diff --git a/backend/go.mod b/backend/go.mod
index cdeaa84..41aaccd 100644
--- a/backend/go.mod
+++ b/backend/go.mod
@@ -7,6 +7,7 @@ require github.com/mark3labs/mcp-go v0.56.0
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/go-chi/chi/v5 v5.3.1 // indirect
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/google/jsonschema-go v0.4.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
diff --git a/backend/go.sum b/backend/go.sum
index 756981d..15b6999 100644
--- a/backend/go.sum
+++ b/backend/go.sum
@@ -8,6 +8,8 @@ github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHk
 github.com/frankban/quicktest v1.14.6/go.mod h1:4ptaffx2x8+WTWXmUCuVU6aPUX1/Mz7zb5vbUoiM6w0=
 github.com/go-chi/chi/v5 v5.3.1 h1:3j4HZLGZQ3JpMCrPJF/Jl3mYJfWLKBfNJ6quurUGCf8=
 github.com/go-chi/chi/v5 v5.3.1/go.mod h1:R+tYY2hNuVUUjxoPtqUdgBqevM9s9njzkTLutVsOCto=
+github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
+github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/google/jsonschema-go v0.4.2 h1:tmrUohrwoLZZS/P3x7ex0WAVknEkBZM46iALbcqoRA8=
diff --git a/backend/internal/auth/session.go b/backend/internal/auth/session.go
new file mode 100644
index 0000000..5332918
--- /dev/null
+++ b/backend/internal/auth/session.go
@@ -0,0 +1,150 @@
+// Package auth implements geniusrun's own login gate: an OIDC Authorization
+// Code flow against an existing Keycloak realm (see oidc.go), backed by a
+// signed session cookie geniusrun mints itself (this file) and a chi
+// middleware that checks it (middleware.go). Keycloak's own tokens are never
+// stored or refreshed -- once HandleCallback verifies the ID token and role,
+// only this package's own cookie matters for subsequent requests.
+package auth
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+const (
+	// SessionCookieName holds the signed proof that a request's caller
+	// completed login and passed the required-role check.
+	SessionCookieName = "geniusrun_session"
+	// TxnCookieName holds the short-lived OIDC login transaction (state +
+	// PKCE verifier) between BeginLogin's redirect and HandleCallback.
+	TxnCookieName = "geniusrun_oidc_txn"
+
+	txnCookieDuration = 10 * time.Minute
+)
+
+// Claims identifies the authenticated user, carried in the signed session
+// cookie.
+type Claims struct {
+	Sub   string
+	Name  string
+	Email string
+}
+
+type sessionClaims struct {
+	Sub   string `json:"sub"`
+	Name  string `json:"name"`
+	Email string `json:"email"`
+	jwt.RegisteredClaims
+}
+
+// TxnState is the OIDC login transaction round-tripped via TxnCookieName
+// between BeginLogin and HandleCallback.
+type TxnState struct {
+	State        string
+	CodeVerifier string
+}
+
+type txnClaims struct {
+	State        string `json:"state"`
+	CodeVerifier string `json:"code_verifier"`
+	jwt.RegisteredClaims
+}
+
+// MintSessionCookie signs claims into a JWT valid for duration and wraps it
+// in a cookie. secure should be true whenever the app is served over HTTPS.
+func MintSessionCookie(claims Claims, secret []byte, duration time.Duration, secure bool) (*http.Cookie, error) {
+	now := time.Now()
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, sessionClaims{
+		Sub:   claims.Sub,
+		Name:  claims.Name,
+		Email: claims.Email,
+		RegisteredClaims: jwt.RegisteredClaims{
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(duration)),
+		},
+	})
+	signed, err := token.SignedString(secret)
+	if err != nil {
+		return nil, fmt.Errorf("sign session token: %w", err)
+	}
+	return &http.Cookie{
+		Name:     SessionCookieName,
+		Value:    signed,
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+		Expires:  now.Add(duration),
+	}, nil
+}
+
+// ParseSessionCookie verifies the cookie's signature and expiry and decodes
+// its Claims.
+func ParseSessionCookie(cookie *http.Cookie, secret []byte) (Claims, error) {
+	if cookie == nil {
+		return Claims{}, fmt.Errorf("no session cookie")
+	}
+	var sc sessionClaims
+	if _, err := jwt.ParseWithClaims(cookie.Value, &sc, keyfunc(secret), jwt.WithValidMethods([]string{"HS256"})); err != nil {
+		return Claims{}, fmt.Errorf("parse session token: %w", err)
+	}
+	return Claims{Sub: sc.Sub, Name: sc.Name, Email: sc.Email}, nil
+}
+
+// MintTxnCookie signs an OIDC login transaction into a short-lived cookie.
