diff --git a/docs/openapi.yaml b/docs/openapi.yaml
index d7a9dca..c1f5977 100644
--- a/docs/openapi.yaml
+++ b/docs/openapi.yaml
@@ -1422,6 +1422,8 @@ paths:
                     additionalProperties:
                       type: string
                       enum: [retiring, start_failed, owner, wake, owner_only, allowlist]
+        '400':
+          $ref: '#/components/responses/BadRequest'
         '401':
           $ref: '#/components/responses/Unauthorized'
 
@@ -1505,6 +1507,8 @@ paths:
                 required: [linked]
                 properties:
                   linked: { type: boolean }
+        '400':
+          $ref: '#/components/responses/BadRequest'
         '401':
           $ref: '#/components/responses/Unauthorized'
 
@@ -1637,6 +1641,8 @@ paths:
                 required: [blacklisted]
                 properties:
                   blacklisted: { type: boolean }
+        '400':
+          $ref: '#/components/responses/BadRequest'
         '401':
           $ref: '#/components/responses/Unauthorized'
 
@@ -4684,6 +4690,8 @@ paths:
                 properties:
                   ok: { type: boolean, const: true }
                   mc_uuid: { type: string, format: uuid }
+        '400':
+          $ref: '#/components/responses/BadRequest'
         '401':
           $ref: '#/components/responses/Unauthorized'
         '403':
diff --git a/internal/api/handlers_account.go b/internal/api/handlers_account.go
index 61fdb73..e437c03 100644
--- a/internal/api/handlers_account.go
+++ b/internal/api/handlers_account.go
@@ -6,6 +6,8 @@ import (
 	"net/http"
 	"strings"
 	"time"
+
+	"github.com/google/uuid"
 )
 
 // Account-linking endpoints (spec §10). The flow is forced by the
@@ -51,6 +53,28 @@ func validAuthSource(s string) bool {
 	return s == authSourceMojang || s == authSourceThirdParty
 }
 
+// errBadMCUUID answers an mc_uuid that is not a UUID. Every mc_uuid column is
+// Postgres's uuid type, which refuses such text with 22P02, and that reached the
+// caller as a 500.
+var errBadMCUUID = newError(http.StatusBadRequest, "bad_mc_uuid",
+	"mc_uuid must be a UUID, such as 069a79f4-44e9-4726-a5be-fca90e38aaf5")
+
+// parseMCUUID reads an mc_uuid from a request: surrounding spaces trimmed, empty
+// → 400 bad_request "mc_uuid is required", not a UUID → errBadMCUUID. It returns
+// the canonical lowercase hyphenated form, the text Postgres gives back for the
+// column, so what a handler stores, echoes and compares is one spelling.
+func parseMCUUID(s string) (string, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return "", newError(http.StatusBadRequest, "bad_request", "mc_uuid is required")
+	}
+	id, err := uuid.Parse(s)
+	if err != nil {
+		return "", errBadMCUUID
+	}
+	return id.String(), nil
+}
+
 // deriveAuthSource infers the auth source from the UUID's version nibble when
 // the minting backend omitted auth_source. Felis-nano rewrites every
 // third-party profile to a name-based UUIDv3 under its namespace before it ever
@@ -104,8 +128,9 @@ func (a *API) handleCreateLinkCode(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	if req.MCUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(req.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 	// Default an omitted source from the UUID's version nibble (v3 = felis-nano
@@ -114,7 +139,7 @@ func (a *API) handleCreateLinkCode(w http.ResponseWriter, r *http.Request) {
 	// stored value the panel will later mislabel.
 	authSource := req.AuthSource
 	if authSource == "" {
-		authSource = deriveAuthSource(req.MCUUID)
+		authSource = deriveAuthSource(mcUUID)
 	}
 	if !validAuthSource(authSource) {
 		writeError(w, r, newError(http.StatusBadRequest, "bad_request",
@@ -127,7 +152,7 @@ func (a *API) handleCreateLinkCode(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	expiresAt := a.now().Add(linkCodeTTL)
-	if err := a.Repo.CreateLinkCode(r.Context(), code, req.MCUUID, authSource, expiresAt); err != nil {
+	if err := a.Repo.CreateLinkCode(r.Context(), code, mcUUID, authSource, expiresAt); err != nil {
 		writeError(w, r, err)
 		return
 	}
@@ -171,12 +196,12 @@ func (a *API) handleCreateLinkCode(w http.ResponseWriter, r *http.Request) {
 // handlers_player_reclaim.go keeps CODE-ONLY (reclaimed_by_user_id stays NULL on
 // the verifiable path); this endpoint reports link completion only, not that choice.
