feat(buzz-acp): route goose systemPrompt via ACP+ _meta extension

For goose agents, systemPrompt must be passed via _meta["systemPrompt"]
rather than as a first-class session/new field, per the ACP extensibility
convention (ACP+ / _meta approach). Buzz-agent native agents continue to
use the top-level systemPrompt field unchanged.

Add session_new_full_goose to AcpClient, which puts systemPrompt inside
_meta instead of at the top level. Add agent_command to PromptContext so
create_session_and_apply_model can detect goose (via
normalize_agent_command_identity) and dispatch to the correct variant.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

crates/buzz-acp/src/acp.rs

@@ -321,6 +321,34 @@ impl AcpClient {
         })
     }
 
+    /// Send `session/new` for a goose agent, passing `systemPrompt` via `_meta`
+    /// per the ACP+ extensibility convention (goose reads it from `_meta["systemPrompt"]`
+    /// rather than as a first-class field).
+    pub async fn session_new_full_goose(
+        &mut self,
+        cwd: &str,
+        mcp_servers: Vec<McpServer>,
+        system_prompt: Option<&str>,
+    ) -> Result<SessionNewResponse, AcpError> {
+        let mut params = serde_json::json!({
+            "cwd": cwd,
+            "mcpServers": mcp_servers,
+        });
+        if let Some(sp) = system_prompt {
+            params["_meta"] = serde_json::json!({ "systemPrompt": sp });
+        }
+        let result = self.send_request("session/new", params).await?;
+        let session_id = result["sessionId"]
+            .as_str()
+            .ok_or_else(|| AcpError::Protocol("session/new response missing sessionId".into()))?
+            .to_owned();
+        tracing::info!(target: "acp::session", "session created: {session_id}");
+        Ok(SessionNewResponse {
+            session_id,
+            raw: result,
+        })
+    }
+
     /// Send `session/new` and return only the `sessionId` string.
     ///
     /// Convenience wrapper around [`session_new_full`].
@@ -2068,4 +2096,67 @@ mod tests {
             "systemPrompt should NOT be in params when value is None"
         );
     }
+
+    #[tokio::test]
+    async fn session_new_full_goose_includes_system_prompt_in_meta_when_some() {
+        // Goose path: systemPrompt must appear in _meta, NOT as a top-level field.
+        let script = r#"
+            read -t 2 _init
+            echo '{"jsonrpc":"2.0","id":0,"result":{"protocolVersion":1,"agentCapabilities":{}}}'
+            read -t 2 REQ
+            echo '{"jsonrpc":"2.0","id":1,"result":{"sessionId":"ses_goose","_receivedRequest":'"$REQ"'}}'
+            sleep 1
+        "#;
+        let mut client = spawn_script(script).await;
+        client
+            .initialize()
+            .await
+            .expect("initialize should succeed");
+
+        let resp = client
+            .session_new_full_goose("/tmp", vec![], Some("Be concise."))
+            .await
+            .expect("session_new_full_goose should succeed");
+
+        assert_eq!(resp.session_id, "ses_goose");
+        let received = &resp.raw["_receivedRequest"];
+        assert_eq!(
+            received["params"]["_meta"]["systemPrompt"].as_str(),
+            Some("Be concise."),
+            "systemPrompt should be in _meta when Some"
+        );
+        assert!(
+            received["params"]["systemPrompt"].is_null(),
+            "systemPrompt should NOT appear as a top-level field"
+        );
+    }
+
+    #[tokio::test]
+    async fn session_new_full_goose_omits_meta_when_none() {
+        // Goose path: when system_prompt is None, _meta should not appear in params.
+        let script = r#"
+            read -t 2 _init
+            echo '{"jsonrpc":"2.0","id":0,"result":{"protocolVersion":1,"agentCapabilities":{}}}'
+            read -t 2 REQ
+            echo '{"jsonrpc":"2.0","id":1,"result":{"sessionId":"ses_goose","_receivedRequest":'"$REQ"'}}'
+            sleep 1
+        "#;
+        let mut client = spawn_script(script).await;
+        client
+            .initialize()
+            .await
+            .expect("initialize should succeed");
+
+        let resp = client
+            .session_new_full_goose("/tmp", vec![], None)
+            .await
+            .expect("session_new_full_goose should succeed");
+
+        assert_eq!(resp.session_id, "ses_goose");
+        let received = &resp.raw["_receivedRequest"];
+        assert!(
+            received["params"]["_meta"].is_null(),
+            "_meta should NOT be in params when system_prompt is None"
+        );
+    }
 }

crates/buzz-acp/src/config.rs

@@ -492,7 +492,7 @@ fn validate_allowlist(entries: &[String]) -> Result<HashSet<String>, ConfigError
     Ok(validated)
 }
 
-fn normalize_agent_command_identity(command: &str) -> String {
+pub(crate) fn normalize_agent_command_identity(command: &str) -> String {
     let normalized = command.trim().replace('\\', "/");
     let trimmed = normalized.trim_end_matches('/');
     let basename = trimmed

crates/buzz-acp/src/lib.rs

@@ -1277,6 +1277,7 @@ async fn tokio_main() -> Result<()> {
             .as_deref()
             .and_then(|hex| nostr::PublicKey::from_hex(hex).ok()),
         memory_enabled: config.memory_enabled,
+        agent_command: config.agent_command.clone(),
     });
 
     if !config.memory_enabled {

crates/buzz-acp/src/pool.rs

@@ -32,7 +32,7 @@ use crate::acp::{
     extract_model_config_options, extract_model_state, resolve_model_switch_method, AcpClient,
     AcpError, McpServer, ModelSwitchMethod, StopReason,
 };
-use crate::config::{DedupMode, PermissionMode};
+use crate::config::{normalize_agent_command_identity, DedupMode, PermissionMode};
 use crate::observer;
 use crate::queue::{
     ContextMessage, ConversationContext, FlushBatch, PromptChannelInfo, PromptProfile,
@@ -250,6 +250,9 @@ pub struct PromptContext {
     /// `[Agent Memory — core]` section. On by default; disabled via
     /// `--no-memory` / `BUZZ_ACP_NO_MEMORY`.
     pub memory_enabled: bool,
+    /// Agent command (binary name or path). Used to detect goose agents and
+    /// route `systemPrompt` via `_meta` rather than as a first-class field.
+    pub agent_command: String,
 }
 
 impl AgentPool {
@@ -438,14 +441,25 @@ async fn create_session_and_apply_model(
         None
     };
 
-    let resp = agent
-        .acp
-        .session_new_full(
-            &ctx.cwd,
-            ctx.mcp_servers.clone(),
-            combined_system_prompt.as_deref(),
-        )
-        .await?;
+    let resp = if normalize_agent_command_identity(&ctx.agent_command) == "goose" {
+        agent
+            .acp
+            .session_new_full_goose(
+                &ctx.cwd,
+                ctx.mcp_servers.clone(),
+                combined_system_prompt.as_deref(),
+            )
+            .await?
+    } else {
+        agent
+            .acp
+            .session_new_full(
+                &ctx.cwd,
+                ctx.mcp_servers.clone(),
+                combined_system_prompt.as_deref(),
+            )
+            .await?
+    };
 
     // Populate model capabilities on first session creation.
     if agent.model_capabilities.is_none() {