feat(helexa-acp): model picker + session/set_model handler

Stage 4. Zed's model dropdown now lists every model from every
configured endpoint, and switching it routes the next prompt to a
new endpoint+model.

- Enable `unstable_session_model` on the agent-client-protocol dep
  so SessionModelState / SetSessionModelRequest / ModelInfo are
  available.
- Agent::new becomes async and calls Provider::list_models on every
  provider at startup; per-endpoint failures warn-and-skip instead
  of aborting the agent.
- With a single endpoint configured, model ids appear bare; with
  multiple endpoints every id carries the `endpoint:` prefix so the
  picker is unambiguous and parse_model_selector routes correctly.
- NewSessionResponse and LoadSessionResponse attach SessionModelState
  with the session's current model id + the aggregated catalogue.
- session/set_model: validates the requested model id against
  resolve_provider, mutates session.model_id, and persists so the
  on-disk transcript reflects the new model.

Three new aggregate_models tests cover the prefixing rule (bare vs
multi-endpoint) and warn-and-skip on a failing endpoint.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/helexa-acp/Cargo.toml

@@ -16,7 +16,12 @@ to cortex (helexa's reverse-proxy / fleet gateway).
 # a painless migration to a dedicated GitHub repo in the future if the
 # project grows beyond helexa's needs. All deps are crates.io.
 [dependencies]
-agent-client-protocol = "0.12"
+# `unstable_session_model` flips on the SessionModelState type and the
+# session/set_model RPC the model-picker dropdown in Zed needs. The
+# feature is upstream-marked unstable; we accept that risk because the
+# model picker is core UX and the alternative (rolling our own
+# extension method) drifts further from spec each time it moves.
+agent-client-protocol = { version = "0.12", features = ["unstable_session_model"] }
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "io-util", "process", "signal"] }
 reqwest = { version = "0.12", features = ["json", "stream", "rustls-tls"], default-features = false }
 serde = { version = "1", features = ["derive"] }

crates/helexa-acp/src/agent.rs

@@ -9,9 +9,10 @@
 //! | `session/prompt`      | tool-call loop: stream → dispatch tools → re-enter, repeat  |
 //! | `session/cancel`      | fire the session's cancellation token                       |
 //! | `session/set_mode`    | mutate the session's mode (gated vs. bypass-permissions)    |
+//! | `session/set_model`   | switch the session's active model (endpoint:model selector) |
 //! | (anything else)       | "not implemented yet" error                                 |
 //!
-//! Stage 4 wires `session/set_model`; Stage 5 flips on image content.
+//! Stage 5 flips on image content.
 
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -19,11 +20,12 @@ use std::sync::atomic::{AtomicU64, Ordering};
 
