diff --git a/src/attributionTracker.ts b/src/attributionTracker.ts
new file mode 100644
index 0000000..0b95a23
--- /dev/null
+++ b/src/attributionTracker.ts
@@ -0,0 +1,85 @@
+/**
+ * AttributionTracker — pure span algebra over offset ranges (spec §6.2/§6.5).
+ * Char-honest (INV-7): attributes exactly the characters each author produced —
+ * split character-precisely, coalesce only adjacent same-author/same-turn
+ * spans. vscode-free; the controller converts vscode change events to TextEdit.
+ */
+import type { TextEdit } from "./anchorer";
+import type { Provenance } from "./model";
+
+export interface LiveSpan {
+  id: string;
+  /** half-open [start, end) character offsets in the live document. */
+  start: number;
+  end: number;
+  author: Provenance;
+  turnId?: string;
+  /** ISO-8601; survives splits/merges (earliest wins on merge). */
+  createdAt: string;
+}
+
+/** Injected effects so the algebra stays pure and deterministic in tests. */
+export interface TrackerCtx {
+  newId: () => string;
+  now: () => string;
+  turnId?: string;
+}
+
+function sameAuthor(a: LiveSpan, b: LiveSpan): boolean {
+  return JSON.stringify(a.author) === JSON.stringify(b.author) && a.turnId === b.turnId;
+}
+
+/** Merge adjacent same-author/same-turn spans; drop empties; sort by start. */
+export function coalesce(spans: LiveSpan[]): LiveSpan[] {
+  const sorted = spans.filter((s) => s.end > s.start).sort((a, b) => a.start - b.start);
+  const out: LiveSpan[] = [];
+  for (const s of sorted) {
+    const last = out[out.length - 1];
+    if (last && last.end === s.start && sameAuthor(last, s)) {
+      last.end = s.end;
+      if (s.createdAt < last.createdAt) last.createdAt = s.createdAt;
+    } else {
+      out.push({ ...s });
+    }
+  }
+  return out;
+}
+
+/**
+ * Apply one document edit (the half-open range [start,end) replaced by
+ * newLength chars) authored by `author`. Existing spans shift/split/clip;
+ * inserted chars become a new span of `author` (INV-7).
+ */
+export function applyChange(
+  spans: LiveSpan[],
+  edit: TextEdit,
+  author: Provenance,
+  ctx: TrackerCtx,
+): LiveSpan[] {
+  const delta = edit.newLength - (edit.end - edit.start);
+  const out: LiveSpan[] = [];
+  for (const s of spans) {
+    if (s.end <= edit.start) {
+      out.push({ ...s });
+    } else if (s.start >= edit.end) {
+      out.push({ ...s, start: s.start + delta, end: s.end + delta });
+    } else {
+      const left = s.start < edit.start ? { ...s, end: edit.start } : null;
+      const right =
+        s.end > edit.end ? { ...s, start: edit.end + delta, end: s.end + delta } : null;
+      if (left) out.push(left);
+      if (right) out.push(left ? { ...right, id: ctx.newId() } : right);
+    }
+  }
+  if (edit.newLength > 0) {
+    out.push({
+      id: ctx.newId(),
+      start: edit.start,
+      end: edit.start + edit.newLength,
+      author,
+      turnId: ctx.turnId,
+      createdAt: ctx.now(),
+    });
+  }
+  return coalesce(out);
+}
diff --git a/test/attributionTracker.test.ts b/test/attributionTracker.test.ts
new file mode 100644
index 0000000..c590a8e
--- /dev/null
+++ b/test/attributionTracker.test.ts
@@ -0,0 +1,127 @@
+import { describe, it, expect } from "vitest";
+import { applyChange, coalesce, type LiveSpan, type TrackerCtx } from "../src/attributionTracker";
+import type { Provenance } from "../src/model";
+
+const HUMAN: Provenance = { kind: "human", id: "ben" };
+const AGENT: Provenance = {
+  kind: "agent", id: "claude",
+  agent: { sdk: "@cline/sdk", model: "sonnet", sessionId: "run-1" },
+};
+
+function ctx(turnId?: string): TrackerCtx {
+  let n = 0;
+  return { newId: () => `n${++n}`, now: () => "2026-06-10T00:00:00.000Z", turnId };
+}
+function span(id: string, start: number, end: number, author: Provenance, turnId?: string): LiveSpan {
+  return { id, start, end, author, turnId, createdAt: "2026-06-09T00:00:00.000Z" };
+}
+const ranges = (s: LiveSpan[]) => s.map((x) => [x.start, x.end]);
+const authors = (s: LiveSpan[]) => s.map((x) => x.author.kind);
+
+describe("applyChange — insertions", () => {
+  it("insertion into empty span list creates one author span", () => {
+    const r = applyChange([], { start: 5, end: 5, newLength: 3 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[5, 8]]);
+    expect(r[0].author).toEqual(HUMAN);
+  });
+  it("insertion before a span shifts it", () => {
+    const r = applyChange([span("a", 10, 14, AGENT)], { start: 0, end: 0, newLength: 2 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 2], [12, 16]]);
+    expect(authors(r)).toEqual(["human", "agent"]);
+  });
+  it("insertion after a span leaves it untouched", () => {
+    const r = applyChange([span("a", 0, 4, AGENT)], { start: 10, end: 10, newLength: 2 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 4], [10, 12]]);
