diff --git a/internal/http/auth/session.go b/internal/http/auth/session.go
index 97a85524..a3e26aff 100644
--- a/internal/http/auth/session.go
+++ b/internal/http/auth/session.go
@@ -113,21 +113,25 @@ func WithSession(mgmt *management.State, signer *SessionSigner) Handler {
 			jwt.WithExpirationRequired(),
 		)
 		if err != nil || !token.Valid {
-			return ctx, ErrUnauthorized
+			return ctx, sessionTokenError(err)
 		}
 
+		// The signature verified, so the token is authentically one of ours and
+		// the remaining failures are safe to describe: they mean a valid session
+		// whose claims we can't use (e.g. an expired token, or one minted under a
+		// session policy that has since been removed).
 		methodID, err := uuid.Parse(claimString(claims, sessionMethodClaim))
 		if err != nil {
-			return ctx, ErrUnauthorized
+			return ctx, rejectSession(`token is missing or has an invalid auth-method ("amid") claim`)
 		}
 		subject := claimString(claims, "sub")
 		if subject == "" {
-			return ctx, ErrUnauthorized
+			return ctx, rejectSession(`token is missing the "sub" subject claim`)
 		}
 
 		method, err := mgmt.GetSessionAuthMethod(methodID)
 		if err != nil {
-			return ctx, ErrUnauthorized
+			return ctx, rejectSession("the session's auth method no longer exists")
 		}
 
 		// The session is only valid on its own project's URL; a token minted for
@@ -147,3 +151,22 @@ func WithSession(mgmt *management.State, signer *SessionSigner) Handler {
 		return rbac.WithActor(ctx, actor), nil
 	}
 }
+
+// sessionTokenError decides what a parse/verify failure surfaces. A signature,
+// signing-method, or format failure stays a generic ErrUnauthorized: it usually
+// means the token simply wasn't minted by us (e.g. it's a trusted-issuer JWT), so
+// the auth chain must fall through to the next handler rather than short-circuit.
+// Only a claim-level failure (ErrTokenInvalidClaims) — which the parser reaches
+// solely after the signature has verified — proves the token is authentically
+// ours, and is surfaced with a precise, debuggable reason (expired, missing exp,
+// ...).
+func sessionTokenError(err error) error {
+	if err == nil || !errors.Is(err, jwt.ErrTokenInvalidClaims) {
+		return ErrUnauthorized
+	}
+	return rejectSession(describeTokenError(err))
+}
+
+func rejectSession(reason string) error {
+	return &rejectedTokenError{msg: "session token rejected: " + reason}
+}
diff --git a/internal/http/auth/session_middleware_test.go b/internal/http/auth/session_middleware_test.go
index 7486d4a4..82d0e153 100644
--- a/internal/http/auth/session_middleware_test.go
+++ b/internal/http/auth/session_middleware_test.go
@@ -78,41 +78,64 @@ func TestWithSession(t *testing.T) {
 		assert.ErrorIs(t, err, ErrUnauthorized)
 	})
 
-	t.Run("an expired token is rejected through the handler", func(t *testing.T) {
+	// A token whose signature does not verify (wrong key, wrong alg, malformed)
+	// stays a generic ErrUnauthorized so the auth chain falls through to the next
+	// handler. Only once the signature proves the token is authentically ours does
+	// the handler surface a precise, debuggable reason for the remaining failures.
+
+	t.Run("an expired token surfaces a debuggable reason", func(t *testing.T) {
 		t.Parallel()
 		signer := testSigner(t, "")
 		token, _, err := signer.Mint(uuid.New(), "user_1", -time.Hour)
 		require.NoError(t, err)
 
 		_, err = WithSession(nil, signer)(context.Background(), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, "expired")
 	})
 
-	t.Run("a wrong issuer is rejected", func(t *testing.T) {
+	t.Run("a missing exp claim surfaces a debuggable reason", func(t *testing.T) {
 		t.Parallel()
-		// Signer expects the default issuer; mint with a different one.
+		signer := testSigner(t, "")
+		token := signES256(t, signer, jwt.MapClaims{
+			"iss":              signer.issuer,
+			"sub":              "user_1",
+			sessionMethodClaim: uuid.New().String(),
+			// no "exp": WithExpirationRequired rejects it
+		})
+		_, err := WithSession(nil, signer)(context.Background(), token)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, `"exp"`)
+	})
+
+	t.Run("a wrong issuer surfaces a debuggable reason", func(t *testing.T) {
+		t.Parallel()
+		// Signer expects the default issuer; mint with a different one. The
+		// signature still verifies (same key), so this is one of our tokens.
 		minter := testSigner(t, "https://evil.example")
 		token, _, err := minter.Mint(uuid.New(), "user_1", time.Hour)
 		require.NoError(t, err)
 
 		verifier := &SessionSigner{key: minter.key, issuer: defaultSessionIssuer}
 		_, err = WithSession(nil, verifier)(context.Background(), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, "issuer")
 	})
 