 use agent_client_protocol::schema::{
     AgentCapabilities, CancelNotification, ContentBlock, InitializeRequest, InitializeResponse,
-    ListSessionsRequest, ListSessionsResponse, LoadSessionRequest, LoadSessionResponse,
-    NewSessionRequest, NewSessionResponse, PromptCapabilities, PromptRequest, PromptResponse,
-    SessionCapabilities, SessionId, SessionInfo, SessionListCapabilities, SessionMode,
-    SessionModeId, SessionModeState, SessionNotification, SessionUpdate, SetSessionModeRequest,
-    SetSessionModeResponse, StopReason, TextContent,
+    ListSessionsRequest, ListSessionsResponse, LoadSessionRequest, LoadSessionResponse, ModelId,
+    ModelInfo as AcpModelInfo, NewSessionRequest, NewSessionResponse, PromptCapabilities,
+    PromptRequest, PromptResponse, SessionCapabilities, SessionId, SessionInfo,
+    SessionListCapabilities, SessionMode, SessionModeId, SessionModeState, SessionModelState,
+    SessionNotification, SessionUpdate, SetSessionModeRequest, SetSessionModeResponse,
+    SetSessionModelRequest, SetSessionModelResponse, StopReason, TextContent,
 };
 use agent_client_protocol::{Agent as AgentRole, Client, ConnectionTo, Dispatch, Stdio};
 use futures::StreamExt;
@@ -73,6 +75,14 @@ struct AgentInner {
     /// fits inside `context_window - max_tokens - safety` tokens.
     /// Absent entry → no compaction (legacy behaviour).
     context_window: std::collections::HashMap<String, usize>,
+    /// Aggregated list of selectable models across every configured
+    /// endpoint, computed once at startup. With a single endpoint
+    /// the model ids appear bare; with multiple endpoints every id
+    /// carries the `endpoint:` prefix so the picker is unambiguous.
+    /// Empty when every provider's `list_models` failed at startup —
+    /// the dropdown then shows nothing and the session keeps using
+    /// the configured `default_model`.
+    available_models: Vec<AcpModelInfo>,
     sessions: SessionStore,
     system_prompt_path: Option<PathBuf>,
     /// Monotonic counter for minting session ids. The wire format is
@@ -85,7 +95,13 @@ struct AgentInner {
 impl Agent {
     /// Construct an agent from a validated [`Config`] and the providers
     /// that were successfully built for each endpoint.
-    pub fn new(cfg: &Config, providers: Vec<Arc<dyn Provider>>) -> anyhow::Result<Self> {
+    ///
+    /// `async` because we call `Provider::list_models` on every
+    /// provider up-front so the model-picker dropdown is populated
+    /// from the very first `session/new`. Per-endpoint failure
+    /// warns and skips rather than aborting startup — a single
+    /// unreachable endpoint shouldn't take down the agent.
+    pub async fn new(cfg: &Config, providers: Vec<Arc<dyn Provider>>) -> anyhow::Result<Self> {
         if providers.is_empty() {
             anyhow::bail!("no usable providers");
         }
@@ -110,6 +126,12 @@ impl Agent {
             .iter()
             .filter_map(|ep| ep.context_window.map(|w| (ep.name.clone(), w)))
             .collect();
+        let available_models = aggregate_models(&providers).await;
+        tracing::info!(
+            models = available_models.len(),
+            endpoints = providers.len(),
+            "model catalogue assembled"
+        );
         Ok(Self {
             inner: Arc::new(AgentInner {
                 providers,
@@ -117,6 +139,7 @@ impl Agent {
                 default_model: default.default_model.clone(),
                 max_tokens,
                 context_window,
+                available_models,
                 sessions: session::new_store(),
                 system_prompt_path: cfg.system_prompt_path.clone(),
                 next_session_id: AtomicU64::new(1),
@@ -207,6 +230,18 @@ impl Agent {
                 },
                 agent_client_protocol::on_receive_request!(),
             )
+            .on_receive_request(
+                {
+                    let inner = inner.clone();
+                    async move |req: SetSessionModelRequest, responder, _cx| {
+                        match handle_set_session_model(&inner, req).await {
+                            Ok(()) => responder.respond(SetSessionModelResponse::new()),
+                            Err(e) => responder.respond_with_internal_error(format!("{e:#}")),
+                        }
+                    }
+                },
+                agent_client_protocol::on_receive_request!(),
+            )
             .on_receive_notification(
                 {
                     let inner = inner.clone();
@@ -300,7 +335,12 @@ async fn handle_new_session(
         cwd = %cwd_display,
         "session created"
     );
-    Ok(NewSessionResponse::new(session_id).modes(default_mode_state()))
+    let resp = NewSessionResponse::new(session_id).modes(default_mode_state());
+    let resp = match session_model_state(inner, &log_model) {
+        Some(models) => resp.models(Some(models)),
+        None => resp,
+    };
+    Ok(resp)
 }
 
 /// Rehydrate a session from disk.
@@ -353,7 +393,12 @@ async fn handle_load_session(
         SessionModeId::new(mode_id),
         default_mode_state().available_modes,
     );
-    Ok((LoadSessionResponse::new().modes(modes), history_for_replay))
+    let resp = LoadSessionResponse::new().modes(modes);
+    let resp = match session_model_state(inner, &model_id) {
+        Some(models) => resp.models(Some(models)),
+        None => resp,
+    };
+    Ok((resp, history_for_replay))
 }
 
 /// Re-emit a session's persisted history as `session/update`
@@ -559,6 +604,108 @@ fn derive_session_title(history: &[Message]) -> Option<String> {
         .filter(|s| !s.is_empty())
 }
 