+  });
+  it("PUC-3: insertion strictly inside a different-author span splits it char-precisely", () => {
+    const r = applyChange([span("a", 0, 10, AGENT)], { start: 4, end: 4, newLength: 3 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 4], [4, 7], [7, 13]]);
+    expect(authors(r)).toEqual(["agent", "human", "agent"]);
+  });
+  it("symmetric: agent insertion inside a human span splits it (INV-7)", () => {
+    const r = applyChange([span("a", 0, 10, HUMAN)], { start: 4, end: 4, newLength: 3 }, AGENT, ctx("t1"));
+    expect(authors(r)).toEqual(["human", "agent", "human"]);
+    expect(r[1].turnId).toBe("t1");
+  });
+  it("insertion inside a SAME-author span coalesces back to one span", () => {
+    const r = applyChange([span("a", 0, 10, HUMAN)], { start: 4, end: 4, newLength: 3 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 13]]);
+  });
+  it("insertion at a span boundary does not split; same author coalesces", () => {
+    const r = applyChange([span("a", 0, 5, HUMAN)], { start: 5, end: 5, newLength: 2 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 7]]);
+  });
+});
+
+describe("applyChange — deletions", () => {
+  it("deletion before a span shifts it left", () => {
+    const r = applyChange([span("a", 10, 14, HUMAN)], { start: 0, end: 4, newLength: 0 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[6, 10]]);
+  });
+  it("deletion entirely inside a span shrinks it (no re-authoring, INV-6)", () => {
+    const r = applyChange([span("a", 0, 10, AGENT)], { start: 3, end: 6, newLength: 0 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 7]]);
+    expect(authors(r)).toEqual(["agent"]);
+  });
+  it("deletion covering a whole span removes it", () => {
+    const r = applyChange([span("a", 3, 6, AGENT)], { start: 0, end: 10, newLength: 0 }, HUMAN, ctx());
+    expect(r).toEqual([]);
+  });
+  it("deletion clipping a span head keeps the tail", () => {
+    const r = applyChange([span("a", 4, 10, AGENT)], { start: 0, end: 6, newLength: 0 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 4]]);
+  });
+  it("deletion clipping a span tail keeps the head", () => {
+    const r = applyChange([span("a", 0, 6, AGENT)], { start: 4, end: 10, newLength: 0 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 4]]);
+  });
+});
+
+describe("applyChange — replacements", () => {
+  it("replacement inside an agent span: human chars in the middle, agent halves keep ids/createdAt", () => {
+    const r = applyChange([span("a", 0, 12, AGENT)], { start: 4, end: 8, newLength: 2 }, HUMAN, ctx());
+    expect(ranges(r)).toEqual([[0, 4], [4, 6], [6, 10]]);
+    expect(authors(r)).toEqual(["agent", "human", "agent"]);
+    expect(r[0].id).toBe("a");
+    expect(r[0].createdAt).toBe("2026-06-09T00:00:00.000Z");
+  });
+  it("replacement spanning two spans clips both and inserts the author's span between", () => {
+    const r = applyChange(
+      [span("a", 0, 6, AGENT), span("b", 6, 12, HUMAN)],
+      { start: 4, end: 8, newLength: 5 }, AGENT, ctx("t2"),
+    );
+    expect(ranges(r)).toEqual([[0, 4], [4, 9], [9, 13]]);
+    expect(authors(r)).toEqual(["agent", "agent", "human"]);
+    expect(r[1].turnId).toBe("t2");
+  });
+});
+
+describe("multi-edit sequences", () => {
+  it("type, agent-replace, type inside agent span — ends char-honest", () => {
+    const c = ctx();
+    let s = applyChange([], { start: 0, end: 0, newLength: 20 }, HUMAN, c);
+    s = applyChange(s, { start: 5, end: 10, newLength: 8 }, AGENT, { ...c, turnId: "t1" });
+    s = applyChange(s, { start: 9, end: 9, newLength: 1 }, HUMAN, c);
+    expect(authors(s)).toEqual(["human", "agent", "human", "agent", "human"]);
+    expect(ranges(s)).toEqual([[0, 5], [5, 9], [9, 10], [10, 14], [14, 24]]);
+  });
+});
+
+describe("coalesce", () => {
+  it("merges adjacent same-author same-turn spans, keeps earliest createdAt", () => {
+    const a = { ...span("a", 0, 3, HUMAN), createdAt: "2026-06-09T00:00:00.000Z" };
+    const b = { ...span("b", 3, 6, HUMAN), createdAt: "2026-06-08T00:00:00.000Z" };
+    const r = coalesce([a, b]);
+    expect(ranges(r)).toEqual([[0, 6]]);
+    expect(r[0].id).toBe("a");
+    expect(r[0].createdAt).toBe("2026-06-08T00:00:00.000Z");
+  });
+  it("does NOT merge different authors, different turnIds, or non-adjacent spans", () => {
+    expect(coalesce([span("a", 0, 3, HUMAN), span("b", 3, 6, AGENT)])).toHaveLength(2);
+    expect(coalesce([span("a", 0, 3, AGENT, "t1"), span("b", 3, 6, AGENT, "t2")])).toHaveLength(2);
+    expect(coalesce([span("a", 0, 3, HUMAN), span("b", 4, 6, HUMAN)])).toHaveLength(2);
+  });
+  it("drops empty spans", () => {
+    expect(coalesce([span("a", 5, 5, HUMAN)])).toEqual([]);
+  });
+});