alknet/docs/architecture/crates/call/operation-registry.md

---
status: draft
last_updated: 2026-06-22
---

# Operation Registry

OperationSpec, Handler, OperationRegistry, AccessControl, service discovery, and irpc integration.

## What

The operation registry maps operation names to specs and handlers. It is the dispatch core of the call protocol — when a `call.requested` event arrives, the registry looks up the operation by name, checks access control, invokes the handler, and returns the result.

The registry is populated at startup by the CLI binary (or by the assembly layer in embedded contexts). Operations cannot be added or removed at runtime. This is consistent with OQ-04 (static registration at startup) and the `HandlerRegistry` model in alknet-core.

## Why

The operation registry provides:
- **Discoverability**: Clients can query `/services/list` and `/services/schema` to learn what operations exist before calling them
- **Access control**: Each operation declares its required scopes and resources; the registry enforces ACL before invoking the handler
- **Type safety**: JSON Schema for input and output enables validation and client code generation
- **Composability**: Handlers can invoke other operations through `OperationEnv` (local dispatch — remote dispatch is a separate architectural concern, see Constraints)

The registry design is informed by the `@alkdev/operations` TypeScript package, which demonstrated the same capabilities in JavaScript runtimes. The Rust implementation in alknet-call is canonical — it preserves the behavioral contract (namespace + operation name → invoke with input, return output) while defining the adapter contract (from_*, to_*) in Rust (see ADR-013).

## Architecture

### OperationSpec

Every registered operation has a spec that declares its name, type, schemas, and access control:

```rust
pub struct OperationSpec {
    pub name: String,              // e.g., "fs/readFile", "agent/chat" (no leading slash)
    pub namespace: String,         // e.g., "fs", "agent"
    pub op_type: OperationType,   // Query, Mutation, Subscription
    pub visibility: Visibility,   // External (wire-callable) or Internal (composition-only)
    pub input_schema: Value,      // JSON Schema for input
    pub output_schema: Value,     // JSON Schema for output
    pub error_schemas: Vec<ErrorDefinition>,  // Declared domain errors (ADR-023)
    pub access_control: AccessControl,
}

pub enum OperationType {
    Query,         // Read-only, idempotent (e.g., "fs/readFile", "services/list")
    Mutation,      // Side effects (e.g., "bash/exec", "github/authenticate")
    Subscription,  // Streaming (e.g., "agent/chat", "events/subscribe")
}

pub enum Visibility {
    External,  // Callable from the wire (call.requested from a client)
    Internal,  // Composition-only (env.invoke from a handler)
}

/// A declared operation-level error. See ADR-023.
pub struct ErrorDefinition {
    pub code: String,           // e.g., "FILE_NOT_FOUND", "RATE_LIMITED"
    pub description: String,    // Human-readable description
    pub schema: Value,          // JSON Schema for the error detail payload
    pub http_status: Option<u16>,  // HTTP status for adapter projection (from_openapi/to_openapi)
}
```

Operation names use slash-based paths without a leading slash, aligned with URL path conventions: `fs/readFile`, `agent/chat`, `services/list`. The leading slash is added when needed for display (`spec.path()` returns `/fs/readFile`) and for wire format (the `call.requested` payload uses `/fs/readFile`). See OQ-13 for the path format decision (single-node `service/op` vs head/worker `node/service/op`).

The `namespace` field is derived from the name: for `fs/readFile` it's `fs`, for `agent/chat` it's `agent`. It's a convenience accessor for ACL matching and service grouping.

Visibility (ADR-015) controls whether an operation is callable from the wire. `External` operations are wire-facing — they appear in `services/list` and accept `call.requested` from clients. `Internal` operations are composition-only — they return `NOT_FOUND` (not `FORBIDDEN`) when called from the wire, and do not appear in `services/list`. The assembly layer declares visibility at registration. All import adapters (`from_openapi`, `from_mcp`, `from_jsonschema`, `from_call`) register operations as `Internal` by default (they're composition material, not directly callable); the handler that composes them is `External`.