+/// Build the model catalogue advertised in `NewSessionResponse.models`
+/// (and the resume equivalent). Walks every provider, calls
+/// `list_models`, and prefixes ids with `endpoint:` when the user has
+/// more than one endpoint configured. A failing endpoint logs and
+/// contributes nothing — losing one endpoint must not blank the whole
+/// dropdown.
+async fn aggregate_models(providers: &[Arc<dyn Provider>]) -> Vec<AcpModelInfo> {
+    let multi_endpoint = providers.len() > 1;
+    let mut out: Vec<AcpModelInfo> = Vec::new();
+    for provider in providers {
+        let endpoint = provider.name().to_string();
+        match provider.list_models().await {
+            Ok(models) => {
+                tracing::info!(
+                    endpoint = %endpoint,
+                    count = models.len(),
+                    "fetched models from endpoint"
+                );
+                for m in models {
+                    let id = if multi_endpoint {
+                        format!("{endpoint}:{}", m.id)
+                    } else {
+                        m.id.clone()
+                    };
+                    let display = m.display_name.unwrap_or_else(|| m.id.clone());
+                    let info = AcpModelInfo::new(ModelId::new(id), display)
+                        .description(Some(format!("endpoint: {endpoint}")));
+                    out.push(info);
+                }
+            }
+            Err(e) => {
+                tracing::warn!(
+                    endpoint = %endpoint,
+                    error = %format!("{e:#}"),
+                    "list_models failed; this endpoint's models won't appear in the picker"
+                );
+            }
+        }
+    }
+    out
+}
+
+/// Build the `SessionModelState` that Zed renders as the
+/// model-picker dropdown. The current model id is exactly what
+/// the session is using right now (already in `endpoint:model`
+/// form if it was set that way). Returns `None` when the
+/// catalogue is empty — no point showing an empty dropdown.
+fn session_model_state(inner: &AgentInner, current: &str) -> Option<SessionModelState> {
+    if inner.available_models.is_empty() {
+        return None;
+    }
+    Some(SessionModelState::new(
+        ModelId::new(current.to_string()),
+        inner.available_models.clone(),
+    ))
+}
+
+async fn handle_set_session_model(
+    inner: &AgentInner,
+    req: SetSessionModelRequest,
+) -> anyhow::Result<()> {
+    let Some(state) = session::get(&inner.sessions, &req.session_id).await else {
+        anyhow::bail!("unknown session id {}", req.session_id.0);
+    };
+    let target = req.model_id.0.as_ref().to_string();
+    // Validate the requested model id resolves to a configured
+    // provider. We don't require it to appear in `available_models`
+    // because the catalogue may be stale (endpoint added a model
+    // after startup) and rejecting unknown ids would be too rigid.
+    // Provider lookup is the actual source of truth.
+    let (_, _) = resolve_provider(&inner.providers, &inner.default_endpoint_name, &target)
+        .map_err(|e| anyhow::anyhow!("set_session_model: {e:#}"))?;
+    // Persist the new model id on the session under the mutex,
+    // then snapshot for disk persistence outside the lock.
+    let snapshot = {
+        let mut s = state.lock().await;
+        s.model_id = target.clone();
+        PersistedSession {
+            session_id: req.session_id.0.as_ref().to_string(),
+            cwd: s.cwd.clone(),
+            model_id: s.model_id.clone(),
+            mode_id: s.mode_id.0.as_ref().to_string(),
+            history: s.history.clone(),
+            created_at: store::now_secs(),
+            updated_at: store::now_secs(),
+        }
+    };
+    if let Err(e) = store::save(&snapshot) {
+        tracing::warn!(
+            session_id = %req.session_id.0,
+            error = %format!("{e:#}"),
+            "session persist after set_model failed; on-disk model id stays stale"
+        );
+    }
+    tracing::info!(
+        session_id = %req.session_id.0,
+        model_id = %target,
+        "session model changed"
+    );
+    Ok(())
+}
+
 /// The three modes every Stage 3 session advertises:
 ///
 /// - **Default** — writes / bash prompt the user.
@@ -1375,6 +1522,55 @@ mod tests {
         }
     }
 
