diff --git a/server/routes/mcp/index.test.ts b/server/routes/mcp/index.test.ts
index b8793575be..f75becd668 100644
--- a/server/routes/mcp/index.test.ts
+++ b/server/routes/mcp/index.test.ts
@@ -1,5 +1,6 @@
 import { Scope, TeamPreference } from "@shared/types";
 import type { ProsemirrorData } from "@shared/types";
+import { Attachment } from "@server/models";
 import {
   buildUser,
   buildAdmin,
@@ -823,6 +824,92 @@ describe("POST /mcp/", () => {
     });
   });
 
+  describe("attachment tools", () => {
+    it("create_attachment returns absolute uploadUrl and proxied attachment url", async () => {
+      const { accessToken } = await buildOAuthUser();
+      const res = await callMcpTool(server, accessToken, "create_attachment", {
+        contentType: "image/png",
+        name: "test.png",
+        size: 1000,
+      });
+
+      expect(res?.result?.isError).toBeFalsy();
+      const data = JSON.parse(res?.result?.content?.[0]?.text ?? "{}");
+
+      expect(data.uploadUrl).toMatch(/^https?:\/\//);
+      expect(data.attachment.url).toMatch(/^https?:\/\//);
+      expect(data.attachment.url).toContain("/api/attachments.redirect?id=");
+      expect(data.curlCommand).toContain(data.uploadUrl);
+    });
+
+    it("create_attachment persists attachment record", async () => {
+      const { user, accessToken } = await buildOAuthUser();
+      const res = await callMcpTool(server, accessToken, "create_attachment", {
+        contentType: "image/png",
+        name: "test.png",
+        size: 1000,
+      });
+
+      const data = JSON.parse(res?.result?.content?.[0]?.text ?? "{}");
+      const attachment = await Attachment.findByPk(data.attachment.id, {
+        rejectOnEmpty: true,
+      });
+      expect(Number(attachment.size)).toEqual(1000);
+      expect(attachment.contentType).toEqual("image/png");
+      expect(attachment.userId).toEqual(user.id);
+      expect(attachment.teamId).toEqual(user.teamId);
+    });
+
+    it("create_attachment rejects size larger than max", async () => {
+      const { accessToken } = await buildOAuthUser();
+      const res = await callMcpTool(server, accessToken, "create_attachment", {
+        contentType: "image/png",
+        name: "huge.png",
+        size: 10_000_000_000,
+      });
+      expect(res?.result?.isError).toBe(true);
+    });
+
+    it("create_attachment rejects negative size", async () => {
+      const { accessToken } = await buildOAuthUser();
+      const res = await callMcpTool(server, accessToken, "create_attachment", {
+        contentType: "image/png",
+        name: "neg.png",
+        size: -1,
+      });
+      expect(res?.error ?? res?.result?.isError).toBeTruthy();
+    });
+
+    it("create_attachment rejects fractional size", async () => {
+      const { accessToken } = await buildOAuthUser();
+      const res = await callMcpTool(server, accessToken, "create_attachment", {
+        contentType: "image/png",
+        name: "frac.png",
+        size: 1.5,
+      });
+      expect(res?.error ?? res?.result?.isError).toBeTruthy();
+    });
+
+    it("read-only token does not have create_attachment tool", async () => {
+      const user = await buildUser();
+      const auth = await buildOAuthAuthentication({
+        user,
+        scope: [Scope.Read],
+      });
+      const res = await callMcpTool(
+        server,
+        auth.accessToken!,
+        "create_attachment",
+        {
+          contentType: "image/png",
+          name: "test.png",
+          size: 1000,
+        }
+      );
+      expect(res?.result?.isError).toBe(true);
+    });
+  });
+
   describe("scope enforcement", () => {
     async function buildScopedOAuthUser(scope: Scope[]) {
       const user = await buildUser();
diff --git a/server/tools/attachments.ts b/server/tools/attachments.ts
index c146d125a5..ab23a802dd 100644
--- a/server/tools/attachments.ts
+++ b/server/tools/attachments.ts
@@ -1,6 +1,7 @@
 import { randomUUID } from "crypto";
 import { z } from "zod";
 import { type McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import { ValidationError } from "@server/errors";
 import { Attachment, Team } from "@server/models";
 import AttachmentHelper from "@server/models/helpers/AttachmentHelper";
 import { authorize } from "@server/policies";
@@ -8,7 +9,14 @@ import presentAttachment from "@server/presenters/attachment";
 import FileStorage from "@server/storage/files";
 import AuthenticationHelper from "@shared/helpers/AuthenticationHelper";
 import { AttachmentPreset } from "@shared/types";
-import { error, success, buildAPIContext, withTracing } from "./util";
+import { bytesToHumanReadable } from "@shared/utils/files";
+import {
+  error,
+  success,
+  buildAPIContext,
+  pathToUrl,
+  withTracing,
+} from "./util";
 
 /**
  * Registers attachment-related MCP tools on the given server, filtered by
@@ -36,7 +44,12 @@ export function attachmentTools(server: McpServer, scopes: string[]) {
           name: z
             .string()
             .describe("The filename including extension, e.g. screenshot.png."),
-          size: z.coerce.number().describe("The file size in bytes."),
+          size: z.coerce
+            .number()
+            .int()
+            .nonnegative()
+            .finite()
+            .describe("The file size in bytes."),
         },
       },
       withTracing(
@@ -53,6 +66,15 @@ export function attachmentTools(server: McpServer, scopes: string[]) {
             const preset = AttachmentPreset.DocumentAttachment;
             const maxUploadSize =
               AttachmentHelper.presetToMaxUploadSize(preset);
+
+            if (size > maxUploadSize) {
+              throw ValidationError(
+                `Sorry, this file is too large – the maximum size is ${bytesToHumanReadable(
+                  maxUploadSize
+                )}`
+              );
+            }
+
             const id = randomUUID();
             const acl = AttachmentHelper.presetToAcl(preset);
             const key = AttachmentHelper.getKey({
@@ -79,7 +101,8 @@ export function attachmentTools(server: McpServer, scopes: string[]) {
               contentType
             );
 
-            const uploadUrl = FileStorage.getUploadUrl();
+            const uploadUrl = new URL(FileStorage.getUploadUrl(), team.url)
+              .href;
             const form = {
               "Cache-Control": "max-age=31557600",
               "Content-Type": contentType,
@@ -97,10 +120,10 @@ export function attachmentTools(server: McpServer, scopes: string[]) {
               form,
               maxUploadSize,
               curlCommand,
-              attachment: {
+              attachment: pathToUrl(team, {
                 ...presentAttachment(attachment),
                 url: attachment.redirectUrl,
-              },
+              }),
             });
           } catch (message) {
             return error(message);
diff --git a/server/tools/util.ts b/server/tools/util.ts
index a18cc7f6af..5350068550 100644
--- a/server/tools/util.ts
+++ b/server/tools/util.ts
@@ -42,6 +42,7 @@ export function buildAPIContext(context: McpContext) {
   return {
     state: { auth },
     context: { auth, ip },
+    cookies: { get: () => undefined, set: () => undefined },
   } as unknown as APIContext;
 }