fix: channels parse-failure path writes the negotiation error frame (R4)

Review #002 R4 — a NegotiateRequest parse failure of the open op's
schema-validated input died silently (log + return, channel teardown,
consumer observed NoExitChunk — indistinguishable from a crashed
producer), while the other post-open failure classes (unknown backend,
allocate_failed, ownership denial) wrote the 0x00-prefixed error frame.

- make_tty_open_handler now accepts the channel's BiStream and writes
  a malformed_negotiation frame via the shared
  crate::adapter::send_negotiation_error (now pub(crate)) before
  returning; the consumer's M1 peek surfaces NegotiationRejected
  unchanged
- the frame type and layout are unchanged (ADR-001 wire-stable
  contract); no new frame type, no wire change
- tests: make_tty_open_handler seam test with a hand-built
  schema-bypassing input (cwd: 42) + a real-registry end-to-end test
  via ChannelClient::open_channel (bypasses open_via_channels's local
  fail-fast parse — R5's path — so it exercises the producer handler)
- docs: ADR-009 amended (Parse-failure error frame section);
  tty-adapter.md malformed_negotiation row covers both paths;
  session.rs post-open failure lists updated; review #002 R4 resolved

Note: the review's "unreachable end-to-end" premise was refined —
open_via_channels parses params locally (fail-fast) so a TtySession
consumer never hits the producer-side parse failure, but direct
ChannelClient callers do; the schema is deliberately partial so a
schema-valid value (cwd typed as a number) reaches the handler.

Verification: cargo test 95 lib (default) / 138 (--all-features);
clippy -D warnings native + wasm clean; fmt clean; doc 0 warnings.

docs/architecture/decisions/009-channels-open-op-is-the-negotiation.md

@@ -6,6 +6,11 @@ Accepted (2026-09-05). Resolves review #001 L1. Prerequisites: alkcall
 0.4.0 (call-time `input_schema` enforcement) and alkcall 0.4.1
 (early-arrival parking for un-adopted channels).
 
+Amended 2026-09-05 (review #002 R4): a `NegotiateRequest` parse failure
+of the schema-validated `input` is now a client-visible
+`malformed_negotiation` error frame, not a silent teardown — see
+§"Parse-failure error frame (R4 amendment, 2026-09-05)".
+
 ## Context
 
 Before this ADR, the channels path carried the negotiation twice. The
@@ -73,9 +78,10 @@ so the consumer's ADR-001 §5 disambiguation read applies unchanged.
 A `NegotiateRequest` parse failure of the schema-validated `input`
 (the schema is deliberately partial — `carriage`/`backend`/`cmd`
 required, `tty`/`cwd`/`env`/backend-params free-form so the opaque
-ADR-053 params pass through) is a handler-side failure; the handler
-logs and returns without writing an error frame (the channel is torn
-down by the wrapper's teardown task).
+ADR-053 params pass through) was originally a handler-side failure
+logged and returned without an error frame. The R4 amendment below
+makes it a client-visible error frame like every other post-open
+failure.
 
 ### Consumer side
 
@@ -102,6 +108,37 @@ prefix starting `0x00`) from a raw chunk (`stream_type` in `{1, 2, 4}`).
   This ADR removes a frame from the channels data stream; it does not
   change any frame that remains.
 
+### Parse-failure error frame (R4 amendment, 2026-09-05)
+
+All three post-open failure classes on the channels path are now
+client-visible through the same `0x00` error-frame peek
+(`from_halves_raw`):
+
+1. **Semantic validation failures** (`carriage != "raw"`, empty `cmd`,
+   unknown backend, `allocate_failed`, ownership denial) — error
+   frames from `validate_and_allocate` (unchanged).
+2. **`NegotiateRequest` parse failure of schema-valid `input`** — the
+   open handler accepts the channel's `BiStream` and writes a
+   `malformed_negotiation` frame
+   (`{"error":"malformed_negotiation","message":"..."}`) via the shared
+   `crate::adapter::send_negotiation_error`, then returns. Reachable
+   despite the registry's schema check because the schema is
+   deliberately partial: e.g. `cwd` typed as a number passes the schema
+   (unknown-key/type fields pass through for the opaque ADR-053
+   params) but fails the typed parse.
+3. **Schema-invalid `input`** — rejected at dispatch by the registry
+   (alkcall 0.4) before any handler runs: a `CallError` on the open
+   op, no channel allocated, no error frame (unchanged).
+
+This replaces the original behavior (log + return, channel teardown,
+consumer observes `NoExitChunk` — indistinguishable from a crashed
+producer). The error-frame layout is unchanged (ADR-001's wire-stable
+contract); no new frame type, no new stream type. Tests: the
+`make_tty_open_handler` seam test (hand-built `input` bypassing the
+schema, `channels.rs::tests`) and the real-registry end-to-end test
+(`testing.rs`); both assert the consumer-side `0x00` peek observes
+`malformed_negotiation`.
+
 ## Consequences
 
