Add Session Stream

README.md

@@ -2,7 +2,7 @@
 
 Reference runtime for the Multi-Agent Coordination Protocol (MACP).
 
-This runtime implements the current MACP core/service surface, the five standards-track modes in the main RFC repository, and one experimental `multi_round` mode that remains available only by explicit canonical name. The focus of this release is freeze-readiness for SDKs and real-world unary integrations: strict `SessionStart`, mode-semantic correctness, authenticated senders, bounded resources, and durable restart recovery.
+This runtime implements the current MACP core/service surface, the five standards-track modes in the main RFC repository, and one experimental `multi_round` mode that remains available only by explicit canonical name. The focus of this release is freeze-readiness for SDKs and real-world unary and streaming integrations: strict `SessionStart`, mode-semantic correctness, authenticated senders, bounded resources, and durable restart recovery.
 
 ## What changed in v0.4.0
 
@@ -28,8 +28,9 @@ This runtime implements the current MACP core/service surface, the five standard
   - per-session append-only log files and session snapshots via `FileBackend`
   - crash recovery with dedup state reconciliation
   - atomic writes (tmp file + rename) prevent partial-write corruption
-- **Unary freeze profile**
-  - `StreamSession` is intentionally disabled in this profile
+- **Authoritative session streaming**
+  - `StreamSession` now emits accepted MACP envelopes for one bound session per stream
+  - mixed-session streams are rejected
   - `WatchModeRegistry` and `WatchRoots` remain unimplemented
 
 ## Implemented modes
@@ -176,7 +177,7 @@ cargo run --bin fuzz_client
 | `GetManifest` | implemented |
 | `ListModes` | implemented |
 | `ListRoots` | implemented |
-| `StreamSession` | intentionally disabled in freeze profile |
+| `StreamSession` | implemented (accepted-envelope session stream) |
 | `WatchModeRegistry` | unimplemented |
 | `WatchRoots` | unimplemented |
 
@@ -205,6 +206,6 @@ runtime/
 - The RFC/spec repository remains the normative source for protocol semantics.
 - This runtime only accepts the canonical standards-track mode identifiers for the five main modes.
 - `multi_round` remains experimental and is not advertised by discovery RPCs.
-- `StreamSession` is intentionally not part of the freeze surface for the first SDKs.
+- `StreamSession` is available for bidirectional session-scoped coordination. Use `Send` when you need per-message negative acknowledgements. The current implementation streams future accepted envelopes from the time the stream binds; it does not backfill earlier accepted history.
 
 See `docs/README.md` and `docs/examples.md` for the updated local development and usage guidance.

docs/README.md

@@ -6,7 +6,7 @@ The RFC/spec repository is still the normative source for MACP semantics. These
 
 ## What is in this runtime profile
 
-- unary-first MACP server over gRPC
+- MACP server over gRPC with unary RPCs and per-session bidirectional streaming
 - five standards-track modes from the main RFC repository
 - one experimental `macp.mode.multi_round.v1` mode kept off discovery surfaces
 - strict canonical `SessionStart` for standards-track modes
@@ -24,21 +24,21 @@ The RFC/spec repository is still the normative source for MACP semantics. These
 
 ## Freeze profile
 
-The current runtime is intended to be the freeze candidate for unary SDKs and reference examples.
+The current runtime is intended to be the freeze candidate for unary and streaming SDKs and reference examples.
 
 Implemented and supported:
 
 - `Initialize`
 - `Send`
+- `StreamSession`
 - `GetSession`
 - `CancelSession`
 - `GetManifest`
 - `ListModes`
 - `ListRoots`
 
-Not part of the freeze surface:
+Not yet implemented:
 
-- `StreamSession` is intentionally disabled in this profile
 - `WatchModeRegistry` is unimplemented
 - `WatchRoots` is unimplemented
 

docs/architecture.md

@@ -93,4 +93,4 @@ Responsibilities:
 
 ## Freeze-profile design choice
 
-The runtime intentionally prioritizes unary correctness over streaming completeness. `StreamSession` is therefore disabled in this release profile so SDKs can target a stable, explicit surface.
+The runtime now exposes `StreamSession` as a per-session accepted-envelope stream. Each gRPC stream binds to one session and receives canonical MACP envelopes in runtime acceptance order. Unary `Send` remains the path for explicit per-message acknowledgement semantics.

docs/examples.md

@@ -1,6 +1,6 @@
 # Examples and local development guide
 
-These examples target `macp-runtime v0.4.0` and the unary freeze profile.
+These examples target `macp-runtime v0.4.0` and the stream-capable freeze profile.
 
 They intentionally use the local-development security shortcut:
 
@@ -141,7 +141,19 @@ cargo run --bin multi_round_client
 
 This mode is still experimental. It remains callable by the explicit canonical name `macp.mode.multi_round.v1`, but it is not advertised by discovery RPCs and it does not use the strict standards-track `SessionStart` contract.
 
-## Example 7: Freeze-check / error-path client
+## Example 7: StreamSession
+
+`StreamSession` emits only accepted canonical MACP envelopes. A single gRPC stream binds to one session. If a client needs negative per-message acknowledgements, it should continue to use `Send`.
+
+Practical notes:
+
+- bind a stream by sending a session-scoped envelope for the target session
+- use `SessionStart` to create a new session over the stream
+- stream attachment starts observing future accepted envelopes from the bind point; it does not replay earlier history
+- use a session-scoped `Signal` envelope with the correct `session_id` and `mode` to attach to an existing session without mutating it
+- mixed-session streams are rejected with `FAILED_PRECONDITION`
+
+## Example 8: Freeze-check / error-path client
 
 Run:
 

docs/protocol.md

@@ -10,22 +10,34 @@ The RFC/spec repository is the normative source for protocol semantics. This fil
 
 Clients should call `Initialize` before using the runtime.
 
