feat(web,server,core): attach files to a message from the composer (#121)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/core/test/input-images.test.ts

@@ -3,13 +3,25 @@
  * images -- data URL images are written to the session scratchpad and their paths appended to the
  * user text; http(s) URLs are referenced as-is; image messages are removed from the input;
  * image-free input is returned unchanged; images that fail to parse are replaced with an explanatory line.
+ *
+ * Plus appendAttachmentLines, the placement rule both attachment-line producers share (the
+ * images above and the server's `[attached file: …]` uploads).
  */
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { mkdtemp, readFile, readdir, rm } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import path from "node:path";
-import { imagesToScratchpadPaths } from "../src/internal/session-support.js";
-import { imageUrlMessage, userText } from "../src/omnimessage/index.js";
+import { appendAttachmentLines, imagesToScratchpadPaths } from "../src/internal/session-support.js";
+import {
+  buildHandoffMessage,
+  buildModelSwitchMessage,
+  buildScheduledMessage,
+  buildSkillsMessage,
+  imageUrlMessage,
+  parseHandoffMessage,
+  parseModelSwitchMessage,
+  userText,
+} from "../src/omnimessage/index.js";
 import type { TextPayload } from "../src/omnimessage/index.js";
 
 const PNG_1X1 = Buffer.from(
@@ -80,3 +92,71 @@ describe("imagesToScratchpadPaths", () => {
     expect(p.text).toContain("could not be saved");
   });
 });
+
+/** Text of the message at `index` of an appendAttachmentLines result. */
+const textAt = (out: ReturnType<typeof appendAttachmentLines>, index: number): string =>
+  (out[index]!.payload as TextPayload).text;
+
+describe("appendAttachmentLines placement", () => {
+  const LINE = "[attached file: /d/scratchpad/s1/report.pdf]";
+
+  it("appends to the last user text message", () => {
+    const out = appendAttachmentLines([userText("first"), userText("second")], [LINE]);
+    expect(out).toHaveLength(2);
+    expect(textAt(out, 0)).toBe("first");
+    expect(textAt(out, 1)).toBe(`second\n\n${LINE}`);
+  });
+
+  it("no user text at all: the lines become a message of their own", () => {
+    const out = appendAttachmentLines([], [LINE]);
+    expect(out).toHaveLength(1);
+    expect(textAt(out, 0)).toBe(LINE);
+  });
+
+  it("a whole-message origin block is left alone; the lines get their own message", () => {
+    // The composer's files-only-plus-staged-handoff shape: the only text message is the origin
+    // block, and both of these parsers require the block to be the WHOLE message — appending to
+    // it would render the raw marker in a user bubble instead of a one-line banner.
+    for (const block of [
+      buildHandoffMessage({ agentId: "alpha", agentName: "Alpha", sessionId: "s0" }),
+      buildModelSwitchMessage({ sessionId: "s0", prevModelId: "m0" }),
+    ]) {
+      const out = appendAttachmentLines([userText(block)], [LINE]);
+      expect(out).toHaveLength(2);
+      expect(textAt(out, 0)).toBe(block);
+      expect(textAt(out, 1)).toBe(LINE);
+    }
+    // …and the blocks still parse afterwards, which is the property that actually matters.
+    const handoff = appendAttachmentLines(
+      [userText(buildHandoffMessage({ agentId: "alpha", sessionId: "s0" }))],
+      [LINE],
+    );
+    expect(parseHandoffMessage(textAt(handoff, 0))?.agentId).toBe("alpha");
+    const switched = appendAttachmentLines(
+      [userText(buildModelSwitchMessage({ sessionId: "s0" }))],
+      [LINE],
+    );
+    expect(parseModelSwitchMessage(textAt(switched, 0))?.sessionId).toBe("s0");
+  });
+
+  it("prefix blocks still take the lines after their body ([use_skills] / [scheduled_task])", () => {
+    // The other direction: these two are parsed at index 0 and keep the body that follows, so a
+    // trailing line is an ordinary footnote to the message and must NOT start a new one.
+    for (const message of [
+      buildSkillsMessage(["web-design"], "fix the layout"),
+      buildScheduledMessage("nightly", "2026-07-29T02:00:00Z", "run the report"),
+    ]) {
+      const out = appendAttachmentLines([userText(message)], [LINE]);
+      expect(out).toHaveLength(1);
+      expect(textAt(out, 0)).toBe(`${message}\n\n${LINE}`);
+    }
+  });
+
+  it("an earlier ordinary message wins over a trailing origin block", () => {
+    const block = buildHandoffMessage({ agentId: "alpha" });
+    const out = appendAttachmentLines([userText("hello"), userText(block)], [LINE]);
+    expect(out).toHaveLength(2);
+    expect(textAt(out, 0)).toBe(`hello\n\n${LINE}`);
+    expect(textAt(out, 1)).toBe(block);
+  });
+});