jwx-guide-v4

This skill helps you assist Go developers who are using github.com/lestrrat-go/jwx/v4 in their own projects. It is scoped to v4 only. There is no equivalent skill for v3 or v2; for those, work from the version's own docs/ directory and pkg.go.dev. Do not apply v4 rules to a v3 or v2 codebase — the jwa identifiers are constants there, not functions, and the jwk and error APIs differ.

Where to look things up

You will not have this repository checked out. To verify an API claim before answering, use these sources, in order of preference:

1. Go module cache — if the user's project depends on jwx, the source is on disk:

   echo "$(go env GOMODCACHE)/github.com/lestrrat-go/jwx/v4@$(go list -m -f '{{.Version}}' github.com/lestrrat-go/jwx/v4)"

   This directory contains the full source tree including docs/, jwt/, jws/, jwe/, jwk/, jwa/.
2. pkg.go.dev — canonical API reference for any exported symbol:
   • https://pkg.go.dev/github.com/lestrrat-go/jwx/v4
   • https://pkg.go.dev/github.com/lestrrat-go/jwx/v4/jwt (etc. per subpackage)
3. GitHub — narrative documentation lives at the repo:
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/01-jwt.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/02-jws.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/03-jwe.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/04-jwk.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/10-extensions.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/99-faq.md
4. Examples repo — runnable usage patterns. The repo's README.md is a topical index that maps "what do I want to do" → "which *_example_test.go file." Fetch the README first when looking for an example by topic; then fetch the linked file:
   • https://github.com/jwx-go/examples/blob/develop/v4/README.md

When the user's question goes beyond the patterns in this skill (custom claim types, JWE recipients with per-recipient headers, nested JWS+JWE serializers, custom key providers, base64 backend swap, performance tuning), fetch from these sources rather than guessing.

Prerequisites

• Go 1.26+ is required. v4 uses generics features and stdlib additions (errors.AsType[T]) that are new in 1.26.
• GOEXPERIMENT=jsonv2 must be set on Go 1.26 for every go build/go test/go run, because v4 depends on encoding/json/v2. Without it → builds fail with build constraints exclude all Go files. NEVER set it on Go 1.27+ → encoding/json/v2 is in the standard library there, and naming the experiment rebuilds the standard library under a non-default configuration.

Sub-package map:

Critical rules (read first)

1. jwt.Parse verifies AND validates by default. A bare jwt.Parse(data) errors because no key was supplied. To intentionally skip both, use jwt.ParseInsecure. To verify but skip claim validation, pass jwt.WithValidate(false). This is deliberately asymmetric vs. jws.Parse/jwe.Parse (which only parse). Do not "correct" it.
2. Always pin the algorithm on the verify side. jwt.WithKey(jwa.RS256(), key), jws.WithKey(jwa.ES256(), key). Never trust the alg from the incoming header alone.
3. Never use jwt.ParseInsecure for tokens received from the network. It is for testing or for extracting claims from a token whose origin is already trusted by other means.
4. jwa algorithms are functions in v4, not constants. Write jwa.RS256(), not jwa.RS256. This trips up users migrating from v2/v3.
5. kid matching is enforced when verifying with a JWK Set. Override the requirement with jwt.WithKeySet(set, jws.WithRequireKid(false)) only when you understand the consequences.
6. jku (key URL in the JWS header) is attacker-controlled. Use jwt.WithVerifyAuto only with a jwkfetch.Client configured with a jwkfetch.NewMapWhitelist() of allowed URLs.
7. HMAC keys are []byte, not string. Pass []byte("secret"), or better, a jwk.Key imported from those bytes.
8. jwk.Import and jwk.Export require explicit type parameters. jwk.Import[jwk.Key](raw), jwk.Export[*rsa.PublicKey](key). Their type argument is not inferable from the call, so bare jwk.Import(raw) does not compile.
9. jwk.ParseKey is not generic. jwk.ParseKey(data) returns (jwk.Key, error). Use jwk.ParseKeyAs[jwk.RSAPublicKey](data) when a concrete JWK type is required.

