diff --git a/src/core/hunkHeader.test.ts b/src/core/hunkHeader.test.ts
new file mode 100644
index 00000000..5cc5ad57
--- /dev/null
+++ b/src/core/hunkHeader.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, test } from "bun:test";
+import type { Hunk } from "@pierre/diffs";
+import { formatHunkHeader } from "./hunkHeader";
+
+/**
+ * Build a synthesized hunk that has no parsed `hunkSpecs`, so `formatHunkHeader`
+ * falls back to building the `@@ -l,s +l,s @@` header from the numeric fields.
+ *
+ * A changeset transform can legally produce such a hunk: validation only
+ * requires `hunkContent` (`summarizeHunk`), and a transform that supplies the
+ * numeric header fields but no raw specs drives this exact fallback path both
+ * in the review stream (`src/ui/diff/pierre.ts`) and in `summarizeHunk`.
+ */
+function createTestSynthesizedHunk(overrides: Partial<Hunk> = {}): Hunk {
+  return {
+    collapsedBefore: 0,
+    // Per-side totals (context + changed), i.e. the `count` of `@@ -l,s +l,s @@`.
+    deletionStart: 10,
+    deletionCount: 4,
+    additionStart: 10,
+    additionCount: 4,
+    // Changed `+`/`-` lines only — strictly less than the per-side total when
+    // the hunk carries context, which is precisely the undercount trap.
+    deletionLines: 1,
+    additionLines: 1,
+    hunkContent: [],
+    ...overrides,
+  } as unknown as Hunk;
+}
+
+describe("formatHunkHeader", () => {
+  test("fallback counts per-side totals (context + changes), not changed lines only", () => {
+    // Edge case: a context-bearing synthesized hunk (3 context + 1 changed per
+    // side) forces the fallback. The header count must be the per-side total.
+    const hunk = createTestSynthesizedHunk();
+
+    expect(formatHunkHeader(hunk)).toBe("@@ -10,4 +10,4 @@");
+  });
+
+  test("fallback appends hunkContext after the well-formed ranges", () => {
+    const hunk = createTestSynthesizedHunk({
+      deletionStart: 5,
+      deletionCount: 3,
+      additionStart: 5,
+      additionCount: 3,
+      deletionLines: 1,
+      additionLines: 1,
+      hunkContext: "render(context)",
+    });
+
+    expect(formatHunkHeader(hunk)).toBe("@@ -5,3 +5,3 @@ render(context)");
+  });
+
+  test("prefers a parsed hunkSpecs verbatim and only appends context when present", () => {
+    // The primary branch is untouched by the fallback fix: a parsed hunk keeps
+    // its raw specs, and hunkContext is appended only when it exists.
+    const withContext = { hunkSpecs: "@@ -1,5 +1,7 @@", hunkContext: "main()" } as unknown as Hunk;
+    const withoutContext = { hunkSpecs: "@@ -0,0 +1,3 @@" } as unknown as Hunk;
+
+    expect(formatHunkHeader(withContext)).toBe("@@ -1,5 +1,7 @@ main()");
+    expect(formatHunkHeader(withoutContext)).toBe("@@ -0,0 +1,3 @@");
+  });
+});
diff --git a/src/core/hunkHeader.ts b/src/core/hunkHeader.ts
index 2faf1358..de76d783 100644
--- a/src/core/hunkHeader.ts
+++ b/src/core/hunkHeader.ts
@@ -4,6 +4,10 @@ import type { Hunk } from "@pierre/diffs";
 export function formatHunkHeader(hunk: Hunk) {
   const specs =
     hunk.hunkSpecs ??
-    `@@ -${hunk.deletionStart},${hunk.deletionLines} +${hunk.additionStart},${hunk.additionLines} @@`;
+    // The `,count` of `@@ -l,s +l,s @@` is the per-side line total (context +
+    // changed), parsed from `-X,count` / `+X,count` as `*Count` — not `*Lines`,
+    // which counts only the changed `+`/`-` lines and would undercount any hunk
+    // that carries context.
+    `@@ -${hunk.deletionStart},${hunk.deletionCount} +${hunk.additionStart},${hunk.additionCount} @@`;
   return hunk.hunkContext ? `${specs} ${hunk.hunkContext}` : specs;
 }