fix: strip leaked thinking tags from openai compat content

pkg/providers/openai_compat/provider.go

@@ -10,6 +10,7 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"regexp"
 	"strings"
 	"time"
 
@@ -40,6 +41,18 @@ type Option func(*Provider)
 
 const defaultRequestTimeout = 120 * time.Second
 
+var (
+	escapedTagReplacer = strings.NewReplacer(
+		`\u003c`, "<",
+		`\u003e`, ">",
+		`\u003C`, "<",
+		`\u003E`, ">",
+	)
+	reasoningTagPattern         = regexp.MustCompile(`(?is)<(?:think|thinking|thought|reasoning)\b[^>]*>.*?</(?:think|thinking|thought|reasoning)\s*>`)
+	trailingReasoningTagPattern = regexp.MustCompile(`(?is)<(?:think|thinking|thought|reasoning)\b[^>]*>.*$`)
+	finalTagPattern             = regexp.MustCompile(`(?is)</?final\b[^>]*>`)
+)
+
 func WithMaxTokensField(maxTokensField string) Option {
 	return func(p *Provider) {
 		p.maxTokensField = maxTokensField
@@ -351,7 +364,7 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 	}
 
 	return &LLMResponse{
-		Content:          choice.Message.Content,
+		Content:          sanitizeAssistantContent(choice.Message.Content),
 		ReasoningContent: choice.Message.ReasoningContent,
 		Reasoning:        choice.Message.Reasoning,
 		ReasoningDetails: choice.Message.ReasoningDetails,
@@ -361,6 +374,19 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 	}, nil
 }
 
+func sanitizeAssistantContent(content string) string {
+	if content == "" {
+		return ""
+	}
+
+	sanitized := escapedTagReplacer.Replace(content)
+	sanitized = reasoningTagPattern.ReplaceAllString(sanitized, "")
+	sanitized = trailingReasoningTagPattern.ReplaceAllString(sanitized, "")
+	sanitized = finalTagPattern.ReplaceAllString(sanitized, "")
+
+	return strings.TrimSpace(sanitized)
+}
+
 // openaiMessage is the wire-format message for OpenAI-compatible APIs.
 // It mirrors protocoltypes.Message but omits SystemParts, which is an
 // internal field that would be unknown to third-party endpoints.

pkg/providers/openai_compat/provider_test.go

@@ -152,6 +152,63 @@ func TestProviderChat_ParsesReasoningContent(t *testing.T) {
 	}
 }
 
+func TestParseResponse_StripsThinkingAndFinalTags(t *testing.T) {
+	body := strings.NewReader(`{
+		"choices": [{
+			"message": {
+				"content": "<think>internal reasoning</think><final>The answer is 2</final>"
+			},
+			"finish_reason": "stop"
+		}]
+	}`)
+
+	out, err := parseResponse(body)
+	if err != nil {
+		t.Fatalf("parseResponse() error = %v", err)
+	}
+	if out.Content != "The answer is 2" {
+		t.Fatalf("Content = %q, want %q", out.Content, "The answer is 2")
+	}
+}
+
+func TestParseResponse_StripsEscapedThinkingAndFinalTags(t *testing.T) {
+	body := strings.NewReader(`{
+		"choices": [{
+			"message": {
+				"content": "\\u003cthink\\u003einternal reasoning\\u003c/think\\u003e\\u003cfinal\\u003eThe answer is 2\\u003c/final\\u003e"
+			},
+			"finish_reason": "stop"
+		}]
+	}`)
+
+	out, err := parseResponse(body)
+	if err != nil {
+		t.Fatalf("parseResponse() error = %v", err)
+	}
+	if out.Content != "The answer is 2" {
+		t.Fatalf("Content = %q, want %q", out.Content, "The answer is 2")
+	}
+}
+
+func TestParseResponse_DropsTrailingUnclosedThinkingBlock(t *testing.T) {
+	body := strings.NewReader(`{
+		"choices": [{
+			"message": {
+				"content": "<final>The answer is 2</final><think>internal reasoning"
+			},
+			"finish_reason": "stop"
+		}]
+	}`)
+
+	out, err := parseResponse(body)
+	if err != nil {
+		t.Fatalf("parseResponse() error = %v", err)
+	}
+	if out.Content != "The answer is 2" {
+		t.Fatalf("Content = %q, want %q", out.Content, "The answer is 2")
+	}
+}
+
 func TestProviderChat_PreservesReasoningContentInHistory(t *testing.T) {
 	var requestBody map[string]any