Verifying a JWT (the 90% case)

import (
    "github.com/lestrrat-go/jwx/v4/jwa"
    "github.com/lestrrat-go/jwx/v4/jwt"
)

tok, err := jwt.Parse(raw, jwt.WithKey(jwa.RS256(), publicKey))
if err != nil {
    // signature failed, claim validation failed, or parse failed
    return err
}
// tok is verified + validated; safe to read claims

publicKey may be:

• a *rsa.PublicKey / *ecdsa.PublicKey / ed25519.PublicKey from crypto/*
• []byte for HMAC algorithms
• a jwk.Key

To validate against expected claim values, pass jwt.WithIssuer(...), jwt.WithAudience(...), jwt.WithSubject(...), jwt.WithJwtID(...). To extend the default clock skew window: jwt.WithAcceptableSkew(30 * time.Second).

Verifying with a JWK Set (kid-based key selection)

set, err := jwk.Parse(jwksBytes)
if err != nil { return err }

tok, err := jwt.Parse(raw, jwt.WithKeySet(set))

If the JWS header has a kid, the matching key is selected from the set. The algorithm comes from each key's alg field. To opt out of kid-required matching: jwt.WithKeySet(set, jws.WithRequireKid(false)).

A key with no alg field is skipped, not guessed at. Inference from the key type is opt-in via jwt.WithKeySet(set, jws.WithInferAlgorithmFromKey(true)), and it is a fallback, not a default. It tries every algorithm compatible with the key type, so it is slower and weaker than an explicit alg; combined with jws.WithRequireKid(false) against a large JWKS it also multiplies out to N_keys × N_algs_per_keytype verification attempts. The right fix is almost always to add alg to the keys in the JWKS. If a user reports that verification against a JWKS silently finds no usable key, check for missing alg fields first.

Verifying via JWKS endpoint

HTTP fetching is not in the core jwk package in v4. It lives in a companion module:

import "github.com/jwx-go/jwkfetch/v4"

client := jwkfetch.NewClient()
set, err := client.Fetch(ctx, "https://issuer.example/jwks.json")
// then: jwt.Parse(raw, jwt.WithKeySet(set))

For repeated fetches with background refresh, use jwkfetch.NewCache. For jku-driven verification, build a jwkfetch.Client with a jwkfetch.NewMapWhitelist() of allowed URLs and pass it to jwt.WithVerifyAuto(client).

Signing a JWT

tok, err := jwt.NewBuilder().
    Issuer("https://issuer.example").
    Audience([]string{"https://api.example"}).
    Subject("user-123").
    IssuedAt(time.Now()).
    Expiration(time.Now().Add(15 * time.Minute)).
    Claim("scope", "read:things").
    Build()
if err != nil { return err }

signed, err := jwt.Sign(tok, jwt.WithKey(jwa.RS256(), privateKey))

privateKey may be a *rsa.PrivateKey/*ecdsa.PrivateKey/ed25519.PrivateKey/HMAC []byte, or a jwk.Key.

Match algorithm to key type:

JWK basics

jwk.Import and jwk.Export require the type parameter. The parsers do not: jwk.ParseKey and jwk.Parse are non-generic, and jwk.ParseKeyAs[T] is the typed variant.

// Parse a single JWK (returns jwk.Key):
key, err := jwk.ParseKey(jwkBytes)

// Parse a single JWK with a concrete type (fails if not that type):
rsaKey, err := jwk.ParseKeyAs[jwk.RSAPublicKey](jwkBytes)

// Parse a JWK Set (returns jwk.Set, not generic):
set, err := jwk.Parse(jwksBytes)

// Wrap an existing crypto.* key as a jwk.Key:
key, err := jwk.Import[jwk.Key](rsaPrivKey)

// Or with a concrete jwk type:
typed, err := jwk.Import[jwk.RSAPrivateKey](rsaPrivKey)

// Export a jwk.Key back to a crypto.* key:
raw, err := jwk.Export[*rsa.PublicKey](key)

jwk.Import[jwk.Key] is the default. Use a concrete type parameter (jwk.RSAPrivateKey, jwk.ECDSAPublicKey, jwk.SymmetricKey, jwk.OKPPublicKey, etc.) only when you want compile-time guarantees and acceptance of failure when the input is anything else.

