feat(api): error codes, idempotency keys, cursor pagination

- Stable machine-readable error codes alongside the error message
  (UNAUTHENTICATED, INVALID_TOKEN, ADMIN_REQUIRED, SCOPE_INSUFFICIENT,
  IDEMPOTENCY_IN_PROGRESS); CLI renders "CODE: message".
- Idempotency middleware on authed/admin groups: POST requests with an
  Idempotency-Key header replay the first response for 24h (Redis-backed,
  caller-scoped); concurrent duplicates get 409. X-Idempotent-Replay
  header marks replays.
- Shared pagination helpers: ?cursor= (opaque, preferred) alongside
  legacy ?page= on projects and deployments lists; pagination meta
  includes next_cursor; invalid cursors fail soft to page 1.
- CLI --idempotency-key flag for safe scripted retries.
- OpenAPI updated: error envelope, cursor params, idempotency contract.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/deployments.go

@@ -6,7 +6,6 @@ import (
 	"context"
 	"fmt"
 	"net/http"
-	"strconv"
 	"strings"
 	"time"
 
@@ -74,14 +73,21 @@ func handleGetDeployments(c *gin.Context) {
 		return
 	}
 
+	offset, limit := pageWindow(c, 50, 100)
+
+	var total int
+	_ = db.(*database.DB).QueryRow(
+		`SELECT COUNT(*) FROM deployments WHERE service_id = $1`, serviceID,
+	).Scan(&total)
+
 	rows, err := db.(*database.DB).Query(
 		`SELECT id, service_id, commit_hash, status, image_name, image_tag,
 		        build_log, runtime_log, error, started_at, completed_at, created_at, updated_at
 		 FROM deployments
 		 WHERE service_id = $1
 		 ORDER BY created_at DESC
-		 LIMIT 50`,
-		serviceID,
+		 LIMIT $2 OFFSET $3`,
+		serviceID, limit, offset,
 	)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to retrieve deployments"})
@@ -104,7 +110,10 @@ func handleGetDeployments(c *gin.Context) {
 		deployments = append(deployments, d)
 	}
 
-	c.JSON(http.StatusOK, gin.H{"deployments": deployments})
+	c.JSON(http.StatusOK, gin.H{
+		"deployments": deployments,
+		"pagination":  paginationMeta(offset, limit, total),
+	})
 }
 
 // RecentDeployment is a deployment row joined with its service and project names.
@@ -132,11 +141,19 @@ func handleGetRecentDeployments(c *gin.Context) {
 	userID := optionalUserUUID(c)
 	isAdmin := contextIsAdmin(c)
 
-	limit := 10
-	if v := c.Query("limit"); v != "" {
-		if n, err := strconv.Atoi(v); err == nil && n > 0 && n <= 50 {
-			limit = n
-		}
+	offset, limit := pageWindow(c, 10, 50)
+
+	var total int
+	if err := db.(*database.DB).QueryRow(
+		`SELECT COUNT(*)
+		 FROM deployments d
+		 JOIN services s ON s.id = d.service_id
+		 JOIN projects p ON p.id = s.project_id
+		 WHERE p.is_approved OR p.owner_id = $1 OR $2::bool
+		    OR EXISTS (SELECT 1 FROM project_members pm WHERE pm.project_id = p.id AND pm.user_id = $1)`,
+		userID, isAdmin,
+	).Scan(&total); err != nil {
+		total = 0
 	}
 
 	rows, err := db.(*database.DB).Query(
@@ -148,8 +165,8 @@ func handleGetRecentDeployments(c *gin.Context) {
 		 WHERE p.is_approved OR p.owner_id = $1 OR $2::bool
 		    OR EXISTS (SELECT 1 FROM project_members pm WHERE pm.project_id = p.id AND pm.user_id = $1)
 		 ORDER BY d.created_at DESC
-		 LIMIT $3`,
-		userID, isAdmin, limit,
+		 LIMIT $3 OFFSET $4`,
+		userID, isAdmin, limit, offset,
 	)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to retrieve deployments"})
@@ -168,7 +185,10 @@ func handleGetRecentDeployments(c *gin.Context) {
 		deployments = append(deployments, d)
 	}
 
