fix(commands): restore /cost slash command and improve command execution errors

The /cost command was listed as built-in but had no handler, causing execution to
fall through to custom command logic and return 400 ("command path is required").

- Add a built-in /cost handler in server/routes/commands.js
- Return the expected payload shape for the chat UI (`action: "cost"`, token usage,
  estimated cost, model)
- Normalize token usage inputs and compute usage percentage
- Add provider-based default pricing for cost estimation
- Fix model selection in command execution context so codex uses `codexModel`
  instead of `claudeModel`
- Improve frontend command error handling by parsing backend error responses and
  showing meaningful error messages instead of a generic failure

server/routes/commands.js

@@ -209,6 +209,86 @@ Custom commands can be created in:
     };
   },
 
+  '/cost': async (args, context) => {
+    const tokenUsage = context?.tokenUsage || {};
+    const provider = context?.provider || 'claude';
+    const model =
+      context?.model ||
+      (provider === 'cursor'
+        ? CURSOR_MODELS.DEFAULT
+        : provider === 'codex'
+          ? CODEX_MODELS.DEFAULT
+          : CLAUDE_MODELS.DEFAULT);
+
+    const used = Number(tokenUsage.used ?? tokenUsage.totalUsed ?? tokenUsage.total_tokens ?? 0) || 0;
+    const total =
+      Number(
+        tokenUsage.total ??
+          tokenUsage.contextWindow ??
+          parseInt(process.env.CONTEXT_WINDOW || '160000', 10),
+      ) || 160000;
+    const percentage = total > 0 ? Number(((used / total) * 100).toFixed(1)) : 0;
+
+    const inputTokensRaw =
+      Number(
+        tokenUsage.inputTokens ??
+          tokenUsage.input ??
+          tokenUsage.cumulativeInputTokens ??
+          tokenUsage.promptTokens ??
+          0,
+      ) || 0;
+    const outputTokens =
+      Number(
+        tokenUsage.outputTokens ??
+          tokenUsage.output ??
+          tokenUsage.cumulativeOutputTokens ??
+          tokenUsage.completionTokens ??
+          0,
+      ) || 0;
+    const cacheTokens =
+      Number(
+        tokenUsage.cacheReadTokens ??
+          tokenUsage.cacheCreationTokens ??
+          tokenUsage.cacheTokens ??
+          tokenUsage.cachedTokens ??
+          0,
+      ) || 0;
+
+    // If we only have total used tokens, treat them as input for display/estimation.
+    const inputTokens =
+      inputTokensRaw > 0 || outputTokens > 0 || cacheTokens > 0 ? inputTokensRaw + cacheTokens : used;
+
+    // Rough default rates by provider (USD / 1M tokens).
+    const pricingByProvider = {
+      claude: { input: 3, output: 15 },
+      cursor: { input: 3, output: 15 },
+      codex: { input: 1.5, output: 6 },
+    };
+    const rates = pricingByProvider[provider] || pricingByProvider.claude;
+
+    const inputCost = (inputTokens / 1_000_000) * rates.input;
+    const outputCost = (outputTokens / 1_000_000) * rates.output;
+    const totalCost = inputCost + outputCost;
+
+    return {
+      type: 'builtin',
+      action: 'cost',
+      data: {
+        tokenUsage: {
+          used,
+          total,
+          percentage,
+        },
+        cost: {
+          input: inputCost.toFixed(4),
+          output: outputCost.toFixed(4),
+          total: totalCost.toFixed(4),
+        },
+        model,
+      },
+    };
+  },
+
   '/status': async (args, context) => {
     // Read version from package.json
     const packageJsonPath = path.join(path.dirname(__dirname), '..', 'package.json');

src/hooks/chat/useChatComposerState.ts

@@ -280,7 +280,7 @@ export function useChatComposerState({
           projectName: selectedProject.name,
           sessionId: currentSessionId,
           provider,
-          model: provider === 'cursor' ? cursorModel : claudeModel,
+          model: provider === 'cursor' ? cursorModel : provider === 'codex' ? codexModel : claudeModel,
           tokenUsage: tokenBudget,
         };
 
@@ -298,7 +298,14 @@ export function useChatComposerState({
         });
 
         if (!response.ok) {
-          throw new Error('Failed to execute command');
+          let errorMessage = `Failed to execute command (${response.status})`;
+          try {
+            const errorData = await response.json();
+            errorMessage = errorData?.message || errorData?.error || errorMessage;
+          } catch {
+            // Ignore JSON parse failures and use fallback message.
+          }
+          throw new Error(errorMessage);
         }
 
         const result = (await response.json()) as CommandExecutionResult;
@@ -324,6 +331,7 @@ export function useChatComposerState({
     },
     [
       claudeModel,
+      codexModel,
       currentSessionId,
       cursorModel,
       handleBuiltInCommand,