diff --git a/cmd/mithril-go/json.go b/cmd/mithril-go/json.go
index b647a47..85a3315 100644
--- a/cmd/mithril-go/json.go
+++ b/cmd/mithril-go/json.go
@@ -3,7 +3,6 @@ package main
 import (
 	"encoding/json"
 	"fmt"
-	"io"
 	"os"
 )
 
@@ -19,12 +18,39 @@ func emitJSON(v any) int {
 	return 0
 }
 
-// emitJSONErr writes a structured error envelope. Mirrors the shape
-// Claude/MCP-friendly consumers want: {"error": {"code":..., "message":...}}.
-func emitJSONErr(w io.Writer, code, msg string) {
-	enc := json.NewEncoder(w)
+// emitJSONErr writes a structured error envelope to stdout in the shape
+// MCP / agent consumers expect:
+//
+//	{"error": {"code": "...", "message": "..."}}
+//
+// Returns the supplied exit code so callers can do `return emitJSONErr(...)`.
+func emitJSONErr(code int, kind, msg string) int {
+	enc := json.NewEncoder(os.Stdout)
 	enc.SetIndent("", "  ")
 	_ = enc.Encode(map[string]any{
-		"error": map[string]string{"code": code, "message": msg},
+		"error": map[string]any{
+			"code":     code,
+			"kind":     kind,
+			"message":  msg,
+		},
 	})
+	return code
+}
+
+// failure routes an error to either stdout-as-JSON (when the user passed
+// -json) or stderr-as-text (default). Returns the supplied exit code.
+//
+// kind is a stable short string ("network", "integrity", "verify",
+// "usage", "internal") — agents can branch on this without parsing
+// human-readable text.
+func failure(asJSON bool, code int, kind, prefix string, err error) int {
+	msg := err.Error()
+	if prefix != "" {
+		msg = prefix + ": " + msg
+	}
+	if asJSON {
+		return emitJSONErr(code, kind, msg)
+	}
+	fmt.Fprintln(os.Stderr, msg)
+	return code
 }
diff --git a/cmd/mithril-go/main.go b/cmd/mithril-go/main.go
index b253c61..635c755 100644
--- a/cmd/mithril-go/main.go
+++ b/cmd/mithril-go/main.go
@@ -16,6 +16,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"io"
 	"net/http"
 	"os"
 	"os/signal"
@@ -34,7 +35,7 @@ import (
 	"git.sulkta.coop/Sulkta-Coop/mithril-go/internal/verify"
 )
 
-const version = "0.0.3-dev"
+const version = "1.0.1"
 
 // Stable exit codes. Any addition goes at the end; existing values
 // don't renumber. LLM/automation-friendly contract.
@@ -132,14 +133,12 @@ func cmdList(ctx context.Context, args []string) int {
 	asJSON := fs.Bool("json", false, "emit structured JSON")
 	n, _, err := resolveNetwork(fs, args)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		return 2
+		return failure(*asJSON, exitUsage, "usage", "", err)
 	}
 	c := aggregator.New(n.AggregatorURL)
 	snaps, err := c.ListCardanoDBSnapshots(ctx)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "list:", err)
-		return exitNetwork
+		return failure(*asJSON, exitNetwork, "network", "list", err)
 	}
 	if *asJSON {
 		return emitJSON(map[string]any{
@@ -167,8 +166,7 @@ func cmdShow(ctx context.Context, args []string) int {
 	asJSON := fs.Bool("json", false, "emit structured JSON")
 	n, rest, err := resolveNetwork(fs, args)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "", err)
 	}
 	hash := "latest"
 	if len(rest) > 0 {
@@ -177,8 +175,7 @@ func cmdShow(ctx context.Context, args []string) int {
 	c := aggregator.New(n.AggregatorURL)
 	snap, err := resolveSnapshot(ctx, c, hash)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "show:", err)
-		return exitNetwork
+		return failure(*asJSON, exitNetwork, "network", "show", err)
 	}
 	if *asJSON {
 		return emitJSON(snap)
@@ -282,12 +279,11 @@ func cmdVerify(ctx context.Context, args []string) int {
 	asJSON := fs.Bool("json", false, "emit structured JSON")
 	n, rest, err := resolveNetwork(fs, args)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "", err)
 	}
 	if len(rest) == 0 {
-		fmt.Fprintln(os.Stderr, "verify: cert hash required (or 'head' / 'genesis')")
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "",
+			fmt.Errorf("verify: cert hash required (or 'head' / 'genesis' / 'chain' / 'manifest <dir>')"))
 	}
 	mode := rest[0] // "head" = verify head cert (STM, not yet), "genesis" = walk chain + verify genesis, or a specific hash
 	c := aggregator.New(n.AggregatorURL)
@@ -309,24 +305,21 @@ func cmdVerify(ctx context.Context, args []string) int {
 
