ADR-003: WASM Plugins Use Extism As The Initial Execution Bridge

This is a restored retroactive record. The original ADR was added under docs/architecture/decisions/adr-003-wasm-extism-plugin-model.md and was dropped during the mdBook migration. It records the historical Extism-based plugin bridge that was accepted on 2026-03-15. The current WIT and direct wasmtime model supersedes it in ADR-009.

Context

ZeroClaw compiled many tools and channels into a single binary. Every user paid the compile time and binary size for capabilities they might never use. Third-party developers could not extend ZeroClaw without forking the repository and writing Rust code against internal APIs.

The Intentional Architecture RFC defined a microkernel target where non-core tools and channels become loadable plugins. That required a sandboxed execution model that:

1. Runs untrusted code without compromising the host process.
2. Works on Linux, macOS, Windows, ARM, and x86_64 targets.
3. Supports capability-based permissions such as HTTP access, environment-variable reads, and file I/O.
4. Allows plugins to be written in any language that compiles to WASM.
5. Adds minimal binary size when the feature is unused.

The original evaluation considered three WASM runtime options:

Runtime	Pros	Cons
Extism	High-level SDK, built-in host function system, PDKs for several guest languages, active maintenance	Adds binary size behind the feature flag
Raw wasmtime	Maximum control, mature runtime	Requires building the ABI, memory protocol, and host function system directly
Wasmer	LLVM and Cranelift backends	Smaller ecosystem and less Rust-native host-function ergonomics

Decision

ZeroClaw would use Extism 1.x as the initial WASM plugin runtime behind the plugins-wasm feature flag.

Plugins were WASM modules exporting two JSON functions:

• tool_metadata(String) -> String, returning JSON with name, description, and parameters_schema fields.
• execute(String) -> String, receiving tool arguments as JSON and returning a JSON result with success, output, and optional error fields.

The runtime provided two permission-gated host functions:

• zc_http_request(String) -> String, gated on PluginPermission::HttpClient.
• zc_env_read(String) -> String, gated on PluginPermission::EnvRead.

Extism's built-in HTTP support was deliberately not used because it would bypass ZeroClaw's permission enforcement.

Each plugin shipped a manifest.toml alongside its .wasm file. The manifest declared name, version, capabilities such as tool, channel, memory, and observer, and required permissions such as http_client, env_read, file_read, file_write, memory_read, and memory_write.

Plugin manifests supported optional Ed25519 signatures with three enforcement modes: disabled, permissive, and strict.

Plugin authors depended on extism-pdk and compiled to wasm32-wasip1. The protocol used documented JSON contracts rather than a ZeroClaw-specific guest SDK crate.

Consequences

Positive:

• WASM linear memory isolation prevented plugins from accessing host memory.
• Permission-gated host functions gave the initial bridge a clear capability model.
• WASM modules could run on any platform that the underlying runtime supported.
• Languages with wasm32-wasip1 targets could produce plugins.
• Users who disabled plugins-wasm did not pay the binary-size or compile-time cost.

Negative:

• Extism added binary size behind the feature flag.
• Plugin authors depended on extism-pdk, an external SDK.
• The initial bridge made tool plugins functional before channel plugins.
• Extism calls were synchronous while ZeroClaw's Tool trait was async, so calls used blocking-task bridging.

Known gaps:

• zc_http_request could forward plugin-supplied URLs without the same private-IP, loopback, and link-local restrictions used by native HTTP tools.
• env_read granted access to any variable by name instead of a per-plugin allowlist.
• CPU execution did not have a fuel limit or epoch interruption.

Those gaps meant the permission model was initially a documented contract, not yet a hardened boundary for plugins from untrusted authors.

References

• ADR-009: WIT components and direct wasmtime plugin execution
• Historical source path: docs/architecture/decisions/adr-003-wasm-extism-plugin-model.md
• Historical implementation paths: crates/zeroclaw-plugins/src/runtime.rs, crates/zeroclaw-plugins/src/wasm_tool.rs, crates/zeroclaw-plugins/src/host.rs, and crates/zeroclaw-plugins/src/signature.rs
• Tracking issues from the original ADR: #5918 and #5919