diff --git a/apps/web/tests/collections/collections-origin.test.ts b/apps/web/tests/collections/collections-origin.test.ts
new file mode 100644
index 0000000..1ba1368
--- /dev/null
+++ b/apps/web/tests/collections/collections-origin.test.ts
@@ -0,0 +1,58 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { call, create, h, ownerKey } from "./collections-harness";
+
+/**
+ * A collection write in a web session (no `Authorization`) rides the ambient
+ * cookie, so it must come from this origin; an API key is never ambient and is
+ * not checked.
+ */
+const session = vi.hoisted(() => ({ user: null as { id: string } | null }));
+vi.mock("~/lib/auth/session-helpers", () => ({
+  getSession: async () => (session.user === null ? null : { user: session.user }),
+}));
+
+beforeEach(() => {
+  session.user = null;
+});
+
+const count = () => h.d1.raw.prepare("SELECT COUNT(*) AS n FROM collections").get() as { n: number };
+
+describe("collections: same-origin web sessions", () => {
+  it("refuses a cross-origin web-session write on every write method, writing nothing", async () => {
+    session.user = { id: "user-a" };
+    for (const headers of [{ Origin: "https://evil.example" }, { Origin: "https://sub.skillx.test" }, { "Sec-Fetch-Site": "same-site" }]) {
+      const refused = await create({}, { key: null, headers });
+      expect(refused.status).toBe(403);
+      expect(refused.body.error.code).toBe("cross_origin");
+    }
+    expect(count().n).toBe(0);
+
+    const { id } = (await create({}, { key: ownerKey })).body.collection;
+    const foreign = { key: null, headers: { Origin: "https://evil.example" } };
+    for (const [path, body, method] of [
+      [`/api/v1/collections/${id}`, { name: "x" }, "PATCH"],
+      [`/api/v1/collections/${id}`, {}, "DELETE"],
+      [`/api/v1/collections/${id}/revisions`, { expectedRevision: 1, members: [] }, "POST"],
+      [`/api/v1/collections/${id}/members`, { expectedRevision: 1, remove: ["@alpha/demo"] }, "POST"],
+      [`/api/v1/collections/${id}/clone`, { slug: "copy" }, "POST"],
+    ] as const) {
+      const refused = await call(path, body, { ...foreign, method });
+      expect(refused.status).toBe(403);
+      expect(refused.body.error.code).toBe("cross_origin");
+    }
+    const after = await call(`/api/v1/collections/${id}`, undefined, { key: ownerKey });
+    expect(after.body.collection).toMatchObject({ name: "Starter", latestRevisionNumber: 1 });
+    expect(count().n).toBe(1);
+  });
+
+  it("accepts a same-origin or header-less web-session write", async () => {
+    session.user = { id: "user-a" };
+    expect((await create({}, { key: null, headers: { Origin: "https://skillx.test" } })).status).toBe(201);
+    expect((await create({ slug: "second" }, { key: null, headers: { "Sec-Fetch-Site": "same-origin" } })).status).toBe(201);
+    expect((await create({ slug: "third" }, { key: null })).status).toBe(201);
+  });
+
+  it("does not check the origin of an API-key write", async () => {
+    expect((await create({}, { headers: { Origin: "https://evil.example" } })).status).toBe(201);
+  });
+});
diff --git a/apps/web/tests/personal-kits/kit-origin.test.ts b/apps/web/tests/personal-kits/kit-origin.test.ts
new file mode 100644
index 0000000..ab3ce4f
--- /dev/null
+++ b/apps/web/tests/personal-kits/kit-origin.test.ts
@@ -0,0 +1,56 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { BODY, createKitHarness, skillText } from "./kit-harness";
+import type { KitHarness } from "./kit-harness";
+
+/**
+ * A kit write in a web session (no `Authorization`) rides the ambient cookie,
+ * so it must come from this origin; an API key is never ambient and is not checked.
