refactor(models): remove responses API model registration hack

Fantasy v0.21.0 natively includes gpt-5.5 and other newer models in
its responsesModelIDs/responsesReasoningModelIDs lists, making our
workaround unnecessary.

- Delete responses_models.go (go:linkname hack + RegisterResponsesModels)
- Delete responses_models_test.go
- Replace isResponsesAPIModel/isResponsesReasoningModel heuristics with
  direct openai.IsResponsesModel/openai.IsResponsesReasoningModel calls
- Remove RegisterResponsesModels calls from registry init/reload
- Remove hack documentation from AGENTS.md
- Update all deps (fantasy v0.21.0, smithy-go, ultraviolet, etc.)

internal/models/providers.go

@@ -309,7 +309,7 @@ func CreateProvider(ctx context.Context, config *ProviderConfig) (*ProviderResul
 			// For OpenAI Responses API models, we skip merging entirely because
 			// ResponsesProviderOptions and ProviderOptions are incompatible types.
 			skipMerge := false
-			if provider == "openai" && isResponsesAPIModel(modelName) {
+			if provider == "openai" && openai.IsResponsesModel(modelName) {
 				skipMerge = true
 			}
 			if !skipMerge {
@@ -549,69 +549,17 @@ func clearConflictingAnthropicSamplingParams(config *ProviderConfig) {
 	}
 }
 
-// isResponsesAPIModel returns true when the model ID should use the OpenAI
-// Responses API. It first consults fantasy's built-in list (which may lag
-// behind new model releases) and falls back to a heuristic based on the
-// model ID prefix. All modern OpenAI models (gpt-4.1+, gpt-4.5+, gpt-5+,
-// o-series, codex, chatgpt) use the Responses API.
-func isResponsesAPIModel(modelName string) bool {
-	if openai.IsResponsesModel(modelName) {
-		return true
-	}
-	// Heuristic: modern OpenAI model families that use the Responses API.
-	// This catches newly released models (e.g. gpt-5.5) before fantasy
-	// adds them to its hardcoded list.
-	for _, prefix := range []string{
-		"gpt-4.1", "gpt-4.5", "gpt-5",
-		"o1", "o3", "o4",
-		"codex",
-		"chatgpt-",
-	} {
-		if strings.HasPrefix(modelName, prefix) {
-			return true
-		}
-	}
-	return false
-}
-
-// isResponsesReasoningModel returns true when the model ID should be treated
-// as an OpenAI Responses API reasoning model. Like isResponsesAPIModel, it
-// supplements fantasy's built-in list with a heuristic for new models.
-func isResponsesReasoningModel(modelName string) bool {
-	if openai.IsResponsesReasoningModel(modelName) {
-		return true
-	}
-	// Heuristic: if it's a responses-API model, check model metadata.
-	// Reasoning models in the gpt-5+ and o-series families have
-	// reasoning=true in models.dev.
-	if !isResponsesAPIModel(modelName) {
-		return false
-	}
-	registry := GetGlobalRegistry()
-	modelInfo := registry.LookupModel("openai", modelName)
-	if modelInfo != nil && modelInfo.Reasoning {
-		return true
-	}
-	// For unknown models in reasoning families, assume reasoning.
-	for _, prefix := range []string{"o1", "o3", "o4", "gpt-5", "codex"} {
-		if strings.HasPrefix(modelName, prefix) {
-			return true
-		}
-	}
-	return false
-}
-
 // buildOpenAIProviderOptions returns fantasy.ProviderOptions configured for
 // OpenAI Responses API models. For reasoning models it sets reasoning_summary
 // to "auto", includes encrypted reasoning content, and maps the ThinkingLevel
 // to an OpenAI ReasoningEffort. For non-responses or non-reasoning models the
 // returned map is nil (no extra options needed).
 func buildOpenAIProviderOptions(config *ProviderConfig, modelName string) fantasy.ProviderOptions {
-	if !isResponsesAPIModel(modelName) {
+	if !openai.IsResponsesModel(modelName) {
 		return nil
 	}
 
