test(mcp): pin RFC #110 cache-prefix claims with 6 unit tests

tests/mcp-reconnect-prefix-invariant.test.ts

@@ -0,0 +1,91 @@
+/** Pins down the cache-prefix claims in RFC #110 (`/mcp reconnect <name>`). */
+
+import { describe, expect, it } from "vitest";
+import { ImmutablePrefix } from "../src/memory/runtime.js";
+import type { ToolSpec } from "../src/types.js";
+
+function tool(name: string, description = "", params: object = { type: "object" }): ToolSpec {
+  return {
+    type: "function",
+    function: { name, description, parameters: params },
+  };
+}
+
+describe("RFC #110 — cache-prefix invariant under MCP reconnect", () => {
+  it("re-bridging an IDENTICAL tool list yields byte-identical fingerprint (the safe case)", () => {
+    const before = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write"), tool("search")],
+    });
+    const after = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write"), tool("search")],
+    });
+    expect(after.fingerprint).toBe(before.fingerprint);
+  });
+
+  it("re-bridging with one ADDED tool changes the fingerprint (cache miss next turn)", () => {
+    const before = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write")],
+    });
+    const after = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write"), tool("delete")],
+    });
+    expect(after.fingerprint).not.toBe(before.fingerprint);
+  });
+
+  it("re-bridging with one REMOVED tool changes the fingerprint", () => {
+    const before = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write"), tool("search")],
+    });
+    const after = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write")],
+    });
+    expect(after.fingerprint).not.toBe(before.fingerprint);
+  });
+
+  it("a description-only change on an existing tool changes the fingerprint", () => {
+    const before = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read", "reads a file")],
+    });
+    const after = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read", "reads a file (utf-8)")],
+    });
+    expect(after.fingerprint).not.toBe(before.fingerprint);
+  });
+
+  it("a parameter-schema change on an existing tool changes the fingerprint", () => {
+    const before = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read", "", { type: "object", properties: { path: { type: "string" } } })],
+    });
+    const after = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [
+        tool("read", "", {
+          type: "object",
+          properties: { path: { type: "string" }, encoding: { type: "string" } },
+        }),
+      ],
+    });
+    expect(after.fingerprint).not.toBe(before.fingerprint);
+  });
+
+  it("REORDERING the same tools changes the fingerprint (array order is part of the prefix)", () => {
+    const a = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("read"), tool("write"), tool("search")],
+    });
+    const b = new ImmutablePrefix({
+      system: "s",
+      toolSpecs: [tool("write"), tool("read"), tool("search")],
+    });
+    expect(b.fingerprint).not.toBe(a.fingerprint);
+  });
+});