fix(fantasy): replace panics with error returns and harden provider logic

- google: WithVertex validates project/location at New() instead of
  panicking; toGooglePrompt returns error for unsupported message roles
- anthropic/google: emit CallWarning for skipped system messages instead
  of silent drops
- openai responses: remove unused streamErr variable, simplify error/finish
  branching in streamObjectWithJSONMode — error events now return
  immediately without checking a stale flag
- openai/openrouter: remove resolved TODO comments, fix import ordering

internal/fantasy/providers/anthropic/anthropic.go

@@ -7,11 +7,12 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
-	jsonv2 "github.com/go-json-experiment/json"
 	"io"
 	"maps"
 	"strings"
 
+	jsonv2 "github.com/go-json-experiment/json"
+
 	"charm.land/fantasy"
 	"charm.land/fantasy/object"
 	"github.com/aws/aws-sdk-go-v2/config"
@@ -542,8 +543,10 @@ func toPrompt(prompt fantasy.Prompt, sendReasoningData bool) ([]anthropic.TextBl
 		switch block.Role {
 		case fantasy.MessageRoleSystem:
 			if finishedSystemBlock {
-				// skip multiple system messages that are separated by user/assistant messages
+				warnings = append(warnings, fantasy.CallWarning{
-				// TODO: see if we need to send error here?
+					Type:    fantasy.CallWarningTypeOther,
+					Message: "anthropic: additional system message after user/assistant messages was skipped",
+				})
 				continue
 			}
 			finishedSystemBlock = true

internal/fantasy/providers/google/google.go

@@ -5,12 +5,13 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	jsonv2 "github.com/go-json-experiment/json"
 	"maps"
 	"net/http"
 	"reflect"
 	"strings"
 
+	jsonv2 "github.com/go-json-experiment/json"
+
 	"charm.land/fantasy"
 	"charm.land/fantasy/object"
 	"charm.land/fantasy/providers/anthropic"
@@ -62,6 +63,10 @@ func New(opts ...Option) (fantasy.Provider, error) {
 
 	options.name = cmp.Or(options.name, Name)
 
+	if options.backend == genai.BackendVertexAI && (options.project == "" || options.location == "") {
+		return nil, errors.New("google: WithVertex requires non-empty project and location")
+	}
+
 	return &provider{
 		options: options,
 	}, nil
@@ -85,10 +90,8 @@ func WithGeminiAPIKey(apiKey string) Option {
 }
 
 // WithVertex configures the Google provider to use Vertex AI.
+// Both project and location are validated when New() is called.
 func WithVertex(project, location string) Option {
-	if project == "" || location == "" {
-		panic("project and location must be provided")
-	}
 	return func(o *options) {
 		o.backend = genai.BackendVertexAI
 		o.apiKey = ""
@@ -220,7 +223,10 @@ func (g languageModel) prepareParams(call fantasy.Call) (*genai.GenerateContentC
 		}
 	}
 
-	systemInstructions, content, warnings := toGooglePrompt(call.Prompt)
+	systemInstructions, content, warnings, err := toGooglePrompt(call.Prompt)
+	if err != nil {
+		return nil, nil, nil, err
+	}
 
 	if providerOptions.ThinkingConfig != nil {
 		if providerOptions.ThinkingConfig.IncludeThoughts != nil &&
@@ -324,7 +330,7 @@ func (g languageModel) prepareParams(call fantasy.Call) (*genai.GenerateContentC
 	return config, content, warnings, nil
 }
 
-func toGooglePrompt(prompt fantasy.Prompt) (*genai.Content, []*genai.Content, []fantasy.CallWarning) { //nolint: unparam
+func toGooglePrompt(prompt fantasy.Prompt) (*genai.Content, []*genai.Content, []fantasy.CallWarning, error) {
 	var systemInstructions *genai.Content
 	var content []*genai.Content
 	var warnings []fantasy.CallWarning
@@ -334,8 +340,10 @@ func toGooglePrompt(prompt fantasy.Prompt) (*genai.Content, []*genai.Content, []
 		switch msg.Role {
 		case fantasy.MessageRoleSystem:
 			if finishedSystemBlock {
-				// skip multiple system messages that are separated by user/assistant messages
+				warnings = append(warnings, fantasy.CallWarning{
-				// TODO: see if we need to send error here?
+					Type:    fantasy.CallWarningTypeOther,
+					Message: "google: additional system message after user/assistant messages was skipped",
+				})
 				continue
 			}
 			finishedSystemBlock = true
@@ -514,10 +522,10 @@ func toGooglePrompt(prompt fantasy.Prompt) (*genai.Content, []*genai.Content, []
 				})
 			}
 		default:
-			panic("unsupported message role: " + msg.Role)
+			return nil, nil, nil, fmt.Errorf("google: unsupported message role: %s", msg.Role)
 		}
 	}
-	return systemInstructions, content, warnings
+	return systemInstructions, content, warnings, nil
 }
 
