Greentic MCP Tool Bridge

greentic-mcp provides the host-side tool registry and WASIX/WASI execution bridge used by flows to invoke MCP tools compiled to WebAssembly. It loads a declarative tool map, resolves tools by logical name, and executes them through Wasmtime with per-tool timeouts, retry hints, and transient error handling.

The crate leans on the shared contracts published in greentic-types and the WIT definitions plus generated bindings in greentic-interfaces. Tools are compiled for the WASI Preview2 target (wasm32-wasip2) and export a canonical entrypoint that accepts and returns UTF-8 JSON payloads, so hosts no longer have to marshal pointer/length pairs manually. When tools additionally export the greentic:component/component@1.0.0 world, the bridge calls describe-json to fetch the node schema/defaults that live with the tool component rather than mirroring them in this repository.

Runtime callbacks (HTTP + KV) are satisfied through the runner-host-v1 helpers shipped by greentic-interfaces, which keeps the host implementation aligned with the latest Greentic runner contracts. Secrets must travel through the greentic:secrets/store@1.0.0 bytes API (no env/host-import fallback); the legacy secret_get hook is removed and callers should provide a store binding that enforces runtime TenantCtx scope.

Tool map configuration

Tool metadata is loaded from JSON or YAML. Each entry records where the tool artifact lives, the export to call, and optional execution hints.

tools:
  - name: echo
    component: ./tools/echo.wasm
    entry: tool_invoke
    timeout_ms: 1000
    max_retries: 2
    retry_backoff_ms: 200

Use greentic_mcp::load_tool_map to parse the file and build a ToolMap.

use greentic_mcp::{invoke_with_map, load_tool_map, WasixExecutor};
use serde_json::json;
use std::path::Path;

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
let map = load_tool_map(Path::new("toolmap.yaml"))?;
let executor = WasixExecutor::new()?;

let echoed = invoke_with_map(&map, &executor, "echo", json!({"hello": "world"}))
    .await?;

assert_eq!(echoed, json!({"hello": "world"}));
# Ok(())
# }

WasixExecutor ensures that traps bubble up as transient errors, applies exponential backoff with jitter between retries, and converts wall-clock timeouts into McpError::Timeout.

ABI contracts

See ABI.md for the exact contract implemented by the integration tests. When a tool exports the richer Greentic component API, use the bindings from greentic-interfaces to avoid manual JSON marshalling and to work with the describe-v1/runner-host-v1 WIT worlds directly.

Secret requirements

describe_tool now normalizes any secret descriptors in tool metadata to Vec<SecretRequirement>:

• Prefer secret_requirements returned by describe-json.
• Fall back to legacy list_secrets payloads and emit a warning.
• Keys validated via greentic-types; unknown formats default to text.
• Default scope uses the runtime sentinel { env: "runtime", tenant: "runtime" } so TenantCtx/session identity can override at execution time.