diff --git a/src/mcp/drift.ts b/src/mcp/drift.ts
new file mode 100644
index 0000000..ff49d44
--- /dev/null
+++ b/src/mcp/drift.ts
@@ -0,0 +1,86 @@
+/** Classifies a tool-list drift across an MCP reconnect. Drives the policy in `/mcp reconnect`. */
+
+import type { ToolSpec } from "../types.js";
+
+/** Ordered by "cache cost" — `identity` and `append` are nearly free; `reorder` is catastrophic. */
+export type DriftKind = "identity" | "append" | "edit" | "reorder" | "remove";
+
+export interface DriftReport {
+  kind: DriftKind;
+  /** Tool names added by the new spec (relative to `before`). */
+  added: string[];
+  /** Tool names removed by the new spec (gone from `after`). */
+  removed: string[];
+  /** Tool names whose name + position match but whose serialized content changed. */
+  edited: string[];
+}
+
+export function classifyToolListDrift(
+  before: readonly ToolSpec[],
+  after: readonly ToolSpec[],
+): DriftReport {
+  const beforeNames = before.map(nameOf);
+  const afterNames = after.map(nameOf);
+  const beforeSet = new Set(beforeNames);
+  const afterSet = new Set(afterNames);
+
+  const added = afterNames.filter((n) => !beforeSet.has(n));
+  const removed = beforeNames.filter((n) => !afterSet.has(n));
+
+  const edited: string[] = [];
+  // Same-position-same-name slots whose serialized content differs.
+  const sharedLen = Math.min(before.length, after.length);
+  for (let i = 0; i < sharedLen; i++) {
+    if (beforeNames[i] === afterNames[i] && hash(before[i]!) !== hash(after[i]!)) {
+      edited.push(beforeNames[i]!);
+    }
+  }
+
+  // Identity: same length, same names in order, same content.
+  if (
+    before.length === after.length &&
+    edited.length === 0 &&
+    beforeNames.every((n, i) => n === afterNames[i])
+  ) {
+    return { kind: "identity", added: [], removed: [], edited: [] };
+  }
+
+  // Remove anywhere → catastrophic regardless of other changes.
+  if (removed.length > 0) {
+    return { kind: "remove", added, removed, edited };
+  }
+
+  // Append: every before-tool stays put with identical content, new ones tacked on the end.
+  if (
+    after.length > before.length &&
+    beforeNames.every((n, i) => n === afterNames[i] && hash(before[i]!) === hash(after[i]!))
+  ) {
+    return { kind: "append", added, removed: [], edited: [] };
+  }
+
+  // Same name set as before? Then positions or content changed.
+  const sameNameSet =
+    beforeSet.size === afterSet.size && [...beforeSet].every((n) => afterSet.has(n));
+  if (sameNameSet) {
+    const positionsMatch = beforeNames.every((n, i) => n === afterNames[i]);
+    if (positionsMatch) {
+      // Names + positions stable, only content edited in place.
+      return { kind: "edit", added: [], removed: [], edited };
+    }
+    // Same set, different order — cache-wise as bad as a structural change.
+    return { kind: "reorder", added: [], removed: [], edited };
+  }
+
+  // Additions present but NOT clean appends (e.g. inserted in the middle, or
+  // appended-but-existing-tools-also-edited). Treat as reorder for safety —
+  // the divergence point is no longer the tail of the list.
+  return { kind: "reorder", added, removed: [], edited };
+}
+
+function nameOf(spec: ToolSpec): string {
+  return spec.function?.name ?? "";
+}
+
+function hash(spec: ToolSpec): string {
+  return JSON.stringify(spec);
+}
diff --git a/tests/mcp-drift.test.ts b/tests/mcp-drift.test.ts
new file mode 100644
index 0000000..2a2b69a
--- /dev/null
+++ b/tests/mcp-drift.test.ts
@@ -0,0 +1,100 @@
+import { describe, expect, it } from "vitest";
+import { classifyToolListDrift } from "../src/mcp/drift.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 },
+  };
+}
+
+const A = tool("read");
+const B = tool("write");
+const C = tool("search");
+const D = tool("delete");
+
+describe("classifyToolListDrift", () => {
+  it("identity: same length, same names, same content → identity", () => {
+    const r = classifyToolListDrift([A, B, C], [A, B, C]);
+    expect(r.kind).toBe("identity");
+    expect(r.added).toEqual([]);
+    expect(r.removed).toEqual([]);
+    expect(r.edited).toEqual([]);
+  });
+
+  it("append: every before-tool unchanged, new tool appended", () => {
+    const r = classifyToolListDrift([A, B, C], [A, B, C, D]);
+    expect(r.kind).toBe("append");
+    expect(r.added).toEqual(["delete"]);
+    expect(r.removed).toEqual([]);
+    expect(r.edited).toEqual([]);
+  });
+
+  it("append: multiple tools appended at the end", () => {
+    const E = tool("touch");
+    const r = classifyToolListDrift([A, B], [A, B, C, D, E]);
+    expect(r.kind).toBe("append");
+    expect(r.added).toEqual(["search", "delete", "touch"]);
+  });
+
+  it("edit: same names + same order, content of one tool changed", () => {
+    const aEdited = tool("read", "now reads UTF-8 only");
+    const r = classifyToolListDrift([A, B, C], [aEdited, B, C]);
+    expect(r.kind).toBe("edit");
+    expect(r.edited).toEqual(["read"]);
+    expect(r.added).toEqual([]);
+    expect(r.removed).toEqual([]);
+  });
+
+  it("edit: schema change on the same tool", () => {
+    const aSchema = tool("read", "", {
+      type: "object",
+      properties: { path: { type: "string" }, encoding: { type: "string" } },
+    });
+    const r = classifyToolListDrift([A], [aSchema]);
+    expect(r.kind).toBe("edit");
+    expect(r.edited).toEqual(["read"]);
+  });
+
+  it("remove: a tool present in before is missing from after", () => {
+    const r = classifyToolListDrift([A, B, C], [A, C]);
+    expect(r.kind).toBe("remove");
+    expect(r.removed).toEqual(["write"]);
+    expect(r.added).toEqual([]);
+  });
+
+  it("remove dominates: even if other tools were added, removal makes it `remove`", () => {
+    // before: A, B, C  → after: A, C, D  (B removed, D added)
+    const r = classifyToolListDrift([A, B, C], [A, C, D]);
+    expect(r.kind).toBe("remove");
+    expect(r.removed).toEqual(["write"]);
+    expect(r.added).toEqual(["delete"]);
+  });
+
+  it("reorder: same name set in a different order", () => {
+    const r = classifyToolListDrift([A, B, C], [B, A, C]);
+    expect(r.kind).toBe("reorder");
+  });
+
+  it("reorder: addition NOT at the end (cache-equivalent to a reorder)", () => {
+    const r = classifyToolListDrift([A, B, C], [A, D, B, C]);
+    expect(r.kind).toBe("reorder");
+  });
+
+  it("identity with empty lists", () => {
+    expect(classifyToolListDrift([], []).kind).toBe("identity");
+  });
+
+  it("append onto an empty list", () => {
+    const r = classifyToolListDrift([], [A, B]);
+    expect(r.kind).toBe("append");
+    expect(r.added).toEqual(["read", "write"]);
+  });
+
+  it("remove down to an empty list", () => {
+    const r = classifyToolListDrift([A, B], []);
+    expect(r.kind).toBe("remove");
+    expect(r.removed).toEqual(["read", "write"]);
+  });
+});