diff --git a/src/indexing/sparse.test.ts b/src/indexing/sparse.test.ts
new file mode 100644
index 0000000..9566359
--- /dev/null
+++ b/src/indexing/sparse.test.ts
@@ -0,0 +1,162 @@
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import path from 'node:path'
+import { afterEach, beforeEach, describe, expect, test } from 'bun:test'
+
+import { Bm25Index, type Chunk, enrichForBm25, selectorToMask } from './sparse.ts'
+
+function makeChunk(overrides: Partial<Chunk> & { filePath: string, content?: string }): Chunk {
+  return {
+    content: overrides.content ?? '',
+    filePath: overrides.filePath,
+    startLine: overrides.startLine ?? 1,
+    endLine: overrides.endLine ?? 1,
+    language: overrides.language ?? null,
+  }
+}
+
+describe('enrichForBm25', () => {
+  test('appends repeated stem and last 3 dir parts (2-part dir)', () => {
+    // Mirrors upstream Python: Path('src/utils/format.ts').parent.parts == ('src', 'utils'),
+    // so last-3 is the full ['src', 'utils'].
+    const out = enrichForBm25(makeChunk({ filePath: 'src/utils/format.ts', content: 'hello world' }))
+    expect(out).toBe('hello world format format src utils')
+  })
+
+  test('trims to the last 3 dir parts (4-part dir)', () => {
+    const out = enrichForBm25(makeChunk({ filePath: 'a/b/c/d/foo.py', content: 'x' }))
+    expect(out).toBe('x foo foo b c d')
+  })
+
+  test('handles a top-level file with no directory components', () => {
+    const out = enrichForBm25(makeChunk({ filePath: 'foo.py', content: 'x' }))
+    expect(out).toBe('x foo foo ')
+  })
+
+  test('drops "." pseudo-segments from relative paths', () => {
+    const out = enrichForBm25(makeChunk({ filePath: './a/b/foo.ts', content: 'x' }))
+    expect(out).toBe('x foo foo a b')
+  })
+
+  test('normalizes backslashes for cross-platform consistency', () => {
+    // Repo-relative paths must produce the same enrichment regardless of
+    // host OS — Windows hosts may surface back-slashes if a caller forgets
+    // to normalize before passing the chunk through.
+    const out = enrichForBm25(makeChunk({ filePath: 'src\\utils\\format.ts', content: 'hello world' }))
+    expect(out).toBe('hello world format format src utils')
+  })
+})
+
+describe('selectorToMask', () => {
+  test('builds a 0/1 mask the same length as `size`', () => {
+    const mask = selectorToMask(new Uint32Array([0, 2, 5]), 6)
+    expect(mask).not.toBeNull()
+    expect(Array.from(mask!)).toEqual([1, 0, 1, 0, 0, 1])
+  })
+
+  test('returns null for a null selector', () => {
+    expect(selectorToMask(null, 6)).toBeNull()
+  })
+
+  test('returns null for an undefined selector', () => {
+    expect(selectorToMask(undefined, 6)).toBeNull()
+  })
+
+  test('ignores indices outside the mask bounds', () => {
+    // Out-of-bounds indices are silently dropped rather than crashing —
+    // upstream relies on the selector being well-formed but we want to be
+    // defensive in the TS port.
+    const mask = selectorToMask(new Uint32Array([0, 10]), 3)
+    expect(Array.from(mask!)).toEqual([1, 0, 0])
+  })
+})
+
+describe('Bm25Index.build / getScores', () => {
+  test('ranks documents containing the query term higher', () => {
+    const index = Bm25Index.build([
+      ['hello', 'world'],
+      ['hello'],
+      ['world'],
+    ])
+    const scores = index.getScores(['hello'])
+    expect(scores).toHaveLength(3)
+    expect(scores[0]).toBeGreaterThan(0)
+    expect(scores[1]).toBeGreaterThan(0)
+    expect(scores[2]).toBe(0)
+  })
+
+  test('returns zero scores for unknown query tokens', () => {
+    const index = Bm25Index.build([['hello'], ['world']])
+    const scores = index.getScores(['unknown'])
+    expect(Array.from(scores)).toEqual([0, 0])
+  })
+
+  test('returns an empty-array-equivalent for an empty corpus', () => {
+    const index = Bm25Index.build([])
+    const scores = index.getScores(['anything'])
+    expect(scores).toHaveLength(0)
+  })
+
+  test('returns zero scores when query tokens are empty', () => {
+    const index = Bm25Index.build([['hello'], ['world']])
+    const scores = index.getScores([])
+    expect(Array.from(scores)).toEqual([0, 0])
+  })
+
+  test('weightMask zeros out masked-out documents', () => {
+    const index = Bm25Index.build([
+      ['hello', 'world'],
+      ['hello'],
+      ['world'],
+    ])
+    // Mask in docs 0 and 2 only.