 func runVerifyManifest(args []string, asJSON bool) int {
 	if len(args) == 0 {
-		fmt.Fprintln(os.Stderr, "verify manifest: needs path to download dir (with digests/ + db/)")
-		return exitUsage
+		return failure(asJSON, exitUsage, "usage", "",
+			fmt.Errorf("verify manifest: needs path to download dir (with digests/ + db/)"))
 	}
 	dir := args[0]
 	digestsPath, err := manifest.LocateDigests(filepath.Join(dir, "digests"))
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "locate digests.json:", err)
-		return exitGeneric
+		return failure(asJSON, exitGeneric, "internal", "locate digests.json", err)
 	}
 	entries, err := manifest.Load(digestsPath)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "load manifest:", err)
-		return exitIntegrity
+		return failure(asJSON, exitIntegrity, "integrity", "load manifest", err)
 	}
 	res, err := manifest.Verify(entries, filepath.Join(dir, "db"))
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "verify manifest:", err)
-		return exitGeneric
+		return failure(asJSON, exitGeneric, "internal", "verify manifest", err)
 	}
 	if asJSON {
 		code := emitJSON(res)
@@ -349,13 +342,11 @@ func runVerifyChain(ctx context.Context, n networks.Network, asJSON bool) int {
 	c := aggregator.New(n.AggregatorURL)
 	snap, err := resolveSnapshot(ctx, c, "latest")
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "resolve:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "resolve", err)
 	}
 	res, err := chain.Verify(ctx, nil, n, snap.CertificateHash, 2048)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "chain verify:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "chain verify", err)
 	}
 	if asJSON {
 		code := emitJSON(res)
@@ -390,22 +381,19 @@ func runVerifyGenesis(ctx context.Context, c *aggregator.Client, n networks.Netw
 	// Find the head snapshot's cert, walk to genesis, verify Ed25519 on the genesis cert.
 	snap, err := resolveSnapshot(ctx, c, "latest")
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "resolve:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "resolve", err)
 	}
-	chain, err := c.CertChain(ctx, snap.CertificateHash, 2048)
+	certs, err := c.CertChain(ctx, snap.CertificateHash, 2048)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "chain:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "chain", err)
 	}
-	if len(chain) == 0 {
-		fmt.Fprintln(os.Stderr, "empty chain")
-		return exitGeneric
+	if len(certs) == 0 {
+		return failure(asJSON, exitGeneric, "internal", "", fmt.Errorf("empty chain"))
 	}
-	gen := chain[len(chain)-1]
+	gen := certs[len(certs)-1]
 	if gen.GenesisSignature == "" {
-		fmt.Fprintln(os.Stderr, "tail of chain is not a genesis certificate")
-		return exitGeneric
+		return failure(asJSON, exitGeneric, "internal", "",
+			fmt.Errorf("tail of chain is not a genesis certificate"))
 	}
 	return verifyGenesisCert(n, gen, asJSON)
 }
@@ -413,8 +401,7 @@ func runVerifyGenesis(ctx context.Context, c *aggregator.Client, n networks.Netw
 func runVerifyHead(ctx context.Context, c *aggregator.Client, n networks.Network, asJSON bool) int {
 	snap, err := resolveSnapshot(ctx, c, "latest")
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "resolve:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "resolve", err)
 	}
 	return runVerifySingle(ctx, c, n, snap.CertificateHash, asJSON)
 }
@@ -422,8 +409,7 @@ func runVerifyHead(ctx context.Context, c *aggregator.Client, n networks.Network
 func runVerifySingle(ctx context.Context, c *aggregator.Client, n networks.Network, hash string, asJSON bool) int {
 	cert, err := c.GetCertificate(ctx, hash)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "cert:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "cert", err)
 	}
 	if cert.GenesisSignature != "" {
 		return verifyGenesisCert(n, cert, asJSON)
@@ -438,18 +424,15 @@ func verifySTMCert(ctx context.Context, c *aggregator.Client, n networks.Network
 	// Re-fetch as raw JSON to access the AVK + params fields.
 	raw, err := fetchCertRaw(ctx, n.AggregatorURL, hash)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "fetch raw cert:", err)
-		return exitNetwork
+		return failure(asJSON, exitNetwork, "network", "fetch raw cert", err)
 	}
 	ms, err := stm.DecodeMultiSig(raw.MultiSignature)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "decode multi_signature:", err)
-		return exitIntegrity
+		return failure(asJSON, exitIntegrity, "integrity", "decode multi_signature", err)
 	}
 	avk, err := stm.DecodeAVK(raw.AggregateVerificationKey)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "decode avk:", err)
-		return exitIntegrity