### AccessControl

```rust
pub struct AccessControl {
    pub required_scopes: Vec<String>,          // AND-checked: caller must have ALL
    pub required_scopes_any: Option<Vec<String>>, // OR-checked: caller must have at LEAST ONE
    pub resource_type: Option<String>,          // e.g., "service"
    pub resource_action: Option<String>,        // e.g., "read"
}
```

When a `call.requested` event arrives:
1. The `CallAdapter` resolves the caller's `Identity` from `AuthContext` (and possibly an `AuthToken` in the payload)
2. The registry checks operation **visibility** — if the operation is `Internal`, returns `call.error` with code `NOT_FOUND` (does not leak existence)
3. The registry checks `access_control.check(identity)` — for external calls (`internal: false`), ACL runs against the **caller's identity**; for internal calls (`internal: true`), ACL runs against the **handler's identity** (ADR-015)
4. If access is denied, the adapter returns `call.error` with code `FORBIDDEN`
5. If the relevant identity is `None` and the operation has restrictions, the adapter returns `call.error` with code `FORBIDDEN` and message `"authentication required"`

Operations with empty `AccessControl` (no required scopes, no resource checks) are accessible to all callers, including unauthenticated ones.

**Internal calls and authority context**: When a handler invokes another operation through `OperationEnv`, the nested call is marked `internal: true`, meaning it originated from composition (not from a wire request). The `internal` flag switches the authority context: the ACL check runs against the composing handler's `handler_identity` (set at registration), not the caller's identity and not as a blanket skip. This prevents privilege escalation through composition — a handler can only compose operations its own identity is authorized for. See ADR-015.

### Handler

```rust
pub type Handler = Arc<dyn Fn(Value, OperationContext) -> Pin<Box<dyn Future<Output = ResponseEnvelope> + Send>> + Send + Sync>;
```

Handlers are async — many operations (file I/O, HTTP service calls, irpc service calls) are inherently asynchronous. The handler receives an `async` runtime context and returns a `Future<Output = ResponseEnvelope>`.

A handler receives:
- `input: Value` — the deserialized `payload` from the `call.requested` event (always `serde_json::Value`)
- `context: OperationContext` — request ID, identity, metadata, env

And returns a `ResponseEnvelope` containing the result or an error. `ResponseEnvelope` is defined in [call-protocol.md](call-protocol.md#responseenvelope) — it carries the request ID and a `Result<Value, CallError>`. Local dispatch produces it with no serialization overhead; the `CallAdapter` converts it to `EventEnvelope` for the wire.

When a handler returns an error, the `CallError.code` is matched against the operation's declared `error_schemas` (ADR-023). If the code matches a declared `ErrorDefinition`, the `call.error` event carries that code and the error's detail payload. If it doesn't match, the `call.error` carries `INTERNAL`. This is how handler failures become typed errors on the wire instead of string-matched messages.

### OperationContext

```rust
pub struct OperationContext {
    pub request_id: String,
    pub parent_request_id: Option<String>,
    pub identity: Option<Identity>,                       // Caller's identity (inbound — who invoked me)
    pub handler_identity: Option<CompositionAuthority>,    // Handler's composition authority (ADR-022)
    pub capabilities: Capabilities,
    pub metadata: HashMap<String, Value>,
    pub env: OperationEnv,
    /// Composition-origin flag. Set by `OperationEnv::invoke()` (true) or the
    /// `CallAdapter` dispatch path (false) — never by handlers. Module-private
    /// for writes; read via `is_internal()`. See ADR-015.
    pub(crate) internal: bool,
}

impl OperationContext {
    pub fn is_internal(&self) -> bool { self.internal }
}
```

