TemplateGate

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Policy-as-code acceptance gate for AI-edited Excel & Word documents.

AI agents (Claude Code, Codex, ChatGPT, ...) are great at editing Excel and Word files — and occasionally destroy a formula, a merged cell, or a print layout while doing it. TemplateGate is a read-only regression test for .xlsx / .xlsm / .docx files: it compares a baseline (before the edit) with a candidate (after the edit) and verifies that only the changes your policy allows were made.

• Default deny / fail closed — any change not explicitly allowed is a violation.
• Deterministic structural checks — values, formulas, formats, merged cells, conditional formatting, data validation, sheet structure, images (count / content hash / position / size), headers & footers, print settings, VBA, Word paragraphs / tables / sections.
• Catches what editing tools silently throw away — charts, pivot tables, comments, embedded objects, custom XML, Excel shapes & textboxes, and every other part of the file besides. Parts are read straight out of the document's internal package, so damage is caught even when the tool that made the edit could not represent those parts in the first place — the failure mode where you get a file back that opens fine and looks fine, minus its charts. A part nobody recognises still counts: unknown is not the same as allowed.
• Catches what you cannot see — a row quietly hidden, a sheet left unlocked, a workbook switched to manual calculation so every formula shows a stale answer, a hyperlink repointed while its display text stays put, body text set to 4pt white. Edits made as tracked changes are visible too: an insertion is text on the page, so it is reported as one.
• Optional semantic checks — off (default; nothing ever leaves your machine), review (AI findings as warnings), or gate (AI findings affect PASS/FAIL). Bring your own model — TemplateGate does not pin a vendor.
• Not an editor — TemplateGate never modifies documents and never auto-repairs. A failed candidate should be discarded; a human makes the final call.

The trust boundary

The agent that edited the document must never be the one that decides what is allowed. An agent may propose a policy, but the actual check runs against a trusted policy that a human (or CI) has reviewed and pinned. TemplateGate only reads the policy file — it has no way to widen it.

This is not a hypothetical risk. Microsoft Research measured frontier LLMs corrupting an average of 25% of document content over long editing workflows — "sparse but severe errors that silently corrupt documents" — and found that agentic tool use alone does not fix it (Laban, Schnabel & Neville, 2026). Verification has to live outside the agent.

Install

TemplateGate is a command-line tool, so install it as one — isolated from your project's own dependencies:

uv tool install templategate

To use the Python API from your own project instead, add it as a dependency:

uv add templategate

Plain pip works too, if you would rather not add a tool:

pip install templategate

To try the latest unreleased revision, replace templategate with git+https://github.com/kousunh/TemplateGate.git in any of the commands.

Requires Python 3.10+. No Office installation and no network access are needed to run a check. Dependencies: openpyxl, python-docx (which brings lxml), Pillow, and PyYAML.

Quick start

# 1. Generate a starter policy and edit it
#    (already have an example edit? `templategate suggest` drafts one from it)
templategate init --target excel

# 2. Let your agent edit a COPY of the document

# 3. Check the result
templategate check \
  --baseline plan_2026.xlsx \
  --candidate plan_2026.edited.xlsx \
  --policy templategate.policy.yaml \
  --report json

Exit codes: 0 = PASS, 1 = FAIL, 2 = execution error.

A candidate that still opens as a package but has lost parts it referenced is damaged, not unreadable — that is a FAIL (exit 1) with the missing parts named, not a tool error.

Exit 2 means the file could not be read unambiguously: it will not open at all, or its package is ambiguous. A zip holding two copies of the same part is refused rather than guessed at, because different readers would open different documents from it. Configuration mistakes are exit 2 as well. Neither review_only mode nor structural: ignore can bless a document that cannot be read.

Between those two there is a middle state worth knowing about. When a file comes from a writer that omits parts the usual readers expect — document properties, for instance — the check falls back to comparing package parts alone. It says so in the report rather than pretending otherwise. That is an honest comparison but a much weaker one: part-level hashes will tell you a sheet changed, not which cell. If you see it, prefer a writer that produces a complete package.

A policy looks like this:

version: 1
target: excel
allow:
  - selector: "Sheet1!B2:B100"   # the agent may change these values...
    attributes: [value]
protect:
  - selector: "*"                 # ...but formulas and print settings, never
    attributes: [formula, print_settings, vba]
structural:
  sheets: strict                  # adding/removing/hiding sheets fails
  images: strict
  charts: strict                  # so does losing a chart, a pivot table,
  pivot_tables: strict            # a comment, an embedded object, custom XML,
  comments: strict                # or an Excel shape / textbox
  parts: strict                   # ...or any other part of the file
  links: strict                   # or a hyperlink repointed elsewhere
semantic:
  mode: "off"                     # off | review | gate

Every structural category is strict unless you say otherwise — deleting the line does not turn it off. Set one to ignore to opt out.

