diff --git a/src/lib/server/collection-ai-pair.test.ts b/src/lib/server/collection-ai-pair.test.ts
new file mode 100644
index 0000000..12ad723
--- /dev/null
+++ b/src/lib/server/collection-ai-pair.test.ts
@@ -0,0 +1,142 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/server/ai-settings-service", () => ({
+  getActiveAiConfig: vi.fn(async () => ({
+    modelId: "MiniMax-M3",
+    maxTokens: 1000,
+    apiKey: "k",
+    baseUrl: "u",
+    provider: "minimax",
+  })),
+  getAiSettings: vi.fn(async () => ({})),
+}));
+
+import { pairDocumentLines, parsePairs, type PairPoLine } from "./collection-ai-pair";
+import type { CollectionDocLine } from "@/lib/types";
+
+const docLine = (n: number, over: Partial<CollectionDocLine> = {}): CollectionDocLine => ({
+  line_no: n,
+  code: `C${n}`,
+  description: `Doc item ${n}`,
+  quantity: 10,
+  unit: "ea",
+  unit_price: 1,
+  line_total: 10,
+  to_follow: null,
+  price_note: "",
+  ...over,
+});
+
+const poLine = (id: string, over: Partial<PairPoLine> = {}): PairPoLine => ({
+  id,
+  po_number: "PO1",
+  description: `Our item ${id}`,
+  collected: true,
+  order_qty: 10,
+  quantity: 10,
+  qty_collected: 10,
+  sku: "SK-" + id,
+  po_description: `PO desc ${id}`,
+  cost_price: 1.5,
+  status: "ordered",
+  ...over,
+});
+
+const docNos = new Set([1, 2, 3]);
+const poIds = new Set(["p1", "p2"]);
+
+describe("parsePairs", () => {
+  it("keeps valid pairs", () => {
+    const m = parsePairs(
+      [{ doc_line_no: 1, po_line_id: "p1", same_item: "sure", reason: " same bolt " }],
+      docNos,
+      poIds,
+    );
+    expect(m.get(1)).toEqual({ doc_line_no: 1, po_line_id: "p1", same_item: "sure", reason: "same bolt" });
+  });
+
+  it("nulls unknown po line ids", () => {
+    const m = parsePairs([{ doc_line_no: 1, po_line_id: "ghost", same_item: "sure", reason: "x" }], docNos, poIds);
+    expect(m.get(1)).toMatchObject({ po_line_id: null, same_item: null });
+  });
+
+  it("drops unknown doc line numbers", () => {
+    const m = parsePairs([{ doc_line_no: 99, po_line_id: "p1", same_item: "sure", reason: "" }], docNos, poIds);
+    expect(m.size).toBe(0);
+  });
+
+  it("keeps the first pairing when a PO line is paired twice", () => {
+    const m = parsePairs(
+      [
+        { doc_line_no: 1, po_line_id: "p1", same_item: "likely", reason: "a" },
+        { doc_line_no: 2, po_line_id: "p1", same_item: "sure", reason: "b" },
+      ],
+      docNos,
+      poIds,
+    );
+    expect(m.get(1)?.po_line_id).toBe("p1");
+    expect(m.get(2)).toMatchObject({ po_line_id: null, same_item: null });
+  });
+
+  it("nulls an invalid same_item and tolerates junk input", () => {
+    const m = parsePairs([{ doc_line_no: 1, po_line_id: "p1", same_item: "maybe", reason: 5 }], docNos, poIds);
+    expect(m.get(1)).toMatchObject({ po_line_id: "p1", same_item: null, reason: "" });
+    expect(parsePairs(undefined, docNos, poIds).size).toBe(0);
+    expect(parsePairs([null, "x", { doc_line_no: "1" }], docNos, poIds).size).toBe(0);
+  });
+});
+
+describe("pairDocumentLines", () => {
+  const toolUse = (input: unknown) => ({ type: "tool_use", id: "t", name: "submit_line_pairs", input });
+  function fake(content: unknown[], stop_reason = "tool_use") {
+    const create = vi.fn<(a: unknown) => Promise<{ content: unknown[]; stop_reason: string }>>(async () => ({
+      content,
+      stop_reason,
+    }));
+    return { client: { messages: { create } } as never, create };
+  }
+
+  beforeEach(() => vi.clearAllMocks());
+
+  it("makes no AI call without PO lines and returns every line unpaired", async () => {
+    const { client, create } = fake([]);
+    const res = await pairDocumentLines([docLine(1), docLine(2)], [], { client });
+    expect(create).not.toHaveBeenCalled();
+    expect([...res.pairs.values()]).toEqual([
+      { doc_line_no: 1, po_line_id: null, same_item: null, reason: "" },
+      { doc_line_no: 2, po_line_id: null, same_item: null, reason: "" },
+    ]);
+  });
+
+  it("sends PO line details, forces submit_line_pairs, and fills unanswered lines", async () => {
+    const { client, create } = fake([
+      toolUse({ pairs: [{ doc_line_no: 1, po_line_id: "p1", same_item: "sure", reason: "ok" }] }),
+    ]);
+    const res = await pairDocumentLines([docLine(1), docLine(2)], [poLine("p1"), poLine("p2")], { client });
+    const args = create.mock.calls[0][0] as {
+      system: string;
