fix(desktop): clear stale instance agent args on harness switch

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

@@ -3,7 +3,7 @@
 //! of `discovery` (split under the desktop file-size cap) so the override
 //! decisions stay next to the resolution ladder they feed.
 
-use super::{effective_agent_command, known_acp_runtime};
+use super::{effective_agent_command, known_acp_runtime, record_agent_command};
 
 /// Decide whether a user-picked harness command is an explicit per-instance
 /// pin or merely the persona's own runtime restated. Returns the override to
@@ -90,12 +90,21 @@ pub fn update_time_agent_command_override(
 /// record the materialized runtime is the only harness source left after the
 /// override clear, so a stray empty `agent_command` from a non-dialog caller
 /// must not change what the agent runs.
+///
+/// When the edit changes the effective command, `record.agent_args` is
+/// cleared: instance args are harness-specific (e.g. Grok Build's
+/// `agent --always-approve stdio`), win over the harness definition's
+/// defaults at spawn, and are passed verbatim to the new binary — carrying
+/// them across a harness switch crash-loops the agent on flags the new
+/// binary does not understand. Callers that supply replacement args in the
+/// same update write them AFTER this apply, so explicit new args survive.
 pub fn apply_agent_command_update(
     record: &mut crate::managed_agents::types::ManagedAgentRecord,
     personas: &[crate::managed_agents::types::AgentDefinition],
     agent_command: &str,
     harness_override: bool,
 ) {
+    let previous_command = record_agent_command(record, personas);
     record.agent_command_override = update_time_agent_command_override(
         record.persona_id.as_deref(),
         personas,
@@ -105,6 +114,9 @@ pub fn apply_agent_command_update(
     if agent_command.trim().is_empty() && record.persona_id.is_some() {
         record.runtime = None;
     }
+    if record_agent_command(record, personas) != previous_command {
+        record.agent_args.clear();
+    }
 }
 
 /// Decide the `agent_command_override` to persist at AGENT CREATE time.

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

@@ -667,6 +667,55 @@ fn apply_agent_command_update_concrete_pin_keeps_materialized_runtime() {
     assert_eq!(record_agent_command(&record, &personas), "codex-acp");
 }
 
+#[test]
+fn apply_agent_command_update_clears_args_when_command_changes() {
+    // The Lens repro: a record carrying Grok Build's args switches to Oh My
+    // Pi. Instance args win over harness defaults at spawn, so keeping them
+    // would pass `agent --always-approve stdio` to `omp` and crash-loop.
+    let personas = vec![persona_with_runtime("p1", Some("grok"))];
+    let mut record = record_with(Some("grok"), Some("p1"), None);
+    record.agent_args = vec![
+        "agent".to_string(),
+        "--always-approve".to_string(),
+        "stdio".to_string(),
+    ];
+
+    apply_agent_command_update(&mut record, &personas, "omp", true);
+
+    assert_eq!(record_agent_command(&record, &personas), "omp");
+    assert!(
+        record.agent_args.is_empty(),
+        "harness-specific instance args must not survive a command change"
+    );
+}
+
+#[test]
+fn apply_agent_command_update_preserves_args_when_command_unchanged() {
+    // Re-saving the same harness (e.g. a name edit that echoes the command)
+    // must not wipe deliberately customized args.
+    let personas = vec![persona_with_runtime("p1", Some("claude"))];
+    let mut record = record_with(Some("claude"), Some("p1"), None);
+    record.agent_args = vec!["--verbose".to_string()];
+
+    apply_agent_command_update(&mut record, &personas, "claude-agent-acp", false);
+
+    assert_eq!(record.agent_args, vec!["--verbose".to_string()]);
+}
+
+#[test]
+fn apply_agent_command_update_inherit_sentinel_clears_args_on_fallback() {
+    // Choosing Inherit drops the pin; when the persona's command differs from
+    // the pinned one, the pin's args must not leak onto the persona harness.
+    let personas = vec![persona_with_runtime("p1", Some("goose"))];
+    let mut record = record_with(Some("claude"), Some("p1"), Some("codex-acp"));
+    record.agent_args = vec!["--codex-flag".to_string()];
+
+    apply_agent_command_update(&mut record, &personas, "", false);
+
+    assert_eq!(record_agent_command(&record, &personas), "goose");
+    assert!(record.agent_args.is_empty());
+}
+
 // ── probe_codex_acp_version ───────────────────────────────────────────────────
 
 mod managed_path_resolution;