+    const mask = new Uint8Array([1, 0, 1])
+    const scores = index.getScores(['hello'], mask)
+    expect(scores[0]).toBeGreaterThan(0)
+    expect(scores[1]).toBe(0)
+    expect(scores[2]).toBe(0) // doc 2 doesn't contain 'hello'
+  })
+
+  test('weightMask only suppresses scores; matched-in docs are unchanged', () => {
+    const index = Bm25Index.build([
+      ['hello', 'world'],
+      ['hello'],
+      ['world'],
+    ])
+    const baseline = index.getScores(['hello'])
+    const masked = index.getScores(['hello'], new Uint8Array([1, 1, 1]))
+    expect(Array.from(masked)).toEqual(Array.from(baseline))
+  })
+
+  test('repeated query tokens do not compound scores', () => {
+    const index = Bm25Index.build([['hello']])
+    const single = index.getScores(['hello'])
+    const repeated = index.getScores(['hello', 'hello', 'hello'])
+    expect(Array.from(repeated)).toEqual(Array.from(single))
+  })
+})
+
+describe('Bm25Index.save / load', () => {
+  let tmp: string
+
+  beforeEach(async () => {
+    tmp = await mkdtemp(path.join(tmpdir(), 'csp-bm25-'))
+  })
+
+  afterEach(async () => {
+    await rm(tmp, { recursive: true, force: true })
+  })
+
+  test('round-trips an index and preserves scores', async () => {
+    const index = Bm25Index.build([
+      ['alpha', 'beta'],
+      ['alpha'],
+      ['beta', 'gamma'],
+    ])
+    await index.save(tmp)
+    const loaded = await Bm25Index.load(tmp)
+    const original = index.getScores(['alpha'])
+    const restored = loaded.getScores(['alpha'])
+    expect(Array.from(restored)).toEqual(Array.from(original))
+  })
+})
diff --git a/src/indexing/sparse.ts b/src/indexing/sparse.ts
new file mode 100644
index 0000000..16a235b
--- /dev/null
+++ b/src/indexing/sparse.ts
@@ -0,0 +1,219 @@
+// Port of src/semble/index/sparse.py
+//
+// Implements the two helpers from the upstream module plus a minimal BM25
+// index (Bm25Index) that stands in for Python's `bm25s` library.
+//
+// BM25 backend choice (see PR body for full discussion):
+//   Option B (inline minimal BM25+ with k1=1.5, b=0.75) was chosen over a
+//   third-party npm such as wink-bm25-text-search because:
+//     - The dependency tree stays self-contained while the project is still
+//       a scaffold (no other indexing deps are pinned yet).
+//     - The required surface is tiny (build / getScores / save / load) and
+//       getScores must respect a weight_mask that maps cleanly to BM25's
+//       per-document scoring loop.
+//     - Replacing this backend later is a localized change because all
+//       callers go through the Bm25Index class.
+
+import { mkdir, readFile, writeFile } from 'node:fs/promises'
+import path from 'node:path'
+
+// Stopgap structural type until ./types.ts lands from Unit 1.
+// Mirrors semble.types.Chunk with camelCase field names per
+// @pleaseai/csp public-API conventions.
+export interface Chunk {
+  content: string
+  filePath: string
+  startLine: number
+  endLine: number
+  language?: string | null
+}
+
+/**
+ * Append file path components to BM25 content to boost path-based queries.
+ *
+ * Assumes `chunk.filePath` is already repo-relative (set during indexing) so
+ * machine-specific directory components are never indexed. The stem is
+ * repeated twice to up-weight file-path matches in BM25.
+ *
+ * Repo-relative paths are normalized to POSIX (forward slashes) before
+ * parsing so Windows-host indexes produce the same enriched text as POSIX
+ * hosts. Without this, `path.parse` on Windows would split on `\\` while
+ * the indexer stores forward-slash paths, leading to inconsistent BM25
+ * tokenization across platforms.
+ */
+export function enrichForBm25(chunk: Chunk): string {
+  const normalized = chunk.filePath.replace(/\\/g, '/')
+  const parsed = path.posix.parse(normalized)
+  const stem = parsed.name
+  const dirParts = parsed.dir
+    .split('/')
+    .filter(part => part !== '' && part !== '.')
+  const dirText = dirParts.slice(-3).join(' ')
+  return `${chunk.content} ${stem} ${stem} ${dirText}`
+}
+
+/**
+ * Convert a selector array of indices into a boolean mask of length `size`.
+ *
+ * Returns `null` when `selector` is null/undefined so callers can skip mask
+ * application entirely (matching the upstream semantics).
+ */
+export function selectorToMask(
+  selector: Uint32Array | null | undefined,
+  size: number,
+): Uint8Array | null {
+  if (selector === null || selector === undefined)
+    return null
+  const mask = new Uint8Array(size)
+  for (const idx of selector) {
+    if (idx < size)
+      mask[idx] = 1
+  }
+  return mask
+}
+
+// ---------------------------------------------------------------------------
+// Minimal BM25 index
+// ---------------------------------------------------------------------------
+
+// Standard Okapi BM25 hyperparameters used by bm25s' default Lucene scorer.
+const K1 = 1.5
+const B = 0.75
+
+interface Bm25State {
+  // Number of documents indexed.
+  numDocs: number
+  // Document length (token count) per document, in doc order.
+  docLengths: Float32Array
+  // Average document length across the corpus.
