diff --git a/packages/compiler/__tests__/directives-id-stability.test.js b/packages/compiler/__tests__/directives-id-stability.test.js
new file mode 100644
index 000000000..759cca4ea
--- /dev/null
+++ b/packages/compiler/__tests__/directives-id-stability.test.js
@@ -0,0 +1,86 @@
+// Server-function ids are keyed on IDENTITY, not position (#3109).
+//
+// The id is the address a deployed client already holds, so an edit that
+// moves one silently re-points a call: before the fix the id was
+// `<hash>-<ordinal>`, appending a function renumbered the rest, and an old
+// tab calling getUser reached deleteUser with a 200. The fixture suite
+// cannot see that — every fixture is a single fixed source, so it pins the
+// id FORMAT and no fixture has a second version to compare against. This
+// is the differential half: compile two versions of one file and require
+// every surviving name to keep its address.
+
+const path = require("path");
+
+const compilerDir = path.resolve(__dirname, "..");
+const { transformDirectives } = require(compilerDir);
+
+const ROOT = "/app";
+const FILENAME = "/app/src/api/users.ts";
+const RUNTIME = "@solidjs/web/server-functions";
+
+/** The extracted functions, as the production build emits them. */
+function compile(source) {
+  const result = transformDirectives(source, {
+    filename: FILENAME,
+    root: ROOT,
+    mode: "server",
+    env: "production",
+    directive: "use server",
+    register: { kind: "named", name: "registerServerReference", source: RUNTIME },
+    create: { kind: "named", name: "createServerReference", source: RUNTIME }
+  });
+  return result.functions;
+}
+
+/** name -> wire id (callers here never register the same name twice). */
+const idsOf = source => Object.fromEntries(compile(source).map(fn => [fn.name, fn.id]));
+
+const fn = (name, body = `return "${name}";`) =>
+  `export async function ${name}(x) { "use server"; ${body} }`;
+
+describe("server-function ids survive an edit to their file", () => {
+  const baseline = idsOf([fn("getUser"), fn("deleteUser")].join("\n"));
+
+  const edits = {
+    "a function appended at the end": [fn("getUser"), fn("deleteUser"), fn("archiveAccount")],
+    "a function inserted at the top": [fn("ping"), fn("getUser"), fn("deleteUser")],
+    "the first function deleted": [fn("deleteUser")],
+    "the two reordered": [fn("deleteUser"), fn("getUser")],
+    "a body edited": [fn("getUser", "return 42;"), fn("deleteUser")]
+  };
+
+  for (const [edit, source] of Object.entries(edits)) {
+    test(`${edit} moves no surviving id`, () => {
+      const after = idsOf(source.join("\n"));
+      for (const [name, id] of Object.entries(after)) {
+        if (name in baseline) expect(id).toBe(baseline[name]);
+      }
+    });
+  }
+
+  // The consequence of keying on the name, stated so it is a decision and
+  // not a surprise: a rename IS a new address, and an old client calling
+  // the old one gets a clean 404 (#3110) rather than another function.
+  test("a rename is a new address, and does not take over another", () => {
+    const after = idsOf([fn("fetchUser"), fn("deleteUser")].join("\n"));
+    expect(after.deleteUser).toBe(baseline.deleteUser);
+    expect(Object.values(after)).not.toContain(baseline.getUser);
+  });
+});
+
+describe("a name that recurs in one file", () => {
+  // The residual: among functions sharing a descriptive name, position
+  // still decides, so deleting the first renumbers the second. Pinned as
+  // known and accepted rather than left for someone to discover.
+  const source = [
+    `export const a = (() => { const handler = async () => { "use server"; return 1; }; return handler; })();`,
+    `export const b = (() => { const handler = async () => { "use server"; return 2; }; return handler; })();`
+  ].join("\n");
+
+  test("disambiguates with a trailing ordinal", () => {
+    const ids = compile(source).map(fn => fn.id);
+    expect(ids).toHaveLength(2);
+    expect(ids[0]).toMatch(/^handler-[0-9a-f]{1,8}$/);
+    expect(ids[1]).toMatch(/^handler-[0-9a-f]{1,8}-1$/);
+  });
+});