-	t.Run("a missing issuer is rejected", func(t *testing.T) {
+	t.Run("a token signed by a different key stays generic (chain falls through)", func(t *testing.T) {
 		t.Parallel()
-		signer := testSigner(t, "")
-		token := signES256(t, signer, jwt.MapClaims{
-			"sub":              "user_1",
-			sessionMethodClaim: uuid.New().String(),
-			"exp":              time.Now().Add(time.Hour).Unix(),
-		})
-		_, err := WithSession(nil, signer)(context.Background(), token)
+		// A token minted by a foreign key (e.g. another scheme's) must not be
+		// described as a session token; it stays ErrUnauthorized so the next
+		// handler gets a turn.
+		minter := testSigner(t, "")
+		token, _, err := minter.Mint(uuid.New(), "user_1", time.Hour)
+		require.NoError(t, err)
+
+		_, err = WithSession(nil, testSigner(t, ""))(context.Background(), token)
 		assert.ErrorIs(t, err, ErrUnauthorized)
 	})
 
-	t.Run("an unparseable method id claim is rejected", func(t *testing.T) {
+	t.Run("an unparseable method id claim surfaces a debuggable reason", func(t *testing.T) {
 		t.Parallel()
 		signer := testSigner(t, "")
 		token := signES256(t, signer, jwt.MapClaims{
@@ -122,10 +145,11 @@ func TestWithSession(t *testing.T) {
 			"exp":              time.Now().Add(time.Hour).Unix(),
 		})
 		_, err := WithSession(nil, signer)(context.Background(), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, "amid")
 	})
 
-	t.Run("a missing method id claim is rejected", func(t *testing.T) {
+	t.Run("a missing method id claim surfaces a debuggable reason", func(t *testing.T) {
 		t.Parallel()
 		signer := testSigner(t, "")
 		token := signES256(t, signer, jwt.MapClaims{
@@ -134,10 +158,11 @@ func TestWithSession(t *testing.T) {
 			"exp": time.Now().Add(time.Hour).Unix(),
 		})
 		_, err := WithSession(nil, signer)(context.Background(), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, "amid")
 	})
 
-	t.Run("an empty subject is rejected", func(t *testing.T) {
+	t.Run("an empty subject surfaces a debuggable reason", func(t *testing.T) {
 		t.Parallel()
 		signer := testSigner(t, "")
 		token := signES256(t, signer, jwt.MapClaims{
@@ -147,7 +172,8 @@ func TestWithSession(t *testing.T) {
 			"exp":              time.Now().Add(time.Hour).Unix(),
 		})
 		_, err := WithSession(nil, signer)(context.Background(), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, `"sub"`)
 	})
 }
 
@@ -161,7 +187,8 @@ func TestWithSessionUnknownMethod(t *testing.T) {
 	require.NoError(t, err)
 
 	_, err = WithSession(mgmt, signer)(context.Background(), token)
-	assert.ErrorIs(t, err, ErrUnauthorized)
+	assert.NotErrorIs(t, err, ErrUnauthorized)
+	assert.ErrorContains(t, err, "auth method")
 }
 
 func TestWithSessionBuildsActor(t *testing.T) {
diff --git a/internal/http/auth/trusted_issuer.go b/internal/http/auth/trusted_issuer.go
index 3fde53fa..45d7b253 100644
--- a/internal/http/auth/trusted_issuer.go
+++ b/internal/http/auth/trusted_issuer.go
@@ -5,6 +5,8 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
+	"fmt"
+	"strings"
 
 	"github.com/golang-jwt/jwt/v5"
 	"github.com/lunogram/platform/internal/jwks"
@@ -48,14 +50,21 @@ func WithTrustedIssuer(mgmt *management.State, cache *jwks.Cache) Handler {
 			return ctx, ErrUnauthorized
 		}
 