 func (a *API) handleLinkStatus(w http.ResponseWriter, r *http.Request) {
-	mcUUID := r.PathValue("mc_uuid")
-	if mcUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(r.PathValue("mc_uuid"))
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
-	_, err := a.Repo.UserByMCUUID(r.Context(), mcUUID)
+	_, err = a.Repo.UserByMCUUID(r.Context(), mcUUID)
 	switch {
 	case errors.Is(err, ErrNotFound):
 		// Not linked yet. For the poller this is simply "keep waiting": velocity
diff --git a/internal/api/handlers_account_migrate.go b/internal/api/handlers_account_migrate.go
index 2d055a0..d62fc9c 100644
--- a/internal/api/handlers_account_migrate.go
+++ b/internal/api/handlers_account_migrate.go
@@ -111,9 +111,9 @@ func (a *API) handleMigrateStart(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	mcUUID := strings.TrimSpace(req.MCUUID)
-	if mcUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(req.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 	sourceUserID, err := a.Repo.UserByMCUUID(r.Context(), mcUUID)
diff --git a/internal/api/handlers_allowlist.go b/internal/api/handlers_allowlist.go
index 2921e4d..52d0ed9 100644
--- a/internal/api/handlers_allowlist.go
+++ b/internal/api/handlers_allowlist.go
@@ -66,7 +66,7 @@ func (a *API) handleAllowlistSetWake(w http.ResponseWriter, r *http.Request) {
 	}
 	id, err := uuid.Parse(r.PathValue("uuid"))
 	if err != nil {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "invalid Minecraft UUID"))
+		writeError(w, r, errBadMCUUID)
 		return
 	}
 	var req allowlistWakeRequest
diff --git a/internal/api/handlers_internal.go b/internal/api/handlers_internal.go
index 07f540d..1423763 100644
--- a/internal/api/handlers_internal.go
+++ b/internal/api/handlers_internal.go
@@ -83,11 +83,12 @@ func (a *API) handleJoinEvent(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	if req.MCUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(req.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
-	if err := a.Repo.RecordJoin(r.Context(), name, req.MCUUID); err != nil {
+	if err := a.Repo.RecordJoin(r.Context(), name, mcUUID); err != nil {
 		a.writeLookupError(w, r, err)
 		return
 	}
@@ -120,8 +121,9 @@ func (a *API) handleInternalWake(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	if req.MCUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(req.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 
@@ -148,7 +150,7 @@ func (a *API) handleInternalWake(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := a.authorizeWakeByUUID(r.Context(), req.MCUUID, info, rec); err != nil {
+	if err := a.authorizeWakeByUUID(r.Context(), mcUUID, info, rec); err != nil {
 		writeError(w, r, err)
 		return
 	}
@@ -235,8 +237,9 @@ func (a *API) handleInternalClaim(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	if req.MCUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(req.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 
@@ -245,7 +248,7 @@ func (a *API) handleInternalClaim(w http.ResponseWriter, r *http.Request) {
 	// no separate IsLinked check (mirrors the external claim's order, link → quota
 	// → write). ErrNotFound here is "claimer not linked" (412), never "server
 	// missing" — that distinction is the claim call's, below.
-	userID, err := a.Repo.UserByMCUUID(r.Context(), req.MCUUID)
+	userID, err := a.Repo.UserByMCUUID(r.Context(), mcUUID)
 	if err != nil {
 		if errors.Is(err, ErrNotFound) {
 			writeError(w, r, newError(http.StatusPreconditionFailed, "not_linked",
@@ -364,9 +367,9 @@ const (
 // since a retry gets past those. Retiring comes first: nobody may start such a
 // server, so it is the reason a stranger is shown too.