Two worked examples live in examples/: an Excel policy that lets an agent update quantity cells and nothing else (excel-quantity-update.policy.yaml), and a Word one that allows body-text rewriting while locking everything else (word-body-rewrite.policy.yaml). They are commented throughout and are the fastest way to see a real policy.

Writing a policy: start from diff

You do not have to guess what to put in a policy. Make the edit you intend to allow — by hand, or by letting the agent do it once — and ask what changed:

templategate diff --baseline plan.xlsx --candidate plan.edited.xlsx

Every change is listed with the exact location and attribute name a policy rule would use. Allow the ones that were the point of the edit, and leave everything else to default deny. Running diff on a legitimate edit is also how you find out what your editing tool damages on the way past.

Or have it draft the policy for you

Reading a diff and deciding which lines belong in a policy is the part that takes experience. suggest does that triage for you. Give it a baseline and an edited copy you have already looked at and are happy with, and it writes a commented policy:

templategate suggest \
  --baseline quote.xlsx \
  --candidate quote.edited.xlsx \
  --output quote.policy.yaml

It sorts every difference into three kinds, and the difference between them is the whole point:

• What the edit did — the cells and paragraphs whose contents changed. Allowed, with tight selectors and a note of what was observed. Neighbouring cells collapse into one range, so you get 見積書!D10:D12 rather than three rules nobody will read.
• What the save did — cached formula results, chart caches, a column width read back, a default font. Allowed too, each with a comment explaining why it is harmless. This is the collateral described above, recognised for you.
• What neither explains — a formula replaced by a number, an image gone, a sheet removed, a package part dropped. Never allowed, no matter that the sample edit did it. These are listed at the top of the draft for you to decide on, one by one.

That last rule has a consequence worth stating plainly: if the edit you drew the draft from contained real damage, the draft will not pass it. Run it on an edit where a formula was overwritten and the header says so — 見積書!D16: formula replaced by a hardcoded value (=D14+D15 -> 1430000) — and the policy refuses to bless it. A draft that quietly allowed that would be worse than no draft at all.

The output is a starting point, not a finished policy. Read it, delete what you did not mean to permit, and pin it yourself. To grow a policy you already have instead of starting over, pass it with --policy and the additions arrive commented out, for you to accept one at a time.

Editing in real Excel

If the edit is made in Excel itself — by a person, or by an agent driving it through COM — then a correct, allowed edit still changes more than the cell you touched. Excel recalculates dependent formulas and refreshes chart caches on every save, so the file legitimately differs in places nobody edited. A policy that allows only the target cells will fail a perfectly good edit.

Editing a single quantity cell in a real workbook produced five changes: the value itself, the cached results of two formulas that depend on it, the result of an array formula, and the chart's cached copy of the series data.

The recipe:

allow:
  - selector: "Sales!B3:B5"        # the cells actually being edited
    attributes: [value]
  - selector: "Sales!D3:D7"        # results of formulas that depend on them
    attributes: [value]
  - selector: "Sales!G3:G5"        # array-formula results — easy to forget
    attributes: [value]
  - selector: "package#charts:*"   # chart caches Excel refreshes on save
    attributes: [charts]
protect:
  - selector: "*"
    attributes: [formula, format, merge, conditional_formatting,
                 data_validation, print_settings, header_footer, vba]

Allowing value generously on computed ranges is safe because formula stays protected: the results may move, but the rules that produce them may not. A formula overwritten with a literal is still caught, under formula.

Allowing package#charts:* does mean a deliberate edit to a chart's own definition would pass. The chart's presence is still guarded — deleting it also removes the drawing that anchors it to the sheet, which drawings catches.

The cascade is not really about Excel. It follows from the baseline and the candidate having been written by different tools, and it happens in both directions.

Run it the other way — a library editing a workbook that Excel authored, which is the common case when an agent touches a real business document — and you get the mirror image. openpyxl does not recalculate, so rather than refreshing the cached results it discards them: every dependent cell reads as empty until Excel next opens the file. One measured edit of three quantity cells produced sixteen changes: the edit itself, seven wiped formula results, a date cell whose value and number format were both rewritten, two hidden columns whose widths read back differently, and a workbook extension block that was dropped.

