agent: bundle sampling fields as SamplingParams on AgentState

Collapse the split `temperature` / `top_p` / `top_k` fields on
AgentState into a single `sampling: SamplingParams` struct, mirroring
how the wire-level fields flow into the Generate RPC. Adds
`max_tokens` to SamplingParams so it's actually plumbed end to end
(previously the client had a hardcoded 4096 fallback inside
`run_session_generate`).

AgentState construction sites now set `sampling: SamplingParams { ...
max_tokens: 4096 }` as the default. The assignment sites in
oneshot.rs / subconscious.rs / unconscious.rs switch from
`st.temperature = X` to `st.sampling.temperature = X`.

`stream_session_mm` takes `SamplingParams` directly; the
`sampling_max_tokens()` helper goes away. `pb::GenerateRequest` is
populated with `sampling.max_tokens` (and the other fields) in
`run_session_generate`. SamplingParams is `pub` so it can be
embedded in the public AgentState without a visibility warning.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -38,6 +38,21 @@ pub struct ReadoutManifest {
 /// from pairing with the manifest fetched at startup.
 pub type TokenReadout = Vec<Vec<f32>>;
 
+/// Client-side sampling state. Mirrors the wire-level fields in
+/// `GenerateRequest` (proto flattened its `SamplingParams` submessage
+/// in so the server handler reads them directly), but stays as a
+/// grouped struct on the client because UI / config / tests pass
+/// these around together.
+#[derive(Clone, Copy)]
+pub struct SamplingParams {
+    pub temperature: f32,
+    pub top_p: f32,
+    pub top_k: u32,
+    /// Decode budget. 0 = prefill only; >0 = decode up to this many
+    /// tokens, stopping early on EOS / stop_token_ids.
+    pub max_tokens: u32,
+}
+
 /// A JoinHandle that aborts its task when dropped.
 pub(crate) struct AbortOnDrop(tokio::task::JoinHandle<()>);
 
@@ -47,13 +62,6 @@ impl Drop for AbortOnDrop {
     }
 }
 
-/// Sampling parameters for model generation.
-#[derive(Clone, Copy)]
-pub(crate) struct SamplingParams {
-    pub temperature: f32,
-    pub top_p: f32,
-    pub top_k: u32,
-}
 
 // ─────────────────────────────────────────────────────────────
 //  Stream events — yielded by backends, consumed by the runner
@@ -288,13 +296,12 @@ async fn run_session_generate(
     } else {
         Vec::new()
     };
-    let max_tokens = sampling_max_tokens(&sampling);
     let req = pb::GenerateRequest {
         session_id: handle.session_id.clone(),
         append_tokens: pending,
         offset: handle.committed_len,
         truncating: false,
-        max_tokens,
+        max_tokens: sampling.max_tokens,
         logprobs_ranges: Vec::new(),
         logprob_top_k: 0,
         readout_ranges,
@@ -422,10 +429,4 @@ async fn flush_pending(
     Ok(())
 }
 
-fn sampling_max_tokens(_sampling: &SamplingParams) -> u32 {
-    // SamplingParams doesn't carry max_tokens today; 4096 mirrors
-    // the old server-side default and is a sensible interactive cap.
-    // TODO: plumb from the caller if we need bigger budgets.
-    4096
-}
 

src/agent/mod.rs

@@ -177,9 +177,7 @@ pub struct AgentState {
     pub think_native: bool,
     /// Tool-based thinking — add a "think" tool for structured reasoning.
     pub think_tool: bool,
-    pub temperature: f32,
-    pub top_p: f32,
-    pub top_k: u32,
+    pub sampling: api::SamplingParams,
     pub activities: Vec<ActivityEntry>,
     next_activity_id: u64,
     pub pending_yield: bool,
@@ -241,9 +239,12 @@ impl Agent {
                 reasoning_effort: "none".to_string(),
                 think_native: true,
                 think_tool: false,
+                sampling: api::SamplingParams {
                     temperature: 0.6,
                     top_p: 0.95,
                     top_k: 20,
+                    max_tokens: 4096,
+                },
                 activities: Vec::new(),
                 next_activity_id: 0,
                 pending_yield: false,
@@ -312,9 +313,7 @@ impl Agent {
                 reasoning_effort: "none".to_string(),
                 think_native: st.think_native,
                 think_tool: st.think_tool,
-                temperature: st.temperature,
-                top_p: st.top_p,
-                top_k: st.top_k,
+                sampling: st.sampling,
                 activities: Vec::new(),
                 next_activity_id: 0,
                 pending_yield: false,
@@ -424,11 +423,7 @@ impl Agent {
                 agent.client.stream_session_mm(
                     agent.grpc_session.clone(),
                     chunks,
-                    api::SamplingParams {
-                        temperature: st.temperature,
-                        top_p: st.top_p,
-                        top_k: st.top_k,
-                    },
+                    st.sampling,
                     st.priority,
                     readout_shape,
                 )

src/agent/oneshot.rs

@@ -269,7 +269,7 @@ impl AutoAgent {
             let mut st = agent.state.lock().await;
             st.provenance = format!("standalone:{}", self.name);
             st.tools = self.tools.clone();
-            st.temperature = self.temperature;
+            st.sampling.temperature = self.temperature;
             st.priority = Some(self.priority);
         }
 

src/mind/subconscious.rs

@@ -631,7 +631,7 @@ impl Subconscious {
             {
                 let mut st = forked.state.lock().await;
                 st.provenance = auto.name.clone();
-                st.temperature = auto.temperature;
+                st.sampling.temperature = auto.temperature;
                 // Surface agent gets near-interactive priority;
                 // other subconscious agents get lower priority.
                 st.priority = Some(if auto.name == "surface" { 1 } else { auto.priority });

src/mind/unconscious.rs

@@ -321,7 +321,7 @@ pub async fn prepare_spawn(
         let mut st = agent.state.lock().await;
         st.provenance = auto.name.clone();
         st.priority = Some(auto.priority);
-        st.temperature = auto.temperature;
+        st.sampling.temperature = auto.temperature;
     }
 
     let agent_clone = agent.clone();

src/subconscious/generate.rs

@@ -43,6 +43,7 @@ where F: FnMut(&AstNode) -> bool,
         temperature: 0.6,
         top_p: 0.95,
         top_k: 20,
+        max_tokens: 4096,
     };
 
     // Ephemeral per-call session — opens on first touch, drops when