fix(acp): reject unattended permission requests

crates/buzz-acp/src/acp.rs

@@ -155,7 +155,7 @@ pub struct AcpClient {
     /// a `cancelled` outcome before the agent returns from `session/prompt`.
     pending_permission_id: Option<serde_json::Value>,
     /// Whether we have already sent a response to the pending permission request.
-    /// Guards against double-response if a timeout fires after the allow_once
+    /// Guards against double-response if a timeout fires after the rejection
     /// response was written but before `pending_permission_id` was cleared.
     permission_responded: bool,
     /// The JSON-RPC id of the most recently sent `session/prompt` request.
@@ -1162,7 +1162,8 @@ impl AcpClient {
     ///
     /// While waiting, handles:
     /// - `session/update` notifications → logged via tracing
-    /// - `session/request_permission` requests → auto-approved with `allow_once`
+    /// - `session/request_permission` requests → rejected unless an owner has
+    ///   already selected a non-interactive permission mode at session setup
     /// - Any other messages → debug-logged and ignored; if they carry an `id`
     ///   (i.e. they are requests, not notifications), a JSON-RPC -32601 error is sent.
     ///
@@ -1870,12 +1871,12 @@ impl AcpClient {
         }
     }
 
-    /// Auto-approve a `session/request_permission` request from the agent.
+    /// Reject a `session/request_permission` request from the agent.
     ///
-    /// Finds the option with `kind == "allow_once"` and responds with its `optionId`.
-    /// If no `allow_once` option exists, falls back to `reject_once`.
-    ///
-    /// **Critical:** Never hardcode `optionId` — always find it dynamically by `kind`.
+    /// Buzz has no human permission prompt in this harness, so selecting
+    /// `allow_once` would turn any admitted prompt into an implicit approval.
+    /// Find `reject_once` by kind when the adapter offers it; otherwise use the
+    /// protocol's cancelled outcome, which is also fail-closed.
     ///
     /// The request `id` is stored as `serde_json::Value` to support both numeric
     /// and string IDs per JSON-RPC 2.0.
@@ -1901,40 +1902,7 @@ impl AcpClient {
             options.len()
         );
 
-        // Find allow_once by kind — NEVER hardcode optionId.
-        let allow_once = options
-            .iter()
-            .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("allow_once"));
-
-        let response = if let Some(opt) = allow_once {
-            let option_id = opt["optionId"]
-                .as_str()
-                .ok_or_else(|| AcpError::Protocol("allow_once option missing optionId".into()))?;
-            tracing::info!(
-                target: "acp::permission",
-                "auto-approving permission id={id} with allow_once optionId={option_id:?}"
-            );
-            permission_response_selected(&id, option_id)
-        } else {
-            // No allow_once — fall back to reject_once.
-            tracing::warn!(
-                target: "acp::permission",
-                "no allow_once option found in permission request id={id}, falling back to reject_once"
-            );
-            let reject = options
-                .iter()
-                .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("reject_once"));
-
-            if let Some(opt) = reject {
-                let option_id = opt["optionId"].as_str().unwrap_or("reject");
-                permission_response_selected(&id, option_id)
-            } else {
-                return Err(AcpError::Protocol(
-                    "no suitable permission option found (neither allow_once nor reject_once)"
-                        .into(),
-                ));
-            }
-        };
+        let response = permission_denial_response(&id, options)?;
 
         // Write the response first, then mark as responded.
         //
@@ -2046,6 +2014,42 @@ fn permission_response_cancelled(id: &serde_json::Value) -> serde_json::Value {
     })
 }
 
+/// Choose the fail-closed response to a `session/request_permission` request.
+///
+/// Buzz has no human permission prompt in this harness, so selecting
+/// `allow_once` would turn any admitted prompt into an implicit approval.
+/// Prefer the adapter's `reject_once` option — matched by `kind`, never by a
+/// hardcoded `optionId` — and fall back to the protocol's cancelled outcome for
+/// adapters that do not offer one. Both answers deny.
+///
+/// Kept free of the client so the decision is testable without an agent
+/// subprocess: `AcpClient` owns a real `Child` and its stdio pipes.
+fn permission_denial_response(
+    id: &serde_json::Value,
+    options: &[serde_json::Value],
+) -> Result<serde_json::Value, AcpError> {
+    let reject_once = options
+        .iter()
+        .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("reject_once"));
+
+    let Some(opt) = reject_once else {
+        tracing::warn!(
+            target: "acp::permission",
+            "no reject_once option found in permission request id={id}, cancelling"
+        );
+        return Ok(permission_response_cancelled(id));
+    };
+
+    let option_id = opt["optionId"]
+        .as_str()
+        .ok_or_else(|| AcpError::Protocol("reject_once option missing optionId".into()))?;
+    tracing::info!(
+        target: "acp::permission",
+        "rejecting permission id={id} with reject_once optionId={option_id:?}"
+    );
+    Ok(permission_response_selected(id, option_id))
+}
+
 /// Full `session/new` response — session ID plus the raw JSON result.
 ///
 /// Callers use the extractor helpers to pull model info from `raw`.
@@ -2300,63 +2304,96 @@ mod tests {
         assert_eq!(StopReason::from_str("Refusal"), Some(StopReason::Refusal));
     }
 