The fix is the same recipe — allow value across the computed ranges, keep formula protected — and it is safe for the same reason: the cached answers may move or vanish, the formulas that produce them may not. Expect to allow a little more than values alone, such as format on a date cell the library cannot round-trip and layout on hidden columns.

Steady state is quiet. When the baseline and the candidate come from the same tool, there is no cascade and a narrow policy is exactly right.

Editing in real Word

Word is well behaved in the steady state: opening a Word-authored document and saving it without editing reports nothing at all. The friction is only on the first save after Word takes over a file that a library produced.

That first save is a one-time re-authoring of the document's scaffolding rather than damage. Word creates the footnote and endnote parts it expects every document to have, drops legacy parts it no longer writes, rewrites the style, numbering, settings and web-settings parts — re-pointing built-in styles to its own identifiers — restamps the image markup in the body, and re-points headers and footers at the renamed styles.

Those are real changes to real parts, which is why they are reported instead of quietly ignored: only a person can say whether a wholesale style rewrite is acceptable. In one measured run the first save produced ten changes across those categories. Do not treat that as a fixed list — the built-in style names Word writes are localized, so the exact set varies with version and locale.

Re-baseline. Save the document once in Word, adopt that file as the baseline, and from then on baseline and candidate share an author and the steady state is exactly zero.

One detail decides whether this works: adopt the file Word actually wrote. Do not regenerate the baseline by converting the original a second time. Each conversion stamps fresh identifiers, so two independent first saves of the same source differ from each other even though neither was edited.

If you cannot re-baseline — the baseline is a fixed template someone else owns — allow the known parts explicitly instead. These are the mechanical ones, safe to allow outright:

allow:
  - selector: "package#parts:word/footnotes.xml"
    attributes: [parts]
  - selector: "package#parts:word/endnotes.xml"
    attributes: [parts]
  - selector: "package#parts:word/numbering.xml"
    attributes: [parts]
  - selector: "package#parts:word/settings.xml"
    attributes: [parts]
  - selector: "package#parts:word/webSettings.xml"
    attributes: [parts]

That is deliberately not the whole residual — on the run measured above it clears five of the ten changes. What remains is the style parts, the headers and footers, and the image markup, and each of those needs a decision from you, because each is somewhere real damage could hide. Derive the list from your own templategate diff output rather than copying this one, since it is specific to the install that produced the file.

Know what finishing the list costs. Adding word/styles.xml to it gives up detection of style-definition tampering: setting every paragraph to 4pt white text through a style change touches nothing but that part, so an allow rule for it lets the change through silently. Re-baselining gives up nothing, which is why it is the recommendation and this is the fallback.

Choosing a mode

mode: normal_input    # review_only | normal_input | page_extension

• normal_input (the default) compares by position: paragraph 12 is expected to still be paragraph 12. Every violation is an error.
• page_extension is for Word documents that are meant to grow. Paragraphs are aligned by content, so inserting one does not report every paragraph after it as changed, and a block that genuinely moved is reported as moved. On the same one-paragraph deletion, normal_input reports ten violations and page_extension reports one.
• review_only reports violations as warnings and exits 0. Useful for seeing what a policy would catch before you enforce it. It cannot bless a document that will not open.

What a policy can name

templategate init writes a starter policy for the target you name, with the attributes and structural keys already filled in — the fastest way to see the current set. The ones worth knowing about:

• Package parts — charts, comments, embedded, custom_xml, parts and links apply to both formats; pivot_tables and drawings (shapes, textboxes and chart frames) are Excel-side, since a Word document keeps its equivalents in the document body where the Word attributes below cover them. parts is the catch-all for everything else in the file, including parts TemplateGate has never heard of, so losing one is damage by default. links compares the external targets of relationships, which is what catches a hyperlink repointed to a new URL while its display text is untouched.
• Excel — layout (hidden rows and columns, and their sizes), protection (sheet and workbook locking), sheet_settings (a workbook switched to manual calculation, or Excel's warning triangles suppressed).
• Word — paragraph_format, field, bookmark, content_control, revision (tracked changes), and moved for a block whose content survived but whose position did not. markup is the backstop: anything inside a block that no other attribute accounts for, which is what catches an unmodelled character style, a deleted footnote reference, a removed comment anchor, or a form field quietly disabled.

The VBA project keeps its own vba selector and attribute rather than living under package#.

