feat(anthropic): add structured output support (#802)

Enables JSON schema-based structured outputs for Anthropic models
using the structured-outputs-2025-11-13 beta feature.

- Remove blocking error and route to Beta API when configured
- Configure OutputFormat via BetaJSONSchemaOutputFormat helper

Closes #802

Signed-off-by: Sadique Azmi <sadiquemobaraka5@gmail.com>

Author: smazmi

pkg/model/provider/anthropic/beta_client.go

@@ -61,6 +61,17 @@ func (c *Client) createBetaStream(
 		params.System = sys
 	}
 
+	// Apply structured output configuration
+	if structuredOutput := c.ModelOptions.StructuredOutput(); structuredOutput != nil {
+		slog.Debug("Anthropic Beta API using structured output", "name", structuredOutput.Name)
+
+		// Add structured outputs beta header
+		params.Betas = append(params.Betas, "structured-outputs-2025-11-13")
+
+		// Configure output format using the SDK helper
+		params.OutputFormat = anthropic.BetaJSONSchemaOutputFormat(structuredOutput.Schema)
+	}
+
 	// For interleaved thinking to make sense, we use a default of 16384 tokens for the thinking budget
 	thinkingTokens := int64(16384)
 	if c.ModelConfig.ThinkingBudget != nil {

pkg/model/provider/anthropic/client.go

@@ -122,10 +122,6 @@ func NewClient(ctx context.Context, cfg *latest.ModelConfig, env environment.Pro
 
 	slog.Debug("Anthropic client created successfully", "model", cfg.Model)
 
-	if globalOptions.StructuredOutput() != nil {
-		return nil, errors.New("anthropic does not support native structured_output")
-	}
-
 	return &Client{
 		Config: base.Config{
 			ModelConfig:  *cfg,
@@ -158,8 +154,11 @@ func (c *Client) CreateChatCompletionStream(
 		return nil, err
 	}
 
-	// Use Beta API with interleaved thinking only when enabled
-	if c.interleavedThinkingEnabled() {
+	// Use Beta API when:
+	// 1. Interleaved thinking is enabled, or
+	// 2. Structured output is configured
+	// Note: Structured outputs require beta header support (only available on BetaMessageNewParams)
+	if c.interleavedThinkingEnabled() || c.ModelOptions.StructuredOutput() != nil {
 		return c.createBetaStream(ctx, client, messages, requestTools, maxTokens)
 	}