Generating keys

import (
    "crypto/rand"
    "crypto/rsa"

    "github.com/lestrrat-go/jwx/v4/jwa"
    "github.com/lestrrat-go/jwx/v4/jwk"
)

raw, _ := rsa.GenerateKey(rand.Reader, 2048)
key, _ := jwk.Import[jwk.Key](raw)
key.Set(jwk.KeyIDKey, "2025-q1")
key.Set(jwk.AlgorithmKey, jwa.RS256())

pubKey, _ := jwk.PublicKeyOf(key)

jwk.KeyIDKey and jwk.AlgorithmKey are string constants for the standard JWK fields kid and alg.

JWS (signing arbitrary payloads)

sig, err := jws.Sign(payload, jws.WithKey(jwa.ES256(), privateKey))
payload, err := jws.Verify(sig, jws.WithKey(jwa.ES256(), publicKey))

jws.Parse only parses the structure — it does not verify. Use jws.Verify (which returns the verified payload) for verification.

The protected alg must match the verifying algorithm exactly

jws.Verify rejects a message whose protected header advertises one algorithm while it is verified under another. The comparison is plain string equality with no aliasing, and it applies to every key source (jws.WithKey, jws.WithKeySet, jws.WithVerifyAuto, custom jws.WithKeyProvider). The check only fires when the protected header actually carries an alg.

The practical consequence involves EdDSA. Per RFC 9864, EdDSA, Ed25519 and Ed448 are three distinct alg values, so a token whose header says alg: Ed25519 does not verify under jws.WithKey(jwa.EdDSA(), key), and vice versa. Match the identifier the producer actually emitted.

jws.WithSkipAlgorithmMatch(true) bypasses the check. It exists for interop with non-conforming producers, and it weakens a real safety guard, so treat it the way you treat jws.WithRequireKid(false).

JWE (encrypting payloads)

enc, err := jwe.Encrypt(
    payload,
    jwe.WithKey(jwa.RSA_OAEP_256(), recipientPublicKey),
    jwe.WithContentEncryption(jwa.A256GCM()),
)

plain, err := jwe.Decrypt(enc, jwe.WithKey(jwa.RSA_OAEP_256(), recipientPrivateKey))

jwe.WithKey(alg, key) is the same on both encrypt and decrypt sides — this symmetry is intentional.

Companion modules

Beyond the core github.com/lestrrat-go/jwx/v4 module, the project ships companion modules under github.com/jwx-go. The agent should know what's available and when to reach for each one — depth lives in each module's godoc.

For algorithm and HPKE modules: import for side effects (import _ "..."). They register themselves in init() and panic at import time if registration fails (intentional — surfaces problems early). The one case that used to panic in normal use no longer does: see the ML-DSA note below.

Signature algorithms (extension)

Key agreement / encryption (extension)

Tooling and backends

Cross-references

• Canonical extension docs (always-current list, with examples): https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/10-extensions.md
• Each companion's godoc: https://pkg.go.dev/github.com/jwx-go/<name>/v4

What to do when the user asks about post-quantum or non-default algorithms

Default jwx supports the common RFC 7518 algorithms (RS*, PS*, ES*, HS*, EdDSA, AGCM, RSA-OAEP-, etc.) out of the box. For everything in the tables above, the user must add the companion module to their go.mod and import it for side effects. If a user reports an algorithm not registered or similar error for ES256K/Ed448/ML-DSA/ML-KEM/X448, they almost certainly missed the side-effect import.