+		return failure(asJSON, exitIntegrity, "integrity", "decode avk", err)
 	}
 	msg := []byte(cert.SignedMessage)
 	params := stm.Parameters{K: raw.Metadata.Parameters.K, M: raw.Metadata.Parameters.M, PhiF: raw.Metadata.Parameters.PhiF}
@@ -501,14 +484,21 @@ func fetchCertRaw(ctx context.Context, aggregatorURL, hash string) (*rawCert, er
 	if err != nil {
 		return nil, err
 	}
+	req.Header.Set("Accept", "application/json")
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return nil, err
 	}
 	defer resp.Body.Close()
+	if resp.StatusCode >= 400 {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 2048))
+		return nil, fmt.Errorf("aggregator GET /certificate/%s: %d: %s", hash, resp.StatusCode, string(body))
+	}
+	// Cap at 16 MiB — current mainnet cert JSON is well under 100 KiB.
+	limited := io.LimitReader(resp.Body, 16<<20)
 	var r rawCert
-	if err := json.NewDecoder(resp.Body).Decode(&r); err != nil {
-		return nil, err
+	if err := json.NewDecoder(limited).Decode(&r); err != nil {
+		return nil, fmt.Errorf("decode cert json: %w", err)
 	}
 	return &r, nil
 }
@@ -516,8 +506,7 @@ func fetchCertRaw(ctx context.Context, aggregatorURL, hash string) (*rawCert, er
 func verifyGenesisCert(n networks.Network, cert *aggregator.Certificate, asJSON bool) int {
 	vk, err := verify.DecodeGenesisVerifyKey(n.GenesisVerifyKey)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "decode genesis key:", err)
-		return exitGeneric
+		return failure(asJSON, exitGeneric, "internal", "decode genesis key", err)
 	}
 	err = verify.GenesisFromJSON(vk, cert.SignedMessage, cert.GenesisSignature, cert.ProtocolMessage)
 	if asJSON {
@@ -552,28 +541,25 @@ func cmdCert(ctx context.Context, args []string) int {
 	asJSON := fs.Bool("json", false, "emit structured JSON")
 	n, rest, err := resolveNetwork(fs, args)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "", err)
 	}
 	if len(rest) == 0 {
-		fmt.Fprintln(os.Stderr, "cert: hash required (or 'head' to use the latest snapshot's cert_hash)")
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "",
+			fmt.Errorf("cert: hash required (or 'head' to use the latest snapshot's cert_hash)"))
 	}
 	head := rest[0]
 	c := aggregator.New(n.AggregatorURL)
 	if head == "head" {
 		snap, err := resolveSnapshot(ctx, c, "latest")
 		if err != nil {
-			fmt.Fprintln(os.Stderr, "resolve head:", err)
-			return exitNetwork
+			return failure(*asJSON, exitNetwork, "network", "resolve head", err)
 		}
 		head = snap.CertificateHash
 	}
 	if *chain {
 		certs, err := c.CertChain(ctx, head, *maxDepth)
 		if err != nil {
-			fmt.Fprintln(os.Stderr, "chain:", err)
-			return exitNetwork
+			return failure(*asJSON, exitNetwork, "network", "chain", err)
 		}
 		if *asJSON {
 			return emitJSON(map[string]any{"chain_length": len(certs), "certs": certs})
@@ -591,8 +577,7 @@ func cmdCert(ctx context.Context, args []string) int {
 	}
 	cert, err := c.GetCertificate(ctx, head)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, "cert:", err)
-		return exitNetwork
+		return failure(*asJSON, exitNetwork, "network", "cert", err)
 	}
 	if *asJSON {
 		return emitJSON(cert)
@@ -619,8 +604,7 @@ func cmdInfo(args []string) int {
 	asJSON := fs.Bool("json", false, "emit structured JSON")
 	n, _, err := resolveNetwork(fs, args)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		return exitUsage
+		return failure(*asJSON, exitUsage, "usage", "", err)
 	}
 	if *asJSON {
 		return emitJSON(map[string]any{
diff --git a/internal/artifact/download.go b/internal/artifact/download.go
index e29546a..575d94c 100644
--- a/internal/artifact/download.go
+++ b/internal/artifact/download.go
@@ -121,6 +121,10 @@ func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progres
 	if expectedSHA256 != "" {
 		got := hex.EncodeToString(h.Sum(nil))
 		if got != expectedSHA256 {
+			// Remove the .part file — leaving it behind would cause every
+			// subsequent retry to resume from the same corrupted bytes and
+			// fail SHA again indefinitely.
+			_ = os.Remove(partPath)
 			return fmt.Errorf("SHA256 mismatch: want %s, got %s", expectedSHA256, got)
 		}
 	}
diff --git a/internal/stm/lottery.go b/internal/stm/lottery.go
index 5c23ebc..6d6bd15 100644
--- a/internal/stm/lottery.go
+++ b/internal/stm/lottery.go
@@ -58,6 +58,12 @@ func IsLotteryWon(phiF float64, ev [64]byte, stake, totalStake uint64) bool {
 	if math.Abs(phiF-1.0) < 1e-15 {
 		return true
 	}
+	// Defensive: zero-stake or zero-total-stake produces nonsense (and
+	// totalStake==0 would panic at SetFrac). Guard at the lottery layer
+	// in addition to AVK-decode-time validation.
+	if stake == 0 || totalStake == 0 {
+		return false
+	}
 
