fix: harvest error-message fixes from PR #2933 — better tool denial + subagent conflict messages (#3020)

Three targeted error-message improvements extracted from community
PR #2933 (author cy2311), with additional model-not-found annotation:

1. dispatch.rs format_tool_error: pass through self-explanatory messages
   that already name the cause (mode switch, allow_shell, feature flag,
   denied by user) instead of appending a conflicting generic suffix.
   Fixes the Plan-mode double-message (#2657).

2. subagent/mod.rs session-name conflict: include elapsed time
   (started Ns ago / NmNs ago) so the parent can distinguish a live
   worker from a stale/failed earlier spawn (#2656).

3. subagent/mod.rs annotate_child_model_error: catch model-not-found
   patterns (Model Not Exist, does not exist, no such model, etc.) in
   the raw error text even when the taxonomy classifies them as
   Internal rather than Authorization/State (#2653).

Closes #2653, #2656, #2657.
Credit: cy2311 for the dispatch.rs and subagent conflict hunks from #2933.

Co-authored-by: cy2311 <29836092+cy2311@users.noreply.github.com>

crates/tui/src/core/engine/dispatch.rs

@@ -117,7 +117,16 @@ pub(super) fn format_tool_error(err: &ToolError, tool_name: &str) -> String {
         ),
         ToolError::NotAvailable { message } => {
             let lower = message.to_ascii_lowercase();
-            if lower.contains("current tool catalog") || lower.contains("did you mean:") {
+            // #3020: Pass through self-explanatory messages that already name the
+            // cause (mode switch, allow_shell, feature flag).  Avoids appending a
+            // conflicting "Check mode, feature flags" suffix on top of
+            // "switch to Agent, Goal, or YOLO mode" which confuses the model.
+            if lower.contains("current tool catalog")
+                || lower.contains("did you mean:")
+                || lower.contains("mode")
+                || lower.contains("allow_shell")
+                || lower.contains("feature flag")
+            {
                 message.clone()
             } else {
                 format!(
@@ -125,9 +134,20 @@ pub(super) fn format_tool_error(err: &ToolError, tool_name: &str) -> String {
                 )
             }
         }
-        ToolError::PermissionDenied { message } => format!(
-            "Tool '{tool_name}' was denied: {message}. Adjust approval mode or request permission."
-        ),
+        ToolError::PermissionDenied { message } => {
+            let lower = message.to_ascii_lowercase();
+            // #3020: Pass through messages that already name the denial cause.
+            if lower.contains("mode")
+                || lower.contains("allow_shell")
+                || lower.contains("denied by user")
+            {
+                message.clone()
+            } else {
+                format!(
+                    "Tool '{tool_name}' was denied: {message}. Adjust approval mode or request permission."
+                )
+            }
+        }
     }
 }
 

crates/tui/src/tools/subagent/mod.rs

@@ -1496,8 +1496,19 @@ impl SubAgentManager {
                 .values()
                 .find(|existing| existing.session_name == name)
             {
+                // #3020: Include elapsed time so the parent can distinguish a
+                // live worker from a stale/failed earlier spawn (#2656).
+                let elapsed = existing.started_at.elapsed();
+                let since = if elapsed.as_secs() < 120 {
+                    format!("{}s ago", elapsed.as_secs())
+                } else {
+                    let mins = elapsed.as_secs() / 60;
+                    let secs = elapsed.as_secs() % 60;
+                    format!("{mins}m{secs}s ago")
+                };
                 return Err(anyhow!(
-                    "Sub-agent session name '{name}' is already in use by agent_id '{}' (status: {}). \
+                    "Sub-agent session name '{name}' is already in use by agent_id '{}' \
+                     (status: {}, started {since}). \
                      Reuse that agent_id with agent_eval/agent_close, or open with a different name.",
                     existing.id,
                     subagent_status_name(&existing.status)
@@ -5619,7 +5630,26 @@ fn annotate_child_model_error(err: &str, model: &str) -> String {
             "{err}\n(child model `{model}` may be unavailable under the current access profile — \
              retry agent_open with a different `model`, or remove `model` to inherit the parent's)"
         ),
-        _ => err.to_string(),
+        _ => {
+            // #3020 (#2653): Provider rejections like "Model Not Exist" or
+            // "does not exist or you do not have access" often classify as
+            // `Internal` rather than `Authorization`/`State`.  Catch these
+            // patterns in the raw error text and annotate anyway.
+            let lower = err.to_ascii_lowercase();
+            if lower.contains("model not exist")
+                || lower.contains("model_not_found")
+                || lower.contains("does not exist")
+                || lower.contains("no such model")
+                || lower.contains("invalid model")
+            {
+                format!(
+                    "{err}\n(child model `{model}` may be unavailable under the current access profile — \
+                     retry agent_open with a different `model`, or remove `model` to inherit the parent's)"
+                )
+            } else {
+                err.to_string()
+            }
+        }
     }
 }