Docs (Context7 Library Lookup)

Fetch fresh, authoritative third-party library documentation via the ctx7 CLI instead of guessing from (possibly stale) training memory.

When to use

USE when:

•The user asks "how do I X with library Y" for a named third-party library or framework.
•You are about to write code against a named third-party library and are not 100% sure the API still matches your training memory.
•A question is version-specific ("in the latest FastAPI…", "post-v18 React…").

SKIP when:

•The question is about a language stdlib (Python stdlib, JS built-ins, Go stdlib, etc.) — you already know these and ctx7 is wasted tokens.
•It's a general CS/programming concept, not a library.
•The code in question lives in the current repo — read the source directly instead.
•You've already fetched the same library+topic earlier in this session — reuse that answer.

Two-step workflow

Context7 splits lookup into a name→ID resolution step and a docs-fetch step.

Step 1: Resolve library name to a Context7 ID

bash

npx ctx7 library <name> "<query>"

Always pass a query — per ctx7 library --help it's positional-optional, but Context7 uses it to rank candidate libraries by relevance and results get noticeably worse without it. The command returns several candidates; pick the first result unless it's obviously wrong for your query (e.g., a fork, translated mirror, or unrelated namespace match).

Example:

bash

npx ctx7 library react "useEffect cleanup function"
# → /reactjs/react.dev  (first result)
# → /websites/react_dev  (second result)
# → /facebook/react      (fifth result — canonical repo, but not always top-ranked)

Step 2: Fetch docs for the resolved ID

bash

npx ctx7 docs <libraryId> "<query>"

Unlike ctx7 library, both arguments are required on ctx7 docs — omitting the query produces error: missing required argument 'query' and exits 1.

Example:

bash

npx ctx7 docs /reactjs/react.dev "useEffect cleanup function"

Returns curated doc snippets ranked for the query, with source URLs.

When to skip Step 1

Only skip the library step when you already resolved the ID earlier in the current session and you're reusing it. Do not guess IDs from intuition — the highest-ranked match is often not what you'd expect (/reactjs/react.dev, not /facebook/react; /pola-rs/polars, not /polars/polars).

Efficiency rules

•Don't re-query the same library+topic twice in one session. Reuse the first result from earlier in the conversation.
•Prefer specific queries over broad ones. "useEffect cleanup" beats "hooks"; "scan_parquet glob pattern" beats "polars io".
•--json is available on both commands if you need structured output, but plain text is fine for reading into your own context.

ctx7 vs WebFetch

•Reach for ctx7 first whenever the target is a named library — it returns curated, query-ranked snippets rather than raw HTML.
•Fall back to WebFetch for arbitrary URLs (blog posts, GitHub issues, RFCs, changelogs not yet indexed by Context7).

Auth / setup

•The first npx ctx7 invocation auto-installs the package; no manual setup needed.
•Works anonymously. If you hit rate limits or auth errors, run npx ctx7 login and retry.

Common mistakes

•Guessing the library ID instead of running ctx7 library first. IDs are not predictable (/reactjs/react.dev, /pola-rs/polars). Resolve first unless you already saw the ID earlier in this session.
•Using ctx7 for stdlib questions. Wasted tokens — you already know list.sort().
•Re-running the same query multiple times in one session instead of reusing the first output.
•Ignoring errors. ctx7 exits 1 and prints ✖ … or error: … on bad IDs, missing queries, or unknown names. If the output looks like an error, it IS — don't proceed as if you got docs. Re-run ctx7 library with a different name or fall back to WebFetch.