+		// Past this point the token has resolved to a configured trusted issuer,
+		// so a failure is almost always a malformed or misconfigured integration
+		// token rather than a probe. Surface a precise reason (which claim is
+		// missing, expired, etc.) instead of a bare "unauthorized": the failures
+		// above stay generic because revealing them would leak whether an issuer
+		// or project exists, but here the caller already proved knowledge of a
+		// real issuer, so a debuggable message is safe and far more useful.
 		claims, err := verifyTrustedIssuerToken(ctx, cache, method, tokenString)
 		if err != nil {
-			return ctx, ErrUnauthorized
+			return ctx, rejectTrustedIssuer(describeTokenError(err))
 		}
 
 		subject := claimString(claims, method.SubjectClaim)
 		if subject == "" {
-			return ctx, ErrUnauthorized
+			return ctx, rejectTrustedIssuer(fmt.Sprintf("token is missing the %q subject claim", subjectClaimName(method.SubjectClaim)))
 		}
 
 		actor := rbac.NewActor(
@@ -106,8 +115,15 @@ func verifyTrustedIssuerToken(ctx context.Context, cache *jwks.Cache, method *ma
 			token, err = jwt.ParseWithClaims(tokenString, claims, refreshed, opts...)
 		}
 	}
-	if err != nil || !token.Valid {
-		return nil, errors.New("auth: invalid trusted-issuer token")
+	// Preserve the underlying cause (a golang-jwt sentinel such as
+	// ErrTokenRequiredClaimMissing or ErrTokenExpired) so the caller can turn it
+	// into a precise, debuggable message; describeTokenError keeps the surfaced
+	// text safe.
+	if err != nil {
+		return nil, err
+	}
+	if !token.Valid {
+		return nil, jwt.ErrTokenUnverifiable
 	}
 	return claims, nil
 }
@@ -147,11 +163,75 @@ func parsePublicKeyPEM(pemData string) (any, error) {
 
 // claimString returns the string value of the named claim (defaulting to "sub").
 func claimString(claims jwt.MapClaims, name string) string {
-	if name == "" {
-		name = "sub"
-	}
-	if v, ok := claims[name].(string); ok {
+	if v, ok := claims[subjectClaimName(name)].(string); ok {
 		return v
 	}
 	return ""
 }
+
+// subjectClaimName resolves the configured subject claim, defaulting to "sub".
+func subjectClaimName(name string) string {
+	if name == "" {
+		return "sub"
+	}
+	return name
+}
+
+// rejectedTokenError carries a human-readable reason a token was rejected.
+// Unlike ErrUnauthorized it is deliberately NOT part of the generic unauthorized
+// set, so the authentication middleware returns it (and its reason) to the caller
+// instead of collapsing it into a bare "unauthorized". It is only produced once a
+// token has proven to be authentically ours (a resolved trusted issuer, or a
+// verified session signature), so the detail aids an integrator debugging their
+// token without leaking anything they don't already know.
+type rejectedTokenError struct{ msg string }
+
+func (e *rejectedTokenError) Error() string { return e.msg }
+
+func rejectTrustedIssuer(reason string) error {
+	return &rejectedTokenError{msg: "trusted-issuer token rejected: " + reason}
+}
+
+// describeTokenError maps a JWT verification failure to a concise reason that is
+// safe to return to the integrator. It names the standard-claim problems an
+// integrator can actually fix (missing/expired/not-yet-valid/issuer/audience);
+// anything else — signature, malformed token, key resolution — collapses to a
+// generic reason so verification internals are never leaked.
+func describeTokenError(err error) string {
+	switch {
+	case errors.Is(err, jwt.ErrTokenRequiredClaimMissing):
+		// Both the mandatory "exp" and an enforced "iss"/"aud" land here; name the
+		// claim golang-jwt actually reported rather than assuming "exp".
+		return requiredClaimReason(err)
+	case errors.Is(err, jwt.ErrTokenExpired):
+		return "token has expired"
+	case errors.Is(err, jwt.ErrTokenNotValidYet):
+		return `token is not valid yet (its "nbf" not-before claim is in the future)`
+	case errors.Is(err, jwt.ErrTokenUsedBeforeIssued):
+		return `token was used before its "iat" issued-at time`
+	case errors.Is(err, jwt.ErrTokenInvalidIssuer):
+		return "token issuer does not match the expected issuer"
+	case errors.Is(err, jwt.ErrTokenInvalidAudience):
+		return "token audience does not match the expected audience"
+	case errors.Is(err, jwt.ErrTokenSignatureInvalid), errors.Is(err, jwt.ErrTokenUnverifiable):
+		return "token signature could not be verified against the issuer's keys"
+	default:
+		return "token could not be verified"
+	}
+}
+
+// requiredClaimReason names the standard claim golang-jwt reported as missing.
+// golang-jwt phrases the leaf as `<claim> claim is required`, so an integrator
+// sees exactly what to add — most often the mandatory "exp" expiry claim. It
+// falls back to a generic (but still exp-centric) message if the library ever
+// changes that phrasing.
+func requiredClaimReason(err error) string {
+	const marker = "missing required claim: "
+	msg := err.Error()
+	if i := strings.LastIndex(msg, marker); i >= 0 {
+		if claim := strings.TrimSuffix(msg[i+len(marker):], " claim is required"); claim != "" && !strings.ContainsRune(claim, ' ') {
+			return fmt.Sprintf("token is missing the required %q claim", claim)
+		}
+	}
+	return `token is missing a required claim (the "exp" expiry claim is mandatory)`
+}
diff --git a/internal/http/auth/trusted_issuer_middleware_test.go b/internal/http/auth/trusted_issuer_middleware_test.go
index 7a65775c..34e69edf 100644
--- a/internal/http/auth/trusted_issuer_middleware_test.go
+++ b/internal/http/auth/trusted_issuer_middleware_test.go
@@ -112,7 +112,11 @@ func TestWithTrustedIssuer(t *testing.T) {
 		assert.ErrorIs(t, err, ErrUnauthorized)
 	})
 
