feat(usage): persist resolvedModel on provider_usage events

internal/providers/anthropic/provider.go

@@ -90,6 +90,7 @@ type Options struct {
 	// When unset, Zero uses providerio.ResolveStreamIdleTimeout — the
 	// ZERO_STREAM_IDLE_TIMEOUT override or providerio.DefaultStreamIdleTimeout.
 	StreamIdleTimeout time.Duration
+	SessionHeader     string
 }
 
 // Provider streams completions from Anthropic's Messages API.
@@ -108,6 +109,7 @@ type Provider struct {
 	httpClient        *http.Client
 	userAgent         string
 	streamIdleTimeout time.Duration
+	sessionHeader     providerio.SessionHeader
 }
 
 // New creates an Anthropic provider.
@@ -143,6 +145,7 @@ func New(options Options) (*Provider, error) {
 		httpClient:        providerio.HTTPClient(options.HTTPClient),
 		userAgent:         options.UserAgent,
 		streamIdleTimeout: providerio.ResolveStreamIdleTimeout(options.StreamIdleTimeout),
+		sessionHeader:     providerio.NewSessionHeader(options.SessionHeader),
 	}, nil
 }
 
@@ -163,12 +166,12 @@ func (provider *Provider) StreamCompletion(
 	events := make(chan zeroruntime.StreamEvent, 16)
 	go func() {
 		defer close(events)
-		provider.stream(ctx, body, events)
+		provider.stream(ctx, body, request.PromptCacheKey, events)
 	}()
 	return events, nil
 }
 
-func (provider *Provider) stream(ctx context.Context, body []byte, events chan<- zeroruntime.StreamEvent) {
+func (provider *Provider) stream(ctx context.Context, body []byte, sessionID string, events chan<- zeroruntime.StreamEvent) {
 	// streamCtx lets the idle watchdog abort an in-flight body read by cancelling
 	// the request, which unblocks the SSE reader goroutine.
 	streamCtx, cancelStream := context.WithCancel(ctx)
@@ -185,6 +188,7 @@ func (provider *Provider) stream(ctx context.Context, body []byte, events chan<-
 		},
 		provider.oauthResolver,
 		func(request *http.Request) {
+			provider.sessionHeader.Apply(request, sessionID)
 			request.Header.Set("Content-Type", "application/json")
 			request.Header.Set("anthropic-version", provider.version)
 			if provider.beta != "" {

internal/providers/factory.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"net/url"
 	"strings"
 
 	"github.com/Gitlawb/zero/internal/config"
@@ -53,6 +54,11 @@ func New(profile config.ProviderProfile, options Options) (zeroruntime.Provider,
 		return newCodexProvider(profile, resolved, options)
 	}
 
+	sessionHeader := sessionHeaderForProfile(profile, resolved)
+	if sessionHeader != "" && strings.TrimSpace(options.UserAgent) == "" {
+		options.UserAgent = "zero"
+	}
+
 	switch resolved.providerKind {
 	case config.ProviderKindOpenAI, config.ProviderKindOpenAICompatible:
 		// prompt_cache_key is an OpenAI-only chat-completions field. Strict
@@ -74,6 +80,7 @@ func New(profile config.ProviderProfile, options Options) (zeroruntime.Provider,
 			UserAgent:             options.UserAgent,
 			ParseThinkTags:        parseThinkTagsForProfile(profile, resolved),
 			DisablePromptCacheKey: resolved.providerKind == config.ProviderKindOpenAICompatible,
+			SessionHeader:         sessionHeader,
 		})
 	case config.ProviderKindAnthropic, config.ProviderKindAnthropicCompat:
 		return anthropic.New(anthropic.Options{
@@ -88,6 +95,7 @@ func New(profile config.ProviderProfile, options Options) (zeroruntime.Provider,
 			MaxTokens:       resolved.maxOutputTokens,
 			HTTPClient:      options.HTTPClient,
 			UserAgent:       options.UserAgent,
+			SessionHeader:   sessionHeader,
 		})
 	case config.ProviderKindGoogle:
 		return gemini.New(gemini.Options{
@@ -345,6 +353,18 @@ func isCodexCatalog(profile config.ProviderProfile, _ resolvedProfile) bool {
 	return providercatalog.NormalizeID(profile.CatalogID) == "chatgpt"
 }
 
+func sessionHeaderForProfile(profile config.ProviderProfile, resolved resolvedProfile) string {
+	switch providercatalog.NormalizeID(profile.CatalogID) {
+	case "opencode", "opencode-go", "opencode-go-anthropic-compatible":
+		return "x-opencode-session"
+	}
+	endpoint, err := url.Parse(resolved.baseURL)
+	if err == nil && strings.EqualFold(endpoint.Hostname(), "opencode.ai") && (endpoint.Path == "/zen" || strings.HasPrefix(endpoint.Path, "/zen/")) {
+		return "x-opencode-session"
+	}
+	return ""
+}
+
 // newCodexProvider builds a Codex-flavored openai provider for the chatgpt
 // catalog. The Codex headers (`originator`, `chatgpt-account-id`) are
 // injected by the CodexProvider wrapper. The `chatgpt-account-id` is read from