Locations are addressable:

protect:
  - selector: "package#*"                              # every package part
  - selector: "package#charts:*"                       # one whole category
  - selector: "package#charts:xl/charts/chart1.xml"    # one specific part
  - selector: "package#links:https://example.com/x"    # one external target
  - selector: "Sheet1!1:10"                            # rows, for layout
  - selector: "'Q1!Q4'!A1"                             # see below

Word content controls and text boxes are sdt1, textbox1; a table nested inside a table cell extends its parent's location, so the first cell of a table nested in cell r1c1 is table1!r1c1!table1!r1c1.

Sheet names are quoted Excel-style when they contain ', ! or #: 'Q1!Q4'!A1 is cell A1 of the sheet named Q1!Q4, where bare Q1!Q4 would mean cell Q4 of a sheet named Q1. You only need the quotes for names containing those characters.

Optional semantic checks

Everything above is deterministic and offline. The semantic block adds judgement calls that structure alone cannot make — "the dates must not contradict each other" — by handing the baseline and candidate text to a command you choose:

semantic:
  mode: "off"           # off (default) | review | gate
  provider: command
  command: "claude -p"  # any CLI that reads a prompt on stdin, prints JSON
  model: ""             # passed to that command as $TEMPLATEGATE_MODEL
  checks:
    - "dates and periods must not contradict each other"

mode: off is the default and makes no network calls at all. review reports findings as warnings that never change PASS/FAIL; gate lets a failed finding fail the run. The command receives a prompt on stdin and must print a JSON array of {check, verdict, message}, where verdict is pass, fail or warning — so any model or vendor works, and TemplateGate pins none. With a non-off mode and no command set, the run reports a configuration error rather than silently skipping the checks.

Reading a report

Reports name what actually changed: a format violation reads font.bold True -> False, not "the format changed". When one edit shifts everything after it, the text and Markdown reports collapse the knock-on changes into a single line — p4..p11: content shifted because 1 paragraph removed at p3 — while the JSON report always keeps every individual change, tied together by a group field.

Add --lang ja to check or diff (or set TEMPLATEGATE_LANG=ja once for a whole job) to read the text and Markdown reports in Japanese, in the register of README.ja.md — plain 事務文書 です・ます, addressed to whoever decides if the document may ship. The flag beats the environment variable, so one run stays readable to somebody else without unsetting a variable the rest of the job depends on. The JSON report never moves: agents and CI parse it as a contract, in English, whatever --lang says.

Other commands:

templategate diff --baseline a.xlsx --candidate b.xlsx   # list every change, no policy
templategate snapshot file.docx                           # dump the structural snapshot

Which editing tools work

All of them, for detection. The gate judges the file, not the editor, so no tool is unsupported. What differs is how much collateral each one leaves behind, and therefore how wide your policy has to be to accept a correct edit.

Measured 2026-07-31 by opening one Excel-authored workbook — formulas with cached results, hidden columns, a seal image, a Japanese date format, theme colours — and saving it again with no edit at all:

Writer	Version	Changes	What it did
Excel / Word via COM (incl. xlwings)	Office 16	0	Word→Word is exactly silent; Excel refreshes caches it can recompute
python-docx	1.2.0	0	silent on a Word-authored document
openpyxl	3.1.5	13	discards 11 cached formula results; hidden-column width; workbook extension block
ExcelJS	4.4.0	17	vertical alignment and theme font colours on 12 cells; Japanese date format down to a bare serial
SheetJS (xlsx)	0.18.5	49	borders, fills, fonts and alignment across 38 cells; the image; the theme and metadata parts
pandas + XlsxWriter	3.0.5 / 3.2.9	113	authoring, not editing — see below

Two caveats on that table. openpyxl needs Pillow: without it, it also deletes images outright rather than preserving them, so install it alongside. And python-docx's zero is for this document — earlier sweeps found it drops comments and VBA, which this one did not contain.

pandas and XlsxWriter are authoring tools, not editors. XlsxWriter cannot open an existing workbook at all; writing "to" a file replaces it. The 113 changes above are not a bug, they are what "rewrite the file from a DataFrame" means — formulas, images, merges and formatting are gone because they were never read. Re-baselining from the writer's own first output is the expected workflow here, not a workaround.

Getting out of the way of it

This kind of damage is deterministic: the same library on the same file makes the same omissions every time. Retrying is pointless. Three things do work:

• Use a richer editor. A document with charts, shapes and real formatting needs a tool that understands them. Driving real Excel or Word through COM — xlwings, or the automation interface directly — is the safest option and is available on any machine with Office installed.
• Edit the package directly. For a value-only change, rewriting the one XML part inside the .xlsx zip touches nothing else by construction.
• Let real Office repair what it can. Opening a library-damaged workbook in Excel and saving restores the cached results it can recompute. In the run above that took openpyxl's 14 changes down to 2 — but the two that remained were the deleted image and the dropped extension block. A resave recomputes; it does not resurrect. Anything actually removed is still gone.