- `request_id`: Correlates with the `call.requested` event's `id` field
- `parent_request_id`: Set when this call was initiated by another operation (via `OperationEnv`). Records the agency chain — the call tree is the principal→agent chain (ADR-015)
- `identity`: The authenticated caller (from `IdentityProvider`) — inbound auth (who is calling me). For external calls, this is who sent the `call.requested`. For internal calls, this is the parent handler's `handler_identity` (propagated through `OperationEnv::invoke()`)
- `handler_identity`: The composition authority of the handler processing this call. `None` for leaves (`FromOpenAPI`, `FromMCP`, `FromCall`) — they don't compose. `Some(...)` for `Local` and `Session` ops that can compose children. For internal calls (`internal: true`), the ACL check runs against this authority (ADR-015, ADR-022). This is NOT a peer `Identity` — it's a declared authority bundle set at registration by the assembly layer
- `capabilities`: Outbound credentials the handler may use (decrypted API keys, scoped vault access) — see [Capability Injection](#capability-injection) below
- `metadata`: Request-scoped context (tracing IDs, connection info). **Must not hold secret material** — see ADR-014. **Does not propagate through `OperationEnv::invoke()`** — nested calls get fresh metadata. The tracing link between parent and child is `parent_request_id`, not metadata propagation. Anything a handler needs to pass to a child goes in the call `input`.
- `env`: The operation environment for composing calls to other operations. Scoped — the handler can only invoke a declared set of operations (ADR-015). `None`/empty for leaves.
- `internal`: When `true`, this call originated from composition (a handler calling another operation via `OperationEnv`), not from a wire request. This switches the authority context: ACL runs against `handler_identity`, not `identity`. The `internal` field uses module-private construction — handlers construct `OperationContext` through `OperationEnv::invoke()` which sets `internal: true`, or through the `CallAdapter` dispatch path which sets `internal: false`. The field is not `pub` for writes; only `pub fn is_internal(&self) -> bool` is exposed for reads. See ADR-015.

`identity` and `capabilities` are orthogonal: identity is inbound (who is calling me), capabilities are outbound (what credentials I can use). `identity` and `handler_identity` are the principal/agent pair: `identity` is the principal (who delegated), `handler_identity` is the agent (who is acting). See ADR-014 for capabilities, ADR-015 for the privilege model, and ADR-022 for the composition authority type.

### OperationRegistry

```rust
pub struct OperationRegistry {
    operations: HashMap<String, HandlerRegistration>,
}
```

The registry maps operation names to `HandlerRegistration` bundles. See ADR-022 for the full registration model. Key methods:

- `register(registration)`: Add an operation at startup
- `registration(name)`: Find a registration by operation name (returns spec, handler, provenance, composition authority, scoped env, capabilities)
- `invoke(name, input, context)`: Look up, check ACL, invoke handler, return result
- `list_operations()`: Return all registered specs (for `/services/list`)

### HandlerRegistration

The registration bundle carries everything the dispatch path needs to construct an `OperationContext`. See ADR-022 for the full rationale.

```rust
pub struct HandlerRegistration {
    pub spec: OperationSpec,
    pub handler: Handler,
    pub provenance: OperationProvenance,
    pub composition_authority: Option<CompositionAuthority>, // None for leaves
    pub scoped_env: Option<ScopedOperationEnv>,               // None for leaves
    pub capabilities: Capabilities,
}
```

- `provenance`: Where the op came from (`Local`, `FromOpenAPI`, `FromMCP`, `FromCall`, `FromJsonSchema`, `Session`). Determines composition capability, default visibility, and trust model. Only `Local` and `Session` ops can compose; leaves get `composition_authority: None` and `scoped_env: None`.
- `composition_authority`: The declared authority (label + scopes + resources) the handler operates under when composing children. `None` for leaves. This replaces ADR-015's `handler_identity: Identity` — it's not a peer identity, it's a declared authority bundle. See ADR-022.
- `scoped_env`: The set of operations this handler may reach via `env.invoke()`. `None` for leaves (empty env). The reachability control from ADR-015.
- `capabilities`: Outbound credentials (decrypted API keys, signing keys). Populated by the assembly layer from the vault at registration time. See [Capability Injection](#capability-injection).