-	if isResponsesReasoningModel(modelName) {
+	if openai.IsResponsesReasoningModel(modelName) {
 		reasoningSummary := "auto"
 		opts := &openai.ResponsesProviderOptions{
 			ReasoningSummary: &reasoningSummary,
@@ -957,7 +905,7 @@ func buildCodexProviderOptions(config *ProviderConfig, modelName string) fantasy
 		opts.Instructions = &config.SystemPrompt
 	}
 
-	if isResponsesReasoningModel(modelName) {
+	if openai.IsResponsesReasoningModel(modelName) {
 		opts.ReasoningEffort = thinkingLevelToReasoningEffort(config.ThinkingLevel)
 	}
 

internal/models/registry.go

@@ -481,13 +481,6 @@ func (r *ModelsRegistry) ValidateModelString(modelString string) error {
 // Global registry instance
 var globalRegistry = NewModelsRegistry()
 
-func init() {
-	// Ensure fantasy's Responses API model lists include any new models
-	// from the model database that were released after the fantasy
-	// dependency was pinned.
-	RegisterResponsesModels()
-}
-
 // GetGlobalRegistry returns the global models registry instance.
 func GetGlobalRegistry() *ModelsRegistry {
 	return globalRegistry
@@ -497,5 +490,4 @@ func GetGlobalRegistry() *ModelsRegistry {
 // data sources (cache → embedded). Call after updating the cache.
 func ReloadGlobalRegistry() {
 	globalRegistry = NewModelsRegistry()
-	RegisterResponsesModels()
 }

internal/models/responses_models.go

@@ -1,58 +0,0 @@
-package models
-
-import (
-	_ "unsafe" // Required for go:linkname.
-)
-
-// responsesModelIDs and responsesReasoningModelIDs are the unexported slices
-// in charm.land/fantasy/providers/openai that gate whether a model uses the
-// Responses API code path vs Chat Completions. When a brand-new model is
-// released (e.g. gpt-5.5) and models.dev is updated via `kit update-models`,
-// Kit recognises the model but fantasy's hardcoded list does not. That causes
-// a type-mismatch crash: Kit builds *ResponsesProviderOptions (correct for
-// the Responses endpoint) but fantasy routes through Chat Completions and
-// rejects the type.
-//
-// RegisterResponsesModels appends model IDs that are missing from fantasy's
-// lists so the provider routes through the correct code path. It is called
-// once during provider creation after loading the model database.
-
-//go:linkname fantasyResponsesModelIDs charm.land/fantasy/providers/openai.responsesModelIDs
-var fantasyResponsesModelIDs []string
-
-//go:linkname fantasyResponsesReasoningModelIDs charm.land/fantasy/providers/openai.responsesReasoningModelIDs
-var fantasyResponsesReasoningModelIDs []string
-
-// RegisterResponsesModels ensures every OpenAI model known to our model
-// database that should use the Responses API is present in fantasy's
-// internal lists. This is a no-op for models already registered.
-func RegisterResponsesModels() {
-	registry := GetGlobalRegistry()
-	providerInfo := registry.GetProviderInfo("openai")
-	if providerInfo == nil {
-		return
-	}
-
-	existing := make(map[string]bool, len(fantasyResponsesModelIDs))
-	for _, id := range fantasyResponsesModelIDs {
-		existing[id] = true
-	}
-	existingReasoning := make(map[string]bool, len(fantasyResponsesReasoningModelIDs))
-	for _, id := range fantasyResponsesReasoningModelIDs {
-		existingReasoning[id] = true
-	}
-
-	for modelID, modelInfo := range providerInfo.Models {
-		if !isResponsesAPIModel(modelID) {
-			continue
-		}
-		if !existing[modelID] {
-			fantasyResponsesModelIDs = append(fantasyResponsesModelIDs, modelID)
-			existing[modelID] = true
-		}
-		if modelInfo.Reasoning && !existingReasoning[modelID] {
-			fantasyResponsesReasoningModelIDs = append(fantasyResponsesReasoningModelIDs, modelID)
-			existingReasoning[modelID] = true
-		}
-	}
-}

