diff --git a/ui/src/ui/chat-markdown.browser.test.ts b/ui/src/ui/chat-markdown.browser.test.ts
index 17a898bac4c..a3499539f79 100644
--- a/ui/src/ui/chat-markdown.browser.test.ts
+++ b/ui/src/ui/chat-markdown.browser.test.ts
@@ -31,7 +31,42 @@ describe("chat markdown rendering", () => {
 
     await app.updateComplete;
 
-    const strong = app.querySelector(".sidebar-markdown strong");
-    expect(strong?.textContent).toBe("world");
+    const strongNodes = Array.from(app.querySelectorAll(".sidebar-markdown strong"));
+    expect(strongNodes.map((node) => node.textContent)).toContain("world");
+  });
+
+  it("shows tool call request parameters in the sidebar", async () => {
+    const app = mountApp("/chat");
+    await app.updateComplete;
+
+    const timestamp = Date.now();
+    app.chatMessages = [
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "toolcall",
+            name: "sessions_spawn",
+            arguments: { agentId: "research", prompt: "hello" },
+          },
+        ],
+        timestamp,
+      },
+    ];
+
+    await app.updateComplete;
+
+    const toolCard = app.querySelector<HTMLElement>(".chat-tool-card");
+    expect(toolCard).not.toBeNull();
+    toolCard?.click();
+
+    await app.updateComplete;
+
+    const sidebar = app.querySelector(".sidebar-markdown");
+    expect(sidebar?.textContent).toContain("Arguments");
+    expect(sidebar?.textContent).toContain("agentId");
+    expect(sidebar?.textContent).toContain("research");
+    expect(sidebar?.textContent).toContain("prompt");
+    expect(sidebar?.textContent).toContain("hello");
   });
 });
diff --git a/ui/src/ui/chat/tool-cards.ts b/ui/src/ui/chat/tool-cards.ts
index acd427b9e77..d5975a16102 100644
--- a/ui/src/ui/chat/tool-cards.ts
+++ b/ui/src/ui/chat/tool-cards.ts
@@ -5,7 +5,7 @@ import type { ToolCard } from "../types/chat-types.ts";
 import { TOOL_INLINE_THRESHOLD } from "./constants.ts";
 import { extractTextCached } from "./message-extract.ts";
 import { isToolResultMessage } from "./message-normalizer.ts";
-import { formatToolOutputForSidebar, getTruncatedPreview } from "./tool-helpers.ts";
+import { buildToolSidebarContent, getTruncatedPreview } from "./tool-helpers.ts";
 
 export function extractToolCards(message: unknown): ToolCard[] {
   const m = message as Record<string, unknown>;
@@ -56,14 +56,14 @@ export function renderToolCardSidebar(card: ToolCard, onOpenSidebar?: (content:
   const canClick = Boolean(onOpenSidebar);
   const handleClick = canClick
     ? () => {
-        if (hasText) {
-          onOpenSidebar!(formatToolOutputForSidebar(card.text!));
-          return;
-        }
-        const info = `## ${display.label}\n\n${
-          detail ? `**Command:** \`${detail}\`\n\n` : ""
-        }*No output — tool completed successfully.*`;
-        onOpenSidebar!(info);
+        onOpenSidebar!(
+          buildToolSidebarContent({
+            title: display.label,
+            detail,
+            args: card.args,
+            output: card.text,
+          }),
+        );
       }
     : undefined;
 
@@ -95,11 +95,7 @@ export function renderToolCardSidebar(card: ToolCard, onOpenSidebar?: (content:
           <span class="chat-tool-card__icon">${icons[display.icon]}</span>
           <span>${display.label}</span>
         </div>
-        ${
-          canClick
-            ? html`<span class="chat-tool-card__action">${hasText ? "View" : ""} ${icons.check}</span>`
-            : nothing
-        }
+        ${canClick ? html`<span class="chat-tool-card__action">View ${icons.check}</span>` : nothing}
         ${isEmpty && !canClick ? html`<span class="chat-tool-card__status">${icons.check}</span>` : nothing}
       </div>
       ${detail ? html`<div class="chat-tool-card__detail">${detail}</div>` : nothing}
diff --git a/ui/src/ui/chat/tool-helpers.test.ts b/ui/src/ui/chat/tool-helpers.test.ts
index f18cd738a7f..18fdc0d72a4 100644
--- a/ui/src/ui/chat/tool-helpers.test.ts
+++ b/ui/src/ui/chat/tool-helpers.test.ts
@@ -1,5 +1,10 @@
 import { describe, it, expect } from "vitest";
-import { formatToolOutputForSidebar, getTruncatedPreview } from "./tool-helpers.ts";
+import {
+  buildToolSidebarContent,
+  formatToolOutputForSidebar,
+  formatToolPayloadForSidebar,
+  getTruncatedPreview,
+} from "./tool-helpers.ts";
 