The `OperationRegistryBuilder` provides a fluent API with convenience methods for common cases:

```rust
let registry = OperationRegistryBuilder::new()
    // Built-in service discovery (Local, no composition)
    .with_local(services_list_spec(), Arc::new(services_list_handler),
                CompositionAuthority::none(), ScopedOperationEnv::empty())
    .with_local(services_schema_spec(), Arc::new(schema_handler),
                CompositionAuthority::none(), ScopedOperationEnv::empty())
    // Agent handler (Local, composes — has authority + scoped env)
    .with_local(agent_chat_spec(), Arc::new(agent_chat_handler),
                CompositionAuthority::new("agent-chat", ["llm:call", "fs:read", "vastai:query"]),
                ScopedOperationEnv::new(["fs/readFile", "vastai/listMachines", "llm/generate"]))
    // Imported ops (leaves — no authority, no scoped env)
    .with_leaf(vastai_listMachines_spec(), Arc::new(vastai_handler), vastai_credentials)
    .build();
```

The CLI binary (or assembly layer) constructs the registry and passes it to the `CallAdapter`. Once built, the registry is immutable.

### OperationEnv

`OperationEnv` is the universal composition mechanism. A handler calls `context.env.invoke("fs", "readFile", input, &context)` and gets a `ResponseEnvelope` back — regardless of whether the operation runs locally, via an irpc service, or on a remote node.

The `parent` parameter propagates the calling context: the nested call gets `parent_request_id: Some(parent.request_id)`, inherits `parent.handler_identity` as the caller identity, and is marked `internal: true`.

**Metadata does not propagate through composition.** Nested calls get fresh metadata (`HashMap::new()`), not the parent's metadata bag. This is a security constraint (ADR-014): `metadata: HashMap<String, Value>` accepts any `serde_json::Value`, including secret material. If metadata propagated through `env.invoke()`, a handler that accidentally placed a secret in metadata would leak it to every child operation — and if a child is a `from_call` operation (ADR-017), the metadata would cross the wire to the remote node. The tracing link between parent and child is `parent_request_id`, not metadata propagation. Anything a handler needs to pass to a child goes in the call `input`, not in ambient context.

**Local dispatch only.** The initial `OperationEnv` implementation dispatches directly through the local `OperationRegistry`:

```rust
pub struct LocalOperationEnv {
    registry: Arc<OperationRegistry>,
}

#[async_trait]
impl OperationEnv for LocalOperationEnv {
    async fn invoke(&self, namespace: &str, operation: &str, input: Value, parent: &OperationContext) -> ResponseEnvelope {
        let name = format!("{namespace}/{operation}");

        // Reachability check (ADR-015, ADR-022): is this op in the parent's
        // scoped env? If not, return NOT_FOUND. This bounds the
        // parameterized-dispatch attack surface — a handler (or an LLM
        // picking tools) can only reach declared ops.
        if !parent.env.allows(&name) {
            return ResponseEnvelope::not_found(name);
        }

        let registration = self.registry.registration(&name);
        let context = OperationContext {
            // Unique per invocation — a counter, UUID, or parent_id + suffix.
            // A deterministic ID (e.g. format!("env-{name}")) collides across
            // concurrent invocations of the same operation, which corrupts
            // PendingRequestMap correlation and the abort-cascade tree
            // (ADR-016), which is indexed by parent_request_id.
            request_id: generate_request_id(),
            parent_request_id: Some(parent.request_id.clone()),
            // Parent's composition authority becomes the caller for the child.
            // This is the authority switch: the child's ACL checks against
            // the parent's authority, not the original wire caller's identity.
            identity: parent.handler_identity.as_identity(),
            // Child's own composition authority (from its registration).
            // None for leaves — they don't compose, so this is never used
            // for ACL on a grandchild.
            handler_identity: registration.composition_authority.clone(),
            capabilities: parent.capabilities.clone(),       // Inherit caller's capabilities
            metadata: HashMap::new(),                         // Fresh — does NOT propagate parent metadata (ADR-014)
            env: registration.scoped_env.clone()
                .unwrap_or_else(ScopedOperationEnv::empty),  // Child's own scoped env (empty for leaves)
            internal: true,                                   // Nested calls use handler authority
        };
        self.registry.invoke(&name, input, context).await
    }
}
```

