diff --git a/internal/tools/pr_files_diff.go b/internal/tools/pr_files_diff.go
index 0400802..12f131a 100644
--- a/internal/tools/pr_files_diff.go
+++ b/internal/tools/pr_files_diff.go
@@ -143,7 +143,13 @@ func splitUnifiedDiff(d []byte) map[string][]byte {
 
 	flush := func() {
 		if currentFile != "" {
-			m[currentFile] = current.Bytes()
+			// Copy: bytes.Buffer.Bytes() returns the internal slice,
+			// which Reset() then reuses. Without the copy, every map
+			// entry ends up aliased to the last file's data.
+			b := current.Bytes()
+			cp := make([]byte, len(b))
+			copy(cp, b)
+			m[currentFile] = cp
 			current.Reset()
 		}
 	}
diff --git a/internal/tools/pr_files_diff_test.go b/internal/tools/pr_files_diff_test.go
index 02bec0f..0e1a61a 100644
--- a/internal/tools/pr_files_diff_test.go
+++ b/internal/tools/pr_files_diff_test.go
@@ -97,6 +97,47 @@ func TestPRFilesDiffSmall(t *testing.T) {
 	assert.ElementsMatch(t, fileNames, paths)
 }
 
+// Regression for issue #25: every file's diff entry must contain its OWN diff,
+// not a shared buffer pointing at the last file. Prior bug: splitUnifiedDiff
+// flushed bytes.Buffer.Bytes() into the map without copying, so every entry
+// aliased the buffer's backing array and showed the last file's content.
+func TestPRFilesDiffPerFileIsolation(t *testing.T) {
+	fileNames := []string{"alpha.go", "beta.go", "gamma.go", "delta.go"}
+	rawDiff := buildDiff(fileNames, 5)
+	filesJSON := buildFilesJSON(fileNames, 5)
+
+	srv := newPRFilesDiffServer(t, filesJSON, rawDiff)
+	defer srv.Close()
+
+	tool := tools.NewPRFilesDiff(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"o"}))
+	result, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"o","name":"r","number":1}`))
+	require.NoError(t, err)
+
+	var out struct {
+		Files []struct {
+			Path string `json:"path"`
+			Diff string `json:"diff"`
+		} `json:"files"`
+	}
+	require.NoError(t, json.Unmarshal(result, &out))
+	require.Len(t, out.Files, len(fileNames))
+
+	for _, f := range out.Files {
+		expected := fmt.Sprintf("diff --git a/%s b/%s", f.Path, f.Path)
+		assert.Contains(t, f.Diff, expected,
+			"file %s diff must contain its own header, got: %.80q", f.Path, f.Diff)
+		// No other file's header should leak in.
+		for _, other := range fileNames {
+			if other == f.Path {
+				continue
+			}
+			otherHeader := fmt.Sprintf("diff --git a/%s b/%s", other, other)
+			assert.NotContains(t, f.Diff, otherHeader,
+				"file %s diff must NOT contain %s's header", f.Path, other)
+		}
+	}
+}
+
 func TestPRFilesDiffPerFileTruncated(t *testing.T) {
 	// One file with a 30KB diff (each "+abcdefghij\n" = 12 bytes; 30KB / 12 ≈ 2560 lines).
 	fileNames := []string{"bigfile.go"}