diff --git a/src/panel/message-renderer.ts b/src/panel/message-renderer.ts
index 55fe2a9b..484fbc4f 100644
--- a/src/panel/message-renderer.ts
+++ b/src/panel/message-renderer.ts
@@ -3,7 +3,7 @@ import { MarkdownRenderer, type App, type Component } from "obsidian";
 import type { DisplayItem } from "../display/types";
 import { copyTextWithNotice } from "../ui/clipboard";
 import { renderTextWithWikiLinks as renderInlineWikiLinks } from "../ui/dom";
-import { messageRenderBlocks, syncMessageRenderBlocks } from "../ui/message-stream";
+import { messageRenderBlocks, notifyMessageContentRendered, syncMessageRenderBlocks } from "../ui/message-stream";
 import { bottomScrollTop, captureScrollAnchor, isNearScrollBottom, restoreScrollAnchor } from "../ui/scroll";
 import type { TurnDiffViewState } from "../ui/turn-diff";
 import { markdownFileLinkTarget } from "./markdown-file-links";
@@ -91,6 +91,7 @@ export class PanelMessageRenderer {
     void MarkdownRenderer.render(this.options.app, text, parent, sourcePath, this.options.owner).then(() => {
       this.bindRenderedWikiLinks(parent, sourcePath);
       this.bindRenderedMarkdownFileLinks(parent, sourcePath);
+      notifyMessageContentRendered(parent);
       this.scrollMarkdownMessageIntoPinnedBottom(parent);
     });
   }
diff --git a/src/ui/message-stream.ts b/src/ui/message-stream.ts
index 8f455216..a714e690 100644
--- a/src/ui/message-stream.ts
+++ b/src/ui/message-stream.ts
@@ -15,6 +15,9 @@ import {
 } from "./work-items";
 import type { TurnDiffViewState } from "./turn-diff";
 
+const USER_MESSAGE_COLLAPSE_HEIGHT_PX = 360;
+const MESSAGE_CONTENT_RENDERED_EVENT = "codex-panel:message-content-rendered";
+
 export interface MessageRenderBlock {
   key: string;
   signature: string;
@@ -226,12 +229,17 @@ function renderDisplayItem(parent: HTMLElement, item: DisplayItem, context: Mess
   if (context.canRollbackItem?.(item)) {
     renderMessageAction(role, "undo-2", "Rollback last turn", "codex-panel__rollback-turn", () => context.onRollbackItem?.(item));
   }
-  const content = messageEl.createDiv({ cls: `codex-panel__message-content ${item.markdown === false ? "" : "markdown-rendered"}` });
+  const collapsible = isCollapsibleUserMessage(item);
+  const contentParent = collapsible ? messageEl.createDiv({ cls: "codex-panel__message-collapse" }) : messageEl;
+  const content = contentParent.createDiv({ cls: `codex-panel__message-content ${item.markdown === false ? "" : "markdown-rendered"}` });
   if (item.markdown === false) {
     context.renderTextWithWikiLinks(content, item.text);
   } else {
     context.renderMarkdown(content, item.text);
   }
+  if (collapsible) {
+    renderUserMessageCollapse(contentParent, content, item.id, context);
+  }
   if (item.kind === "message" && item.editedFiles && item.editedFiles.length > 0) {
     renderEditedFiles(messageEl, item, context);
   }
@@ -251,6 +259,43 @@ function renderDisplayItem(parent: HTMLElement, item: DisplayItem, context: Mess
   }
 }
 
