A tool/protocol call failed on its SCHEMA or FORMAT — wrong request shape, wrong network/scheme identifier, a protocol-level auth/handshake problem, or a malformed payment payload. Distinct from hallucinated_tool_args (invented values) — here the shape/protocol/config is wrong. Includes MCP protocol, x402 payload, and RPC method-not-found errors — see subclasses.
mcp_stdout_corruption
MCP stdio server broke its JSON-RPC stream by writing non-protocol output (a console.log) to stdout (-32700 parse error).
mcp_transport_mismatch
MCP client/server transport mismatch (stdio vs SSE vs Streamable HTTP) or an initialize handshake timeout.
mcp_auth_discovery
MCP 401 without the RFC 9728 WWW-Authenticate / resource_metadata pointer, or an audience (RFC 8707) mismatch — OAuth can't start.
mcp_session_id
MCP Streamable HTTP request missing the Mcp-Session-Id header ('no valid session').
x402_invalid_payload
x402 payment rejected as invalid_payload/invalid_network — malformed body, below-minimum amount, or wrong CAIP-2 network/scheme identifier.
rpc_method_not_found
RPC -32601/-32004 — the provider doesn't expose that method on this chain/plan.
wallet_unrecognized_chain
Wallet 4902 unrecognized chain (call wallet_addEthereumChain; pass a 0x-hex chainId).
solana_versioned_tx
Solana Address-Lookup-Table tx built as legacy instead of a versioned (v0) tx.
api_insufficient_scope
API 403/insufficient_scope — a valid token issued with scopes too narrow for this operation.
How this class maps to the two leading agent-failure taxonomies.
MAST
Task specification disobedience
TRAIL
Incorrect tool arguments, Tool/API errors
Send your agent's trace to the ingest endpoint. Snapback diagnoses it — including this class — and returns a structured verdict with the root cause and a concrete fix. Pre-flight screening is free; you only draw credit on a full diagnosis.
curl -X POST "https://ingest.snapback.sh/v1/traces" \
-H "Authorization: Bearer vdk_YOUR_TOKEN" \
-H "Content-Type: application/json" \
-d '{
"trace_id": "run-42",
"steps": [
{ "agent": "planner", "action": "...", "output": "..." },
{ "agent": "executor", "tool": "...", "error": "..." }
]
}'Get your ingest token when you create a source. Already emitting OpenTelemetry? Point your exporter at /v1/traces/otlp — no re-instrumentation. AI agents can call the same pipeline over MCP (diagnose_trace).
What comes back
Illustrative — your verdict is diagnosed from your actual trace.
{
"failure_class": "tool_schema",
"failed_at_step": 2,
"failure_agent": "executor",
"root_cause": "<the specific cause, inferred from your trace>",
"fix_suggestion": "<a concrete, applicable fix>",
"confidence": 0.9
}Catch this failure automatically
Snapback watches your agents and diagnoses this class the moment it happens — with the specific trace, the likely cause, and the fix.
Start free