Use sepia to Write, Review, and Refactor More Human Prose

What is sepia?

sepia is a portable Agent Skill for improving AI-assisted fiction and professional prose. It works with Claude Code, Codex, Grok Build, and Antigravity from one canonical SKILL.md, avoiding separate instructions for each platform.

Unlike tools that concentrate on vocabulary and sentence syntax, sepia begins with the structural patterns that can make writing feel machine-generated. For fiction, that includes overly tidy causal chains, narrator-explained themes, linear time, limited emotional representation, absent real-world references, and endings resolved entirely through growth or acceptance.

For professional documents, sepia looks for different problems: filler without information, unnecessary hedging, chatbot remnants, inappropriate register, repetitive formatting, and failure to meet the expectations of a document's venue.

Core principle: calibrate writing toward the human distribution rather than mechanically reversing every perceived AI tendency.

Key features

Four writing operations

sepia exposes four operations suited to different stages of a draft:

• write: Create new prose using the relevant fiction or professional-writing rules.
• review: Diagnose problems without editing the text.
• refactor: Make minimal, targeted edits while preserving as much of the draft as possible.
• recreate: Fully rewrite the material when local edits are insufficient.

A three-pass fiction protocol

1. Narrative architecture: Address theme explanation, causal neatness, revelation timing, emotional modes, character networks, and references to real things.
2. Discourse flow: Break repetitive paragraph-question templates, repair mid-story sag, and vary rhythm and information placement.
3. Surface style: Revise clichés, syntax templates, vocabulary, and register after the larger structural issues have been handled.

The skill also includes a 30-feature fiction diagnosis rubric and corrections for patterns associated with Claude, GPT, Gemini, DeepSeek, and Kimi.

Rules for professional documents

Professional prose uses a shared checklist plus a thin rule file for each supported venue:

• Release notes and announcements: Put user impact first, support claims with artifacts, and avoid marketing inflation.
• Pull request and issue replies: Answer first, cite file:line, avoid reflexive praise, and match length to the stakes.
• Postmortems: Remain blameless toward people while examining mechanisms rigorously; include timestamps, dead ends, and owned action items.
• Tickets and work orders: Express the outcome in the title, provide testable acceptance criteria, and link instead of repeating information.
• Technical articles: Open with the problem, include one real dead end and one committed opinion, and qualify numbers with their conditions.

Install sepia

The default installation scope is the user account, so one installation can be used across projects. Choose the instructions for your agent platform.

Claude Code

# install
claude plugin marketplace add Nanako0129/sepia
claude plugin install sepia@sepia --scope user

# update
claude plugin marketplace update sepia
claude plugin update sepia

If you install from Claude Code's in-session plugin dialog, select User when prompted for a scope.

Codex

# install
codex plugin marketplace add Nanako0129/sepia
codex plugin add sepia@sepia

# update
codex plugin marketplace upgrade sepia
codex plugin add sepia@sepia

Updating Codex requires refreshing the marketplace snapshot and adding the plugin again to obtain the new version.

Grok Build

# install
grok plugin install Nanako0129/sepia --trust

# update
grok plugin update

Grok Build can also discover an existing Claude Code installation of sepia automatically.

Antigravity

Antigravity does not use a marketplace for this skill. Clone the repository, copy the skill directory, and install the global /sepia workflow:

# install
git clone https://github.com/Nanako0129/sepia.git ~/.sepia
mkdir -p ~/.gemini/config/skills ~/.gemini/antigravity/global_workflows
cp -R ~/.sepia/skills/sepia ~/.gemini/config/skills/sepia
cp ~/.sepia/.agents/workflows/sepia.md ~/.gemini/antigravity/global_workflows/sepia.md

# update
git -C ~/.sepia pull
rm -rf ~/.gemini/config/skills/sepia && cp -R ~/.sepia/skills/sepia ~/.gemini/config/skills/sepia
cp ~/.sepia/.agents/workflows/sepia.md ~/.gemini/antigravity/global_workflows/sepia.md

Install for all four platforms

The repository provides a script that installs sepia at user scope for all four supported platforms:

curl -fsSL https://raw.githubusercontent.com/Nanako0129/sepia/main/install.sh | bash

The script clones the project to ~/.sepia. Set SEPIA_HOME to use another location. Running the same command again updates the installation. To inspect the script before executing it, clone the repository and run ./install.sh from the checkout.