-	c.JSON(http.StatusOK, gin.H{"deployments": deployments})
+	c.JSON(http.StatusOK, gin.H{
+		"deployments": deployments,
+		"pagination":  paginationMeta(offset, limit, total),
+	})
 }
 
 func handleCreateDeployment(c *gin.Context) {

app/backend/internal/api/errors.go

@@ -0,0 +1,29 @@
+package api
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// Stable machine-readable error codes for agent clients. The "error" field
+// stays human-readable; "code" is the contract the CLI/MCP rely on.
+const (
+	CodeUnauthorized       = "UNAUTHENTICATED"
+	CodeInvalidToken       = "INVALID_TOKEN"
+	CodeForbidden          = "FORBIDDEN"
+	CodeAdminRequired      = "ADMIN_REQUIRED"
+	CodeScopeInsufficient  = "SCOPE_INSUFFICIENT"
+	CodeNotFound           = "NOT_FOUND"
+	CodeValidation         = "VALIDATION"
+	CodeConflict           = "CONFLICT"
+	CodeInternal           = "INTERNAL"
+	CodeUnavailable        = "DEPENDENCY_UNAVAILABLE"
+	CodeIdempotencyRunning = "IDEMPOTENCY_IN_PROGRESS"
+)
+
+// respondError emits the standard error envelope. Existing handlers return
+// bare {"error": msg} — this adds the stable code for new and
+// agent-facing paths.
+func respondError(c *gin.Context, status int, code, message string) {
+	c.JSON(status, gin.H{"error": message, "code": code})
+	c.Abort()
+}

app/backend/internal/api/pagination.go

@@ -0,0 +1,52 @@
+package api
+
+import (
+	"encoding/base64"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+// pageWindow parses pagination from either ?page=&limit= (legacy) or
+// ?cursor=&limit= (agent-friendly). The cursor is an opaque base64 offset —
+// cheap to implement over the existing offset queries while the public
+// contract stays cursor-shaped for clients.
+func pageWindow(c *gin.Context, defaultLimit, maxLimit int) (offset, limit int) {
+	limit = defaultLimit
+	if v := c.Query("limit"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 && n <= maxLimit {
+			limit = n
+		}
+	}
+	if cur := c.Query("cursor"); cur != "" {
+		if raw, err := base64.RawURLEncoding.DecodeString(strings.TrimPrefix(cur, "o_")); err == nil {
+			if n, err := strconv.Atoi(string(raw)); err == nil && n >= 0 {
+				return n, limit
+			}
+		}
+		return 0, limit // invalid cursor → first page, fail soft
+	}
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	if page < 1 {
+		page = 1
+	}
+	return (page - 1) * limit, limit
+}
+
+// paginationMeta renders the standard envelope: legacy page fields plus
+// next_cursor when another page exists.
+func paginationMeta(offset, limit, total int) gin.H {
+	page := offset/limit + 1
+	meta := gin.H{
+		"page":  page,
+		"limit": limit,
+		"total": total,
+		"pages": (total + limit - 1) / limit,
+	}
+	if offset+limit < total {
+		meta["next_cursor"] = "o_" + base64.RawURLEncoding.EncodeToString([]byte(fmt.Sprint(offset+limit)))
+	}
+	return meta
+}

