🤖 fix: enable visible reasoning for Opus 4.5 by adding thinking parameter

Opus 4.5 requires both parameters to show reasoning traces in the UI:
- thinking: { type: 'enabled', budgetTokens } - enables visible reasoning
- effort: controls how tokens are spent across all output

Previously we only passed effort, which controls token spend but doesn't
enable the reasoning traces to be returned from the API.

_Generated with mux_

Change-Id: I3aa6492d8028fd6ddebaac2123cedd07ae32d4f9
Signed-off-by: Thomas Kosiewski <tk@coder.com>

Author: ThomasK33

src/common/utils/ai/providerOptions.ts

@@ -93,17 +93,29 @@ export function buildProviderOptions(
     const isOpus45 = modelName?.includes("opus-4-5") ?? false;
 
     if (isOpus45) {
-      // Opus 4.5: Use effort parameter for reasoning control
+      // Opus 4.5: Use effort parameter AND thinking to enable visible reasoning
+      // effort controls HOW MUCH thinking happens (token spend)
+      // thinking enables the reasoning traces to be returned
       const effort = ANTHROPIC_EFFORT[effectiveThinking];
+      const budgetTokens = ANTHROPIC_THINKING_BUDGETS[effectiveThinking];
       log.debug("buildProviderOptions: Anthropic Opus 4.5 config", {
         effort,
+        budgetTokens,
         thinkingLevel: effectiveThinking,
       });
 
       const options: ProviderOptions = {
         anthropic: {
           disableParallelToolUse: false, // Always enable concurrent tool execution
           sendReasoning: true, // Include reasoning traces in requests sent to the model
+          // Enable thinking to get visible reasoning traces
+          // budgetTokens sets the ceiling; effort controls how eagerly tokens are spent
+          ...(budgetTokens > 0 && {
+            thinking: {
+              type: "enabled",
+              budgetTokens,
+            },
+          }),
           // Use effort parameter (Opus 4.5 only) to control token spend
           // SDK auto-adds beta header "effort-2025-11-24" when effort is set
           ...(effort && { effort }),