diff --git a/src/lib/agent/resolve.test.ts b/src/lib/agent/resolve.test.ts
new file mode 100644
index 0000000..5c11cb7
--- /dev/null
+++ b/src/lib/agent/resolve.test.ts
@@ -0,0 +1,70 @@
+import { describe, it, expect } from "vitest";
+import { resolveSite, type SiteRef } from "./resolve";
+
+const SITES: SiteRef[] = [
+  { id: "aaa111", name: "The Hills", projectCode: "C26/09" },
+  { id: "bbb222", name: "Hillcrest Extension", projectCode: "C26/11" },
+  { id: "ccc333", name: "N1 Plant Yard", projectCode: "C25/04" },
+];
+
+describe("resolveSite", () => {
+  it("matches an exact record id", () => {
+    const r = resolveSite("bbb222", SITES);
+    expect(r.kind).toBe("exact");
+    if (r.kind === "exact") expect(r.site.id).toBe("bbb222");
+  });
+
+  it("matches a project code case-insensitively", () => {
+    const r = resolveSite("c26/09", SITES);
+    expect(r.kind).toBe("exact");
+    if (r.kind === "exact") expect(r.site.id).toBe("aaa111");
+  });
+
+  it("matches a full name case-insensitively", () => {
+    const r = resolveSite("the hills", SITES);
+    expect(r.kind).toBe("exact");
+    if (r.kind === "exact") expect(r.site.id).toBe("aaa111");
+  });
+
+  it("ignores surrounding whitespace", () => {
+    const r = resolveSite("  The Hills  ", SITES);
+    expect(r.kind).toBe("exact");
+  });
+
+  it("falls back to a unique substring match", () => {
+    const r = resolveSite("n1 plant", SITES);
+    expect(r.kind).toBe("exact");
+    if (r.kind === "exact") expect(r.site.id).toBe("ccc333");
+  });
+
+  it("reports ambiguity rather than guessing", () => {
+    const r = resolveSite("hill", SITES);
+    expect(r.kind).toBe("ambiguous");
+    if (r.kind === "ambiguous") {
+      expect(r.matches.map((m) => m.id).sort()).toEqual(["aaa111", "bbb222"]);
+    }
+  });
+
+  it("prefers an exact name over a substring of another site", () => {
+    // "The Hills" is an exact name and also a substring of nothing else,
+    // but this pins the precedence explicitly.
+    const r = resolveSite("The Hills", SITES);
+    expect(r.kind).toBe("exact");
+    if (r.kind === "exact") expect(r.site.id).toBe("aaa111");
+  });
+
+  it("returns none for no match", () => {
+    const r = resolveSite("Rivonia", SITES);
+    expect(r.kind).toBe("none");
+  });
+
+  it("returns none for an empty query", () => {
+    const r = resolveSite("   ", SITES);
+    expect(r.kind).toBe("none");
+  });
+
+  it("returns none when there are no sites", () => {
+    const r = resolveSite("anything", []);
+    expect(r.kind).toBe("none");
+  });
+});
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diff --git a/src/lib/agent/resolve.ts b/src/lib/agent/resolve.ts
new file mode 100644
index 0000000..6c74542
--- /dev/null
+++ b/src/lib/agent/resolve.ts
@@ -0,0 +1,53 @@
+/**
+ * Resolve a free-text site query to a site.
+ *
+ * Callers are humans (via an LLM) who say "the Hills" or "C26/09", not
+ * record ids. Ambiguity is returned explicitly so the tool can ask a
+ * follow-up question instead of silently picking the wrong contract —
+ * these numbers are money.
+ */
+
+export interface SiteRef {
+  id: string;
+  name: string;
+  projectCode: string;
+}
+
+export type ResolveSiteResult =
+  | { kind: "exact"; site: SiteRef }
+  | { kind: "ambiguous"; matches: SiteRef[] }
+  | { kind: "none" };
+
+export function resolveSite(query: string, sites: SiteRef[]): ResolveSiteResult {
+  const q = query.trim().toLowerCase();
+  if (!q) return { kind: "none" };
+
+  // 1. Exact record id (case-sensitive — PB ids are lowercase alnum).
+  const byId = sites.find((s) => s.id === query.trim());
+  if (byId) return { kind: "exact", site: byId };
+
+  // 2. Exact project code.
+  const byCode = sites.filter((s) => s.projectCode.toLowerCase() === q);
+  if (byCode.length === 1) return { kind: "exact", site: byCode[0] };
+  if (byCode.length > 1) return { kind: "ambiguous", matches: byCode };
+
+  // 3. Exact name.
+  const byName = sites.filter((s) => s.name.toLowerCase() === q);
+  if (byName.length === 1) return { kind: "exact", site: byName[0] };
+  if (byName.length > 1) return { kind: "ambiguous", matches: byName };
+
+  // 4. Substring across name and code.
+  const bySubstring = sites.filter(
+    (s) =>
+      s.name.toLowerCase().includes(q) || s.projectCode.toLowerCase().includes(q),
+  );
+  if (bySubstring.length === 1) return { kind: "exact", site: bySubstring[0] };
+  if (bySubstring.length > 1) return { kind: "ambiguous", matches: bySubstring };
+
+  return { kind: "none" };
+}
+
+/** Human-readable disambiguation line for an ambiguous result. */
+export function describeAmbiguity(matches: SiteRef[]): string {
+  return matches.map((m) => `${m.name} (${m.projectCode})`).join(", ");
+}
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