Two things happen in `invoke()`:

1. **Reachability check**: before constructing the child context, `invoke()` checks whether the requested op is in the parent's scoped env. If not, `NOT_FOUND`. This is the reachability control — a handler can only compose declared ops.
2. **Authority propagation**: the child's `identity` is the parent's `handler_identity` (the parent's composition authority becomes the caller). The child's `handler_identity` is the child's own registration's `composition_authority` — so if the child itself composes further, its children inherit the child's authority. This is the principal/agent chain from ADR-015, now wired via ADR-022.

Future work may add irpc service dispatch and remote call protocol dispatch as additional backends. The handler-facing API stays the same.

**`OperationEnv` must remain a trait.** This is a constraint, not a suggestion. The trait-based design enables session-scoped registries (OQ-19) — a session env wraps the global env (check session registry first, fall through to global). Making `OperationEnv` concrete or hardcoding the global registry into the dispatch path would close the session-overlay pattern. See OQ-19.

### Service Discovery

Two built-in operations expose what the node offers:

| Operation name | Display path | Type | Description |
|---------------|-------------|------|-------------|
| `services/list` | `/services/list` | Query | List registered operation names and metadata |
| `services/schema` | `/services/schema` | Query | Get the `OperationSpec` for a specific operation |

These are read-only — no admin operations are exposed through the call protocol itself.

`services/list` only returns `External` operations to remote callers. `Internal` operations are not part of the wire-facing API surface — they're implementation details of composition. A remote client cannot enumerate the internal call tree. See ADR-015.

`services/list` returns:

```json
{
  "operations": [
    { "name": "fs/readFile", "namespace": "fs", "op_type": "query" },
    { "name": "agent/chat", "namespace": "agent", "op_type": "subscription" },
    { "name": "events/subscribe", "namespace": "events", "op_type": "subscription" }
  ]
}
```

`services/schema` accepts `{ "name": "fs/readFile" }` and returns the full `OperationSpec` including input/output JSON Schemas and declared `error_schemas` (ADR-023). This enables client code generation: a client reading the schema can produce typed error enums instead of generic error handling.

### irpc Integration

irpc and the operation registry serve different scopes:

| Layer | Mechanism | Serialization | Scope |
|-------|-----------|---------------|-------|
| Call protocol (external) | `EventEnvelope` over QUIC streams | JSON | Cross-language, cross-node |
| irpc services (internal) | `VaultProtocol` derive macro, `Service` trait | postcard (binary) | Rust-to-Rust, in-process or in-cluster |
| Local dispatch (in-process) | Direct function call through `OperationRegistry` | None | Same process |

irpc services are an internal dispatch mechanism — they are not directly exposed on the call protocol. The vault's `VaultProtocol` uses irpc for in-process, type-safe dispatch via `VaultServiceHandle` (postcard serialization for in-cluster, direct calls for in-process). The vault is accessed by the assembly layer (CLI binary) at startup, not by handlers at call time. See ADR-008 and ADR-014.

If a handler internally uses an irpc-based service, the handler bridges the two: it receives JSON input from the call protocol, calls the irpc service in-process (postcard, type-safe), and serializes the result back to JSON for the call protocol response. This layering preserves irpc's type safety for internal calls while keeping the external interface cross-language.

