diff --git a/src/reporters/site-api.test.ts b/src/reporters/site-api.test.ts
new file mode 100644
index 0000000..725e04f
--- /dev/null
+++ b/src/reporters/site-api.test.ts
@@ -0,0 +1,183 @@
+import { test, expect, describe } from "vitest";
+import {
+  compareRuns,
+  type CiQueryPayload,
+  type PreviousRun,
+} from "./site-api.ts";
+
+function makeQuery(hash: string, cost: number = 100): CiQueryPayload {
+  return {
+    hash,
+    query: `SELECT * FROM t WHERE id = $1`,
+    formattedQuery: `SELECT *\nFROM t\nWHERE id = $1`,
+    optimization: {
+      state: "no_improvement_found",
+      cost,
+      indexesUsed: [],
+    },
+  };
+}
+
+function makePreviousRun(queries: CiQueryPayload[]): PreviousRun {
+  return {
+    id: "prev-run-1",
+    repo: "test/repo",
+    branch: "main",
+    commitSha: "abc123",
+    queries,
+  };
+}
+
+describe("compareRuns", () => {
+  describe("new query detection via previousRun", () => {
+    test("when previousRun has no queries, all current queries are new", () => {
+      const queries = [makeQuery("hash-a"), makeQuery("hash-b")];
+      const previousRun = makePreviousRun([]);
+
+      const result = compareRuns(queries, previousRun, 10);
+
+      expect(result.newQueries).toHaveLength(2);
+      expect(result.newQueries.map((q) => q.hash)).toEqual([
+        "hash-a",
+        "hash-b",
+      ]);
+      expect(result.regressed).toHaveLength(0);
+      expect(result.improved).toHaveLength(0);
+    });
+
+    test("when previousRun contains hashes, only non-seen queries are new", () => {
+      const queries = [
+        makeQuery("hash-a"),
+        makeQuery("hash-b"),
+        makeQuery("hash-c"),
+      ];
+      const previousRun = makePreviousRun([
+        makeQuery("hash-a"),
+        makeQuery("hash-c"),
+      ]);
+
+      const result = compareRuns(queries, previousRun, 10);
+
+      expect(result.newQueries).toHaveLength(1);
+      expect(result.newQueries[0].hash).toBe("hash-b");
+    });
+
+    test("query in previousRun with increased cost is flagged as regression", () => {
+      const currentQueries = [makeQuery("hash-a", 500)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 100)]);
+
+      const result = compareRuns(currentQueries, previousRun, 10);
+
+      expect(result.newQueries).toHaveLength(0);
+      expect(result.regressed).toHaveLength(1);
+      expect(result.regressed[0].hash).toBe("hash-a");
+      expect(result.regressed[0].previousCost).toBe(100);
+      expect(result.regressed[0].currentCost).toBe(500);
+      expect(result.regressed[0].regressionPercentage).toBe(400);
+    });
+  });
+
+  describe("regression threshold", () => {
+    test("cost change within threshold is not flagged", () => {
+      const currentQueries = [makeQuery("hash-a", 115)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 100)]);
+
+      const result = compareRuns(currentQueries, previousRun, 20);
+
+      expect(result.regressed).toHaveLength(0);
+      expect(result.improved).toHaveLength(0);
+      expect(result.newQueries).toHaveLength(0);
+    });
+
+    test("cost decrease beyond threshold is flagged as improved", () => {
+      const currentQueries = [makeQuery("hash-a", 30)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 100)]);
+
+      const result = compareRuns(currentQueries, previousRun, 10);
+
+      expect(result.improved).toHaveLength(1);
+      expect(result.improved[0].hash).toBe("hash-a");
+      expect(result.improved[0].previousCost).toBe(100);
+      expect(result.improved[0].currentCost).toBe(30);
+    });
+
+    test("acknowledged regression goes to acknowledgedRegressed, not regressed", () => {
+      const currentQueries = [makeQuery("hash-a", 500)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 100)]);
+
+      const result = compareRuns(
+        currentQueries,
+        previousRun,
+        10,
+        0,
+        ["hash-a"],
+      );
+
+      expect(result.regressed).toHaveLength(0);
+      expect(result.acknowledgedRegressed).toHaveLength(1);
+      expect(result.acknowledgedRegressed[0].hash).toBe("hash-a");
+    });
+  });
+
+  describe("minimumCost filtering", () => {
+    test("regression where both costs are below minimumCost is skipped", () => {
+      const currentQueries = [makeQuery("hash-a", 8)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 2)]);
+
+      const result = compareRuns(currentQueries, previousRun, 10, 50);
+
+      expect(result.regressed).toHaveLength(0);
+    });
+
+    test("regression where current cost exceeds minimumCost is reported", () => {
+      const currentQueries = [makeQuery("hash-a", 200)];
+      const previousRun = makePreviousRun([makeQuery("hash-a", 10)]);
+
+      const result = compareRuns(currentQueries, previousRun, 10, 50);
+
+      expect(result.regressed).toHaveLength(1);
+    });
+  });
+
+  describe("disappeared queries", () => {
+    test("queries in previousRun but not in current are disappeared", () => {
+      const currentQueries = [makeQuery("hash-a")];
+      const previousRun = makePreviousRun([
+        makeQuery("hash-a"),
+        makeQuery("hash-b"),
+        makeQuery("hash-c"),
+      ]);
+
+      const result = compareRuns(currentQueries, previousRun, 10);
+
+      expect(result.disappearedHashes).toEqual(["hash-b", "hash-c"]);
+    });
+  });
+
+  describe("mixed scenarios", () => {
+    test("new, regressed, improved, and disappeared queries together", () => {
+      const currentQueries = [
+        makeQuery("hash-a", 500), // regressed (was 100)
+        makeQuery("hash-b", 30),  // improved (was 100)
+        makeQuery("hash-c", 100), // unchanged
+        makeQuery("hash-d", 200), // new
+      ];
+      const previousRun = makePreviousRun([
+        makeQuery("hash-a", 100),
+        makeQuery("hash-b", 100),
+        makeQuery("hash-c", 100),
+        makeQuery("hash-e", 100), // disappeared
+      ]);
+
+      const result = compareRuns(currentQueries, previousRun, 10);
+
+      expect(result.newQueries).toHaveLength(1);
+      expect(result.newQueries[0].hash).toBe("hash-d");
+      expect(result.regressed).toHaveLength(1);
+      expect(result.regressed[0].hash).toBe("hash-a");
+      expect(result.improved).toHaveLength(1);
+      expect(result.improved[0].hash).toBe("hash-b");
+      expect(result.disappearedHashes).toEqual(["hash-e"]);
+    });
+  });
+});