feat(desktop): support MCP sidecars for custom harnesses

desktop/src-tauri/src/commands/agent_discovery.rs

@@ -173,7 +173,7 @@ pub async fn save_custom_harness(
         command: command_opt,
         binary_path,
         default_args,
-        mcp_command: None,
+        mcp_command: definition.mcp_command.clone(),
         model_env_var: None,
         provider_env_var: None,
         thinking_env_var: None,

desktop/src-tauri/src/managed_agents/custom_harnesses.rs

@@ -56,6 +56,9 @@ pub(crate) struct HarnessDefinition {
     /// Default CLI arguments passed to the command (array, not split-string).
     #[serde(default)]
     pub args: Vec<String>,
+    /// Optional MCP sidecar passed to `buzz-acp` for this harness.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub mcp_command: Option<String>,
     /// Environment variables injected at spawn time. Definition env is applied
     /// first and LOSES on conflict with Buzz-injected vars — `BUZZ_MANAGED_AGENT`
     /// is always authoritative and cannot be overridden here.
@@ -173,6 +176,9 @@ fn validate_harness_definition(def: &HarnessDefinition) -> Result<(), String> {
     if def.label.trim().is_empty() {
         return Err("label must not be empty".into());
     }
+    if matches!(def.mcp_command.as_deref(), Some(command) if command.trim().is_empty()) {
+        return Err("mcpCommand must not be blank when provided".into());
+    }
     // Args travel to the harness through the comma-delimited
     // `BUZZ_ACP_AGENT_ARGS` env transport (clap `value_delimiter = ','` on the
     // buzz-acp side), so a literal comma inside one argument would silently
@@ -733,18 +739,13 @@ mod tests {
         assert_eq!(loaded[0].id, "custom-dup");
     }
 
-    // ── Round-trip via save_custom_harness_to_dir (B-4) ─────────────────────
-    //
-    // These tests exercise the REAL persistence helper, not raw fs::write.
-    // They prove: create, same-ID edit (backup-swap), rename (old file removed),
-    // backup file cleaned up on success.
-
     fn make_def(id: &str, label: &str) -> HarnessDefinition {
         HarnessDefinition {
             id: id.to_string(),
             label: label.to_string(),
             command: format!("{id}-bin"),
             args: vec![],
+            mcp_command: None,
             env: BTreeMap::new(),
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -773,11 +774,9 @@ mod tests {
         let v1 = make_def("my-harness", "V1 Label");
         save_custom_harness_to_dir(dir.path(), &v1, None).unwrap();
 
-        // Same-ID edit: label changes.
         let v2 = make_def("my-harness", "V2 Label");
         let outcome = save_custom_harness_to_dir(dir.path(), &v2, None).unwrap();
 
-        // No old-path reported (id unchanged).
         assert!(outcome.removed_old_path.is_none());
 
         let loaded = load_custom_harnesses(dir.path());
@@ -794,7 +793,6 @@ mod tests {
         let v2 = make_def("my-harness", "V2");
         save_custom_harness_to_dir(dir.path(), &v2, None).unwrap();
 
-        // .bak file must be gone after a successful commit.
         let bak = dir.path().join("my-harness.json.bak");
         assert!(
             !bak.exists(),
@@ -808,19 +806,16 @@ mod tests {
         let old_def = make_def("old-id", "Old");
         save_custom_harness_to_dir(dir.path(), &old_def, None).unwrap();
 
-        // Rename: new id, old_id supplied.
         let new_def = make_def("new-id", "New");
         let outcome = save_custom_harness_to_dir(dir.path(), &new_def, Some("old-id")).unwrap();
 
-        // The outcome carries the old path that was removed.
         let expected_old = dir.path().join("old-id.json");
         assert_eq!(
             outcome.removed_old_path,
             Some(expected_old.clone()),
             "removed_old_path must be the old file"
         );
 