app/backend/internal/api/pagination_test.go

@@ -0,0 +1,55 @@
+package api
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func ctxWithQuery(q string) *gin.Context {
+	gin.SetMode(gin.TestMode)
+	c, _ := gin.CreateTestContext(httptest.NewRecorder())
+	c.Request = httptest.NewRequest("GET", "/x?"+q, nil)
+	return c
+}
+
+func TestPageWindowDefaults(t *testing.T) {
+	offset, limit := pageWindow(ctxWithQuery(""), 10, 100)
+	if offset != 0 || limit != 10 {
+		t.Fatalf("expected 0/10, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowLegacyPage(t *testing.T) {
+	offset, limit := pageWindow(ctxWithQuery("page=3&limit=20"), 10, 100)
+	if offset != 40 || limit != 20 {
+		t.Fatalf("expected 40/20, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowCursor(t *testing.T) {
+	// cursor for offset 50 produced by paginationMeta
+	meta := paginationMeta(0, 50, 200)
+	cursor := meta["next_cursor"].(string)
+	offset, limit := pageWindow(ctxWithQuery("cursor="+cursor+"&limit=50"), 10, 100)
+	if offset != 50 || limit != 50 {
+		t.Fatalf("expected 50/50, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowInvalidCursorFailsSoft(t *testing.T) {
+	offset, _ := pageWindow(ctxWithQuery("cursor=garbage"), 10, 100)
+	if offset != 0 {
+		t.Fatalf("expected offset 0 on invalid cursor, got %d", offset)
+	}
+}
+
+func TestPaginationMetaNextCursorOnlyWhenMore(t *testing.T) {
+	if _, ok := paginationMeta(0, 10, 10)["next_cursor"]; ok {
+		t.Fatal("no next page should omit next_cursor")
+	}
+	if _, ok := paginationMeta(0, 10, 25)["next_cursor"]; !ok {
+		t.Fatal("expected next_cursor when more pages exist")
+	}
+}

app/backend/internal/api/projects.go

@@ -8,7 +8,6 @@ import (
 	"database/sql"
 	"errors"
 	"net/http"
-	"strconv"
 	"strings"
 	"time"
 
@@ -55,18 +54,9 @@ func handleGetProjects(c *gin.Context) {
 	db := c.MustGet("db").(*database.DB)
 	queries := sqlcdb.New(db.DB)
 
-	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
-	limit, _ := strconv.Atoi(c.DefaultQuery("limit", "10"))
+	offset, limit := pageWindow(c, 10, 100)
 	search := strings.TrimSpace(c.DefaultQuery("search", ""))
 
-	if page < 1 {
-		page = 1
-	}
-	if limit > 100 || limit < 1 {
-		limit = 10
-	}
-	offset := (page - 1) * limit
-
 	searchParam := sql.NullString{}
 	if search != "" {
 		searchParam = sql.NullString{String: search, Valid: true}
@@ -99,15 +89,9 @@ func handleGetProjects(c *gin.Context) {
 		return
 	}
 
-	totalCount := int(total)
 	c.JSON(http.StatusOK, gin.H{
-		"projects": projects,
-		"pagination": gin.H{
-			"page":  page,
-			"limit": limit,
-			"total": totalCount,
-			"pages": (totalCount + limit - 1) / limit,
-		},
+		"projects":   projects,
+		"pagination": paginationMeta(offset, limit, int(total)),
 	})
 }
 

app/backend/internal/api/routes.go

@@ -256,6 +256,7 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 		// enforce ownership; platform admins additionally pass every check.
 		authed := v1.Group("/")
 		authed.Use(middleware.Auth(cfg.JWTSecret))
+		authed.Use(middleware.Idempotency())
 		{
 			authed.GET("/user/profile", handleGetProfile)
 			authed.PUT("/user/profile", handleUpdateProfile)
@@ -357,6 +358,7 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 		// Admin routes — platform-wide controls and cross-user management.
 		admin := v1.Group("/")
 		admin.Use(middleware.Auth(cfg.JWTSecret))
+		admin.Use(middleware.Idempotency())
 		admin.Use(middleware.RequireAdmin())
 		{
 			admin.GET("/admin/overview", handleAdminOverview)