 func (a *API) handleInternalMenuAccess(w http.ResponseWriter, r *http.Request) {
-	mcUUID := r.PathValue("mc_uuid")
-	if mcUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(r.PathValue("mc_uuid"))
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 	infos, err := a.Cluster.ListServers(r.Context())
diff --git a/internal/api/handlers_mcuuid_test.go b/internal/api/handlers_mcuuid_test.go
new file mode 100644
index 0000000..8a2acca
--- /dev/null
+++ b/internal/api/handlers_mcuuid_test.go
@@ -0,0 +1,70 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+)
+
+// Every door that takes an mc_uuid refuses text that is not a UUID with 400
+// bad_mc_uuid. The columns are Postgres uuids, and such text used to reach the
+// database, fail there with 22P02 and come back as a 500. A UUID in another
+// spelling is stored and echoed in the one Postgres gives back.
+func TestMCUUIDMustBeAUUID(t *testing.T) {
+	repo := newFakeRepo()
+	repo.byName["survival"] = &ServerRecord{Name: "survival", OwnerID: "u1"}
+	repo.claimOK["survival"] = true
+	repo.seedUser(UserView{ID: "u2", Username: "alice", Role: "user"})
+	cl := newFakeCluster()
+	cl.byName["survival"] = &ServerInfo{Name: "survival", AutostartPolicy: "public"}
+	api := newTestAPI(repo, cl)
+	api.External = staticExternal{p: &Principal{UserID: "owner1", Role: "owner", ViaAdminAccess: true}}
+	ih, eh := api.InternalHandler(), api.ExternalHandler()
+
+	const bad = "not-a-uuid"
+	body := `{"mc_uuid":"` + bad + `"}`
+	for _, c := range []struct {
+		h                  http.Handler
+		method, path, body string
+	}{
+		{ih, "POST", "/api/v1/internal/servers/survival/join-event", body},
+		{ih, "POST", "/api/v1/internal/servers/survival/wake", body},
+		{ih, "POST", "/api/v1/internal/servers/survival/claim", body},
+		{ih, "GET", "/api/v1/internal/player/menu-access/" + bad, ""},
+		{ih, "POST", "/api/v1/internal/account/link/code", body},
+		{ih, "GET", "/api/v1/internal/account/link/status/" + bad, ""},
+		{ih, "POST", "/api/v1/internal/account/migrate/start", body},
+		{ih, "GET", "/api/v1/internal/player/blacklist/" + bad, ""},
+		{eh, "PUT", "/api/v1/servers/survival/allowlist/" + bad, `{"can_wake":true}`},
+		{eh, "DELETE", "/api/v1/users/u2/links/" + bad, ""},
+		{eh, "POST", "/api/v1/users/u2/links", body},
+	} {
+		w := do(c.h, c.method, c.path, c.body, jsonHeader)
+		if w.Code != http.StatusBadRequest || decodeErr(t, w) != "bad_mc_uuid" {
+			t.Errorf("%s %s with %q: code = %d body %s, want 400 bad_mc_uuid", c.method, c.path, bad, w.Code, w.Body.String())
+		}
+	}
+	if len(repo.joins) != 0 || len(repo.linkCodes) != 0 || len(repo.links) != 0 || len(repo.audits) != 0 {
+		t.Fatalf("a refused mc_uuid wrote joins=%v codes=%v links=%v audits=%v", repo.joins, repo.linkCodes, repo.links, repo.audits)
+	}
+
+	const spelled, canonical = " 069A79F444E94726A5BEFCA90E38AAF5 ", "069a79f4-44e9-4726-a5be-fca90e38aaf5"
+	w := do(eh, "POST", "/api/v1/users/u2/links", `{"mc_uuid":"`+spelled+`"}`, jsonHeader)
+	var linked struct {
+		MCUUID string `json:"mc_uuid"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &linked); err != nil || w.Code != http.StatusOK || linked.MCUUID != canonical {
+		t.Fatalf("link of %q: code = %d body %s, want 200 echoing %s", spelled, w.Code, w.Body.String(), canonical)
+	}
+	if repo.links[canonical] != "u2" || len(repo.links) != 1 {
+		t.Fatalf("links = %v, want only %s → u2", repo.links, canonical)
+	}
+	if w := do(ih, "POST", "/api/v1/internal/account/link/code", `{"mc_uuid":"`+spelled+`"}`, jsonHeader); w.Code != http.StatusCreated {
+		t.Fatalf("link code for %q: code = %d body %s, want 201", spelled, w.Code, w.Body.String())
+	}
+	for _, lc := range repo.linkCodes {
+		if lc.mcUUID != canonical {
+			t.Fatalf("link code minted for %q, want %s", lc.mcUUID, canonical)
+		}
+	}
+}
diff --git a/internal/api/handlers_player_reclaim.go b/internal/api/handlers_player_reclaim.go
index 8405c48..bea1658 100644
--- a/internal/api/handlers_player_reclaim.go
+++ b/internal/api/handlers_player_reclaim.go
@@ -140,9 +140,9 @@ func (a *API) handleReclaimUsername(w http.ResponseWriter, r *http.Request) {
 // squatter before letting them in; the genuine Mojang UUID (same name, different
 // UUID) is never on the list, so it always passes.