When the gate fails

TemplateGate never writes to your documents. There is no repair mode and there will not be one — a tool that can edit the file it is judging is a tool you have to trust twice. Recovery is deliberately outside the tool, and you already hold everything it needs:

The baseline is the backup. It is the untouched original, and a failed candidate is a scratch file. The loop:

1. Read the violations. They name what broke and where.
2. Throw the candidate away. Do not patch it, and above all do not re-save it — whatever dropped a part on the first save will drop more on the second.
3. Copy the baseline again and retry, handing the violation report back to the agent as feedback. The JSON report is built to be fed straight in.
4. If the same violation survives two attempts, the tool doing the editing cannot preserve that part. Change the approach, not the policy.

In CI the baseline lives in git history, so every commit is another generation of backup and you can always compare against the last version that passed.

Never widen the policy to make a failing check pass. That turns a caught error into a silent one, which is the exact failure this tool exists to prevent.

Python API

import templategate

result = templategate.check("baseline.xlsx", "candidate.xlsx", "policy.yaml")
if not result.passed:
    for v in result.violations:
        print(v.change.location, v.change.attribute, v.message)

For AI agents

The skills/office-document-regression/ directory contains an agent skill that teaches an agent the safe workflow: edit a copy, run templategate check, interpret the JSON report, and never edit the policy to make a failing check pass.

It uses the open Agent Skills format (SKILL.md), supported natively by Claude Code, Codex, ChatGPT / ChatGPT Work and a growing list of other agents (Gemini CLI, Cursor, GitHub Copilot, ...). Installing it is copying one directory; after that it triggers by itself whenever the agent is asked to edit an Office document — the frontmatter description is the trigger, no prompt or AGENTS.md wiring needed.

• Claude Code — .claude/skills/ in the project, or ~/.claude/skills/ for every project:

  cp -r skills/office-document-regression .claude/skills/

• Codex — .agents/skills/ in the repository (auto-discovered from the working directory up to the repo root), or ~/.codex/skills/ for every project:

  cp -r skills/office-document-regression .agents/skills/

• ChatGPT / ChatGPT Work — add the skill folder through the Skills feature; a workspace admin can share it with the whole team.

• Other agents — most read either .agents/skills/ or their own equivalent of the same directory; check the tool's skills documentation.

The strongest deployment is committing the skill into the document repository itself, next to the baseline and the policy:

cp -r skills/office-document-regression <your-docs-repo>/.agents/skills/

.agents/skills/ is the vendor-neutral location. Anyone who opens that repository with a skills-capable agent gets the safe-edit workflow from their first prompt — nothing to install per machine, and the rules travel with the documents they protect.

GitHub Action

action.yml lives in the action/ subdirectory, so the uses: path has to include it. The runner fetches TemplateGate itself — you only check out your repository so the documents and the policy are on disk.

- uses: actions/checkout@v4
- uses: actions/setup-python@v5
  with:
    python-version: "3.12"
- uses: kousunh/TemplateGate/action@v1
  with:
    baseline: docs/plan_baseline.xlsx
    candidate: docs/plan.xlsx
    policy: .templategate/plan.policy.yaml

The step fails the job when the check fails, and appends the report to the job summary either way. It exposes two outputs, passed (true / false) and report-path; to read them instead of failing the job, add continue-on-error: true to the step.

@v1 is a floating tag that follows the latest release. For CI you audit, pin to a full commit SHA instead (kousunh/TemplateGate/action@<sha>) — the same practice you would apply to any third-party action.

What TemplateGate is not

• Not an editor, converter, or auto-repair tool. See When the gate fails.
• Changes inside a part that is not separately modelled are reported as that part changing, without naming the property — you learn word/header1.xml was modified, not which run turned white.
• No round-trip normalization (differences introduced by re-saving in another Office application are out of scope).
• No visual/PDF regression.
• Semantic mode off performs zero network calls; TemplateGate itself never uploads your documents anywhere.

License

MIT. See LICENSE.

TemplateGate is an independent open-source project and is not affiliated with, endorsed by, or sponsored by Microsoft. "Microsoft", "Office", "Excel" and "Word" are trademarks of Microsoft Corporation, used here only to identify file formats.