### Operation Registration at Startup

The CLI binary (or assembly layer) constructs `HandlerRegistration` bundles with provenance, composition authority, scoped env, and capabilities (from the vault — see [Capability Injection](#capability-injection)), then registers them before starting the endpoint:

```rust
// Assembly layer: unlock vault, derive credentials
let vault = VaultServiceHandle::new();
vault.unlock(&mnemonic, passphrase.as_deref())?;
let google_api_key = vault.decrypt(&google_key_blob)?;
let github_signing_key = vault.derive_ed25519(PATHS::GITHUB_SIGNING)?;
let vastai_credentials = Capabilities::new().with_http_token("vastai", vastai_token);

// Register operations — vault operations are NOT registered here
let registry = OperationRegistryBuilder::new()
    // Built-in service discovery (Local, no composition)
    .with_local(services_list_spec(), Arc::new(services_list_handler),
                CompositionAuthority::none(), ScopedOperationEnv::empty())
    .with_local(services_schema_spec(), Arc::new(schema_handler),
                CompositionAuthority::none(), ScopedOperationEnv::empty())
    // Agent handler (Local, composes — has authority + scoped env + capabilities)
    .with(HandlerRegistration {
        spec: agent_chat_spec(),
        handler: Arc::new(agent_chat_handler),
        provenance: OperationProvenance::Local,
        composition_authority: Some(CompositionAuthority::new(
            "agent-chat", ["llm:call", "fs:read", "vastai:query"])),
        scoped_env: Some(ScopedOperationEnv::new(
            ["fs/readFile", "vastai/listMachines", "llm/generate"])),
        capabilities: Capabilities::new().with_api_key("google", google_api_key),
    })
    // Vastai ops (FromOpenAPI, leaves — no authority, no scoped env)
    .with_leaf(vastai_listMachines_spec(), Arc::new(vastai_listMachines_handler),
               vastai_credentials.clone())
    .build();

let call_adapter = CallAdapter::new(Arc::new(registry), identity_provider);
```

The vault is used at construction time to populate `capabilities` in the registration bundle, not registered as call protocol operations. The registry is immutable after construction. Adding operations requires restarting the process. This is consistent with OQ-04, ADR-008, ADR-014, and ADR-022.

### Capability Injection

Handlers that need outbound credentials (LLM provider API keys, signing keys, HTTP service tokens) receive them through the `Capabilities` type on `OperationContext`, not by calling vault operations over the wire and not from environment variables. This is the mechanism that ADR-008 described in prose ("derived keys and decrypted credentials are injected into operation contexts at the assembly layer") and that ADR-014 specifies as a one-way door. ADR-022 specifies the registration path: capabilities live on the `HandlerRegistration` bundle, and the dispatch path populates `OperationContext.capabilities` from the bundle at call time.

The flow is:

```
Assembly layer (CLI startup):
  1. Unlock vault (local, mnemonic from secure prompt or file)
  2. Derive / decrypt the credentials each handler needs
  3. Construct HandlerRegistration bundles with capabilities from the vault
  4. Register the bundles in the OperationRegistry
  5. Start the endpoint

Handler invocation (at call time):
  call.requested → CallAdapter looks up registration by op name
  → build_root_context populates OperationContext.capabilities from registration.capabilities
  → handler reads context.capabilities → uses the credential for its outbound call
```

The handler closure does **not** capture capabilities — that was the pre-ADR-022 "Model A" that created a circular dependency with per-request `OperationContext.capabilities`. Capabilities live on the registration bundle, and the dispatch path populates the context from the bundle. One model, one wiring path. See ADR-022 Decision 6.

The `Capabilities` type holds non-serializable, zeroized secret material. It does not implement `Serialize` — it cannot cross the call protocol wire even by accident. The concrete shape of the type (a typed map, a struct with named fields, a trait object) is a two-way door for implementation. The one-way constraints are fixed by ADR-014:

