fix(coderd/x/chatd/chatprovider): keep gateway model prefix in ResolveModelWithProviderHint (#25725)

For `vercel`, `openrouter`, and `openai-compat`, the
`<provider>/<model>` slash is part of the upstream model ID rather than
a hint. `ResolveModelWithProviderHint` was running
`parseCanonicalModelRef` before honoring `providerHint`, so a config
like `(provider=vercel, model=anthropic/claude-4-5-sonnet)` resolved to
`provider=anthropic, model=claude-4-5-sonnet` and the prefix-less model
name was forwarded to Vercel, which returned `Model 'claude-4-5-sonnet'
not found`.

Honor an explicit gateway provider hint before attempting canonical-ref
parsing. Non-gateway hints (anthropic, openai, etc.) keep the existing
canonical-ref-first behavior so `anthropic/claude-...` still has its
prefix stripped when routed directly to Anthropic.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

coderd/x/chatd/chatprovider/chatprovider.go

@@ -573,6 +573,21 @@ func orderProviders(providerSet map[string]struct{}) []string {
 	return ordered
 }
 
+// isGatewayProvider reports whether the provider routes requests to
+// multiple upstream model providers using a "<provider>/<model>" model
+// identifier, where the slash is part of the upstream model ID rather
+// than a hint.
+func isGatewayProvider(provider string) bool {
+	switch provider {
+	case fantasyvercel.Name,
+		fantasyopenrouter.Name,
+		fantasyopenaicompat.Name:
+		return true
+	default:
+		return false
+	}
+}
+
 // NormalizeProvider canonicalizes a provider name.
 func NormalizeProvider(provider string) string {
 	switch strings.ToLower(strings.TrimSpace(provider)) {
@@ -603,6 +618,15 @@ func ResolveModelWithProviderHint(modelName, providerHint string) (provider stri
 		return "", "", xerrors.New("model is required")
 	}
 
+	// Gateway providers (vercel, openrouter, openai-compat) treat the
+	// "<provider>/<model>" slash as part of the upstream model ID, so
+	// parseCanonicalModelRef would incorrectly strip the prefix and
+	// route to the embedded provider name instead. Honor an explicit
+	// gateway hint before attempting canonical-ref parsing.
+	if normalized := NormalizeProvider(providerHint); normalized != "" && isGatewayProvider(normalized) {
+		return normalized, modelName, nil
+	}
+
 	if provider, modelID, ok := parseCanonicalModelRef(modelName); ok {
 		return provider, modelID, nil
 	}

coderd/x/chatd/chatprovider/chatprovider_test.go

@@ -12,6 +12,7 @@ import (
 	fantasyanthropic "charm.land/fantasy/providers/anthropic"
 	fantasybedrock "charm.land/fantasy/providers/bedrock"
 	fantasyopenai "charm.land/fantasy/providers/openai"
+	fantasyopenaicompat "charm.land/fantasy/providers/openaicompat"
 	fantasyopenrouter "charm.land/fantasy/providers/openrouter"
 	fantasyvercel "charm.land/fantasy/providers/vercel"
 	"github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream"
@@ -1439,3 +1440,73 @@ func TestMergeMissingProviderOptions_OpenRouterNested(t *testing.T) {
 	require.Equal(t, []string{"int8"}, options.OpenRouter.Provider.Quantizations)
 	require.Equal(t, "latency", *options.OpenRouter.Provider.Sort)
 }
+
+func TestResolveModelWithProviderHint(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		modelName    string
+		providerHint string
+		wantProvider string
+		wantModel    string
+		wantErr      bool
+	}{
+		{
+			name:         "VercelHintPreservesPrefixedModelID",
+			modelName:    "anthropic/claude-4-5-sonnet",
+			providerHint: fantasyvercel.Name,
+			wantProvider: fantasyvercel.Name,
+			wantModel:    "anthropic/claude-4-5-sonnet",
+		},
+		{
+			name:         "OpenRouterHintPreservesPrefixedModelID",
+			modelName:    "anthropic/claude-3.5-haiku",
+			providerHint: fantasyopenrouter.Name,
+			wantProvider: fantasyopenrouter.Name,
+			wantModel:    "anthropic/claude-3.5-haiku",
+		},
+		{
+			name:         "OpenAICompatHintPreservesPrefixedModelID",
+			modelName:    "anthropic/claude-4-5-sonnet",
+			providerHint: fantasyopenaicompat.Name,
+			wantProvider: fantasyopenaicompat.Name,
+			wantModel:    "anthropic/claude-4-5-sonnet",
+		},
+		{
+			name:         "AnthropicHintStripsCanonicalPrefix",
+			modelName:    "anthropic/claude-4-5-sonnet",
+			providerHint: fantasyanthropic.Name,
+			wantProvider: fantasyanthropic.Name,
+			wantModel:    "claude-4-5-sonnet",
+		},
+		{
+			name:         "NoHintUsesCanonicalRef",
+			modelName:    "anthropic/claude-4-5-sonnet",
+			providerHint: "",
+			wantProvider: fantasyanthropic.Name,
+			wantModel:    "claude-4-5-sonnet",
+		},
+		{
+			name:         "VercelHintWithoutSlashPasses",
+			modelName:    "claude-4-5-sonnet",
+			providerHint: fantasyvercel.Name,
+			wantProvider: fantasyvercel.Name,
+			wantModel:    "claude-4-5-sonnet",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			provider, model, err := chatprovider.ResolveModelWithProviderHint(tt.modelName, tt.providerHint)
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			require.Equal(t, tt.wantProvider, provider)
+			require.Equal(t, tt.wantModel, model)
+		})
+	}
+}