+function isCollapsibleUserMessage(item: DisplayItem): boolean {
+  return item.kind === "message" && item.role === "user";
+}
+
+function renderUserMessageCollapse(parent: HTMLElement, content: HTMLElement, itemId: string, context: MessageStreamContext): void {
+  const key = `message:${itemId}:expanded`;
+  const details = parent.createEl("details", { cls: "codex-panel__message-collapse-details" });
+  details.createEl("summary", { text: "Show more" });
+
+  const update = () => {
+    const overflows = content.scrollHeight > userMessageCollapseHeight(content) + 1;
+    const expanded = context.openDetails.has(key);
+    parent.classList.toggle("codex-panel__message-collapse--overflow", overflows);
+    parent.classList.toggle("codex-panel__message-collapse--expanded", overflows && expanded);
+    content.classList.toggle("codex-panel__message-content--collapsed", overflows && !expanded);
+    details.hidden = !overflows || expanded;
+  };
+
+  details.ontoggle = () => {
+    if (!details.open) return;
+    details.open = false;
+    context.openDetails.add(key);
+    update();
+    context.onDetailsToggle?.();
+  };
+
+  content.addEventListener(MESSAGE_CONTENT_RENDERED_EVENT, update);
+  update();
+  content.win.requestAnimationFrame(update);
+}
+
+function userMessageCollapseHeight(element: HTMLElement): number {
+  const viewportHeight = element.win.innerHeight;
+  if (viewportHeight <= 0) return USER_MESSAGE_COLLAPSE_HEIGHT_PX;
+  return Math.min(USER_MESSAGE_COLLAPSE_HEIGHT_PX, viewportHeight * 0.45);
+}
+
 function renderMessageAction(parent: HTMLElement, icon: string, label: string, className: string, onClick: () => void): HTMLButtonElement {
   const button = createIconButton(parent, icon, label, `codex-panel__message-action ${className}`);
   button.onclick = (event) => {
@@ -303,6 +348,10 @@ function renderEditedFiles(parent: HTMLElement, item: Extract<DisplayItem, { kin
   }
 }
 
+export function notifyMessageContentRendered(element: HTMLElement): void {
+  element.dispatchEvent(new Event(MESSAGE_CONTENT_RENDERED_EVENT));
+}
+
 function renderMentionedFiles(parent: HTMLElement, item: Extract<DisplayItem, { kind: "message" }>, context: MessageStreamContext): void {
   const mentionedFiles = item.mentionedFiles ?? [];
   const label = mentionedFiles.length === 1 ? "Mentioned 1 file" : `Mentioned ${mentionedFiles.length} files`;
diff --git a/styles.css b/styles.css
index 28c9600b..25ee7f47 100644
--- a/styles.css
+++ b/styles.css
@@ -27,6 +27,7 @@
   --codex-panel-toolbar-padding-y: var(--size-4-2, 8px);
   --codex-panel-toolbar-button-gap: var(--size-2-1, 2px);
   --codex-panel-message-padding-y: var(--size-4-3, 12px);
+  --codex-panel-message-collapse-height: min(360px, 45vh);
   --codex-panel-composer-padding-y: var(--size-4-2, 8px);
   --codex-panel-section-padding: var(--codex-panel-edge-padding-x);
   --codex-panel-section-gap: var(--size-4-2, 8px);
@@ -1010,6 +1011,43 @@
   white-space: pre-wrap;
 }
 
+.codex-panel__message-collapse {
+  position: relative;
+}
+
+.codex-panel__message-content--collapsed {
+  position: relative;
+  max-height: var(--codex-panel-message-collapse-height);
+  overflow: hidden;
+}
+
+.codex-panel__message-content--collapsed::after {
+  content: "";
+  position: absolute;
+  right: 0;
+  bottom: 0;
+  left: 0;
+  height: 72px;
+  background: linear-gradient(to bottom, transparent, var(--background-primary));
+  pointer-events: none;
+}
+
+.codex-panel__message-collapse-details {
+  position: relative;
+  z-index: 1;
+  margin-top: var(--size-2-3, 6px);
+}
+
+.codex-panel__message-collapse-details > summary {
+  cursor: pointer;
+  color: var(--text-muted);
+  font-size: var(--font-ui-smaller);
+}
+
+.codex-panel__message-collapse-details > summary:hover {
+  color: var(--text-normal);
+}
+
 .codex-panel__edited-files,
 .codex-panel__referenced-thread,
 .codex-panel__mentioned-files,
diff --git a/tests/ui/view-renderers.test.ts b/tests/ui/view-renderers.test.ts
index 1c92d9fe..30e33ddb 100644
--- a/tests/ui/view-renderers.test.ts
+++ b/tests/ui/view-renderers.test.ts
@@ -21,6 +21,25 @@ import { composerSuggestionScrollFixture, installObsidianDomShims, topLevelDetai
 
 installObsidianDomShims();
 