 func (a *API) handleCheckBlacklist(w http.ResponseWriter, r *http.Request) {
-	mcUUID := r.PathValue("mc_uuid")
-	if mcUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "mc_uuid is required"))
+	mcUUID, err := parseMCUUID(r.PathValue("mc_uuid"))
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 	blacklisted, err := a.Repo.IsUsernameBlacklisted(r.Context(), mcUUID)
diff --git a/internal/api/handlers_users.go b/internal/api/handlers_users.go
index 2be6d04..6077356 100644
--- a/internal/api/handlers_users.go
+++ b/internal/api/handlers_users.go
@@ -447,11 +447,15 @@ func (a *API) handleUnbindUserPasskeys(w http.ResponseWriter, r *http.Request) {
 // (DELETE /users/{id}/links/{mc_uuid}).
 func (a *API) handleUnlinkAccount(w http.ResponseWriter, r *http.Request) {
 	userID := r.PathValue("id")
-	mcUUID := r.PathValue("mc_uuid")
-	if userID == "" || mcUUID == "" {
+	if userID == "" {
 		writeError(w, r, errBadRequest)
 		return
 	}
+	mcUUID, err := parseMCUUID(r.PathValue("mc_uuid"))
+	if err != nil {
+		writeError(w, r, err)
+		return
+	}
 
 	if err := a.Repo.UnlinkAccount(r.Context(), userID, mcUUID); err != nil {
 		if errors.Is(err, ErrNotFound) {
@@ -484,16 +488,16 @@ func (a *API) handleLinkAccount(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
-	if body.MCUUID == "" {
-		writeError(w, r, newError(http.StatusBadRequest, "bad_request",
-			"mc_uuid is required"))
+	mcUUID, err := parseMCUUID(body.MCUUID)
+	if err != nil {
+		writeError(w, r, err)
 		return
 	}
 	if body.AuthSource == "" {
 		// Same version-nibble inference as the mint path (handlers_account.go):
 		// defaulting to mojang here would leave a force-linked thirdparty UUID
 		// outside the reclaim guard.
-		body.AuthSource = deriveAuthSource(body.MCUUID)
+		body.AuthSource = deriveAuthSource(mcUUID)
 	}
 	if !validAuthSource(body.AuthSource) {
 		writeError(w, r, newError(http.StatusBadRequest, "bad_request",
@@ -501,7 +505,7 @@ func (a *API) handleLinkAccount(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := a.Repo.LinkAccount(r.Context(), userID, body.MCUUID, body.AuthSource); err != nil {
+	if err := a.Repo.LinkAccount(r.Context(), userID, mcUUID, body.AuthSource); err != nil {
 		if errors.Is(err, ErrConflict) {
 			writeError(w, r, newError(http.StatusConflict, "already_linked",
 				"this UUID is already linked to a different user"))
@@ -518,7 +522,7 @@ func (a *API) handleLinkAccount(w http.ResponseWriter, r *http.Request) {
 	a.audit(r, "user.link_account", userID)
 	writeJSON(w, http.StatusOK, map[string]any{
 		"ok":          true,
-		"mc_uuid":     body.MCUUID,
+		"mc_uuid":     mcUUID,
 		"auth_source": body.AuthSource,
 	})
 }
diff --git a/panel/src/i18n/resources/en-US/errors.json b/panel/src/i18n/resources/en-US/errors.json
index 7e6548c..a8eeacd 100644
--- a/panel/src/i18n/resources/en-US/errors.json
+++ b/panel/src/i18n/resources/en-US/errors.json
@@ -4,6 +4,7 @@
   "not_linked": "Link your Minecraft account before claiming (Account → Link).",
   "invalid_code": "That code is invalid or expired — request a fresh one and try again.",
   "already_linked": "That Minecraft account is already linked to another user.",
+  "bad_mc_uuid": "That isn't a Minecraft UUID. It looks like 069a79f4-44e9-4726-a5be-fca90e38aaf5, with or without the dashes.",