 	// ev as big int (LE interpretation)
 	evInt := evAsBigInt(ev)
diff --git a/internal/stm/merkle.go b/internal/stm/merkle.go
index 51c4211..9984997 100644
--- a/internal/stm/merkle.go
+++ b/internal/stm/merkle.go
@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/binary"
 	"fmt"
-	"sort"
 
 	"golang.org/x/crypto/blake2b"
 )
@@ -78,16 +77,31 @@ func VerifyMerkleBatch(root []byte, nrLeaves int, leafValues [][]byte, indices [
 	if len(leafValues) != len(indices) {
 		return fmt.Errorf("leaves/indices count mismatch: %d vs %d", len(leafValues), len(indices))
 	}
-	// Must be sorted ascending
+	if nrLeaves <= 0 {
+		return fmt.Errorf("nrLeaves must be positive, got %d", nrLeaves)
+	}
+	// Validate every proof node is a 32-byte BLAKE2b-256 digest. Anything
+	// shorter or longer is malformed and Rust would reject it.
+	for i, v := range proofValues {
+		if len(v) != 32 {
+			return fmt.Errorf("proof value [%d]: got %d bytes, want 32", i, len(v))
+		}
+	}
+	// Indices must be strictly ascending — duplicates would create
+	// double-claiming under the same leaf and the algorithm doesn't expect
+	// them. (Rust uses sort_unstable + equality compare against the input;
+	// equivalent to "non-decreasing" but doesn't reject equal-adjacent.
+	// We're stricter than upstream here on purpose.)
 	ordered := make([]int, len(indices))
 	for i, v := range indices {
+		if v >= uint64(nrLeaves) {
+			return fmt.Errorf("index [%d]=%d out of range (nr_leaves=%d)", i, v, nrLeaves)
+		}
 		ordered[i] = int(v)
 	}
-	sortedCopy := append([]int(nil), ordered...)
-	sort.Ints(sortedCopy)
-	for i := range ordered {
-		if ordered[i] != sortedCopy[i] {
-			return fmt.Errorf("indices not sorted ascending: %v", indices)
+	for i := 1; i < len(ordered); i++ {
+		if ordered[i] <= ordered[i-1] {
+			return fmt.Errorf("indices not strictly ascending at [%d]: %v", i, indices)
 		}
 	}
 
