Spreadsheet importer returns shared-string index for an empty XLSX shared string

What issue are you seeing?

SpreadsheetFile.importXlsx in the Codex Spreadsheets runtime resolves an XLSX shared-string cell incorrectly when the referenced shared string is empty.

For this valid OOXML:

<!-- xl/worksheets/sheet1.xml -->
<c r="A2" t="s"><v>1</v></c>
<c r="B2" t="s"><v>0</v></c>

<!-- xl/sharedStrings.xml -->
<sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"
     count="2" uniqueCount="2">
  <si><t>visible</t></si>
  <si><t/></si>
</sst>

A2 references shared-string index 1, whose resolved content is an empty string. Codex instead returns the raw index text "1". B2 correctly resolves index 0 to "visible".

This can silently turn blank spreadsheet cells into valid-looking numeric strings. Downstream data transformations can therefore write incorrect values without any parse error.

What steps can reproduce the bug?

Environment:

• Codex App: 26.803.61601 (build 6396)
• Workspace dependencies bundle: 26.805.11740
• @oai/artifact-tool: 2.8.39
• Platform: Darwin 25.5.0 arm64 arm

1. Decode the sanitized workbook included below.
2. Import it with the bundled runtime:

import { FileBlob, SpreadsheetFile } from "@oai/artifact-tool";

const workbook = await SpreadsheetFile.importXlsx(
  await FileBlob.load("codex-empty-shared-string-repro.xlsx"),
);

const result = await workbook.inspect({
  kind: "table",
  sheetId: "Repro",
  range: "A1:B2",
  include: "values,formulas",
});

console.log(result.ndjson);

1. Observe the imported values:

[["empty_shared_string","control"],["1","visible"]]

1. Open the same workbook in WPS/Excel-compatible software, or export it with LibreOffice Calc:

empty_shared_string,control
,visible

The independent reader correctly leaves A2 blank.

What is the expected behavior?

A2 should resolve to an empty string (or the importer's documented blank-cell representation), never to the shared-string table index.

Expected values:

[["empty_shared_string","control"],["","visible"]]

Additional information

• The workbook is fully synthetic and contains no user, repository, company, or business data.
• The XLSX ZIP package passes integrity validation and contains only eight standard OOXML entries.
• There are no formulas or external links.
• A structurally identical real-world workbook used a different shared-string index and showed the same failure mode.
• The behavior is consistent with a falsy empty-string resolution falling back to the raw index. This is an inference from the observed boundary, not a source-code-confirmed root cause.

<details>
<summary>Sanitized minimal XLSX (base64, 3.6 KB)</summary>

SHA-256: aa567082dafc74bd1f2c4ab1d124a9917101a66c4777b572c8645a075e287524

Save the block as repro.b64, then decode it:

python3 -c 'import base64,pathlib; pathlib.Path("codex-empty-shared-string-repro.xlsx").write_bytes(base64.b64decode(pathlib.Path("repro.b64").read_text()))'

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</details>