   "quota_exceeded": "You have reached your server quota.",
   "already_claimed": "Someone else just claimed this server.",
   "image_not_whitelisted": "That image is not on the whitelist.",
diff --git a/panel/src/i18n/resources/zh-CN/errors.json b/panel/src/i18n/resources/zh-CN/errors.json
index b894e93..baa140c 100644
--- a/panel/src/i18n/resources/zh-CN/errors.json
+++ b/panel/src/i18n/resources/zh-CN/errors.json
@@ -4,6 +4,7 @@
   "not_linked": "请先关联 Minecraft 账户（账户页 → 关联）。",
   "invalid_code": "代码无效或已过期——请重新获取后再试。",
   "already_linked": "该 Minecraft 账户已关联至其他用户。",
+  "bad_mc_uuid": "这不是 Minecraft UUID。它的样子是 069a79f4-44e9-4726-a5be-fca90e38aaf5，短横线可带可不带。",
   "quota_exceeded": "服务器数量已达配额上限。",
   "already_claimed": "该服务器已被他人抢先认领。",
   "image_not_whitelisted": "该镜像未在白名单中。",
diff --git a/panel/src/lib/api.test.ts b/panel/src/lib/api.test.ts
index 25f2717..894f4fa 100644
--- a/panel/src/lib/api.test.ts
+++ b/panel/src/lib/api.test.ts
@@ -438,6 +438,12 @@ describe("api access-control wire shapes", () => {
     );
   });
 
+  it("says what a Minecraft UUID looks like when the typed one is not", () => {
+    expect(humanizeError({ status: 400, code: "bad_mc_uuid" })).toBe(
+      "That isn't a Minecraft UUID. It looks like 069a79f4-44e9-4726-a5be-fca90e38aaf5, with or without the dashes.",
+    );
+  });
+
   it("says email codes are off when the install has no mail relay", async () => {
     const { humanizeError } = await import("./api");
     expect(humanizeError({ status: 503, code: "mail_unavailable" })).toBe(
diff --git a/panel/src/lib/api.ts b/panel/src/lib/api.ts
index b2ac34e..1d8028c 100644
--- a/panel/src/lib/api.ts
+++ b/panel/src/lib/api.ts
@@ -1002,6 +1002,8 @@ export function humanizeError(e: unknown): string {
       return t("invalid_code");
     case "already_linked":
       return t("already_linked");
+    case "bad_mc_uuid":
+      return t("bad_mc_uuid");
     case "otp_resend_cooldown":
       return t("otp_resend_cooldown");
     case "otp_locked":
diff --git a/panel/src/lib/openapi.gen.ts b/panel/src/lib/openapi.gen.ts
index 1be2b46..fcf8bf5 100644
--- a/panel/src/lib/openapi.gen.ts
+++ b/panel/src/lib/openapi.gen.ts
@@ -3427,6 +3427,7 @@ export interface operations {
                     };
                 };
             };
+            400: components["responses"]["BadRequest"];
             401: components["responses"]["Unauthorized"];
         };
     };
@@ -3491,6 +3492,7 @@ export interface operations {
                     };
                 };
             };
+            400: components["responses"]["BadRequest"];
             401: components["responses"]["Unauthorized"];
         };
     };
@@ -3615,6 +3617,7 @@ export interface operations {
                     };
                 };
             };
+            400: components["responses"]["BadRequest"];
             401: components["responses"]["Unauthorized"];
         };
     };
@@ -6651,6 +6654,7 @@ export interface operations {
                     };
                 };
             };
+            400: components["responses"]["BadRequest"];
             401: components["responses"]["Unauthorized"];
             403: components["responses"]["Forbidden"];
             /** @description No linked account for this UUID. */