app/backend/internal/api/user_tokens.go

@@ -72,36 +72,36 @@ func handleCreateUserToken(c *gin.Context) {
 
 	req.Name = strings.TrimSpace(req.Name)
 	if req.Name == "" {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "name is required"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": "name is required", "code": "VALIDATION"})
 		return
 	}
 	if req.Scope == "" {
 		req.Scope = "write"
 	}
 	if !userTokenScopes[req.Scope] {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "scope must be one of: read, write, admin"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": "scope must be one of: read, write, admin", "code": "VALIDATION"})
 		return
 	}
 	// A token can never outrank its owner: non-admin users cannot mint
 	// admin-scope tokens even for themselves.
 	if req.Scope == "admin" && !c.GetBool("is_admin") {
-		c.JSON(http.StatusForbidden, gin.H{"error": "admin scope requires a platform admin account"})
+		c.JSON(http.StatusForbidden, gin.H{"error": "admin scope requires a platform admin account", "code": "FORBIDDEN"})
 		return
 	}
 	if req.ExpiresInDays < 0 || req.ExpiresInDays > 3650 {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "expires_in_days must be between 0 (never) and 3650"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": "expires_in_days must be between 0 (never) and 3650", "code": "VALIDATION"})
 		return
 	}
 
 	token, prefix, hash, err := generateUserToken()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to generate token"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to generate token", "code": "INTERNAL"})
 		return
 	}
 
 	ownerUUID, err := uuid.Parse(userID)
 	if err != nil {
-		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context"})
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context", "code": "INVALID_TOKEN"})
 		return
 	}
 
@@ -119,7 +119,7 @@ func handleCreateUserToken(c *gin.Context) {
 		ExpiresAt: expiresAt,
 	})
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to store token"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to store token", "code": "INTERNAL"})
 		return
 	}
 
@@ -140,13 +140,13 @@ func handleListUserTokens(c *gin.Context) {
 
 	ownerUUID, err := uuid.Parse(userID)
 	if err != nil {
-		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context"})
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context", "code": "INVALID_TOKEN"})
 		return
 	}
 
 	rows, err := queries.ListUserTokens(c.Request.Context(), ownerUUID)
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch tokens"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch tokens", "code": "INTERNAL"})
 		return
 	}
 
@@ -167,12 +167,12 @@ func handleDeleteUserToken(c *gin.Context) {
 
 	ownerUUID, err := uuid.Parse(userID)
 	if err != nil {
-		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context"})
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid user context", "code": "INVALID_TOKEN"})
 		return
 	}
 	tokenUUID, err := uuid.Parse(c.Param("id"))
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid token ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid token ID", "code": "VALIDATION"})
 		return
 	}
 
@@ -181,11 +181,11 @@ func handleDeleteUserToken(c *gin.Context) {
 		UserID: ownerUUID,
 	})
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to revoke token"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to revoke token", "code": "INTERNAL"})
 		return
 	}
 	if affected == 0 {
-		c.JSON(http.StatusNotFound, gin.H{"error": "Token not found"})
+		c.JSON(http.StatusNotFound, gin.H{"error": "Token not found", "code": "NOT_FOUND"})
 		return
 	}
 

app/backend/internal/cli/commands/client.go

@@ -8,6 +8,8 @@ import (
 	"net/http"
 	"strings"
 	"time"
+
+	"github.com/spf13/viper"
 )
 
 // Exit codes — stable for scripting and agent use.
@@ -109,6 +111,9 @@ func (c *Client) Do(method, path string, body interface{}) ([]byte, error) {
 	if body != nil {
 		req.Header.Set("Content-Type", "application/json")
 	}
+	if k := viper.GetString("idempotency-key"); k != "" && method == http.MethodPost {
+		req.Header.Set("Idempotency-Key", k)
+	}
 	req.Header.Set("Accept", "application/json")
 
 	hc := c.HTTPClient
@@ -134,9 +139,13 @@ func (c *Client) Do(method, path string, body interface{}) ([]byte, error) {
 		msg := strings.TrimSpace(string(data))
 		var envelope struct {
 			Error string `json:"error"`
+			Code  string `json:"code"`
 		}
 		if json.Unmarshal(data, &envelope) == nil && envelope.Error != "" {
 			msg = envelope.Error
+			if envelope.Code != "" {
+				msg = envelope.Code + ": " + msg
+			}
 		}
 		return nil, &APIError{
 			Status:   resp.StatusCode,