-        // Old file gone, new file present.
         assert!(!expected_old.exists(), "old-id.json must be removed");
         let loaded = load_custom_harnesses(dir.path());
         assert_eq!(loaded.len(), 1);
@@ -829,13 +824,11 @@ mod tests {
 
     #[test]
     fn save_to_dir_rename_nonexistent_old_id_is_non_fatal() {
-        // rename_old_id pointing to a file that does not exist must succeed
-        // (NotFound is silently ignored by the helper).
+        // NotFound for rename_old_id is ignored.
         let dir = tempfile::tempdir().unwrap();
         let def = make_def("alpha", "Alpha");
         let outcome = save_custom_harness_to_dir(dir.path(), &def, Some("ghost-id")).unwrap();
 
-        // New file created, no old path removed.
         assert_eq!(outcome.target_path, dir.path().join("alpha.json"));
         assert!(
             outcome.removed_old_path.is_none(),
@@ -854,6 +847,7 @@ mod tests {
             label: "Env Harness".to_string(),
             command: "env-bin".to_string(),
             args: vec!["--flag".to_string()],
+            mcp_command: Some("mcp-sidecar".to_string()),
             env,
             install_instructions_url: "https://example.com".to_string(),
             install_hint: "Install from example.com".to_string(),
@@ -864,15 +858,14 @@ mod tests {
         let loaded = load_custom_harnesses(dir.path());
         assert_eq!(loaded.len(), 1);
         assert_eq!(loaded[0].args, vec!["--flag"]);
+        assert_eq!(loaded[0].mcp_command.as_deref(), Some("mcp-sidecar"));
         assert_eq!(
             loaded[0].env.get("MY_KEY").map(String::as_str),
             Some("my_value"),
             "env must round-trip through save_custom_harness_to_dir"
         );
     }
 
-    // ── B-3: env validation boundary (validate_harness_definition_pub integration) ──
-
     #[test]
     fn validate_rejects_malformed_key_with_equals_sign() {
         // BUZZ_AUTH_TAG=x is the documented reserved-key bypass shape:
@@ -885,6 +878,7 @@ mod tests {
             label: "Bad".to_string(),
             command: "bad-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -914,6 +908,7 @@ mod tests {
             label: "Bad".to_string(),
             command: "bad-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -935,6 +930,7 @@ mod tests {
             label: "CI".to_string(),
             command: "ci-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -956,6 +952,7 @@ mod tests {
             label: "NUL".to_string(),
             command: "nul-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -978,6 +975,7 @@ mod tests {
             label: "Big".to_string(),
             command: "big-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),
@@ -999,6 +997,7 @@ mod tests {
             label: "Good".to_string(),
             command: "good-bin".to_string(),
             args: vec![],
+            mcp_command: None,
             env,
             install_instructions_url: String::new(),
             install_hint: String::new(),

desktop/src-tauri/src/managed_agents/discovery.rs

@@ -1565,9 +1565,7 @@ pub fn discover_acp_runtimes_from(
                 command,
                 binary_path,
                 default_args,
-                // Custom harnesses are plain ACP — no MCP sidecar, no env-var
-                // model switching, no thinking knobs.
-                mcp_command: None,
+                mcp_command: def.mcp_command.clone(),
                 model_env_var: None,
                 provider_env_var: None,
                 thinking_env_var: None,

desktop/src-tauri/src/managed_agents/discovery/presets.rs

@@ -184,6 +184,7 @@ pub(crate) fn preset_harness_definitions(
                 label: preset.label.to_string(),
                 command: preset.command.to_string(),
                 args: preset.args.iter().map(|arg| arg.to_string()).collect(),
+                mcp_command: None,
                 env: Default::default(),
                 install_instructions_url: preset.install_instructions_url.to_string(),
                 install_hint: preset.install_hint.to_string(),

desktop/src-tauri/src/managed_agents/discovery/tests.rs

@@ -1622,6 +1622,7 @@ fn deleted_harness_summary_display_and_spawn_sentence_agree() {
         label: "Doomed".to_string(),
         command: "doomed-bin".to_string(),
         args: vec![],
+        mcp_command: None,
         env: Default::default(),
         install_instructions_url: String::new(),
         install_hint: String::new(),
@@ -1652,34 +1653,25 @@ fn deleted_harness_summary_display_and_spawn_sentence_agree() {
 