+      max_tokens: number;
+      tool_choice: unknown;
+      messages: Array<{ content: string }>;
+    };
+    expect(args.tool_choice).toEqual({ type: "tool", name: "submit_line_pairs" });
+    expect(args.max_tokens).toBe(4096);
+    expect(args.system).toContain(
+      "pair only when it is the same product; wording differs between supplier and customer — judge the item, not the words",
+    );
+    const text = args.messages[0].content;
+    for (const s of ["p1", "Our item p1", "PO desc p1", "SK-p1", "1.5", "Doc item 1"]) expect(text).toContain(s);
+    expect(res.model).toBe("MiniMax-M3");
+    expect(res.pairs.get(1)?.po_line_id).toBe("p1");
+    expect(res.pairs.get(2)).toEqual({ doc_line_no: 2, po_line_id: null, same_item: null, reason: "" });
+  });
+
+  it("throws on max_tokens and on a missing tool call", async () => {
+    await expect(
+      pairDocumentLines([docLine(1)], [poLine("p1")], { client: fake([toolUse({ pairs: [] })], "max_tokens").client }),
+    ).rejects.toThrow("AI response was cut off (max_tokens)");
+    await expect(
+      pairDocumentLines([docLine(1)], [poLine("p1")], { client: fake([{ type: "text", text: "x" }]).client }),
+    ).rejects.toThrow("AI returned no submit_line_pairs tool call");
+  });
+});
diff --git a/src/lib/server/collection-ai-pair.ts b/src/lib/server/collection-ai-pair.ts
new file mode 100644
index 0000000..eee78f9
--- /dev/null
+++ b/src/lib/server/collection-ai-pair.ts
@@ -0,0 +1,171 @@
+/**
+ * collection-ai-pair — text-only AI call that pairs a supplier document's
+ * lines with our PO lines. The AI only *chooses* among the PO line ids we
+ * hand it; ticks are computed elsewhere, in code.
+ */
+import Anthropic from "@anthropic-ai/sdk";
+import type { CollectionDocLine, SameItem } from "@/lib/types";
+import { getActiveAiConfig } from "./ai-settings-service";
+import type { PoLineLite } from "./collection-ai-findings";
+
+export type PairPoLine = PoLineLite & {
+  sku: string;
+  po_description: string;
+  cost_price: number | null;
+  status: string;
+};
+
+export type PairDecision = {
+  doc_line_no: number;
+  po_line_id: string | null;
+  same_item: SameItem | null;
+  reason: string;
+};
+
+const TOOL_NAME = "submit_line_pairs" as const;
+
+/** Not operator-editable: the pairing contract is fixed in code. */
+export const PAIRING_SYSTEM_PROMPT = `You pair the lines of a supplier's invoice or delivery note with the lines of our purchase orders.
+
+Rules:
+1. For every document line, return the po_line_id of the PO line it supplies, or null when none of the supplied PO lines is the same product.
+2. Be strict: pair only when it is the same product; wording differs between supplier and customer — judge the item, not the words. Compare size, grade, finish, material and pack size.
+3. Use only po_line_id values from the list you are given. Never invent an id.
+4. Pair each PO line to at most one document line.
+5. same_item: "sure" when you are certain it is the same product, "likely" when it probably is, "different" when it is a different product (then still say which PO line you compared it with, or use null if none fits).
+6. reason: one short plain sentence. Call submit_line_pairs exactly once, with one entry per document line.`;
+
+export const SUBMIT_LINE_PAIRS_TOOL = {
+  name: TOOL_NAME,
+  description:
+    "Submit the pairing of every document line to a PO line. Call exactly once with one entry per document line.",
+  input_schema: {
+    type: "object" as const,
+    properties: {
+      pairs: {
+        type: "array",
+        items: {
+          type: "object",
+          properties: {
+            doc_line_no: { type: "number", description: "The document line number, as given." },
+            po_line_id: {
+              type: ["string", "null"],
+              description: "The id of the matching PO line from the list, or null.",
+            },
+            same_item: { type: ["string", "null"], enum: ["sure", "likely", "different", null] },
+            reason: { type: "string" },
+          },
+          required: ["doc_line_no", "po_line_id", "same_item", "reason"],
+        },
+      },
+    },
+    required: ["pairs"],
+  },
+};
+
+const SAME_ITEMS: readonly string[] = ["sure", "likely", "different"];
+
+/**
+ * Defensive parse. Unknown doc line numbers are dropped; unknown PO line ids
+ * become unpaired; a PO line already paired keeps its first pairing.
