review classifier: every ACP run gets one shared, empty cwd

opencode writes the session's cwd into the system message's environment
block ("Working directory: …", "Workspace root folder: …"), ahead of the
PR's user message. Each ACP classification ran with its per-call scratch
directory, /tmp/review-classifier-<pid>-<counter>, as cwd, so every call
sent a different system message and the provider's prompt cache could not
match past it.

The session's cwd is now one fixed directory, <tmp>/review-classifier-acp-cwd,
shared by every run. HOME (the config, system.md and opencode's own state)
stays a directory per run, removed after it, as before. With no tool and
no project config nothing writes into the cwd: a live opencode 1.15.10 run
through initialize, session/new and a prompt left it empty, with its state
all under HOME. So concurrent runs can share it, and no state carries from
one run to the next.

Refs #5245

docs/swarm/review-classifier.md

@@ -124,7 +124,11 @@ resume, no tools (`--tools ""`), no MCP server (`--strict-mcp-config` and no
 config) and an empty directory of its own.
 
 On the ACP runtime, each classification is one prompt on a new `opencode acp`
-session in an empty directory of its own. The service drives it through
+session. Its config and opencode's state live in a directory of its own,
+removed after it. Its working directory is one empty directory that every
+classification shares: opencode writes that path into the system message, so
+one path keeps the system message the same from one classification to the
+next, and the provider's prompt cache can hold. The service drives it through
 [`hive-runtime`](../../hive-runtime/README.md)'s `AcpRuntime`, as the subagent
 daemon does a one-run subagent, and stops the agent when the classification
 ends. The agent has no tool. opencode runs some tools, `read` among them,

swarm-review-classifier/src/acp.rs

@@ -1,6 +1,8 @@
 //! One classification on the ACP runtime: `opencode acp`, driven through
-//! [`hive_runtime::AcpRuntime`] for one prompt on a new session, in a
+//! [`hive_runtime::AcpRuntime`] for one prompt on a new session. Its config,
-//! directory made for it and removed after it.
+//! system prompt and opencode's state live in a directory made for it and
+//! removed after it, its `HOME`; its working directory is [`Acp::cwd`],
+//! shared by every run.
 //!
 //! The agent has no tool at all. opencode runs some tools, `read` among them,
 //! without asking, so a deny-all permission policy alone would never see them:
@@ -68,6 +70,11 @@ pub struct Acp<S> {
     /// [`CANCEL_WAIT`] outside the tests.
     pub cancel_wait: Duration,
     pub cost: S,
+    /// The empty working directory every run's session gets. opencode writes
+    /// it into the system message, so one path for every run keeps that
+    /// message the same and the provider's prompt cache usable; with no tool
+    /// and no project config, nothing writes into it.
+    pub cwd: PathBuf,
 }
 
 impl<S: CostSink> crate::run::Model for Acp<S> {
@@ -97,6 +104,7 @@ impl<S: CostSink> Acp<S> {
         let config_file = dir.join("opencode.json");
         tokio::fs::create_dir_all(dir)
             .await
+            .and(tokio::fs::create_dir_all(&self.cwd).await)
             .and(tokio::fs::write(&system, crate::prompt::SYSTEM).await)
             .and(tokio::fs::write(&config_file, opencode_config(provider).to_string()).await)
             .map_err(|e| format!("preparing the run's directory: {e}"))?;
@@ -112,7 +120,7 @@ impl<S: CostSink> Acp<S> {
             .ok_or("the ACP runtime offers no cancel handle")?;
         let config = Config {
             effort: run.effort.map(|e| e.as_str().to_owned()),
-            cwd: Some(dir.to_owned()),
+            cwd: Some(self.cwd.clone()),
             idle_timeout: Some(IDLE_TIMEOUT),
             ..Config::default()
         };
@@ -271,8 +279,10 @@ mod tests {
 