+ */
+const session = vi.hoisted(() => ({ user: null as { id: string } | null }));
+vi.mock("~/lib/auth/session-helpers", () => ({
+  getSession: async () => (session.user === null ? null : { user: session.user }),
+}));
+
+let k: KitHarness;
+const MINE = { "SKILL.md": skillText(BODY.replace("two\n", "two (mine)\n")), "notes.md": "my notes\n" };
+
+beforeEach(async () => {
+  session.user = null;
+  k = await createKitHarness();
+  await k.publish("@alpha/demo", "1.0.0", { "SKILL.md": skillText(BODY) });
+});
+
+describe("personal kits: same-origin web sessions", () => {
+  it("refuses a cross-origin web-session write, then accepts the same write from this origin", async () => {
+    session.user = { id: "user-a" };
+    const write = async (headers: Record<string, string>) =>
+      k.call("revisions", { expectedGeneration: 0, idempotencyKey: k.idem(), document: k.doc([await k.entry("@alpha/demo", "1.0.0", MINE)]) }, { key: null, headers });
+    for (const headers of [{ Origin: "https://evil.example" }, { "Sec-Fetch-Site": "same-site" }]) {
+      const refused = await write(headers);
+      expect(refused.status).toBe(403);
+      expect(refused.body.error.code).toBe("cross_origin");
+    }
+    // Nothing was written: generation 0 is still the current one.
+    expect((await write({ Origin: "https://skillx.test" })).status).toBe(201);
+  });
+
+  it("refuses a cross-origin web-session write on every write sub-route", async () => {
+    session.user = { id: "user-a" };
+    for (const sub of ["proposals", "proposals/validate", "preview-update", "activate", "rollback", "rebase", "conflicts/c-1/resolve", "revisions/rev-1/reject", "revisions/rev-1/review", "revisions/rev-1/quarantine"]) {
+      const refused = await k.call(sub, {}, { key: null, headers: { Origin: "https://evil.example" } });
+      expect(refused.status, sub).toBe(403);
+      expect(refused.body.error.code, sub).toBe("cross_origin");
+    }
+  });
+
+  it("accepts a header-less web-session write", async () => {
+    session.user = { id: "user-a" };
+    const body = { expectedGeneration: 0, idempotencyKey: k.idem(), document: k.doc([await k.entry("@alpha/demo", "1.0.0", MINE)]) };
+    expect((await k.call("revisions", body, { key: null })).status).toBe(201);
+  });
+
+  it("does not check the origin of an API-key write", async () => {
+    const body = { expectedGeneration: 0, idempotencyKey: k.idem(), document: k.doc([await k.entry("@alpha/demo", "1.0.0", MINE)]) };
+    expect((await k.call("revisions", body, { headers: { Origin: "https://evil.example" } })).status).toBe(201);
+  });
+});
diff --git a/apps/web/tests/personal-kits/kit-revisions.test.ts b/apps/web/tests/personal-kits/kit-revisions.test.ts
index b7add9d..c11c7f1 100644
--- a/apps/web/tests/personal-kits/kit-revisions.test.ts
+++ b/apps/web/tests/personal-kits/kit-revisions.test.ts
@@ -75,19 +75,6 @@ describe("personal kit revisions", () => {
     expect(lock.body.documents).toHaveLength(1);
   });
 
-  it("refuses a cross-origin web-session write, then accepts the same write from this origin", async () => {
-    session.user = { id: "user-a" };
-    const write = async (headers: Record<string, string>) =>
-      k.call("revisions", { expectedGeneration: 0, idempotencyKey: k.idem(), document: k.doc([await k.entry("@alpha/demo", "1.0.0", MINE)]) }, { key: null, headers });
-    for (const headers of [{ Origin: "https://evil.example" }, { "Sec-Fetch-Site": "same-site" }]) {
-      const refused = await write(headers);
-      expect(refused.status).toBe(403);
-      expect(refused.body.error.code).toBe("cross_origin");
-    }
-    // Nothing was written: generation 0 is still the current one.
-    expect((await write({ Origin: "https://skillx.test" })).status).toBe(201);
-  });
-
   it("refuses a client compositionDigest that the pinned bytes do not give, writing nothing", async () => {
     const bad = { ...(await k.entry("@alpha/demo", "1.0.0", MINE)), compositionDigest: `sha256:${"1".repeat(64)}` };
     const res = await k.call("revisions", { expectedGeneration: 0, idempotencyKey: k.idem(), document: k.doc([bad]) });