Fix Anthropic thinking budgets

Normalize Anthropic thinking budgets while building message parameters so budget_tokens stays below max_tokens with response headroom. When the configured max token budget is too small to support Anthropic's minimum thinking budget, omit thinking for that request instead of sending an invalid payload.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Sacha Al Himdani <SachaProbo@users.noreply.github.com>

pkg/llm/anthropic/provider.go

@@ -47,6 +47,11 @@ type (
 	}
 )
 
+const (
+	anthropicThinkingMinBudgetTokens    = 1024
+	anthropicThinkingOutputTokenReserve = 1024
+)
+
 func WithHTTPClient(c *http.Client) Option {
 	return func(cfg *config) { cfg.httpClient = c }
 }
@@ -163,8 +168,8 @@ func buildParams(req *llm.ChatCompletionRequest) (anthropic.MessageNewParams, er
 		params.ToolChoice = buildToolChoice(req.ToolChoice)
 	}
 
-	if req.Thinking != nil && req.Thinking.Enabled {
-		params.Thinking = anthropic.ThinkingConfigParamOfEnabled(int64(req.Thinking.BudgetTokens))
+	if budgetTokens, ok := resolveThinkingBudget(*req.MaxTokens, req.Thinking); ok {
+		params.Thinking = anthropic.ThinkingConfigParamOfEnabled(int64(budgetTokens))
 	}
 
 	if req.ResponseFormat != nil {
@@ -192,6 +197,28 @@ func buildParams(req *llm.ChatCompletionRequest) (anthropic.MessageNewParams, er
 	return params, nil
 }
 
+func resolveThinkingBudget(maxTokens int, thinking *llm.ThinkingConfig) (int, bool) {
+	if thinking == nil || !thinking.Enabled || thinking.BudgetTokens <= 0 {
+		return 0, false
+	}
+
+	if maxTokens <= anthropicThinkingMinBudgetTokens {
+		return 0, false
+	}
+
+	maxBudgetTokens := maxTokens - anthropicThinkingOutputTokenReserve
+	if maxBudgetTokens < anthropicThinkingMinBudgetTokens {
+		maxBudgetTokens = maxTokens - 1
+	}
+
+	budgetTokens := max(min(thinking.BudgetTokens, maxBudgetTokens), anthropicThinkingMinBudgetTokens)
+	if budgetTokens >= maxTokens {
+		return 0, false
+	}
+
+	return budgetTokens, true
+}
+
 func extractSystem(messages []llm.Message) (system []string, rest []llm.Message) {
 	for _, msg := range messages {
 		if msg.Role == llm.RoleSystem {