Decode raw calldata without requiring ABI using Heimdall
AI agents call heimdall_decode to retrieve information from Foundry MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
Decoding calldata is purely an informational retrieval operation that inspects and interprets existing blockchain transaction data. It does not create, modify, delete, or execute any operations on-chain or off-chain. While it provides visibility into transaction details, this is a read-only analysis tool consistent with the Read category.
From the tool's definition Tool name 'heimdall_decode' and description 'Decode raw calldata without requiring ABI using Heimdall' indicate a decoding/parsing operation with no side effects.
Documented attack patterns abuse exactly the kind of access heimdall_decode gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Foundry MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for heimdall_decode:
{
"version": "1",
"default": "deny",
"tools": {
"heimdall_decode": {}
}
}heimdall_decode is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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Decode raw calldata without requiring ABI using Heimdall. It is categorised as a Read tool in the Foundry MCP Server MCP Server, which means it retrieves data without modifying state.
Register the Foundry MCP Server MCP server in PolicyLayer and add a rule for heimdall_decode: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Foundry MCP Server. Nothing to install.
heimdall_decode is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the heimdall_decode rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for heimdall_decode. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
heimdall_decode is provided by the Foundry MCP Server MCP server (praneshasp/foundry-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 16 Foundry MCP Server tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
16 Foundry MCP Server tools catalogued and risk-classified — across an index of 42,500+ MCP servers.