 // ── I2: custom catalog entry carries definition_env for the edit round-trip ───
 
-/// A custom harness definition that includes env vars must surface those vars
-/// in the `definition_env` field of the resulting `AcpRuntimeCatalogEntry`.
-///
-/// This proves the edit-form round-trip: the backend carries env into the
-/// catalog, the frontend reads it back when opening the edit form, and Save
-/// therefore preserves existing env vars rather than silently erasing them.
+/// Custom definition fields must surface in the catalog for edit round-trips.
 #[test]
-fn custom_catalog_entry_carries_definition_env_for_edit_roundtrip() {
+fn custom_catalog_entry_carries_definition_fields_for_edit_roundtrip() {
     use crate::managed_agents::custom_harnesses::registry_test_lock;
     use crate::managed_agents::discovery::discover_acp_runtimes_from;
     use std::{collections::BTreeMap, fs};
     use tempfile::tempdir;
 
-    // Discovery's auth probes read/warm the process-global PATH and
-    // login-shell-PATH caches, and its final step publishes to the global
-    // harness registry — hold both guards so parallel tests (e.g. the
-    // PATH-swapping resolution tests) can't observe or absorb torn state.
-    // Lock order for tests that need both: path lock first, then registry.
+    // Discovery probes PATH and publishes the global harness registry.
     let _path_guard = crate::managed_agents::lock_path_mutex();
     let _lock = registry_test_lock();
     let dir = tempdir().unwrap();
-    // Write a custom definition with two env vars.
     fs::write(
         dir.path().join("env-harness.json"),
         r#"{
             "id": "env-harness",
             "label": "Env Harness",
             "command": "env-harness-bin",
+            "mcpCommand": "read-only-mcp",
             "args": [],
             "env": { "CURSOR_ACP": "1", "MY_TOKEN": "abc" }
         }"#,
@@ -1703,6 +1695,11 @@ fn custom_catalog_entry_carries_definition_env_for_edit_roundtrip() {
         entry.definition_env, expected,
         "catalog entry must carry definition env vars so the edit form can read them back"
     );
+    assert_eq!(
+        entry.mcp_command.as_deref(),
+        Some("read-only-mcp"),
+        "catalog entry must carry the MCP command so the edit form preserves it"
+    );
 }
 
 /// A builtin catalog entry must have an empty `definition_env` — their env
@@ -1766,6 +1763,7 @@ fn harness_def(
         label: label.to_string(),
         command: command.to_string(),
         args: vec![],
+        mcp_command: None,
         env: Default::default(),
         install_instructions_url: String::new(),
         install_hint: String::new(),

desktop/src-tauri/src/managed_agents/readiness.rs

@@ -78,18 +78,7 @@ pub(crate) struct EffectiveAgentEnv {
     pub effective_command: String,
 }
 
-// ── Typed effective-harness descriptor ───────────────────────────────────────
-//
-// A single owned type that fully describes what a spawn would run.  Produced
-// by `resolve_effective_harness_descriptor` and consumed by spawn_agent_child,
-// spawn_config_hash, build_managed_agent_summary, get_agent_models, and
-// agent_readiness — so the harness-definition lookup and arg/env resolution
-// happen exactly once, in one place.
-
-/// The complete effective description of a harness spawn: resolved command,
-/// args, and layered env.  This is the single source of truth for what will
-/// actually run — computed once and shared across every consumer that needs
-/// the effective values.
+/// Complete effective description of a harness spawn.
 #[derive(Debug, Clone)]
 pub(crate) struct EffectiveHarnessDescriptor {
     /// The raw effective command string (e.g. `"buzz-agent"`, `"my-acp-agent"`).
@@ -98,15 +87,17 @@ pub(crate) struct EffectiveHarnessDescriptor {
     /// Normalized effective args.  Instance args win when non-empty; otherwise
     /// the harness definition's args apply.
     pub args: Vec<String>,
+    /// MCP sidecar from the custom definition or compiled runtime metadata.
+    pub mcp_command: Option<String>,
     /// The full layered process env: baked floor → runtime metadata → definition
     /// env → global → persona → agent.
     pub env: BTreeMap<String, String>,
 }
 
