diff --git a/internal/msgs/en_error_messages.go b/internal/msgs/en_error_messages.go
index 1b99587..7d047c6 100644
--- a/internal/msgs/en_error_messages.go
+++ b/internal/msgs/en_error_messages.go
@@ -83,11 +83,12 @@ var (
 	MsgFailedToBuildExistingConfirmationInvalid = ffe("FF23063", "Failed to build confirmations, existing confirmations are not valid")
 	MsgFromBlockInvalid                         = ffe("FF23064", "From block invalid. Must be 'earliest', 'latest' or a decimal: %s", http.StatusBadRequest)
 	MsgInvalidJSONFormatOptions                 = ffe("FF23065", "The JSON formatting options must be a valid set of key=value pairs in URL query string format '%s'")
-	MsgUnknownJSONFormatOptions                 = ffe("FF23066", "JSON formatting option unknown %s=%s")
+	MsgUnknownJSONFormatOption                  = ffe("FF23066", "Unknown JSON formatting option '%s=%s'. Options must be URL query string key=value pairs (e.g. mode=object&number=hex-0x). Supported options and values: %s")
 	MsgObservedPanic                            = ffe("FF23067", "Observed panic: %v")
 	MsgReturnedBlockHashMismatch                = ffe("FF23068", "Returned block %d hash %s does not match requested hash %s")
 	MsgInvalidChainTrackingMode                 = ffe("FF23069", "Invalid chain tracking mode '%s': must be 'light' or 'full'")
 	MsgTransactionNotIncludedInChainHead        = ffe("FF23070", "Transaction '%s' cannot be reconciled because chain head %d is before receipt block %s")
 	MsgTransactionEstimateTooLargeForBlock      = ffe("FF23071", "Gas estimate %s (scaled at %.2f from estimate %s) too large for the current block gas limit %s")
 	MsgMonitoredHeadLengthInvalid               = ffe("FF23072", "Monitored head length must be greater than or equal to 1 value=%d")
+	MsgUnknownJSONFormatOptionValue             = ffe("FF23073", "Unknown value '%s' for JSON formatting option '%s'. Supported values: %s")
 )
diff --git a/pkg/ethrpc/jsonformatoptions.go b/pkg/ethrpc/jsonformatoptions.go
index 7f5a393..b44235d 100644
--- a/pkg/ethrpc/jsonformatoptions.go
+++ b/pkg/ethrpc/jsonformatoptions.go
@@ -19,6 +19,7 @@ package ethrpc
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"math/big"
 	"net/url"
 	"strings"
@@ -53,6 +54,32 @@ const optChecksum = "checksum"
 const optSelfDescribing = "self-describing"
 const optPretty = "pretty"
 