- Capabilities are populated by the assembly layer at registration (on the `HandlerRegistration` bundle). They are never populated from call protocol inputs.
- Capabilities hold secret material that does not implement `Serialize` and does not appear in `EventEnvelope` payloads.
- The call protocol carries no secret material. See [call-protocol.md](call-protocol.md) for the wire-level constraint.
- **Capabilities are `Clone` and cloned through composition.** `OperationEnv::invoke()` calls `parent.capabilities.clone()` to pass capabilities to nested calls. This is intentional: a child handler needs the same outbound credentials as its parent (e.g., the `/agent/chat` handler composing `/fs/readFile` may need the same API key for an outbound LLM call). The security implication is that each composition step duplicates the secret material reference — but capabilities are scoped (the handler can only use what the assembly layer declared on the registration bundle), and children run under the parent's composition authority (ADR-015, ADR-022). A clone is the same scoped handle, not a widening of scope. The concrete cloning semantics (reference-counted `Arc` vs deep copy of zeroized material) is a two-way door for implementation, but `Capabilities: Clone` is required by the composition model.

**No vault operations are registered in the call protocol.** The vault is assembly-layer only (ADR-008, ADR-014). A handler that needs a child key for a specific operation (e.g., signing for GitHub auth) receives a scoped capability that performs the derivation in-process — it never holds the master seed and never calls a network-exposed vault operation.

**Adapters take credential sources.** All import adapters (`from_openapi`, `from_mcp`, `from_jsonschema`, `from_call` — see ADR-017, constrained by ADR-014) register HTTP-backed, MCP-backed, or remote-call-backed operations. The credential each service needs (bearer token, API key, TLS identity for the remote connection) is provided by the assembly layer at registration time — the adapter receives a credential source, not a static token string. This is the integration point where the vault feeds credentials into backed operations, including LLM providers that expose OpenAPI-compatible endpoints. Adapter-registered operations are `Internal` by default (ADR-015) — they're composition material, not directly callable from the wire.

**`from_call` imports remote operations.** The `from_call` adapter (ADR-017) discovers operations on a remote call protocol endpoint via `services/list` and `services/schema`, then registers them with handlers that forward calls over the QUIC connection. This makes cross-node composition transparent — a handler calling `env.invoke("worker", "exec", ...)` doesn't know whether the operation is local or remote. Connection direction (who opened the QUIC connection) is independent of call direction (who calls whom) — both sides can call each other once connected.

**`from_call` trust is transitive.** A `from_call`-imported operation executes the remote node's code, not yours. The scoped env (ADR-015) bounds *which* operations are reachable, but not *what* they do. A compromised remote node can do anything its operations are declared to do (and anything its handler bugs allow). This is inherent to remote composition — same as trusting any RPC endpoint — but it must be explicit in the threat model. `from_call` means "I trust the remote node as much as my own handlers." The scoping protects the caller from reaching arbitrary ops; it does not protect against what the reached op does.

**Scoped composition env.** The `OperationEnv` given to a handler is scoped — it can only invoke a declared set of operations, set at registration on the `HandlerRegistration` bundle by the assembly layer (ADR-022). This bounds the parameterized-dispatch attack surface: a handler (or an LLM picking tools, or a quickjs sandbox) can only reach declared operations, not the entire registry. The scoped env is the reachability control; the composition authority is the authority control. Both are needed for least privilege. See ADR-015 and ADR-022.