+  avgDocLength: number
+  // Term -> array of [docId, termFreq] entries (postings list).
+  postings: Map<string, Array<[number, number]>>
+  // Term -> document frequency (count of docs containing the term).
+  docFreq: Map<string, number>
+}
+
+/**
+ * Minimal BM25 index supporting build / getScores / save / load.
+ *
+ * Documents are passed pre-tokenized (callers use `tokenize(enrichForBm25(...))`).
+ * `getScores` returns a Float32Array of per-document scores in doc order,
+ * matching the bm25s.BM25.get_scores contract used by upstream.
+ */
+export class Bm25Index {
+  // Exposed only for save() — kept private to consumers.
+  readonly #state: Bm25State
+
+  private constructor(state: Bm25State) {
+    this.#state = state
+  }
+
+  /** Build an index from an array of pre-tokenized documents. */
+  static build(documents: string[][]): Bm25Index {
+    const numDocs = documents.length
+    const docLengths = new Float32Array(numDocs)
+    const postings = new Map<string, Array<[number, number]>>()
+    const docFreq = new Map<string, number>()
+
+    let totalLen = 0
+    for (let docId = 0; docId < numDocs; docId++) {
+      const tokens = documents[docId] ?? []
+      docLengths[docId] = tokens.length
+      totalLen += tokens.length
+
+      // Term frequencies for this document.
+      const tf = new Map<string, number>()
+      for (const token of tokens)
+        tf.set(token, (tf.get(token) ?? 0) + 1)
+
+      for (const [term, freq] of tf) {
+        let list = postings.get(term)
+        if (list === undefined) {
+          list = []
+          postings.set(term, list)
+        }
+        list.push([docId, freq])
+        docFreq.set(term, (docFreq.get(term) ?? 0) + 1)
+      }
+    }
+
+    const avgDocLength = numDocs > 0 ? totalLen / numDocs : 0
+
+    return new Bm25Index({ numDocs, docLengths, avgDocLength, postings, docFreq })
+  }
+
+  /**
+   * Compute BM25 scores for the given query tokens.
+   *
+   * Returns a Float32Array of length numDocs, in document order. When
+   * `weightMask` is provided, documents with mask[i] === 0 receive a score
+   * of 0 (matching bm25s.BM25.get_scores(..., weight_mask=mask) semantics).
+   */
+  getScores(queryTokens: string[], weightMask?: Uint8Array | null): Float32Array {
+    const { numDocs, docLengths, avgDocLength, postings, docFreq } = this.#state
+    const scores = new Float32Array(numDocs)
+    if (queryTokens.length === 0 || numDocs === 0)
+      return scores
+
+    // De-duplicate query tokens — repeated terms shouldn't compound BM25 scores.
+    const uniqueTerms = new Set(queryTokens)
+
+    for (const term of uniqueTerms) {
+      const list = postings.get(term)
+      if (list === undefined)
+        continue
+      const df = docFreq.get(term) ?? 0
+      // Lucene/Robertson IDF: log(1 + (N - df + 0.5) / (df + 0.5)).
+      const idf = Math.log(1 + (numDocs - df + 0.5) / (df + 0.5))
+
+      for (const [docId, freq] of list) {
+        // Skip masked-out documents inside the posting-list iteration so we
+        // avoid the work entirely; Float32Array entries default to 0 so the
+        // final scores match the post-loop zeroing approach.
+        if (weightMask && !weightMask[docId])
+          continue
+        const dl = docLengths[docId] ?? 0
+        const denom = freq + K1 * (1 - B + (B * dl) / (avgDocLength || 1))
+        const contrib = (idf * (freq * (K1 + 1))) / (denom || 1)
+        scores[docId] = (scores[docId] ?? 0) + contrib
+      }
+    }
+
+    return scores
+  }
+
+  /** Persist the index to `dir`. Creates the directory if it doesn't exist. */
+  async save(dir: string): Promise<void> {
+    await mkdir(dir, { recursive: true })
+    const { numDocs, docLengths, avgDocLength, postings, docFreq } = this.#state
+    const serialized = {
+      version: 1,
+      numDocs,
+      avgDocLength,
+      docLengths: Array.from(docLengths),
+      postings: Array.from(postings.entries()),
+      docFreq: Array.from(docFreq.entries()),
+    }
+    await writeFile(path.join(dir, 'bm25.json'), JSON.stringify(serialized))
+  }
+
+  /** Load an index previously persisted with `save`. */
+  static async load(dir: string): Promise<Bm25Index> {
+    const raw = await readFile(path.join(dir, 'bm25.json'), 'utf8')
+    const parsed = JSON.parse(raw) as {
+      version: number
+      numDocs: number
+      avgDocLength: number
+      docLengths: number[]
+      postings: Array<[string, Array<[number, number]>]>
+      docFreq: Array<[string, number]>
+    }
+    return new Bm25Index({
+      numDocs: parsed.numDocs,
+      docLengths: Float32Array.from(parsed.docLengths),
+      avgDocLength: parsed.avgDocLength,
+      postings: new Map(parsed.postings),
+      docFreq: new Map(parsed.docFreq),
+    })
+  }
+}