+func MintTxnCookie(txn TxnState, secret []byte, secure bool) (*http.Cookie, error) {
+	now := time.Now()
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, txnClaims{
+		State:        txn.State,
+		CodeVerifier: txn.CodeVerifier,
+		RegisteredClaims: jwt.RegisteredClaims{
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(txnCookieDuration)),
+		},
+	})
+	signed, err := token.SignedString(secret)
+	if err != nil {
+		return nil, fmt.Errorf("sign txn token: %w", err)
+	}
+	return &http.Cookie{
+		Name:     TxnCookieName,
+		Value:    signed,
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+		Expires:  now.Add(txnCookieDuration),
+	}, nil
+}
+
+// ParseTxnCookie verifies and decodes a login transaction cookie.
+func ParseTxnCookie(cookie *http.Cookie, secret []byte) (TxnState, error) {
+	if cookie == nil {
+		return TxnState{}, fmt.Errorf("no txn cookie")
+	}
+	var tc txnClaims
+	if _, err := jwt.ParseWithClaims(cookie.Value, &tc, keyfunc(secret), jwt.WithValidMethods([]string{"HS256"})); err != nil {
+		return TxnState{}, fmt.Errorf("parse txn token: %w", err)
+	}
+	return TxnState{State: tc.State, CodeVerifier: tc.CodeVerifier}, nil
+}
+
+// ClearCookie returns a cookie that immediately expires the named cookie.
+func ClearCookie(name string, secure bool) *http.Cookie {
+	return &http.Cookie{
+		Name:     name,
+		Value:    "",
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+		MaxAge:   -1,
+	}
+}
+
+func keyfunc(secret []byte) jwt.Keyfunc {
+	return func(*jwt.Token) (any, error) { return secret, nil }
+}
diff --git a/backend/internal/auth/session_test.go b/backend/internal/auth/session_test.go
new file mode 100644
index 0000000..7b0f3f1
--- /dev/null
+++ b/backend/internal/auth/session_test.go
@@ -0,0 +1,84 @@
+package auth
+
+import (
+	"testing"
+	"time"
+)
+
+const testSecret = "test-secret-at-least-32-bytes-long!"
+
+func TestMintAndParseSessionCookie(t *testing.T) {
+	claims := Claims{Sub: "u1", Name: "Alice", Email: "alice@example.com"}
+	cookie, err := MintSessionCookie(claims, []byte(testSecret), time.Hour, true)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	if cookie.Name != SessionCookieName || !cookie.HttpOnly || !cookie.Secure {
+		t.Fatalf("cookie = %+v, want name=%s HttpOnly+Secure", cookie, SessionCookieName)
+	}
+
+	got, err := ParseSessionCookie(cookie, []byte(testSecret))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if got != claims {
+		t.Fatalf("got %+v, want %+v", got, claims)
+	}
+}
+
+func TestParseSessionCookie_Expired(t *testing.T) {
+	cookie, err := MintSessionCookie(Claims{Sub: "u1"}, []byte(testSecret), -time.Hour, false)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	if _, err := ParseSessionCookie(cookie, []byte(testSecret)); err == nil {
+		t.Fatal("expected error for expired cookie")
+	}
+}
+
+func TestParseSessionCookie_Tampered(t *testing.T) {
+	cookie, err := MintSessionCookie(Claims{Sub: "u1"}, []byte(testSecret), time.Hour, false)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	cookie.Value = cookie.Value[:len(cookie.Value)-1] + "x"
+	if _, err := ParseSessionCookie(cookie, []byte(testSecret)); err == nil {
+		t.Fatal("expected error for tampered cookie")
+	}
+}
+
+func TestParseSessionCookie_WrongSecret(t *testing.T) {
+	cookie, err := MintSessionCookie(Claims{Sub: "u1"}, []byte(testSecret), time.Hour, false)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	if _, err := ParseSessionCookie(cookie, []byte("a-completely-different-secret!!")); err == nil {
+		t.Fatal("expected error for wrong secret")
+	}
+}
+
+func TestMintAndParseTxnCookie(t *testing.T) {
+	txn := TxnState{State: "abc123", CodeVerifier: "verifier-xyz"}
+	cookie, err := MintTxnCookie(txn, []byte(testSecret), false)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	if cookie.Name != TxnCookieName {
+		t.Fatalf("cookie name = %q, want %q", cookie.Name, TxnCookieName)
+	}
+
+	got, err := ParseTxnCookie(cookie, []byte(testSecret))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if got != txn {
+		t.Fatalf("got %+v, want %+v", got, txn)
+	}
+}
+
+func TestClearCookie(t *testing.T) {
+	c := ClearCookie(SessionCookieName, true)
+	if c.Value != "" || c.MaxAge >= 0 {
+		t.Fatalf("ClearCookie = %+v, want empty value and negative MaxAge", c)
+	}
+}