-## Implemented unary RPCs
+## Implemented RPCs
 
 - `Initialize`
 - `Send`
+- `StreamSession`
 - `GetSession`
 - `CancelSession`
 - `GetManifest`
 - `ListModes`
 - `ListRoots`
 
-## Not in the freeze surface
+## Still unimplemented
 
-- `StreamSession` exists in the protobuf surface but is intentionally disabled in this runtime profile
 - `WatchModeRegistry` is unimplemented
 - `WatchRoots` is unimplemented
 
+## StreamSession profile
+
+`StreamSession` is session-scoped and authoritative for accepted envelopes:
+
+- one gRPC stream binds to one non-empty `session_id`
+- the server emits only accepted canonical MACP envelopes
+- stream attachment observes future accepted envelopes from the bind point; it does not backfill earlier history
+- accepted envelope order matches runtime admission order for that session
+- mixed-session streams are rejected with `FAILED_PRECONDITION`
+- stream-level validation failures terminate the stream with a gRPC status; use `Send` if you need explicit per-message negative acknowledgements
+- to attach to an existing session without mutating it, send a session-scoped `Signal` envelope with the correct `session_id` and `mode`
+
 ## Standards-track mode rules
 
 For these modes:
@@ -81,4 +93,4 @@ For standards-track modes, `CommitmentPayload` must carry version fields that ma
 
 ## Discovery notes
 
-`ListModes` returns the five standards-track modes. `GetManifest` exposes a freeze-profile manifest that matches the implemented unary capabilities.
+`ListModes` returns the five standards-track modes. `GetManifest` exposes a manifest that matches the implemented unary and streaming capabilities.

src/lib.rs

@@ -29,5 +29,6 @@ pub mod registry;
 pub mod runtime;
 pub mod session;
 pub mod storage;
+pub mod stream_bus;
 
 pub mod security;

src/runtime.rs

@@ -18,6 +18,7 @@ use crate::session::{
     validate_standard_session_start_payload, Session, SessionState,
 };
 use crate::storage::StorageBackend;
+use crate::stream_bus::SessionStreamBus;
 
 const EXPERIMENTAL_DEFAULT_TTL_MS: i64 = 60_000;
 
@@ -31,6 +32,7 @@ pub struct Runtime {
     pub storage: Arc<dyn StorageBackend>,
     pub registry: Arc<SessionRegistry>,
     pub log_store: Arc<LogStore>,
+    stream_bus: Arc<SessionStreamBus>,
     modes: HashMap<String, Box<dyn Mode>>,
 }
 
@@ -52,6 +54,7 @@ impl Runtime {
             storage,
             registry,
             log_store,
+            stream_bus: Arc::new(SessionStreamBus::default()),
             modes,
         }
     }
@@ -64,6 +67,19 @@ impl Runtime {
             .collect()
     }
 
+    pub fn subscribe_session_stream(
+        &self,
+        session_id: &str,
+    ) -> tokio::sync::broadcast::Receiver<Envelope> {
+        self.stream_bus.subscribe(session_id)
+    }
+
+    fn publish_accepted_envelope(&self, env: &Envelope) {
+        if !env.session_id.is_empty() {
+            self.stream_bus.publish(&env.session_id, env.clone());
+        }
+    }
+
     fn make_incoming_entry(env: &Envelope) -> LogEntry {
         LogEntry {
             message_id: env.message_id.clone(),
@@ -259,6 +275,7 @@ impl Runtime {
         // 3. Best-effort session save
         self.save_session_to_storage(&session).await;
         guard.insert(env.session_id.clone(), session);
+        self.publish_accepted_envelope(env);
 
         Ok(ProcessResult {
             session_state: result_state,
@@ -323,6 +340,7 @@ impl Runtime {
 
         // 3. Best-effort session save
         self.save_session_to_storage(session).await;
+        self.publish_accepted_envelope(env);
 
         Ok(ProcessResult {
             session_state: result_state,
@@ -714,6 +732,45 @@ mod tests {
         assert_eq!(result.session_state, SessionState::Expired);
     }
 
+    #[tokio::test]
+    async fn accepted_envelopes_are_published_in_order() {
+        let rt = make_runtime();
+        let mut events = rt.subscribe_session_stream("s1");
+
+        let start = env(
+            "macp.mode.decision.v1",
+            "SessionStart",
+            "m1",
+            "s1",
+            "agent://orchestrator",
+            session_start(vec!["agent://fraud".into()]),
+        );
+        rt.process(&start, None).await.unwrap();
+        let first = events.recv().await.unwrap();
+        assert_eq!(first.message_id, "m1");
+        assert_eq!(first.message_type, "SessionStart");
+
+        let proposal = ProposalPayload {
+            proposal_id: "p1".into(),
+            option: "step-up".into(),
+            rationale: "risk".into(),
+            supporting_data: vec![],
+        }
+        .encode_to_vec();
+        let proposal_env = env(
+            "macp.mode.decision.v1",
+            "Proposal",
+            "m2",
+            "s1",
+            "agent://orchestrator",
+            proposal,
+        );
+        rt.process(&proposal_env, None).await.unwrap();
+        let second = events.recv().await.unwrap();
+        assert_eq!(second.message_id, "m2");
+        assert_eq!(second.message_type, "Proposal");
+    }
+
     #[tokio::test]
     async fn commitment_versions_are_carried_into_resolution() {
         let rt = make_runtime();