-	t.Run("rejects when verification fails (wrong key)", func(t *testing.T) {
+	// Failures before the issuer resolves stay a generic ErrUnauthorized (above);
+	// once the token resolves to a configured issuer, the middleware surfaces a
+	// precise, debuggable reason instead so an integrator can fix their token.
+
+	t.Run("rejects when verification fails (wrong key) with a debuggable reason", func(t *testing.T) {
 		_, method := issuerMethod(t)
 		other, err := rsa.GenerateKey(rand.Reader, 2048)
 		require.NoError(t, err)
@@ -122,10 +126,35 @@ func TestWithTrustedIssuer(t *testing.T) {
 			"exp": time.Now().Add(time.Hour).Unix(),
 		})
 		_, err = WithTrustedIssuer(newIssuerState(method), cache)(clientRequestCtx(method.ProjectID.String()), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized, "a resolved issuer surfaces a reason, not a bare unauthorized")
+		assert.ErrorContains(t, err, "signature could not be verified")
+	})
+
+	t.Run("rejects a missing exp claim with a debuggable reason", func(t *testing.T) {
+		key, method := issuerMethod(t)
+		token := signRS256(t, key, jwt.MapClaims{
+			"iss": method.Issuer,
+			"sub": "user_123",
+			// no "exp": WithExpirationRequired rejects it
+		})
+		_, err := WithTrustedIssuer(newIssuerState(method), cache)(clientRequestCtx(method.ProjectID.String()), token)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, `"exp"`, "the reason names the missing claim")
+	})
+
+	t.Run("rejects an expired token with a debuggable reason", func(t *testing.T) {
+		key, method := issuerMethod(t)
+		token := signRS256(t, key, jwt.MapClaims{
+			"iss": method.Issuer,
+			"sub": "user_123",
+			"exp": time.Now().Add(-time.Hour).Unix(),
+		})
+		_, err := WithTrustedIssuer(newIssuerState(method), cache)(clientRequestCtx(method.ProjectID.String()), token)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, "expired")
 	})
 
-	t.Run("rejects an empty subject claim", func(t *testing.T) {
+	t.Run("rejects an empty subject claim with a debuggable reason", func(t *testing.T) {
 		key, method := issuerMethod(t)
 		token := signRS256(t, key, jwt.MapClaims{
 			"iss": method.Issuer,
@@ -133,7 +162,21 @@ func TestWithTrustedIssuer(t *testing.T) {
 			"exp": time.Now().Add(time.Hour).Unix(),
 		})
 		_, err := WithTrustedIssuer(newIssuerState(method), cache)(clientRequestCtx(method.ProjectID.String()), token)
-		assert.ErrorIs(t, err, ErrUnauthorized)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, `"sub"`, "the reason names the missing subject claim")
+	})
+
+	t.Run("names a custom subject claim in the rejection reason", func(t *testing.T) {
+		key, method := issuerMethod(t)
+		method.SubjectClaim = "user_id"
+		token := signRS256(t, key, jwt.MapClaims{
+			"iss": method.Issuer,
+			// no "user_id": the configured subject claim is absent
+			"exp": time.Now().Add(time.Hour).Unix(),
+		})
+		_, err := WithTrustedIssuer(newIssuerState(method), cache)(clientRequestCtx(method.ProjectID.String()), token)
+		assert.NotErrorIs(t, err, ErrUnauthorized)
+		assert.ErrorContains(t, err, `"user_id"`)
 	})
 
 	t.Run("builds an end-user actor on success", func(t *testing.T) {