Install through Skills CLI

The Skills CLI provides another route for sepia and supports more than 77 agents:

npx skills add Nanako0129/sepia -g
npx skills update -g

The -g option selects user scope. Without it, the CLI defaults to project scope.

Basic usage

After installation, ask your agent to apply sepia and specify one of its four operations. The examples below illustrate request wording rather than platform-specific shell commands.

Write a new document

Use write when starting from a brief. Identify the target venue so sepia can select the correct rules.

Use sepia's write operation to draft release notes from these changes.
Put user impact first and support each claim with a concrete artifact.

For fiction, provide the premise, constraints, and intended form without prescribing every structural move:

Use sepia's write operation to create a short story from this premise.
Apply the fiction protocol, but preserve ambiguity and avoid explaining the theme.

Diagnose without changing the draft

Choose review when you want findings before deciding what to revise. Explicitly request diagnosis only so the original remains untouched.

Review this story with sepia. Diagnose only; do not rewrite it.
Prioritize narrative architecture, then discourse flow and surface style.

For a professional document, name its type:

Review this incident postmortem with sepia.
Check timestamps, dead ends, mechanism analysis, and owned action items.
Do not edit the draft.

Make minimal changes

Use refactor when the draft is basically sound and should retain its voice, facts, and organization wherever possible.

Refactor this technical article with sepia using minimal edits.
Preserve its technical claims and voice while removing filler and fixing venue mismatch.

Rewrite from the ground up

Use recreate when structural problems cannot be fixed cleanly through local edits.

Recreate this release announcement with sepia.
Keep all verified facts, lead with user impact, and remove marketing inflation.

Because recreation is a full rewrite, state which facts, quotations, references, or constraints must survive.

Choose the right operation

• Choose write when you have source material or a brief but no usable draft.
• Choose review when you need a diagnosis, editorial plan, or second opinion without changes.
• Choose refactor when preserving the existing draft matters more than achieving a completely new structure.
• Choose recreate when the draft's architecture, organization, or register is fundamentally unsuitable.

Advanced tips

Do not apply every fiction rule

sepia is designed to select approximately three to five useful moves for a story and leave slack elsewhere. Applying every correction can replace one recognizable pattern with another. Ask the agent to prioritize the highest-impact findings rather than maximizing rule coverage.

Review this story with sepia and identify the five highest-impact issues.
Refactor only the top three, leaving intentional irregularities intact.

Revise in structural order

For fiction, avoid polishing sentences before settling the narrative architecture. A better sequence is to review the architecture, correct discourse flow, and only then adjust surface style. This prevents sentence-level polish from being discarded during larger revisions.

Supply venue and evidence

Professional-writing rules depend on context. Tell sepia whether a draft is a ticket, pull request reply, postmortem, release note, or technical article. Include relevant files, line references, timestamps, acceptance criteria, links, measurements, and conditions so the output can remain specific.

Pin sepia to a project

Use project scope when a repository needs to preserve a known copy of the skill. Commit skills/sepia/ as .agents/skills/sepia for Codex and Antigravity, or as .claude/skills/sepia for Claude Code.

This approach is useful when a team wants the repository to carry its own skill version instead of depending on each contributor's user-scoped installation.

Inspect the reference layers

The canonical skill is in skills/sepia/SKILL.md. Its reference files separate narrative architecture, discourse flow, surface style, diagnosis, model fingerprints, shared professional rules, and venue-specific domains. The research/ directory contains the project's digested evidence base and source links.

Keep installations current

Use the update commands for your chosen platform. If you installed all platforms with install.sh, rerun the same installation command. Antigravity's manual setup requires pulling the repository and copying the updated skill and workflow files again.

Conclusion

sepia treats human-sounding writing as more than a vocabulary problem. Its fiction workflow starts with narrative architecture, proceeds through discourse flow, and finishes with surface style, while its professional-writing rules adapt to the expectations of specific document types. Install it in user or project scope, choose the operation that matches your editing goal, and apply a small number of high-impact changes rather than every available rule.

sepia is released under the MIT License. Its source, reference files, installation script, and research digests are available in the GitHub repository.