## Constraints

- The registry is immutable after construction. No runtime registration or deregistration. Two-way door — `ArcSwap<OperationRegistry>` can be added later.
- Operation specs use JSON Schema. The call protocol's external interface is always JSON. irpc's postcard serialization is internal only.
- `OperationEnv::invoke()` dispatches through the local registry. Remote dispatch (federation, head/worker routing) would be a separate mechanism at a different layer — not a prefix added to operation paths. irpc service dispatch is contracted but not built.
- The call protocol does not depend on any database. Operation specs are in-memory, populated at startup.
- `OperationContext.internal` is set by `OperationEnv`, not by callers. A handler cannot mark its own call as internal. The `internal` flag switches authority context (composition authority for ACL), it does not skip ACL — see ADR-015, ADR-022.
- **Operations have External/Internal visibility.** `Internal` operations return `NOT_FOUND` when called from the wire and are excluded from `services/list`. The assembly layer declares visibility at registration. See ADR-015.
- **The composition env is scoped.** A handler can only invoke operations declared in its scoped env (on the `HandlerRegistration` bundle). This bounds parameterized-dispatch attack surface. See ADR-015, ADR-022.
- **No vault operations are registered in the call protocol.** The vault is assembly-layer only (ADR-008, ADR-014). Handlers receive secret material through `OperationContext.capabilities`, not by calling vault operations over the wire.
- **The call protocol carries no secret material.** Secret material (private keys, API keys, mnemonics, decrypted credentials) must not appear in `call.requested` payloads, `call.responded` payloads, or `OperationContext.metadata`. See ADR-014.
- **Metadata does not propagate through composition.** `OperationEnv::invoke()` constructs fresh metadata for nested calls (`HashMap::new()`), not the parent's metadata. This prevents a handler that accidentally places a secret in metadata from leaking it to child operations — and if a child is a `from_call` operation (ADR-017), across the wire to a remote node. The tracing link is `parent_request_id`, not metadata propagation. See ADR-014.
- **Provenance determines composition capability.** Only `Local` and `Session` ops can compose. Leaves (`FromOpenAPI`, `FromMCP`, `FromCall`) get `composition_authority: None` and `scoped_env: None` — they don't compose, so they don't need authority or reachability bounds. See ADR-022.

## Design Decisions

| Decision | ADR | Summary |
|----------|-----|---------|
| irpc as call protocol foundation | [ADR-005](../../decisions/005-irpc-as-call-protocol-foundation.md) | irpc provides framing and service dispatch |
| Call protocol stream model | [ADR-012](../../decisions/012-call-protocol-stream-model.md) | Bidirectional streams, EventEnvelope, ID-based correlation |
| Static handler registration | [ADR-010](../../decisions/010-alpn-router-and-endpoint.md) | Registry is immutable after construction |
| Vault integration via assembly layer | [ADR-008](../../decisions/008-secret-service-integration.md) | Vault is a capability source, accessed at assembly time |
| Secret material flow and capability injection | [ADR-014](../../decisions/014-secret-material-flow-and-capability-injection.md) | Capabilities carry outbound credentials; call protocol carries no secret material |
| Privilege model and authority context | [ADR-015](../../decisions/015-privilege-model-and-authority-context.md) | `internal` = authority switch not ACL skip; External/Internal visibility; composition authority + scoped env |
| Handler registration, provenance, and composition authority | [ADR-022](../../decisions/022-handler-registration-provenance-and-composition-authority.md) | Registration bundle carries provenance, composition authority, scoped env, capabilities; dispatch path reads from bundle |
| Operation error schemas | [ADR-023](../../decisions/023-operation-error-schemas.md) | Operations declare domain errors; `call.error` carries typed `details`; adapter fidelity for `from_openapi`/`to_openapi` |

## Open Questions

See [open-questions.md](../../open-questions.md) for full details.

- **OQ-13** (resolved): Operation path format is `/{service}/{op}`. Remote dispatch is a separate mechanism, not a path prefix.
- **OQ-14** (resolved): Batch is a client-side pattern of correlated `call.requested` events, not a protocol primitive.
- **OQ-16** (resolved by ADR-014): No vault operations are exposed over the call protocol for now.
- **OQ-19** (resolved): Session-scoped operation registries — agent-written operations overlaid on the global registry via `OperationEnv` trait layering. Protocol doesn't need changes; `OperationEnv` must remain a trait. Session ops are `Session` provenance (ADR-022) — always `Internal`, compose under restricted authority scoped down at sandbox creation.

## References

- [call-protocol.md](call-protocol.md) — CallAdapter, EventEnvelope, stream model, PendingRequestMap
- ADR-005: irpc as call protocol foundation
- ADR-008: Vault integration point
- ADR-010: ALPN router and endpoint (static registration)
- ADR-012: Call protocol stream model
- Reference implementation: `/workspace/@alkdev/alknet-main/crates/alknet-core/src/call/`