Cobalt vs Deepstash: Idea Cards or Real Book Summaries, Which to Choose?

Let's say it upfront: this article is published on the Cobalt blog. We are judge and party, and you deserve to know it before reading a single line. But a comparison is only useful if it is honest: Deepstash is a polished app, loved by millions of users, and caricaturing it would serve no one, starting with us.

The real question lies elsewhere: Deepstash and Cobalt are not exactly in the same business. One slices knowledge into collectible idea cards, the other builds structured book summaries designed for retention. Here is what brings them together, what separates them, and how to choose according to the way you learn.

Deepstash: knowledge as idea cards

Launched in 2019 by a Romanian team based between Bucharest and the United States, Deepstash claims several million users worldwide. Its concept: condensing books, articles, podcasts and videos into ideas, short cards readable in seconds, organized into thousands of topics, from leadership to philosophy.

The app has a genuine social dimension: you follow topics and curators, save cards into your stashes, share your finds. It is a daily inspiration feed, designed to replace social media scrolling with a more nourishing scroll. On that ground, Deepstash is probably the best app on the market.

Its limit is the flip side of its format: an idea card is a fragment. It sparks curiosity but rarely carries the book's complete reasoning, its structure and its nuances. And both content and interface are essentially in English: the team has mentioned a French translation, but as of July 2026 the experience remains largely English-speaking.

Cobalt: summaries built for retention

Cobalt is a French book summary app, available on iOS and Android, with the opposite stance: instead of fragmenting knowledge, it rebuilds each book into a structured summary of 8 minutes on average, broken down into key concepts with highlighted core ideas. The goal is not to collect ideas but to understand and retain a book's full argument.

Its most distinctive feature is the adjustable summary length: a setting lets you lengthen or shorten the text in real time, depending on your available time and desired level of detail. Five minutes before a meeting or twenty minutes on the train are not the same summary, and that is precisely the point.

Another structural difference: part of the Cobalt catalog is freely accessible on the web, without registration. About forty summaries (Atomic Habits, The Four Agreements, Influence...) can be read for free in a browser, which lets you judge the quality before installing anything. And the summaries exist in six languages: French, English, Portuguese, Spanish, German and Italian.

Format: idea card versus structured summary

This is the real dividing line. Deepstash excels at discovery: in five minutes of feed, you cross ten ideas from ten different sources. Cobalt excels at depth: in eight minutes, you follow the complete reasoning of a single book, from the problem it raises to the solutions it proposes.

Put differently, Deepstash is an inspiration feed, Cobalt a revision sheet. Inspiration makes you want to act, structure helps you remember. Depending on whether your priority is discovering new ideas or mastering reference books, the right tool is not the same.

Language: English alone versus six languages

This is often the deciding factor. Deepstash offers an ocean of content, but almost exclusively in English. Cobalt offers its summaries in six languages: French, English, Portuguese, Spanish, German and Italian. You read in your own language, and you can even reread a summary in another one to practice a foreign language. If English alone does not slow you down, the gap narrows; for everyone else, it is a deal breaker.

Price: two different logics

Deepstash is free to download and reading cards is open, with ads. The Pro plan, around 12.99 dollars per month or about 90 dollars per year according to prices observed in July 2026, unlocks unlimited saving, offline mode and removes ads. These prices may change: check their website for up-to-date pricing.

Cobalt works differently: the summaries published on the website are free, with no limit and no account creation. The app offers a Premium plan (all summaries, adjustable length, offline mode, personalized recommendations) with a 7-day free trial, at 5.99 euros per month (3.99 euros per month with the welcome promo).

The comparison table

Verdict: two apps, two needs

Choose Deepstash if you read English effortlessly and want a daily inspiration feed: short ideas from varied sources, a community of curators, and a format that pleasantly replaces social media scrolling.

Choose Cobalt if you want to read in your own language and retain whole books: summaries structured into highlighted concepts, length adjustable in real time, and summaries you can test for free on the web before installing anything. Fewer fragments, more depth.

We applied the same reading grid to the other players of the sector in our comparisons between Cobalt and Blinkist: https://www.cobaltapp.io/fr/blog/cobalt-vs-blinkist-application-resumes-livres and between Cobalt and Koober, the French pioneer: https://www.cobaltapp.io/fr/blog/cobalt-vs-koober-comparatif

The simplest way remains to make up your own mind: our summaries are freely available here https://www.cobaltapp.io/fr/resumes and the app is a free download on iOS and Android here https://www.cobaltapp.io/download . And if the idea card format appeals to you more, Deepstash is a fine app: that too is what an honest comparison looks like.