ML-DSA is the one exception, and it depends on the toolchain. From Go 1.27 on, crypto/mldsa is in the standard library, so jwx registers jwa.MLDSA44()/MLDSA65()/MLDSA87() natively and no companion module or side-effect import is needed. On Go 1.26 the algorithms are not registered at all, and github.com/jwx-go/mldsa/v4 is still required.

Keeping the extension imported on Go 1.27 is harmless. From jwx-go/mldsa v4.0.5 on it detects jwx's native registration and bridges filippo.io/mldsa keys onto it instead of registering the algorithms a second time, so code mid-migration keeps working. Only versions before v4.0.5 panic at startup on that combination, because the duplicate registration is rejected. If a user hits that panic, tell them to upgrade the extension, not to drop the import. Canonical owner: docs/10-extensions.md, section "Which implementation you get".

Errors

JWT/JWS/JWE/JWK errors are struct types with named fields. Use errors.Is with a zero-value struct to test for kind, or Go 1.26's errors.AsType[T] to recover the fields:

import "errors"

tok, err := jwt.Parse(raw, jwt.WithKey(jwa.RS256(), key))
if err != nil {
    if errors.Is(err, jwt.TokenExpiredError{}) {
        // exp claim failed
    }
    if e, ok := errors.AsType[jwt.InvalidAudienceError](err); ok {
        log.Printf("audience mismatch: %+v", e)
    }
}

Common types:

• jwt: TokenExpiredError, TokenNotYetValidError, InvalidIssuerError, InvalidAudienceError, MissingRequiredClaimError, ValidationError, ParseError
• jws: VerificationError() factory (signature verification failed)
• jwe: DecryptError() factory, AlgorithmMismatchError
• jwk: KeyTypeMismatchError, ImportError(), ParseError()

Do not match errors by string contents — messages may change.

Token field access

Standard claims have dedicated typed accessors (returning (value, ok)):

exp, ok := tok.Expiration()  // time.Time
iss, ok := tok.Issuer()      // string
aud, ok := tok.Audience()    // []string
sub, ok := tok.Subject()     // string
nbf, ok := tok.NotBefore()
iat, ok := tok.IssuedAt()
jti, ok := tok.JwtID()

For private claims, use tok.Field(name) which returns (any, bool). For type-safe access, use jwt.Get[T](tok, name).

Common mistakes to flag in user code

When reviewing or writing jwx-using code, watch for these:

1. jwt.Parse(data) with no key option — errors out; if the intent was to read claims without verifying, that's a security bug unless the source is already trusted, in which case use jwt.ParseInsecure.
2. jwa.RS256 instead of jwa.RS256() — these are functions in v4.
3. jwk.Import(raw) or jwk.Export(key) without the type parameter — won't compile.
4. jwk.ParseKey[jwk.RSAPublicKey](data) — ParseKey is not generic; the typed parser is jwk.ParseKeyAs[T].
5. Type-asserting a jwk.Key to a crypto.* type. Use jwk.Export[*rsa.PublicKey](key) instead.
6. Hardcoding the alg from the JWS header (or token contents) to pick a verifier — always pin the expected algorithm on the verify side.
7. Reusing a single jwt.Builder across goroutines — builders aren't safe to share.
8. Verifying by passing a private key — works but leaks intent. Use the public key on the verify side.
9. Manually building JWS compact strings via concatenation — always go through jws.Sign/jwt.Sign.
10. tok.Get("exp") — the method is tok.Field("exp") (or just tok.Expiration()).
11. Disabling kid matching as a "make it work" shortcut. Understand why the kid doesn't match before adding jws.WithRequireKid(false).

What NOT to suggest

• Disabling signature verification to "make it work."
• Forking or vendoring jwx to add an algorithm — recommend the extension modules listed above.
• Decoding the JWT envelope with encoding/json directly — always go through jwt.Parse / jws.Verify.
• Recommending a different JWT library — out of scope for this skill.