Fix truthy checks for zero-valued params and empty Write content

Use null checks instead of truthiness for offset/limit (preserves 0)
and Write content (shows "(empty file)" marker for empty strings).
Add tests for both edge cases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

frontend/src/lib/utils/tool-params.test.ts

@@ -140,6 +140,19 @@ describe("extractToolParamMeta", () => {
     ).toBeNull();
   });
 
+  it("preserves zero-valued offset and limit", () => {
+    const meta = extractToolParamMeta("Read", {
+      file_path: "/src/app.ts",
+      offset: 0,
+      limit: 0,
+    });
+    expect(meta).toEqual([
+      { label: "file", value: "/src/app.ts" },
+      { label: "offset", value: "0" },
+      { label: "limit", value: "0" },
+    ]);
+  });
+
   it("truncates long file paths", () => {
     const longPath = "/a".repeat(50);
     const meta = extractToolParamMeta("Read", {
@@ -207,6 +220,15 @@ describe("generateFallbackContent", () => {
     expect(result.length).toBeLessThanOrEqual(501);
   });
 
+  it("shows empty-file marker for Write with empty content", () => {
+    expect(
+      generateFallbackContent("Write", {
+        file_path: "/src/empty.ts",
+        content: "",
+      }),
+    ).toBe("(empty file)");
+  });
+
   it("falls back to generic display for Write without content", () => {
     expect(
       generateFallbackContent("Write", {

frontend/src/lib/utils/tool-params.ts

@@ -26,12 +26,12 @@ export function extractToolParamMeta(
         label: "file",
         value: truncate(String(params.file_path), 80),
       });
-    if (params.offset)
+    if (params.offset != null)
       meta.push({
         label: "offset",
         value: String(params.offset),
       });
-    if (params.limit)
+    if (params.limit != null)
       meta.push({
         label: "limit",
         value: String(params.limit),
@@ -119,8 +119,9 @@ export function generateFallbackContent(
     }
     return lines.length ? lines.join("\n") : null;
   }
-  if (toolName === "Write" && params.content) {
-    return truncate(String(params.content), 500);
+  if (toolName === "Write" && params.content != null) {
+    const text = String(params.content);
+    return text ? truncate(text, 500) : "(empty file)";
   }
   const lines: string[] = [];
   for (const [key, value] of Object.entries(params)) {