 **Positive:**

docs/architecture/tty-adapter.md

@@ -175,7 +175,7 @@ entering raw mode. The error response shape:
 | Error | When | Shape |
 |-------|------|------|
 | `unknown_backend` | the `backend` string is not in the adapter's backend map | `{"error":"unknown_backend","backend":"..."}` |
-| `malformed_negotiation` | the negotiation frame failed to parse as JSON or failed `NegotiateRequest` validation | `{"error":"malformed_negotiation","message":"..."}` |
+| `malformed_negotiation` | the negotiation frame failed to parse as JSON or failed `NegotiateRequest` validation — on the direct path the wire frame, on the channels path the open op's `input` (schema-valid values can still fail the typed parse, e.g. `cwd` typed as a number, because the schema is deliberately partial) | `{"error":"malformed_negotiation","message":"..."}` |
 | `allocate_failed` | `backend.allocate()` returned a `TtyError` | `{"error":"allocate_failed","message":"..."}` |
 
 After sending the error response, the adapter closes the write half of

docs/reviews/002-post-session-review.md

@@ -175,6 +175,22 @@ Option 1 is preferred if a test can be written for it (feed a handler
 a schema-bypassing value directly — the unit-testable seam is
 `make_tty_open_handler` with a hand-built `input`).
 
+**Resolution (2026-09-05)**: option 1 implemented. `make_tty_open_handler`
+accepts the channel's `BiStream` and writes a `malformed_negotiation`
+error frame via the shared `crate::adapter::send_negotiation_error`
+(now `pub(crate)`) before returning — all three post-open failure
+classes are client-visible through the unchanged M1 peek. The
+review's reachability analysis is refined by the R5 fail-fast
+discovery: `TtySession::open_via_channels` parses `params` locally
+before opening, so the consumer never sends a value it can't parse
+itself — but the producer-side handler is still the reachable seam for
+direct `ChannelClient` callers (the schema is deliberately partial; a
+schema-valid `cwd: 42` fails the typed parse). Tests: the
+`make_tty_open_handler` seam test (channels.rs) and a real-registry
+end-to-end test via `ChannelClient::open_channel` (testing.rs). ADR-009
+amended (§"Parse-failure error frame"); `tty-adapter.md` error table
+updated.
+
 ---
 
 ### R5. `open_via_channels` fail-fast surfaces as `NegotiationSerialize`
@@ -257,13 +273,14 @@ version-skew note in ADR-009); option 1 at that point.
 | R1 | ADR-009 never written | write the ADR | small | none | ✅ resolved (`37ae07a`) |
 | R2 | stale docs from the L1 redesign | align with ADR-009 | trivial | none | ✅ resolved (`37ae07a`) |
 | R3 | install-time identity snapshot | accepted design (hub-proxy rationale) | none | none | ✅ closed as intended |
-| R4 | silent death on parse-failure path | error frame or documented asymmetry | small | low | ⬜ open (deferred) |
+| R4 | silent death on parse-failure path | error frame or documented asymmetry | small | low | ✅ resolved (option 1) |
 | R5 | `NegotiationSerialize` mislabel on fail-fast | additive variant (with `#[non_exhaustive]` decision) | small | medium (semver) | ⬜ open (deferred) |
 
-R4/R5 are deferred deliberately: both touch the consumer-facing error
-surface, both are cheap, and neither is reachable-by-design today.
-Batch them with the first post-1.0 API decision rather than churning
-the error enum before a consumer exists.
+R5 is deferred deliberately: it touches the consumer-facing error
+surface, it is cheap, and the producer-side parse-failure arm (R4's
+concern) is now client-visible regardless. Batch it with the first
+post-1.0 API decision rather than churning the error enum before a
+consumer exists.
 