+    /// Provider stub whose `list_models` returns canned results.
+    /// Used by the `aggregate_models` tests.
+    struct ModelProvider {
+        name: &'static str,
+        models: anyhow::Result<Vec<crate::provider::ModelInfo>>,
+    }
+
+    impl ModelProvider {
+        fn ok(name: &'static str, ids: &[&str]) -> Arc<dyn Provider> {
+            let models = ids
+                .iter()
+                .map(|id| crate::provider::ModelInfo {
+                    id: (*id).to_string(),
+                    display_name: None,
+                })
+                .collect();
+            Arc::new(Self {
+                name,
+                models: Ok(models),
+            })
+        }
+        fn err(name: &'static str, msg: &'static str) -> Arc<dyn Provider> {
+            Arc::new(Self {
+                name,
+                models: Err(anyhow::anyhow!(msg)),
+            })
+        }
+    }
+
+    #[async_trait]
+    impl Provider for ModelProvider {
+        fn name(&self) -> &str {
+            self.name
+        }
+        async fn list_models(&self) -> anyhow::Result<Vec<crate::provider::ModelInfo>> {
+            match &self.models {
+                Ok(v) => Ok(v.clone()),
+                Err(e) => Err(anyhow::anyhow!("{e:#}")),
+            }
+        }
+        async fn complete(
+            &self,
+            _request: CompletionRequest,
+            _cancel: CancellationToken,
+        ) -> anyhow::Result<BoxStream<'static, anyhow::Result<CompletionEvent>>> {
+            unimplemented!()
+        }
+    }
+
     fn providers() -> Vec<Arc<dyn Provider>> {
         vec![
             Arc::new(StubProvider("helexa")),
@@ -1431,6 +1627,47 @@ mod tests {
         assert_eq!(prompt_budget(1_000, Some(8_192)), 0);
     }
 
+    // ── aggregate_models ────────────────────────────────────────────
+
+    #[tokio::test]
+    async fn aggregate_models_single_endpoint_has_bare_ids() {
+        let providers = vec![ModelProvider::ok(
+            "helexa",
+            &["helexa/large", "helexa/small"],
+        )];
+        let models = aggregate_models(&providers).await;
+        let ids: Vec<&str> = models.iter().map(|m| m.model_id.0.as_ref()).collect();
+        assert_eq!(ids, vec!["helexa/large", "helexa/small"]);
+    }
+
+    #[tokio::test]
+    async fn aggregate_models_multi_endpoint_prefixes_every_id() {
+        let providers = vec![
+            ModelProvider::ok("helexa", &["helexa/large"]),
+            ModelProvider::ok("openrouter", &["anthropic/claude-opus-4"]),
+        ];
+        let models = aggregate_models(&providers).await;
+        let ids: Vec<&str> = models.iter().map(|m| m.model_id.0.as_ref()).collect();
+        assert_eq!(
+            ids,
+            vec!["helexa:helexa/large", "openrouter:anthropic/claude-opus-4"]
+        );
+    }
+
+    #[tokio::test]
+    async fn aggregate_models_skips_failing_endpoint() {
+        let providers = vec![
+            ModelProvider::err("flaky", "boom"),
+            ModelProvider::ok("openrouter", &["gpt-9"]),
+        ];
+        let models = aggregate_models(&providers).await;
+        let ids: Vec<&str> = models.iter().map(|m| m.model_id.0.as_ref()).collect();
+        // Multi-endpoint case → prefix survives even when one
+        // endpoint dropped out. flaky's models are absent, not
+        // null-filled.
+        assert_eq!(ids, vec!["openrouter:gpt-9"]);
+    }
+
     #[test]
     fn maps_known_finish_reasons() {
         assert!(matches!(

crates/helexa-acp/src/main.rs

@@ -142,6 +142,7 @@ async fn main() -> Result<()> {
     }
 
     let agent = Agent::new(&cfg, providers)
+        .await
         .map_err(|e| agent_client_protocol::util::internal_error(format!("agent: {e:#}")))?;
     agent.serve(Stdio::new()).await
 }