+    fn options(json: &str) -> Vec<serde_json::Value> {
+        serde_json::from_str(json).expect("option list")
+    }
+
+    fn outcome(response: &serde_json::Value) -> Option<&str> {
+        response["result"]["outcome"]["outcome"].as_str()
+    }
+
+    /// The offered `allow_once` and `allow_always` options must be ignored:
+    /// there is no human to click them, so choosing either would make every
+    /// admitted prompt an implicit approval. `optionId`s are deliberately
+    /// non-obvious to prove they are matched by `kind`, never hardcoded.
     #[test]
-    fn find_allow_once_by_kind_not_by_option_id() {
-        // optionId values are intentionally non-obvious to prove we don't hardcode them.
-        let options: Vec<serde_json::Value> = serde_json::from_str(
+    fn permission_requests_select_reject_once_not_allow_once() {
+        let options = options(
             r#"[
             {"optionId": "opt-reject-42",  "name": "Reject",       "kind": "reject_once"},
             {"optionId": "opt-allow-99",   "name": "Allow once",   "kind": "allow_once"},
             {"optionId": "opt-always-7",   "name": "Always allow", "kind": "allow_always"}
         ]"#,
-        )
-        .unwrap();
+        );
 
-        let allow_once = options
-            .iter()
-            .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("allow_once"));
+        let response =
+            permission_denial_response(&serde_json::json!(7), &options).expect("denial response");
 
-        assert!(allow_once.is_some(), "should find allow_once option");
-        let opt = allow_once.unwrap();
-        // Found by kind, not by hardcoded optionId
-        assert_eq!(opt["kind"].as_str(), Some("allow_once"));
-        assert_eq!(opt["optionId"].as_str(), Some("opt-allow-99"));
+        assert_eq!(outcome(&response), Some("selected"));
+        assert_eq!(
+            response["result"]["outcome"]["optionId"].as_str(),
+            Some("opt-reject-42"),
+            "must select reject_once even when allow options are offered"
+        );
     }
 
+    /// Fail-closed backstop: an adapter that offers no `reject_once` must still
+    /// be denied, via the protocol's cancelled outcome rather than an error or
+    /// an approval.
     #[test]
-    fn find_allow_once_returns_none_when_absent() {
-        let options: Vec<serde_json::Value> = serde_json::from_str(
+    fn permission_request_without_reject_once_is_cancelled() {
+        let options = options(
             r#"[
-            {"optionId": "reject-1",      "name": "Reject",        "kind": "reject_once"},
-            {"optionId": "reject-always", "name": "Always reject", "kind": "reject_always"}
+            {"optionId": "opt-allow-99", "name": "Allow once",   "kind": "allow_once"},
+            {"optionId": "opt-always-7", "name": "Always allow", "kind": "allow_always"}
         ]"#,
-        )
-        .unwrap();
+        );
 
-        let allow_once = options
-            .iter()
-            .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("allow_once"));
+        let response = permission_denial_response(&serde_json::json!("req-1"), &options)
+            .expect("cancelled response");
 
-        assert!(allow_once.is_none());
+        assert_eq!(outcome(&response), Some("cancelled"));
+        assert_eq!(
+            response["id"].as_str(),
+            Some("req-1"),
+            "string ids must round-trip per JSON-RPC 2.0"
+        );
     }
 
+    /// An empty option list is the degenerate form of the same backstop.
     #[test]
-    fn find_reject_once_fallback_when_no_allow_once() {
-        let options: Vec<serde_json::Value> = serde_json::from_str(
-            r#"[{"optionId": "rej-x", "name": "Reject", "kind": "reject_once"}]"#,
-        )
-        .unwrap();
+    fn permission_request_with_no_options_is_cancelled() {
+        let response =
+            permission_denial_response(&serde_json::json!(1), &[]).expect("cancelled response");
 
-        let allow_once = options
-            .iter()
-            .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("allow_once"));
-        assert!(allow_once.is_none());
+        assert_eq!(outcome(&response), Some("cancelled"));
+    }
 
-        let reject_once = options
-            .iter()
-            .find(|opt| opt.get("kind").and_then(|k| k.as_str()) == Some("reject_once"));
-        assert!(reject_once.is_some());
-        assert_eq!(reject_once.unwrap()["optionId"].as_str(), Some("rej-x"));
+    /// A `reject_once` option missing its `optionId` is a protocol violation.
+    /// Erroring propagates to the caller, which tears the turn down — still no
+    /// approval is ever sent.
+    #[test]
+    fn reject_once_without_option_id_is_a_protocol_error() {
+        let options = options(r#"[{"name": "Reject", "kind": "reject_once"}]"#);
+
+        let err = permission_denial_response(&serde_json::json!(1), &options)
+            .expect_err("missing optionId must error");
+
+        assert!(matches!(err, AcpError::Protocol(_)), "got {err:?}");
+    }
+
+    #[test]
+    fn find_reject_once_by_kind() {
+        let options =
+            options(r#"[{"optionId": "rej-x", "name": "Reject", "kind": "reject_once"}]"#);
+
+        let response =
+            permission_denial_response(&serde_json::json!(1), &options).expect("denial response");
+
+        assert_eq!(
+            response["result"]["outcome"]["optionId"].as_str(),
+            Some("rej-x")
+        );
     }
 
     #[test]