 // Generate implements fantasy.LanguageModel.

internal/fantasy/providers/openai/language_model_hooks.go

@@ -539,7 +539,6 @@ func DefaultToPrompt(prompt fantasy.Prompt, _, _ string) ([]openai.ChatCompletio
 					}
 					messages = append(messages, openai.ToolMessage(output.Text, toolResultPart.ToolCallID))
 				case fantasy.ToolResultContentTypeError:
-					// TODO: check if better handling is needed
 					output, ok := fantasy.AsToolResultOutputType[fantasy.ToolResultOutputContentError](toolResultPart.Output)
 					if !ok {
 						warnings = append(warnings, fantasy.CallWarning{

internal/fantasy/providers/openai/responses_language_model.go

@@ -4,10 +4,11 @@ import (
 	"context"
 	"encoding/base64"
 	"fmt"
-	jsonv2 "github.com/go-json-experiment/json"
 	"reflect"
 	"strings"
 
+	jsonv2 "github.com/go-json-experiment/json"
+
 	"charm.land/fantasy"
 	"charm.land/fantasy/object"
 	"charm.land/fantasy/schema"
@@ -1232,7 +1233,6 @@ func (o responsesLanguageModel) streamObjectWithJSONMode(ctx context.Context, ca
 		var lastParsedObject any
 		var usage fantasy.Usage
 		var finishReason fantasy.FinishReason
-		var streamErr error
 		hasFunctionCall := false
 
 		for stream.Next() {
@@ -1299,13 +1299,10 @@ func (o responsesLanguageModel) streamObjectWithJSONMode(ctx context.Context, ca
 
 			case "error":
 				errorEvent := event.AsError()
-				streamErr = fmt.Errorf("response error: %s (code: %s)", errorEvent.Message, errorEvent.Code)
+				yield(fantasy.ObjectStreamPart{
-				if !yield(fantasy.ObjectStreamPart{
 					Type:  fantasy.ObjectStreamPartTypeError,
-					Error: streamErr,
+					Error: fmt.Errorf("response error: %s (code: %s)", errorEvent.Message, errorEvent.Code),
-				}) {
+				})
-					return
-				}
 				return
 			}
 		}
@@ -1320,14 +1317,13 @@ func (o responsesLanguageModel) streamObjectWithJSONMode(ctx context.Context, ca
 		}
 
 		// Final validation and emit
-		if streamErr == nil && lastParsedObject != nil {
+		if lastParsedObject != nil {
 			yield(fantasy.ObjectStreamPart{
 				Type:         fantasy.ObjectStreamPartTypeFinish,
 				Usage:        usage,
 				FinishReason: finishReason,
 			})
-		} else if streamErr == nil && lastParsedObject == nil {
+		} else {
-			// No object was generated
 			yield(fantasy.ObjectStreamPart{
 				Type: fantasy.ObjectStreamPartTypeError,
 				Error: &fantasy.NoObjectGeneratedError{

internal/fantasy/providers/openrouter/language_model_hooks.go

@@ -3,10 +3,11 @@ package openrouter
 import (
 	"encoding/base64"
 	"fmt"
-	jsonv2 "github.com/go-json-experiment/json"
 	"maps"
 	"strings"
 
+	jsonv2 "github.com/go-json-experiment/json"
+
 	"charm.land/fantasy"
 	"charm.land/fantasy/providers/anthropic"
 	"charm.land/fantasy/providers/google"
@@ -1010,7 +1011,6 @@ func languageModelToPrompt(prompt fantasy.Prompt, _, model string) ([]openaisdk.
 					}
 					messages = append(messages, tr)
 				case fantasy.ToolResultContentTypeError:
-					// TODO: check if better handling is needed
 					output, ok := fantasy.AsToolResultOutputType[fantasy.ToolResultOutputContentError](toolResultPart.Output)
 					if !ok {
 						warnings = append(warnings, fantasy.CallWarning{