 describe("tool-helpers", () => {
   describe("formatToolOutputForSidebar", () => {
@@ -138,4 +143,54 @@ describe("tool-helpers", () => {
       expect(result.endsWith("…")).toBe(true);
     });
   });
+
+  describe("formatToolPayloadForSidebar", () => {
+    it("formats structured arguments as JSON", () => {
+      const result = formatToolPayloadForSidebar({ agentId: "research", count: 2 });
+
+      expect(result).toBe(`\`\`\`json
+{
+  "agentId": "research",
+  "count": 2
+}
+\`\`\``);
+    });
+
+    it("returns null for undefined arguments", () => {
+      expect(formatToolPayloadForSidebar(undefined)).toBeNull();
+    });
+
+    it("returns null for null arguments", () => {
+      expect(formatToolPayloadForSidebar(null)).toBeNull();
+    });
+  });
+
+  describe("buildToolSidebarContent", () => {
+    it("includes arguments and output when both are present", () => {
+      const result = buildToolSidebarContent({
+        title: "Session Spawn",
+        detail: "sessions_spawn research",
+        args: { agentId: "research" },
+        output: '{"status":"ok"}',
+      });
+
+      expect(result).toContain("## Session Spawn");
+      expect(result).toContain("**Command:** `sessions_spawn research`");
+      expect(result).toContain("**Arguments**");
+      expect(result).toContain('"agentId": "research"');
+      expect(result).toContain("**Output**");
+      expect(result).toContain('"status": "ok"');
+    });
+
+    it("shows completion text when output is absent", () => {
+      const result = buildToolSidebarContent({
+        title: "Session Spawn",
+        args: { agentId: "research" },
+      });
+
+      expect(result).toContain("**Arguments**");
+      expect(result).toContain('"agentId": "research"');
+      expect(result).toContain("*No output - tool completed successfully.*");
+    });
+  });
 });
diff --git a/ui/src/ui/chat/tool-helpers.ts b/ui/src/ui/chat/tool-helpers.ts
index 322b6058f6a..0ceb6351c6b 100644
--- a/ui/src/ui/chat/tool-helpers.ts
+++ b/ui/src/ui/chat/tool-helpers.ts
@@ -22,6 +22,46 @@ export function formatToolOutputForSidebar(text: string): string {
   return text;
 }
 
+/**
+ * Format tool-call input/arguments for display in the sidebar.
+ * Uses JSON code blocks for structured values and preserves plain text input.
+ */
+export function formatToolPayloadForSidebar(value: unknown): string | null {
+  if (value === undefined || value === null) {
+    return null;
+  }
+  if (typeof value === "string") {
+    return formatToolOutputForSidebar(value);
+  }
+  try {
+    return "```json\n" + JSON.stringify(value, null, 2) + "\n```";
+  } catch {
+    return String(value);
+  }
+}
+
+export function buildToolSidebarContent(params: {
+  title: string;
+  detail?: string;
+  args?: unknown;
+  output?: string;
+}): string {
+  const sections = [`## ${params.title}`];
+  if (params.detail) {
+    sections.push(`**Command:** \`${params.detail}\``);
+  }
+  const args = formatToolPayloadForSidebar(params.args);
+  if (args !== null) {
+    sections.push(`**Arguments**\n${args}`);
+  }
+  if (typeof params.output === "string") {
+    sections.push(`**Output**\n${formatToolOutputForSidebar(params.output)}`);
+  } else {
+    sections.push("*No output - tool completed successfully.*");
+  }
+  return sections.join("\n\n");
+}
+
 /**
  * Get a truncated preview of tool output text.
  * Truncates to first N lines or first N characters, whichever is shorter.