 ## Notes
 

src/adapter.rs

@@ -168,7 +168,14 @@ fn has_scope(identity: &Option<Identity>, scope: &str) -> bool {
 /// Send a negotiation error frame and close the write half. Consumes the
 /// writer so the underlying transport's shutdown runs after the frame is
 /// flushed.
-async fn send_negotiation_error<W: AsyncWrite + Unpin>(
+///
+/// Shared by the direct path's failure arms and the channels open
+/// handler (`make_tty_open_handler` writes the same `malformed_negotiation`
+/// frame on a `NegotiateRequest` parse failure of the open op's `input` —
+/// the pre-negotiated driver has no writer to consume, so the handler
+/// builds its own over the channel's write half). `pub(crate)` so the
+/// channels handler reuses it; not public API.
+pub(crate) async fn send_negotiation_error<W: AsyncWrite + Unpin>(
     mut writer: NegotiationWriter<W>,
     error: &str,
     fields: &[(&str, &str)],

src/channels.rs

@@ -43,7 +43,7 @@ use tracing::debug;
 
 use crate::adapter::{drive_session_pre_negotiated, TTY_OPEN_SCOPE};
 use crate::backend::TtyBackend;
-use crate::negotiation::NegotiateRequest;
+use crate::negotiation::{NegotiateRequest, NegotiationWriter};
 
 /// The per-ALPN open operation name (`channels/<alpn>/sub` convention,
 /// ADR-047). TTY is consumer-opens (the client requests a shell), so
@@ -184,12 +184,27 @@ pub fn tty_open_spec() -> OperationSpec {
 /// three-pump session driver as the direct-ALPN path, minus the
 /// wire-frame negotiation phase.
 ///
-/// Parse or validation failures (unknown backend, `allocate_failed`,
-/// ADR-050 ownership denial) are reported to the client as a
-/// negotiation error frame on the channel stream (the same
-/// `0x00`-prefixed framing the direct path uses), so a consumer's
-/// negotiation-error disambiguation read applies unchanged. The
-/// `tty:open` scope gate is enforced by the registry's `AccessControl`
+/// All failure classes write a negotiation error frame on the channel
+/// stream (the same `0x00`-prefixed framing the direct path uses), so a
+/// consumer's negotiation-error disambiguation read applies unchanged:
+///
+/// - The registry rejects schema-invalid `input` at dispatch (alkcall
+///   0.4) — the open op fails, no channel is ever allocated, and the
+///   failure is a `CallError` on the open op, not a channel-stream
+///   error frame. But the schema is deliberately partial (the opaque
+///   ADR-053 backend params pass through), so a schema-valid value can
+///   still fail the full `NegotiateRequest` parse — e.g. `cwd` typed
+///   as a number — and that failure is post-open: the handler writes a
+///   `malformed_negotiation` error frame (the same
+///   [`crate::adapter::send_negotiation_error`] the other failure
+///   classes use) and the consumer's peek surfaces
+///   `NegotiationRejected`.
+/// - Semantic validation failures (unknown backend, `carriage != "raw"`,
+///   empty `cmd`, `allocate_failed`, ADR-050 ownership denial) are
+///   post-open error frames written by `validate_and_allocate` inside
+///   [`crate::adapter::drive_session_pre_negotiated`].
+///
+/// The `tty:open` scope gate is enforced by the registry's `AccessControl`
 /// before this handler runs — the handler does not re-check it.
 ///
 /// The handler's `JoinHandle` is recorded by the channels wrapper for
@@ -214,6 +229,20 @@ fn make_tty_open_handler(
                     Ok(r) => r,
                     Err(e) => {
                         debug!("tty: channels open: invalid negotiate params: {e}");
+                        let stream = match channel_conn.accept_bi().await {
+                            Ok(s) => s,
+                            Err(e) => {
+                                debug!("tty: channels open: accept_bi failed: {e}");
+                                return;
+                            }
+                        };
+                        let (_, mut client_write) = tokio::io::split(stream);
+                        crate::adapter::send_negotiation_error(
+                            NegotiationWriter::new(&mut client_write),
+                            "malformed_negotiation",
+                            &[("message", &e.to_string())],
+                        )
+                        .await;
                         return;
                     }
                 };
@@ -250,6 +279,91 @@ mod tests {
     use std::collections::HashMap as StdHashMap;
     use tokio::io::duplex;
 