app/backend/internal/cli/root.go

@@ -55,12 +55,14 @@ func init() {
 	pf.StringP("profile", "p", "", "config profile to use (default current_profile)")
 	pf.Bool("json", false, "machine-readable JSON output")
 	pf.BoolP("yes", "y", false, "skip confirmation prompts (required for destructive ops in scripts)")
+	pf.String("idempotency-key", "", "Idempotency-Key header for POST mutations (safe retries)")
 
 	viper.BindPFlag("api-url", pf.Lookup("api-url"))
 	viper.BindPFlag("token", pf.Lookup("token"))
 	viper.BindPFlag("profile", pf.Lookup("profile"))
 	viper.BindPFlag("json", pf.Lookup("json"))
 	viper.BindPFlag("yes", pf.Lookup("yes"))
+	viper.BindPFlag("idempotency-key", pf.Lookup("idempotency-key"))
 
 	rootCmd.AddCommand(commands.AuthCmd)
 	rootCmd.AddCommand(commands.TokensCmd)

app/backend/internal/middleware/idempotency.go

@@ -0,0 +1,132 @@
+package middleware
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"net/http"
+	"time"
+
+	"containr/internal/database"
+
+	"github.com/gin-gonic/gin"
+	"github.com/go-redis/redis/v8"
+)
+
+// Idempotency replays POST responses for a repeated Idempotency-Key header
+// so agents can safely retry mutations. The first request wins; concurrent
+// duplicates get 409 IDEMPOTENCY_IN_PROGRESS; completed responses replay
+// verbatim for 24h. Keys are scoped to the authenticated identity.
+func Idempotency() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if c.Request.Method != http.MethodPost {
+			c.Next()
+			return
+		}
+		key := c.GetHeader("Idempotency-Key")
+		if key == "" || len(key) > 128 {
+			c.Next()
+			return
+		}
+		dbValue, exists := c.Get("redis")
+		if !exists {
+			c.Next()
+			return
+		}
+		rdb, ok := dbValue.(*database.Redis)
+		if !ok || rdb == nil || rdb.Client == nil {
+			c.Next()
+			return
+		}
+
+		// Scope by caller identity so a stolen key can't replay across users.
+		actor := c.GetString("user_id")
+		if actor == "" {
+			actor = c.GetHeader("Authorization")
+		}
+		sum := sha256.Sum256([]byte(actor + ":" + c.Request.URL.Path + ":" + key))
+		redisKey := "idem:" + hex.EncodeToString(sum[:16])
+
+		ctx, cancel := context.WithTimeout(c.Request.Context(), 3*time.Second)
+		defer cancel()
+
+		stored, err := rdb.Client.Get(ctx, redisKey).Result()
+		switch {
+		case err == nil:
+			if stored == "processing" {
+				c.JSON(http.StatusConflict, gin.H{
+					"error": "a request with this idempotency key is still in progress",
+					"code":  "IDEMPOTENCY_IN_PROGRESS",
+				})
+				c.Abort()
+				return
+			}
+			var rec idemRecord
+			if json.Unmarshal([]byte(stored), &rec) == nil {
+				c.Header("X-Idempotent-Replay", "true")
+				c.Data(rec.Status, rec.ContentType, rec.Body)
+				c.Abort()
+				return
+			}
+		case err != redis.Nil:
+			// Redis hiccup — degrade to pass-through rather than failing the request.
+			c.Next()
+			return
+		}
+
+		// Claim the key for the duration of the request.
+		if set, serr := rdb.Client.SetNX(ctx, redisKey, "processing", 2*time.Minute).Result(); serr != nil || !set {
+			c.JSON(http.StatusConflict, gin.H{
+				"error": "a request with this idempotency key is still in progress",
+				"code":  "IDEMPOTENCY_IN_PROGRESS",
+			})
+			c.Abort()
+			return
+		}
+
+		recorder := &responseRecorder{ResponseWriter: c.Writer, status: http.StatusOK, body: &bytes.Buffer{}}
+		c.Writer = recorder
+		c.Next()
+
+		// Cache terminal responses only — 5xx is retryable and must not be replayed.
+		if recorder.status < 500 && recorder.body.Len() <= 1<<20 {
+			rec := idemRecord{
+				Status:      recorder.status,
+				ContentType: recorder.Header().Get("Content-Type"),
+				Body:        recorder.body.Bytes(),
+			}
+			if rec.ContentType == "" {
+				rec.ContentType = "application/json; charset=utf-8"
+			}
+			if raw, merr := json.Marshal(rec); merr == nil {
+				_ = rdb.Client.Set(context.Background(), redisKey, raw, 24*time.Hour).Err()
+			}
+		} else {
+			_ = rdb.Client.Del(context.Background(), redisKey).Err()
+		}
+	}
+}
+
+type idemRecord struct {
+	Status      int    `json:"s"`
+	ContentType string `json:"ct"`
+	Body        []byte `json:"b"`
+}
+
+type responseRecorder struct {
+	gin.ResponseWriter
+	status int
+	body   *bytes.Buffer
+}
+
+func (r *responseRecorder) WriteHeader(code int) {
+	r.status = code
+	r.ResponseWriter.WriteHeader(code)
+}
+
+func (r *responseRecorder) Write(b []byte) (int, error) {
+	r.body.Write(b)
+	return r.ResponseWriter.Write(b)
+}