+var jsonFormatOptionSupportedValues = map[string][]string{
+	"mode":    {optObject, optArray, optSelfDescribing},
+	"number":  {optString, optHex0x, optHex, optJSONNumber},
+	"bytes":   {optHex0x, optHex, optHexPlain, optBase64},
+	"address": {optHex0x, optHex, optHexPlain, optChecksum},
+	optPretty: {"true", "false"},
+}
+
+func unknownJSONFormatOptionNameError(ctx context.Context, option, value string) error {
+
+	var jsonFormatOptionNames []string
+	for name := range jsonFormatOptionSupportedValues {
+		jsonFormatOptionNames = append(jsonFormatOptionNames, name)
+	}
+	parts := make([]string, len(jsonFormatOptionNames))
+	for i, name := range jsonFormatOptionNames {
+		parts[i] = fmt.Sprintf("%s (%s)", name, strings.Join(jsonFormatOptionSupportedValues[name], ", "))
+	}
+	return i18n.NewError(ctx, msgs.MsgUnknownJSONFormatOption, option, value, strings.Join(parts, "; "))
+}
+
+func unknownJSONFormatOptionValueError(ctx context.Context, option, value string) error {
+	values := jsonFormatOptionSupportedValues[strings.ToLower(option)]
+	return i18n.NewError(ctx, msgs.MsgUnknownJSONFormatOptionValue, value, option, strings.Join(values, ", "))
+}
+
 func (jfo JSONFormatOptions) GetABISerializer(ctx context.Context) (serializer *abi.Serializer, err error) {
 	return jfo.getABISerializer(ctx, false)
 }
@@ -94,7 +121,7 @@ func (jfo JSONFormatOptions) GetSerializerSet(ctx context.Context, skipErrors bo
 					ss.Mode = abi.FormatAsSelfDescribingArrays
 				default:
 					if !skipErrors {
-						return nil, i18n.WrapError(ctx, err, msgs.MsgUnknownJSONFormatOptions, option, v)
+						return nil, unknownJSONFormatOptionValueError(ctx, option, v)
 					}
 				}
 			case "number":
@@ -107,7 +134,7 @@ func (jfo JSONFormatOptions) GetSerializerSet(ctx context.Context, skipErrors bo
 					ss.Integer = abi.JSONNumberIntSerializer
 				default:
 					if !skipErrors {
-						return nil, i18n.WrapError(ctx, err, msgs.MsgUnknownJSONFormatOptions, option, v)
+						return nil, unknownJSONFormatOptionValueError(ctx, option, v)
 					}
 				}
 			case "bytes":
@@ -119,7 +146,9 @@ func (jfo JSONFormatOptions) GetSerializerSet(ctx context.Context, skipErrors bo
 				case optBase64:
 					ss.Bytes = abi.Base64ByteSerializer
 				default:
-					return nil, i18n.WrapError(ctx, err, msgs.MsgUnknownJSONFormatOptions, option, v)
+					if !skipErrors {
+						return nil, unknownJSONFormatOptionValueError(ctx, option, v)
+					}
 				}
 			case "address":
 				switch strings.ToLower(v) {
@@ -130,13 +159,15 @@ func (jfo JSONFormatOptions) GetSerializerSet(ctx context.Context, skipErrors bo
 				case optChecksum:
 					ss.Address = abi.ChecksumAddrSerializer
 				default:
-					return nil, i18n.WrapError(ctx, err, msgs.MsgUnknownJSONFormatOptions, option, v)
+					if !skipErrors {
+						return nil, unknownJSONFormatOptionValueError(ctx, option, v)
+					}
 				}
 			case optPretty:
 				ss.Pretty = (v != "false")
 			default:
 				if !skipErrors {
-					return nil, i18n.WrapError(ctx, err, msgs.MsgUnknownJSONFormatOptions, option, v)
+					return nil, unknownJSONFormatOptionNameError(ctx, option, v)
 				}
 			}
 		}
diff --git a/pkg/ethrpc/jsonformatoptions_test.go b/pkg/ethrpc/jsonformatoptions_test.go
index 3e0bddd..d724e88 100644
--- a/pkg/ethrpc/jsonformatoptions_test.go
+++ b/pkg/ethrpc/jsonformatoptions_test.go
@@ -239,18 +239,29 @@ func TestJSONFormatOptionErrors(t *testing.T) {
 
 	_, err = JSONFormatOptions("color=blue").GetABISerializer(ctx)
 	assert.Regexp(t, "FF23066", err)
+	assert.Contains(t, err.Error(), "color=blue")
+	assert.Contains(t, err.Error(), "Supported options and values:")
+	assert.Contains(t, err.Error(), "mode (object, array, self-describing)")
 
 	_, err = JSONFormatOptions("mode=blue").GetABISerializer(ctx)
-	assert.Regexp(t, "FF23066", err)
+	assert.Regexp(t, "FF23073", err)
+	assert.Contains(t, err.Error(), "mode")
+	assert.Contains(t, err.Error(), "Supported values: object, array, self-describing")
 
 	_, err = JSONFormatOptions("number=blue").GetABISerializer(ctx)
-	assert.Regexp(t, "FF23066", err)
+	assert.Regexp(t, "FF23073", err)
+	assert.Contains(t, err.Error(), "number")
+	assert.Contains(t, err.Error(), "Supported values: string, hex-0x, hex, json-number")
 
 	_, err = JSONFormatOptions("bytes=blue").GetABISerializer(ctx)
-	assert.Regexp(t, "FF23066", err)
+	assert.Regexp(t, "FF23073", err)
+	assert.Contains(t, err.Error(), "bytes")
+	assert.Contains(t, err.Error(), "Supported values: hex-0x, hex, hex-plain, base64")
 
 	_, err = JSONFormatOptions("address=blue").GetABISerializer(ctx)
-	assert.Regexp(t, "FF23066", err)
+	assert.Regexp(t, "FF23073", err)
+	assert.Contains(t, err.Error(), "address")
+	assert.Contains(t, err.Error(), "Supported values: hex-0x, hex, hex-plain, checksum")
 
 	s := JSONFormatOptions("this;is;ignored").GetABISerializerIgnoreErrors(ctx)
 	assert.NotNil(t, s)