+    /// The R4 unit-testable seam: feed `make_tty_open_handler` a
+    /// hand-built `input` the (partial) schema would accept but the full
+    /// `NegotiateRequest` parse rejects (`cwd` typed as a number). The
+    /// handler must write a `0x00`-prefixed `malformed_negotiation` error
+    /// frame on the channel stream — the same failure class as the
+    /// post-open semantic failures, not a silent teardown. The read side
+    /// is the consumer's exact post-open sequence (`from_halves_raw`):
+    /// peek the first byte, see `0x00`, read the length-prefixed error
+    /// frame.
+    #[tokio::test]
+    async fn open_handler_writes_error_frame_on_schema_bypassing_input() {
+        use alkcall::core::types::{BiStream, BidiStreamSource, StreamError};
+        use tokio::io::AsyncReadExt;
+
+        struct YieldOnce {
+            stream: tokio::sync::Mutex<Option<BiStream>>,
+        }
+        #[async_trait::async_trait]
+        impl BidiStreamSource for YieldOnce {
+            async fn accept_bi(&self) -> Result<BiStream, StreamError> {
+                self.stream
+                    .lock()
+                    .await
+                    .take()
+                    .ok_or(StreamError::ConnectionClosed)
+            }
+            async fn open_bi(&self) -> Result<BiStream, StreamError> {
+                Err(StreamError::StreamClosed)
+            }
+            fn remote_addr(&self) -> Option<std::net::SocketAddr> {
+                None
+            }
+            fn close(&self, _code: u32, _reason: &str) {
+                // The take-on-drop of the wrapped stream is not needed
+                // for this test; the connection is never closed.
+            }
+        }
+
+        let (client_end, server_end) = duplex(64 * 1024);
+        let (server_read, server_write) = tokio::io::split(server_end);
+        let bidi = BiStream::from_joined(server_read, server_write);
+        let channel_conn = Connection::from_source(
+            YieldOnce {
+                stream: tokio::sync::Mutex::new(Some(bidi)),
+            },
+            TTY_ALPN.as_bytes().to_vec(),
+        );
+
+        let mut backends: HashMap<String, Arc<dyn TtyBackend>> = HashMap::new();
+        backends.insert("mock".to_string(), Arc::new(MockBackend::with_exit_code(0)));
+        let handler = make_tty_open_handler(Arc::new(backends), None, None);
+
+        let task = handler(
+            serde_json::json!({
+                "carriage": "raw",
+                "backend": "mock",
+                "cmd": ["true"],
+                "cwd": 42
+            }),
+            channel_conn,
+            AuthContext::anonymous(b"test"),
+        );
+        task.await.expect("handler task");
+
+        let mut read = client_end;
+        let mut first = [0u8; 1];
+        tokio::time::timeout(
+            std::time::Duration::from_secs(5),
+            read.read_exact(&mut first),
+        )
+        .await
+        .expect("no byte from handler")
+        .expect("read first byte");
+        assert_eq!(first[0], 0x00, "error frame length prefix starts with 0x00");
+
+        let mut len_rest = [0u8; 3];
+        read.read_exact(&mut len_rest).await.expect("read len rest");
+        let len = u32::from_be_bytes([first[0], len_rest[0], len_rest[1], len_rest[2]]) as usize;
+        let mut body = vec![0u8; len];
+        read.read_exact(&mut body).await.expect("read error body");
+        let v: serde_json::Value = serde_json::from_slice(&body).expect("parse error frame");
+        assert_eq!(v["error"], "malformed_negotiation");
+        assert!(v["message"].as_str().is_some_and(|m| !m.is_empty()));
+    }
+
     #[test]
     fn op_tty_open_is_channels_tty_sub() {
         assert_eq!(OP_TTY_OPEN, "channels/tty/sub");