+ */
+export function parsePairs(
+  raw: unknown,
+  docLineNos: ReadonlySet<number>,
+  poLineIds: ReadonlySet<string>,
+): Map<number, PairDecision> {
+  const out = new Map<number, PairDecision>();
+  if (!Array.isArray(raw)) return out;
+  const used = new Set<string>();
+  for (const item of raw) {
+    if (!item || typeof item !== "object") continue;
+    const r = item as Record<string, unknown>;
+    const no = r.doc_line_no;
+    if (typeof no !== "number" || !docLineNos.has(no) || out.has(no)) continue;
+    const reason = typeof r.reason === "string" ? r.reason.trim() : "";
+    const id = typeof r.po_line_id === "string" ? r.po_line_id : null;
+    if (id === null || !poLineIds.has(id) || used.has(id)) {
+      out.set(no, { doc_line_no: no, po_line_id: null, same_item: null, reason });
+      continue;
+    }
+    used.add(id);
+    const same =
+      typeof r.same_item === "string" && SAME_ITEMS.includes(r.same_item) ? (r.same_item as SameItem) : null;
+    out.set(no, { doc_line_no: no, po_line_id: id, same_item: same, reason });
+  }
+  return out;
+}
+
+function ordered(l: PairPoLine): number {
+  return l.order_qty ? l.order_qty : l.quantity;
+}
+
+function buildUserText(docLines: CollectionDocLine[], poLines: PairPoLine[]): string {
+  const out: string[] = ["Document lines:"];
+  for (const d of docLines) {
+    out.push(
+      `- doc_line_no=${d.line_no} | code ${d.code || "-"} | ${d.description} | qty ${d.quantity ?? "?"} ${d.unit}`.trimEnd(),
+    );
+  }
+  out.push("", "Our PO lines:");
+  for (const p of poLines) {
+    out.push(
+      `- po_line_id=${p.id} | ${p.po_number} | description: ${p.description} | PO description: ${p.po_description || "-"} | SKU ${p.sku || "-"} | ordered ${ordered(p)} | collected ${p.qty_collected} | cost ${p.cost_price ?? "-"} | status ${p.status || "-"}`,
+    );
+  }
+  return out.join("\n");
+}
+
+function newAnthropic(aiConfig: { apiKey: string; baseUrl: string }): Anthropic {
+  return new Anthropic({
+    apiKey: aiConfig.apiKey,
+    ...(aiConfig.baseUrl ? { baseURL: aiConfig.baseUrl } : {}),
+  });
+}
+
+export async function pairDocumentLines(
+  docLines: CollectionDocLine[],
+  poLines: PairPoLine[],
+  deps: { client?: Pick<Anthropic, "messages"> } = {},
+): Promise<{ pairs: Map<number, PairDecision>; model: string }> {
+  const unpaired = (no: number): PairDecision => ({
+    doc_line_no: no,
+    po_line_id: null,
+    same_item: null,
+    reason: "",
+  });
+
+  if (poLines.length === 0 || docLines.length === 0) {
+    return { pairs: new Map(docLines.map((d) => [d.line_no, unpaired(d.line_no)])), model: "" };
+  }
+
+  const aiConfig = await getActiveAiConfig();
+  const client = deps.client ?? newAnthropic(aiConfig);
+  const response = await client.messages.create({
+    model: aiConfig.modelId,
+    max_tokens: 4096,
+    tools: [SUBMIT_LINE_PAIRS_TOOL],
+    tool_choice: { type: "tool", name: TOOL_NAME },
+    system: PAIRING_SYSTEM_PROMPT,
+    messages: [{ role: "user", content: buildUserText(docLines, poLines) }],
+  });
+
+  if (response.stop_reason === "max_tokens") {
+    throw new Error("AI response was cut off (max_tokens)");
+  }
+  const toolBlock = response.content.find((b) => b.type === "tool_use" && b.name === TOOL_NAME);
+  if (!toolBlock || toolBlock.type !== "tool_use") {
+    throw new Error("AI returned no submit_line_pairs tool call");
+  }
+
+  const toolInput = toolBlock.input as { pairs?: unknown } | null;
+  const parsed = parsePairs(
+    toolInput?.pairs,
+    new Set(docLines.map((d) => d.line_no)),
+    new Set(poLines.map((p) => p.id)),
+  );
+  // Lines the AI skipped are explicitly unpaired, so callers see every line.
+  const pairs = new Map<number, PairDecision>();
+  for (const d of docLines) pairs.set(d.line_no, parsed.get(d.line_no) ?? unpaired(d.line_no));
+  return { pairs, model: aiConfig.modelId };
+}