     /// An ACP agent in plain `sh`, logging to `<log>.*`. It writes its
     /// environment and copies of its config and the system prompt file
-    /// beside it at start, appends each method it is sent to
+    /// beside it and appends its `$HOME` to `<log>.homes` at start, appends
-    /// `<log>.methods` and each prompt request to `<log>.prompts`, and answers a prompt with a text chunk
+    /// each method it is sent to `<log>.methods`, each `session/new` request
+    /// to `<log>.sessions` and each prompt request to `<log>.prompts`, and
+    /// answers a prompt with a text chunk
     /// holding a verdict, then `end_turn` with 1000 input and 200 output
     /// tokens. A prompt holding `ASK-READ` first asks permission for a
     /// `read` and appends the reply to `<log>.permission`; one holding
@@ -283,6 +293,7 @@ log="$1"
 env > "$log.env"
 cp "$OPENCODE_CONFIG" "$log.config"
 cp "${OPENCODE_CONFIG%/*}/system.md" "$log.system"
+printf '%s\n' "$HOME" >> "$log.homes"
 stalled=
 while IFS= read -r line; do
   id=${line#*\"id\":}; id=${id%%[,\}]*}
@@ -292,6 +303,7 @@ while IFS= read -r line; do
     initialize)
       printf '{"jsonrpc":"2.0","id":%s,"result":{"protocolVersion":1,"agentCapabilities":{"mcpCapabilities":{"http":true}}}}\n' "$id" ;;
     session/new)
+      printf '%s\n' "$line" >> "$log.sessions"
       printf '{"jsonrpc":"2.0","id":%s,"result":{"sessionId":"s1"}}\n' "$id" ;;
     session/cancel)
       [ -n "$stalled" ] && printf '{"jsonrpc":"2.0","id":%s,"result":{"stopReason":"cancelled"}}\n' "$stalled" ;;
@@ -338,6 +350,7 @@ done
             deadline,
             cancel_wait: Duration::from_millis(500),
             cost: Arc::clone(&costs),
+            cwd: dir.join("cwd"),
         };
         (acp, costs)
     }
@@ -507,4 +520,45 @@ done
         assert!(acp.ask("classify", "p", &claude).await.is_err());
         assert_eq!(read(dir.path(), "methods"), "", "no agent started");
     }
+
+    /// The cwd of each session the agent was asked for, in order.
+    fn cwds(dir: &Path) -> Vec<String> {
+        read(dir, "sessions")
+            .lines()
+            .map(|l| {
+                let request: Value = serde_json::from_str(l).expect("JSON");
+                request["params"]["cwd"].as_str().expect("a cwd").to_owned()
+            })
+            .collect()
+    }
+
+    #[tokio::test]
+    async fn every_run_gets_the_same_cwd_even_at_once() {
+        let dir = tempfile::tempdir().expect("a dir");
+        let (acp, _) = acp(dir.path(), Duration::from_mins(1));
+
+        let (run_one, run_two) = (run(), run());
+        let (one, two) = tokio::join!(acp.ask("one", "p", &run_one), acp.ask("two", "p", &run_two));
+        one.expect("an answer");
+        two.expect("an answer");
+        acp.ask("three", "p", &run()).await.expect("an answer");
+        let cwd = dir.path().join("cwd").display().to_string();
+        assert_eq!(cwds(dir.path()), [cwd.clone(), cwd.clone(), cwd]);
+    }
+
+    #[tokio::test]
+    async fn each_run_has_a_home_of_its_own_removed_after_it() {
+        let dir = tempfile::tempdir().expect("a dir");
+        let (acp, _) = acp(dir.path(), Duration::from_mins(1));
+
+        acp.ask("one", "p", &run()).await.expect("an answer");
+        acp.ask("two", "p", &run()).await.expect("an answer");
+        let homes = read(dir.path(), "homes");
+        let homes: Vec<&str> = homes.lines().collect();
+        assert_eq!(homes.len(), 2, "{homes:?}");
+        assert_ne!(homes[0], homes[1]);
+        for home in homes {
+            assert!(!Path::new(home).exists(), "{home}");
+        }
+    }
 }

swarm-review-classifier/src/main.rs

@@ -70,6 +70,7 @@ async fn serve() -> Result<()> {
             deadline: swarm_queue_client::review_classifier::CLASSIFICATION_DEADLINE,
             cancel_wait: acp::CANCEL_WAIT,
             cost: cost::Otel,
+            cwd: std::env::temp_dir().join("review-classifier-acp-cwd"),
         },
     };
     let secret = client_secret_with_retry(&client_id).await?;

swarm-review-classifier/src/run.rs

@@ -3,11 +3,11 @@
 //!
 //! On the API key runtime each classification is one `claude --print` with
 //! no session, no tools and no MCP server; on the ACP runtime it is one
-//! prompt on a new ACP session ([`crate::acp`]). Either way it runs in a
+//! prompt on a new ACP session ([`crate::acp`]). Either way its files live
-//! directory made for it and removed after it, so nothing carries from one
+//! in a directory made for it and removed after it, so nothing carries from
-//! run to the next. The key is read from the store for every run and reaches
+//! one run to the next. The key is read from the store for every run and
-//! only the child's environment, which keeps nothing of the daemon's beyond
+//! reaches only the child's environment, which keeps nothing of the daemon's
-//! [`crate::child_env`]'s allowlist.
+//! beyond [`crate::child_env`]'s allowlist.
 
 use std::future::Future;
 use std::path::{Path, PathBuf};