@@ -155,6 +169,12 @@ func VerifyMerkleBatch(root []byte, nrLeaves int, leafValues [][]byte, indices [
 	if len(currentLayer) != 1 {
 		return fmt.Errorf("verification ended with %d nodes, want 1", len(currentLayer))
 	}
+	// All proof values must be consumed. Trailing bytes mean the proof
+	// shipped extra nodes the algorithm didn't need — likely malformed
+	// or attacker-padded.
+	if len(values) > 0 {
+		return fmt.Errorf("proof has %d unconsumed values — malformed", len(values))
+	}
 	if !bytes.Equal(currentLayer[0], root) {
 		return fmt.Errorf("root mismatch: got %x, want %x", currentLayer[0], root)
 	}
diff --git a/internal/stm/types.go b/internal/stm/types.go
index dd86488..6873067 100644
--- a/internal/stm/types.go
+++ b/internal/stm/types.go
@@ -116,6 +116,12 @@ func DecodeAVK(rawJSON []byte) (*AVK, error) {
 	if len(wire.MTCommitment.Root) != 32 {
 		return nil, fmt.Errorf("AVK root: got %d bytes, want 32", len(wire.MTCommitment.Root))
 	}
+	if wire.TotalStake == 0 {
+		return nil, fmt.Errorf("AVK total_stake is zero")
+	}
+	if wire.MTCommitment.NrLeaves == 0 {
+		return nil, fmt.Errorf("AVK nr_leaves is zero")
+	}
 	return &AVK{
 		MerkleRoot: wire.MTCommitment.Root,
 		NumLeaves:  wire.MTCommitment.NrLeaves,
diff --git a/internal/verify/verify.go b/internal/verify/verify.go
index ab02fce..3fce58e 100644
--- a/internal/verify/verify.go
+++ b/internal/verify/verify.go
@@ -27,7 +27,6 @@ var (
 	ErrNotGenesis        = errors.New("certificate is not a genesis certificate")
 	ErrBadSignature      = errors.New("genesis signature verification failed")
 	ErrSignedMessageHash = errors.New("signed_message does not match SHA256(protocol_message)")
-	ErrSTMNotImplemented = errors.New("STM signature verification not implemented yet")
 )
 
 // The Mithril enum order on ProtocolMessagePartKey — BTreeMap iteration
@@ -182,9 +181,5 @@ func GenesisFromJSON(verifyKey ed25519.PublicKey, signedMessageHex, genesisSigna
 	return Genesis(verifyKey, signedMessageHex, genesisSignatureHex, pm)
 }
 
-// STM verifies a non-genesis certificate's aggregate BLS signature.
-// Stub — target is Mithril STM paper §5 (signing) + §6 (aggregation)
-// using gnark-crypto's bls12-381 primitives.
-func STM(protocolMessageJSON, multiSignature []byte, avk any) error {
-	return ErrSTMNotImplemented
-}
+// STM verification lives in the sibling internal/stm package — see
+// stm.Verify(). This file is genesis-Ed25519-only.