+function withMessageContentScrollHeight<T>(scrollHeight: number, fn: () => T): T {
+  const descriptor = Object.getOwnPropertyDescriptor(HTMLElement.prototype, "scrollHeight");
+  Object.defineProperty(HTMLElement.prototype, "scrollHeight", {
+    configurable: true,
+    get() {
+      return this.classList.contains("codex-panel__message-content") ? scrollHeight : 0;
+    },
+  });
+  try {
+    return fn();
+  } finally {
+    if (descriptor) {
+      Object.defineProperty(HTMLElement.prototype, "scrollHeight", descriptor);
+    } else {
+      Reflect.deleteProperty(HTMLElement.prototype, "scrollHeight");
+    }
+  }
+}
+
 describe("message stream block identity and message actions", () => {
   it("invalidates message blocks when markdown rendering mode changes", () => {
     const context = { busy: false, activeTurnId: null, displayItems: [] };
@@ -526,6 +545,86 @@ describe("message stream block identity and message actions", () => {
     expect(onRollbackItem).toHaveBeenCalledWith(expect.objectContaining({ id: "u1" }));
   });
 
+  it("collapses tall user messages without changing the copy payload", () => {
+    withMessageContentScrollHeight(500, () => {
+      const copyText = vi.fn();
+      const openDetails = new Set<string>();
+      const onDetailsToggle = vi.fn();
+      const block = messageRenderBlocks({
+        activeThreadId: "thread",
+        activeTurnId: null,
+        historyCursor: null,
+        loadingHistory: false,
+        busy: false,
+        displayItems: [
+          { id: "u1", kind: "message", role: "user", text: "visible text", copyText: "full copied text", turnId: "turn-1", markdown: true },
+        ],
+        openDetails,
+        onDetailsToggle,
+        loadOlderTurns: vi.fn(),
+        renderMarkdown: (parent, text) => parent.createDiv({ text }),
+        renderTextWithWikiLinks: (parent, text) => parent.createDiv({ text }),
+        copyText,
+      })[0];
+
+      const element = block.render();
+      const content = element.querySelector<HTMLElement>(".codex-panel__message-content");
+      const details = element.querySelector<HTMLDetailsElement>(".codex-panel__message-collapse-details");
+
+      expect(content?.classList.contains("codex-panel__message-content--collapsed")).toBe(true);
+      expect(details?.hidden).toBe(false);
+      expect(details?.querySelector("summary")?.textContent).toBe("Show more");
+
+      if (details) {
+        details.open = true;
+        details.dispatchEvent(new Event("toggle"));
+      }
+      expect(openDetails.has("message:u1:expanded")).toBe(true);
+      expect(content?.classList.contains("codex-panel__message-content--collapsed")).toBe(false);
+      expect(details?.hidden).toBe(true);
+      expect(onDetailsToggle).toHaveBeenCalled();
+
+      element.querySelector<HTMLButtonElement>(".codex-panel__copy-message")?.click();
+      expect(copyText).toHaveBeenCalledWith("full copied text");
+    });
+  });
+
+  it("does not show the collapse control for short user messages or assistant messages", () => {
+    withMessageContentScrollHeight(120, () => {
+      const shortUser = messageRenderBlocks({
+        activeThreadId: "thread",
+        activeTurnId: null,
+        historyCursor: null,
+        loadingHistory: false,
+        busy: false,
+        displayItems: [{ id: "u1", kind: "message", role: "user", text: "short", turnId: "turn-1", markdown: true }],
+        openDetails: new Set(),
+        loadOlderTurns: vi.fn(),
+        renderMarkdown: (parent, text) => parent.createDiv({ text }),
+        renderTextWithWikiLinks: (parent, text) => parent.createDiv({ text }),
+      })[0].render();
+
+      expect(shortUser.querySelector<HTMLDetailsElement>(".codex-panel__message-collapse-details")?.hidden).toBe(true);
+    });
+
+    withMessageContentScrollHeight(500, () => {
+      const assistant = messageRenderBlocks({
+        activeThreadId: "thread",
+        activeTurnId: null,
+        historyCursor: null,
+        loadingHistory: false,
+        busy: false,
+        displayItems: [{ id: "a1", kind: "message", role: "assistant", text: "long", turnId: "turn-1", markdown: true }],
+        openDetails: new Set(),
+        loadOlderTurns: vi.fn(),
+        renderMarkdown: (parent, text) => parent.createDiv({ text }),
+        renderTextWithWikiLinks: (parent, text) => parent.createDiv({ text }),
+      })[0].render();
+
+      expect(assistant.querySelector(".codex-panel__message-collapse-details")).toBeNull();
+    });
+  });
+
   it("does not render rollback action when no item is eligible", () => {
     const block = messageRenderBlocks({
       activeThreadId: "thread",