Context refusal in llama-server's exceed_context_size_error shape; README for v2.3

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/proxy/ctxguard.go

@@ -155,10 +155,21 @@ func largestSlotCtx(hosts []string, h Health, model string) int {
 	return best
 }
 
-// refuseCtx answers the 400 the guard's rule 4: the JSON body carries the
-// estimate and the largest available per-slot context, plus the error text. It
-// records the accounting row (status 400, Err "prompt too large") and never
-// marks the host down.
+// ctxErrorBody is llama-server's shape for a prompt that exceeds a host's
+// context. A client that already handles the server's own overflow error keys
+// on error.type and so recognises crossbar's refusal too. See refuseCtx.
+type ctxErrorBody struct {
+	Code          int    `json:"code"`
+	Type          string `json:"type"`
+	Message       string `json:"message"`
+	NPromptTokens int    `json:"n_prompt_tokens"`
+	NCtx          int    `json:"n_ctx"`
+}
+
+// refuseCtx answers the 400 the guard's rule 4: the body is llama-server's
+// exceed_context_size_error, with the estimate as n_prompt_tokens and the
+// largest available per-slot context as n_ctx. It records the accounting row
+// (status 400, Err "prompt too large") and never marks the host down.
 func (p *Handler) refuseCtx(w http.ResponseWriter, host, route, model, fp string, started time.Time, estimate, maxSlot int) {
 	p.writeRecord(store.Request{
 		Route:   route,
@@ -173,9 +184,13 @@ func (p *Handler) refuseCtx(w http.ResponseWriter, host, route, model, fp string
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusBadRequest)
 	_ = json.NewEncoder(w).Encode(map[string]any{
-		"error":    "prompt too large",
-		"estimate": estimate,
-		"max":      maxSlot,
+		"error": ctxErrorBody{
+			Code:          http.StatusBadRequest,
+			Type:          "exceed_context_size_error",
+			Message:       "prompt too large",
+			NPromptTokens: estimate,
+			NCtx:          maxSlot,
+		},
 	})
 }