src/session.rs

@@ -196,11 +196,13 @@ impl TtySession {
     ///
     /// Failures before the channel opens (ACL denial, unknown op,
     /// channel cap, invalid params) surface as
-    /// [`TtySessionError::ChannelsOpen`]. Post-open failures (unknown
-    /// backend, allocate failure, ownership denial) arrive as a
-    /// negotiation error frame on the channel stream — the session
-    /// surfaces those as [`TtySessionError::NegotiationRejected`] via
-    /// the same `0x00` disambiguation read the direct path uses.
+    /// [`TtySessionError::ChannelsOpen`]. Post-open failures (a
+    /// `NegotiateRequest` parse failure of a schema-valid-but-unparseable
+    /// params value, unknown backend, allocate failure, ownership
+    /// denial) arrive as a negotiation error frame on the channel
+    /// stream — the session surfaces those as
+    /// [`TtySessionError::NegotiationRejected`] via the same `0x00`
+    /// disambiguation read the direct path uses.
     pub async fn open_via_channels(
         client: &ChannelClient,
         params: serde_json::Value,
@@ -292,9 +294,10 @@ impl TtySession {
     /// Core inner for the channels path (ADR-009): the negotiation
     /// already happened in the open op — the stream is already in
     /// raw-chunk mode. The peek still applies: the producer sends a
-    /// `0x00`-prefixed error frame on post-open failures (unknown
-    /// backend, allocate failure, ownership denial), and a raw chunk
-    /// (`stream_type` in `{1, 2, 4}`) on success.
+    /// `0x00`-prefixed error frame on any post-open failure (a
+    /// `NegotiateRequest` parse failure of the open op's `input`,
+    /// unknown backend, allocate failure, ownership denial), and a raw
+    /// chunk (`stream_type` in `{1, 2, 4}`) on success.
     async fn from_halves_raw<R, W>(read: R, write: W) -> Result<Self, TtySessionError>
     where
         R: AsyncRead + Send + Unpin + 'static,

src/testing.rs

@@ -206,4 +206,64 @@ mod tests {
         assert_eq!(first[0], crate::wire::STREAM_STDOUT);
         drop(send);
     }
+
+    /// End-to-end (R4): `input` that passes the registry's (partial)
+    /// schema but fails the handler's full `NegotiateRequest` parse —
+    /// `cwd` typed as a number. The open op succeeds (the schema is
+    /// deliberately partial), the producer-side handler writes a
+    /// `0x00`-prefixed `malformed_negotiation` error frame on the
+    /// channel stream, and the consumer-side post-open read sequence
+    /// (`from_halves_raw`'s peek + error-frame parse) observes it — the
+    /// same failure class as the post-open semantic failures, not a
+    /// silent teardown. Uses `ChannelClient::open_channel` directly
+    /// (the consumer's local fail-fast parse in `open_via_channels`
+    /// would reject these params before the open op runs — R5's
+    /// fail-fast path).
+    #[tokio::test]
+    async fn schema_valid_but_unparseable_input_gets_malformed_negotiation_frame() {
+        use tokio::io::AsyncReadExt;
+
+        let mut backends: HashMap<String, Arc<dyn TtyBackend>> = HashMap::new();
+        backends.insert("mock".to_string(), Arc::new(MockBackend::with_exit_code(0)));
+        let client =
+            wire_client_and_server(Arc::new(backends), None, Some(tty_identity("alice"))).await;
+
+        let (_channel_id, _send, mut recv) = tokio::time::timeout(
+            std::time::Duration::from_secs(5),
+            client.open_channel(
+                OP_TTY_OPEN,
+                serde_json::json!({
+                    "carriage": "raw",
+                    "backend": "mock",
+                    "cmd": ["true"],
+                    "cwd": 42
+                }),
+                crate::channels::TTY_ALPN,
+            ),
+        )
+        .await
+        .expect("open_channel timed out")
+        .expect("open_channel — the partial schema accepts cwd: 42");
+
+        // The consumer-side post-open sequence (`from_halves_raw`):
+        // peek the first byte; `0x00` = negotiation error frame.
+        let mut first = [0u8; 1];
+        tokio::time::timeout(
+            std::time::Duration::from_secs(5),
+            recv.read_exact(&mut first),
+        )
+        .await
+        .expect("no response byte from producer")
+        .expect("read first byte");
+        assert_eq!(first[0], 0x00, "error frame length prefix starts with 0x00");
+
+        let mut len_rest = [0u8; 3];
+        recv.read_exact(&mut len_rest).await.expect("read len rest");
+        let len = u32::from_be_bytes([first[0], len_rest[0], len_rest[1], len_rest[2]]) as usize;
+        let mut body = vec![0u8; len];
+        recv.read_exact(&mut body).await.expect("read error body");
+        let v: serde_json::Value = serde_json::from_slice(&body).expect("parse error frame");
+        assert_eq!(v["error"], "malformed_negotiation");
+        assert!(v["message"].as_str().is_some_and(|m| !m.is_empty()));
+    }
 }