "https://example.com/guide.pdf",
  "offsets": [12345, 12980],
  "tokens": 365,
  "hash": "sha1:cafebabe…",
  "block_type": "paragraph|table|code|formula",
  "anchor": "first salient assertion in this chunk"
}

offsets are byte or character positions in the canonical text layer.
anchor is used by cite-then-explain prompts and for ΔS triangulation.

The `chunk_id` format

Recommended canonical string

<doc_uid>|r=<rev8>|s=<sect_path>|p=<page3>|b=<block3>

doc_uid Stable document identifier. Prefer a repository or CMS id. If unknown, use sha1_8(normalize(source_url)).
rev8 Eight hex chars from sha1(normalize_text(doc)). Changes only when the whole document text meaningfully changes.
sect_path Dot path from the section tree like 2.4.1. If unknown, set to p<page3> and still fill p.
page3 Zero padded page number when page concepts exist. For HTML-only docs, set 000.
block3 Zero padded index of the block within the section or page after applying block-aware splitting.

Human alias If you also want a readable alias for logs, you may attach #s02-04-01#p002-b007. The canonical id remains the pipe form above.

How to compute the parts

doc_uid
- If your source has a stable id, use it.
- Else compute doc_uid = sha1_8(canonicalize_url(source_url)). Canonicalization removes querystrings except version pins and normalizes scheme and host.
rev8
- Compute sha1_8(normalize_text(doc)).
- normalize_text collapses whitespace, strips page headers and footers, and fixes OCR line breaks. See pdf_layouts_and_ocr.md.
sect_path
- Build the hierarchy with heading detection. See title_hierarchy.md and section_detection.md.
- If headings are unreliable, synthesize sect_path as page-major like p002 and keep p identical.
block3
- Within each section or page, enumerate blocks after block-aware splitting. Paragraphs, tables, code, formulas are atomic. See code_tables_blocks.md.
- Use zero padding to 3 digits for stability in lexicographic sort.
offsets
- Record start and end offsets in the canonical text layer for trace and repair. Never omit.

Examples

PDF with headings

doc_uid = "whitepaperX"
rev8    = "8c1e9a02"
sect    = "3.2"
page    = 014
block   = 006

chunk_id = "whitepaperX|r=8c1e9a02|s=3.2|p=014|b=006"

HTML article without reliable headings

doc_uid = sha1_8(url) = "b17a2c55"
rev8    = "e1d0aa91"
sect    = "p005"   // synthesized
page    = 005
block   = 004

chunk_id = "b17a2c55|r=e1d0aa91|s=p005|p=005|b=004"

Notebook with code cells

doc_uid = "nb_231107"
rev8    = "9af03bcd"
sect    = "2.1"
page    = 000
block   = 012
block_type = "code"

chunk_id = "nb_231107|r=9af03bcd|s=2.1|p=000|b=012"

Collision and drift policy

Hash collision If a collision is detected for doc_uid|r, extend rev8 to rev12. Log a warning and continue.
Minor edits If text within a section changes but the section path and block indices remain, keep chunk_id. Update hash and offsets.
Section renumbering If sect_path changes due to new headings, create a redirect map from old ids to new ids during reindex. See reindex_migration.md.
OCR repairs If OCR fixes alter page counts, keep doc_uid and rev but synthesize sect_path using old to new page mapping and emit redirects.

60-second validator

Pass when the following hold on a 20-chunk sample:

uniq(chunk_id) == len(sample)
Sorting by chunk_id equals the natural reading order inside each section or page.
citation_match ≥ 0.90 comparing offsets to ground truth citations.
Stable under re-ingest of the same source: all chunk_id values unchanged.

Copy-paste generator (pseudocode)

def make_chunk_id(doc_uid, rev8, sect_path, page_no, block_ix):
    page3  = f"{int(page_no):03d}"
    block3 = f"{int(block_ix):03d}"
    return f"{doc_uid}|r={rev8}|s={sect_path}|p={page3}|b={block3}"

Use this builder only after your pipeline has produced the section tree and block enumeration.

What to do when audits fail

Top-k returns the right area but citations miss the span → Rebuild the section tree and verify offsets. Open: retrieval-traceability.md.
Hybrid reranker prefers chunks from adjacent sections → Tighten block boundaries. Open: code_tables_blocks.md.
Many ids change after a small doc edit → Check rev8 computation and block numbering stability. Open: reindex_migration.md.

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