app/backend/internal/middleware/idempotency_test.go

@@ -0,0 +1,60 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestIdempotencyPassThroughWithoutKey(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.POST("/x", func(c *gin.Context) { called++; c.JSON(200, gin.H{"ok": true}) })
+
+	req := httptest.NewRequest(http.MethodPost, "/x", nil)
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 || rec.Code != 200 {
+		t.Fatalf("expected pass-through, called=%d code=%d", called, rec.Code)
+	}
+}
+
+func TestIdempotencyPassThroughWithoutRedis(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.POST("/x", func(c *gin.Context) { called++; c.JSON(200, gin.H{"ok": true}) })
+
+	// Key present but no redis in context — must not block the request.
+	req := httptest.NewRequest(http.MethodPost, "/x", nil)
+	req.Header.Set("Idempotency-Key", "abc-123")
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 || rec.Code != 200 {
+		t.Fatalf("expected pass-through without redis, called=%d code=%d", called, rec.Code)
+	}
+}
+
+func TestIdempotencySkipsNonPOST(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.GET("/x", func(c *gin.Context) { called++; c.String(200, "ok") })
+
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req.Header.Set("Idempotency-Key", "abc")
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 {
+		t.Fatalf("GET should bypass idempotency, called=%d", called)
+	}
+}

app/backend/internal/middleware/middleware.go

@@ -212,15 +212,15 @@ func Auth(jwtSecret string) gin.HandlerFunc {
 		}
 
 		if tokenErr != "" {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": tokenErr})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": tokenErr, "code": "INVALID_TOKEN"})
 			c.Abort()
 			return
 		}
 
 		if hasToken {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid token"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid token", "code": "INVALID_TOKEN"})
 		} else {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required", "code": "UNAUTHENTICATED"})
 		}
 		c.Abort()
 	}
@@ -245,12 +245,12 @@ func OptionalAuth(jwtSecret string) gin.HandlerFunc {
 func RequireAdmin() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		if _, exists := c.Get("user_id"); !exists {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required", "code": "UNAUTHENTICATED"})
 			c.Abort()
 			return
 		}
 		if !c.GetBool("is_admin") {
-			c.JSON(http.StatusForbidden, gin.H{"error": "Admin access required"})
+			c.JSON(http.StatusForbidden, gin.H{"error": "Admin access required", "code": "ADMIN_REQUIRED"})
 			c.Abort()
 			return
 		}