# Sponge vs Readwise Reader for AI agent research

Readwise Reader is a paid reading app for pages, feeds, newsletters, and video. Sponge is a private library of pages and PDFs your AI agent can cite.

By Hraness · Published 2026-09-26

Drafted with AI from the source code and reviewed by Claude Opus 5.5 (claude-opus-5-5) editorial review.

Readwise Reader is the read-it-later app that comes with the full Readwise subscription. It is a hosted library for web pages, PDFs, EPUBs, newsletters, RSS feeds, and video, with highlights, notes, tags, and filtered views. Sponge is a private library for articles and PDFs. It keeps a readable copy of each page you save, attaches your highlights and notes to it, and lets your own AI agent work from those sources through a scoped HTTP API or a local MCP server.

Both apps keep a personal library of saved reading, but they are built for different work. Reader is built for reading, collecting, and resurfacing what you saved. Sponge is built so an agent can quote the saved text in its research and you can check what it wrote against the original.

Status: Preview. To try Sponge, sign in at sponge.computer and save a public link or a PDF. [Status and limits](https://sponge.computer/docs/status) lists which parts are verified and which are still being checked.

## Where Reader and Sponge differ

- **What you can save.** Reader stores documents in categories: article, email, RSS, highlight, PDF, EPUB, tweet, and video. Each document has a location (new, later, archive, or feed) and tags. Sponge saves public web pages, PDFs with extractable text, and supported text files up to 12 MiB each. It does not capture newsletters, feeds, tweets, or video.
- **What an agent can do.** The Reader API saves, lists, updates, deletes, and tags document records. Sponge’s API saves, reads, searches, and annotates the items in a private library, and it adds document editing with revision headers and a history of each edit. An agent’s work stays reviewable, and a stale write cannot silently replace a newer version.
- **Credentials and rate limits.** Reader authenticates with one account token and limits each endpoint to between 20 and 50 requests per minute, after which it answers 429 with a `Retry-After` header. Sponge issues scoped credentials that expire: `library:read` and `library:write` for the library and `document:write` for documents. Each credential has a request budget, and every write carries an idempotency key.
- **Highlights.** In Reader, a highlight is its own document record: `parent_id` links it to the source, `content` holds the passage, and `notes` holds your note. In Sponge, a highlight anchors to the saved copy’s text by passage, offset, and surrounding context, and Sponge refuses a selection that does not match the saved text.
- **Exports.** Reader exports a document’s annotations as Markdown through a Jinja2 template you can edit, exports the library as a CSV, and emails a ZIP link with full files and articles. Sponge’s API returns every item’s record, captures, reader text, and annotations, and its export commands produce one bundle per source with the original bytes, the capture records, and your annotations, each file verified by checksum.
- **Price.** Reader is part of the full Readwise plan, which costs $12.99 a month, or $9.99 a month billed annually, after a 30-day trial. The $5.59 Lite plan does not include Reader. Sponge has no subscription today.

## Who should pick Readwise Reader

- You want a finished reading app on the web and on mobile, with newsletters, feeds, EPUBs, and video in one place.
- You collect highlights from other tools. Readwise syncs highlights from Kindle, Instapaper, and other sources into one review feed, and exports them to Notion, Obsidian, and other notes apps.
- You want exports that work without writing a script.

## Who should pick Sponge

- You want your own agent, such as Codex or Claude Code, to research from saved copies. It reads the saved text, cites the passages it uses in a document, and you check each claim against the original.
- You want to give that agent credentials you can scope and revoke, and keep a record of each change it makes.
- You mostly read public web pages and PDFs, and you do not need feeds, newsletters, or video.

## Moving a Reader library to Sponge

Switching takes a script over the Reader API: list your documents, save each link to Sponge, then re-create the highlights and notes whose text still matches Sponge’s copy. [Move a Readwise Reader library to Sponge](https://sponge.computer/migrate/readwise-reader) walks through each step.

## Sources

- [Readwise Reader API](https://readwise.io/reader_api), checked 2026-09-26
- [Exporting FAQs](https://docs.readwise.io/reader/docs/faqs/exporting), checked 2026-09-26
- [Pricing](https://readwise.io/pricing), checked 2026-09-26
- [Sponge agent research skill](https://sponge.computer/skills/sponge-research/SKILL.md), checked 2026-09-26