diff --git a/x/observer/keeper/grpc_query_prove_test.go b/x/observer/keeper/grpc_query_prove_test.go
new file mode 100644
index 0000000000..7eb11b00f5
--- /dev/null
+++ b/x/observer/keeper/grpc_query_prove_test.go
@@ -0,0 +1,72 @@
+package keeper_test
+
+import (
+	"testing"
+
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	"github.com/stretchr/testify/require"
+	"github.com/zeta-chain/zetacore/pkg/proofs"
+	keepertest "github.com/zeta-chain/zetacore/testutil/keeper"
+	"github.com/zeta-chain/zetacore/testutil/sample"
+	"github.com/zeta-chain/zetacore/x/observer/types"
+)
+
+func TestKeeper_Prove(t *testing.T) {
+	t.Run("should error if req is nil", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeper(t)
+		wctx := sdk.WrapSDKContext(ctx)
+
+		res, err := k.Prove(wctx, nil)
+		require.Nil(t, res)
+		require.Error(t, err)
+	})
+
+	t.Run("should error if invalid hash", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeper(t)
+		wctx := sdk.WrapSDKContext(ctx)
+
+		res, err := k.Prove(wctx, &types.QueryProveRequest{
+			ChainId:   987,
+			BlockHash: sample.Hash().String(),
+		})
+		require.Nil(t, res)
+		require.Error(t, err)
+	})
+
+	t.Run("should error if header not found", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeper(t)
+		wctx := sdk.WrapSDKContext(ctx)
+
+		res, err := k.Prove(wctx, &types.QueryProveRequest{
+			ChainId:   5,
+			BlockHash: sample.Hash().String(),
+		})
+		require.Nil(t, res)
+		require.Error(t, err)
+	})
+
+	t.Run("should error if proof not valid", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeper(t)
+		wctx := sdk.WrapSDKContext(ctx)
+
+		hash := sample.Hash()
+		bh := proofs.BlockHeader{
+			Height:     1,
+			Hash:       hash.Bytes(),
+			ParentHash: sample.Hash().Bytes(),
+			ChainId:    1,
+			Header:     proofs.HeaderData{},
+		}
+		k.SetBlockHeader(ctx, bh)
+
+		res, err := k.Prove(wctx, &types.QueryProveRequest{
+			ChainId:   5,
+			BlockHash: hash.String(),
+			Proof:     &proofs.Proof{},
+		})
+		require.Nil(t, res)
+		require.Error(t, err)
+	})
+
+	// TODO: // https://github.com/zeta-chain/node/issues/1875 add more tests
+}
diff --git a/x/observer/keeper/vote_inbound_test.go b/x/observer/keeper/vote_inbound_test.go
index 41a3999b75..5b1c09e81b 100644
--- a/x/observer/keeper/vote_inbound_test.go
+++ b/x/observer/keeper/vote_inbound_test.go
@@ -258,6 +258,57 @@ func TestKeeper_VoteOnInboundBallot(t *testing.T) {
 		require.True(t, isNew)
 	})
 
+	t.Run("fail if can not add vote", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeperWithMocks(t, keepertest.ObserverMocksAll)
+
+		observer := sample.AccAddress()
+		stakingMock := keepertest.GetObserverStakingMock(t, k)
+		slashingMock := keepertest.GetObserverSlashingMock(t, k)
+
+		k.SetCrosschainFlags(ctx, types.CrosschainFlags{
+			IsInboundEnabled: true,
+		})
+		k.SetChainParamsList(ctx, types.ChainParamsList{
+			ChainParams: []*types.ChainParams{
+				{
+					ChainId:     getValidEthChainIDWithIndex(t, 0),
+					IsSupported: true,
+				},
+				{
+					ChainId:     getValidEthChainIDWithIndex(t, 1),
+					IsSupported: true,
+				},
+			},
+		})
+		k.SetObserverSet(ctx, types.ObserverSet{
+			ObserverList: []string{observer},
+		})
+		stakingMock.MockGetValidator(sample.Validator(t, sample.Rand()))
+		slashingMock.MockIsTombstoned(false)
+		ballot := types.Ballot{
+			Index:            "index",
+			BallotIdentifier: "index",
+			VoterList:        []string{observer},
+			// already voted
+			Votes:           []types.VoteType{types.VoteType_SuccessObservation},
+			BallotStatus:    types.BallotStatus_BallotInProgress,
+			BallotThreshold: sdk.NewDec(2),
+		}
+		k.SetBallot(ctx, &ballot)
+		isFinalized, isNew, err := k.VoteOnInboundBallot(
+			ctx,
+			getValidEthChainIDWithIndex(t, 0),
+			getValidEthChainIDWithIndex(t, 1),
+			coin.CoinType_ERC20,
+			observer,
+			"index",
+			"inTxHash",
+		)
+		require.Error(t, err)
+		require.False(t, isFinalized)
+		require.False(t, isNew)
+	})
+
 	t.Run("can add vote and create ballot without finalizing ballot", func(t *testing.T) {
 		k, ctx, _, _ := keepertest.ObserverKeeperWithMocks(t, keepertest.ObserverMocksAll)
 
diff --git a/x/observer/keeper/vote_outbound_test.go b/x/observer/keeper/vote_outbound_test.go
index fa34a11ca6..c92fec5927 100644
--- a/x/observer/keeper/vote_outbound_test.go
+++ b/x/observer/keeper/vote_outbound_test.go
@@ -133,6 +133,48 @@ func TestKeeper_VoteOnOutboundBallot(t *testing.T) {
 		require.Equal(t, expectedBallot, ballot)
 	})
 
+	t.Run("fail if can not add vote", func(t *testing.T) {
+		k, ctx, _, _ := keepertest.ObserverKeeperWithMocks(t, keepertest.ObserverMocksAll)
+
+		observer := sample.AccAddress()
+		stakingMock := keepertest.GetObserverStakingMock(t, k)
+		slashingMock := keepertest.GetObserverSlashingMock(t, k)
+
+		k.SetChainParamsList(ctx, types.ChainParamsList{
+			ChainParams: []*types.ChainParams{
+				{
+					ChainId:     getValidEthChainIDWithIndex(t, 0),
+					IsSupported: true,
+				},
+			},
+		})
+		k.SetObserverSet(ctx, types.ObserverSet{
+			ObserverList: []string{observer},
+		})
+		stakingMock.MockGetValidator(sample.Validator(t, sample.Rand()))
+		slashingMock.MockIsTombstoned(false)
+		ballot := types.Ballot{
+			Index:            "index",
+			BallotIdentifier: "index",
+			VoterList:        []string{observer},
+			// already voted
+			Votes:           []types.VoteType{types.VoteType_SuccessObservation},
+			BallotStatus:    types.BallotStatus_BallotInProgress,
+			BallotThreshold: sdk.NewDec(2),
+		}
+		k.SetBallot(ctx, &ballot)
+		isFinalized, isNew, ballot, _, err := k.VoteOnOutboundBallot(
+			ctx,
+			"index",
+			getValidEthChainIDWithIndex(t, 0),
+			chains.ReceiveStatus_Success,
+			observer,
+		)
+		require.Error(t, err)
+		require.False(t, isFinalized)
+		require.False(t, isNew)
+	})
+
 	t.Run("can add vote and create ballot without finalizing ballot", func(t *testing.T) {
 		k, ctx, _, _ := keepertest.ObserverKeeperWithMocks(t, keepertest.ObserverMocksAll)