internal/models/responses_models_test.go

@@ -1,123 +0,0 @@
-package models
-
-import (
-	"testing"
-
-	"charm.land/fantasy/providers/openai"
-)
-
-func TestIsResponsesAPIModel(t *testing.T) {
-	tests := []struct {
-		modelID  string
-		expected bool
-	}{
-		// Already in fantasy's list — always true
-		{"gpt-5", true},
-		{"gpt-4.1", true},
-		{"o3", true},
-		{"o4-mini", true},
-		{"codex-mini-latest", true},
-
-		// NOT in fantasy's list but matches our heuristic
-		{"gpt-5.5", true},
-		{"gpt-5.6-turbo", true},
-		{"gpt-4.1-ultra", true},
-		{"o3-jumbo", true},
-		{"o4-mega", true},
-
-		// Should NOT match
-		{"gpt-3.5-turbo", true}, // actually IS in fantasy's responses list (legacy Chat Completions compat)
-		{"llama-3", false},
-		{"claude-opus-4-6", false},
-		{"gemini-2.5-pro", false},
-		{"random-model", false},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.modelID, func(t *testing.T) {
-			got := isResponsesAPIModel(tt.modelID)
-			if got != tt.expected {
-				t.Errorf("isResponsesAPIModel(%q) = %v, want %v", tt.modelID, got, tt.expected)
-			}
-		})
-	}
-}
-
-func TestIsResponsesReasoningModel(t *testing.T) {
-	tests := []struct {
-		modelID  string
-		expected bool
-	}{
-		// In fantasy's reasoning list
-		{"gpt-5", true},
-		{"o3", true},
-		{"o4-mini", true},
-
-		// NOT in fantasy's list but matches reasoning heuristic (gpt-5 prefix)
-		{"gpt-5.5", true},
-		{"gpt-5.6-turbo", true},
-
-		// Responses API but NOT reasoning
-		{"gpt-4.1", false},
-		{"gpt-4.1-mini", false},
-
-		// Not OpenAI at all
-		{"claude-opus-4-6", false},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.modelID, func(t *testing.T) {
-			got := isResponsesReasoningModel(tt.modelID)
-			if got != tt.expected {
-				t.Errorf("isResponsesReasoningModel(%q) = %v, want %v", tt.modelID, got, tt.expected)
-			}
-		})
-	}
-}
-
-func TestRegisterResponsesModels(t *testing.T) {
-	// After RegisterResponsesModels() (called in init()),
-	// any model matching our heuristic that's in the model database
-	// should be queryable via openai.IsResponsesModel.
-
-	// Models in the embedded database that are also in fantasy's list
-	// should remain accessible.
-	if !openai.IsResponsesModel("gpt-5") {
-		t.Error("gpt-5 should be a responses model after registration")
-	}
-
-	// The registration should not break existing models.
-	if openai.IsResponsesModel("random-nonexistent-model") {
-		t.Error("random model should NOT be a responses model")
-	}
-}
-
-func TestBuildOpenAIProviderOptions_NewModel(t *testing.T) {
-	// A model like gpt-5.5 that isn't in fantasy's hardcoded list
-	// but matches our heuristic should get ResponsesProviderOptions.
-	config := &ProviderConfig{
-		ModelString: "openai/gpt-5.5",
-	}
-	opts := buildOpenAIProviderOptions(config, "gpt-5.5")
-	if opts == nil {
-		t.Fatal("buildOpenAIProviderOptions should return non-nil for gpt-5.5")
-	}
-	v, ok := opts[openai.Name]
-	if !ok {
-		t.Fatal("should have openai key in provider options")
-	}
-	if _, ok := v.(*openai.ResponsesProviderOptions); !ok {
-		t.Errorf("expected *ResponsesProviderOptions, got %T", v)
-	}
-}
-
-func TestBuildOpenAIProviderOptions_NonResponsesModel(t *testing.T) {
-	// A model that doesn't match any heuristic should get nil.
-	config := &ProviderConfig{
-		ModelString: "openai/some-old-model",
-	}
-	opts := buildOpenAIProviderOptions(config, "some-old-model")
-	if opts != nil {
-		t.Errorf("buildOpenAIProviderOptions should return nil for unknown model, got %v", opts)
-	}
-}