 /// Resolve the complete harness descriptor from a record + context — the single
-/// authoritative path for command, args, and env.
+/// authoritative path for command, args, MCP sidecar, and env.
 ///
-/// This is the only place where harness-definition lookup and arg/env layering
+/// This is the only place where harness lookup and capability/env layering
 /// happen; spawn, hash, summary, and both model-probe paths all consume this.
 ///
 /// Returns `Err("DANGLING_HARNESS_ID:<id>")` when the record (or its linked
@@ -161,14 +152,23 @@ pub(crate) fn resolve_effective_harness_descriptor(
         }
     };
 
-    // Env: full layered resolution (same as resolve_effective_agent_env).
-    // Pass harness_def directly to avoid a second lookup.
+    // Custom definition wins; built-ins fall back to compiled metadata.
+    let mcp_command = harness_def
+        .as_ref()
+        .and_then(|definition| definition.mcp_command.clone())
+        .or_else(|| {
+            runtime_meta
+                .and_then(|runtime| runtime.mcp_command)
+                .map(str::to_string)
+        });
+
     let effective_env =
         resolve_effective_agent_env_with_def(record, personas, runtime_meta, global, harness_def);
 
     Ok(EffectiveHarnessDescriptor {
         command: effective_command,
         args,
+        mcp_command,
         env: effective_env.env,
     })
 }

desktop/src-tauri/src/managed_agents/runtime.rs

@@ -289,13 +289,11 @@ pub fn build_managed_agent_summary(
         crate::managed_agents::readiness::EffectiveHarnessDescriptor {
             command: cmd,
             args,
+            mcp_command: None,
             env: Default::default(),
         }
     });
-    let effective_mcp_command = known_acp_runtime(&descriptor.command)
-        .and_then(|r| r.mcp_command)
-        .unwrap_or("")
-        .to_string();
+    let effective_mcp_command = descriptor.mcp_command.clone().unwrap_or_default();
 
     Ok(ManagedAgentSummary {
         pubkey: record.pubkey.clone(),
@@ -464,9 +462,9 @@ pub fn spawn_agent_child(
     let runtime_key = ManagedAgentRuntimeKey::new(record.pubkey.clone(), relay_url)?;
     // Resolve the effective harness (agent command) from the linked persona, so
     // persona harness edits propagate on the next spawn; an explicit per-agent
-    // override wins. `agent_args` and `mcp_command` are pure derivations of the
-    // command, so we recompute them from the effective value rather than the
-    // frozen record snapshot. Mirrors the model resolution below.
+    // override wins. `agent_args` and `mcp_command` are resolved from the
+    // effective harness descriptor rather than the frozen record snapshot.
+    // Mirrors the model resolution below.
     let personas = super::load_personas(app).unwrap_or_default();
     let teams = super::load_teams(app).unwrap_or_default();
     // Load global config once; used for runtime_metadata_env_vars (model/provider fallback)
@@ -524,9 +522,7 @@ pub fn spawn_agent_child(
         .map_err(|error| format!("failed to clone log handle: {error}"))?;
     let resolved_acp_command = resolve_command(&record.acp_command)
         .ok_or_else(|| missing_command_message(&record.acp_command, "ACP harness command"))?;
-    let effective_mcp_command = known_acp_runtime(effective_command)
-        .and_then(|r| r.mcp_command)
-        .unwrap_or("");
+    let effective_mcp_command = descriptor.mcp_command.as_deref().unwrap_or("");
     let resolved_mcp_command: Option<std::path::PathBuf> = if effective_mcp_command.is_empty() {
         None
     } else {