fix(copilot): prevent premium request count inflation for Claude models

> Copilot Premium usage significantly amplified when using amp

- Add X-Initiator header (user/agent) based on last message role to
  prevent Copilot from billing all requests as premium user-initiated
- Add flattenAssistantContent() to convert assistant content from array
  to string, preventing Claude from re-answering all previous prompts
- Align Copilot headers (User-Agent, Editor-Version, Openai-Intent) with
  pi-ai reference implementation

Closes #113

Amp-Thread-ID: https://ampcode.com/threads/T-019c392b-736e-7489-a06b-f94f7c75f7c0
Co-authored-by: Amp <amp@ampcode.com>

internal/runtime/executor/github_copilot_executor.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strings"
 	"sync"
 	"time"
 
@@ -33,11 +34,11 @@ const (
 	maxScannerBufferSize = 20_971_520
 
 	// Copilot API header values.
-	copilotUserAgent     = "GithubCopilot/1.0"
-	copilotEditorVersion = "vscode/1.100.0"
-	copilotPluginVersion = "copilot/1.300.0"
+	copilotUserAgent     = "GitHubCopilotChat/0.35.0"
+	copilotEditorVersion = "vscode/1.107.0"
+	copilotPluginVersion = "copilot-chat/0.35.0"
 	copilotIntegrationID = "vscode-chat"
-	copilotOpenAIIntent  = "conversation-panel"
+	copilotOpenAIIntent  = "conversation-edits"
 )
 
 // GitHubCopilotExecutor handles requests to the GitHub Copilot API.
@@ -77,7 +78,7 @@ func (e *GitHubCopilotExecutor) PrepareRequest(req *http.Request, auth *cliproxy
 	if errToken != nil {
 		return errToken
 	}
-	e.applyHeaders(req, apiToken)
+	e.applyHeaders(req, apiToken, nil)
 	return nil
 }
 
@@ -120,6 +121,7 @@ func (e *GitHubCopilotExecutor) Execute(ctx context.Context, auth *cliproxyauth.
 	originalTranslated := sdktranslator.TranslateRequest(from, to, req.Model, originalPayload, false)
 	body := sdktranslator.TranslateRequest(from, to, req.Model, bytes.Clone(req.Payload), false)
 	body = e.normalizeModel(req.Model, body)
+	body = flattenAssistantContent(body)
 	requestedModel := payloadRequestedModel(opts, req.Model)
 	body = applyPayloadConfigWithRoot(e.cfg, req.Model, to.String(), "", body, originalTranslated, requestedModel)
 	body, _ = sjson.SetBytes(body, "stream", false)
@@ -133,7 +135,7 @@ func (e *GitHubCopilotExecutor) Execute(ctx context.Context, auth *cliproxyauth.
 	if err != nil {
 		return resp, err
 	}
-	e.applyHeaders(httpReq, apiToken)
+	e.applyHeaders(httpReq, apiToken, body)
 
 	// Add Copilot-Vision-Request header if the request contains vision content
 	if detectVisionContent(body) {
@@ -225,6 +227,7 @@ func (e *GitHubCopilotExecutor) ExecuteStream(ctx context.Context, auth *cliprox
 	originalTranslated := sdktranslator.TranslateRequest(from, to, req.Model, originalPayload, false)
 	body := sdktranslator.TranslateRequest(from, to, req.Model, bytes.Clone(req.Payload), true)
 	body = e.normalizeModel(req.Model, body)
+	body = flattenAssistantContent(body)
 	requestedModel := payloadRequestedModel(opts, req.Model)
 	body = applyPayloadConfigWithRoot(e.cfg, req.Model, to.String(), "", body, originalTranslated, requestedModel)
 	body, _ = sjson.SetBytes(body, "stream", true)
@@ -242,7 +245,7 @@ func (e *GitHubCopilotExecutor) ExecuteStream(ctx context.Context, auth *cliprox
 	if err != nil {
 		return nil, err
 	}
-	e.applyHeaders(httpReq, apiToken)
+	e.applyHeaders(httpReq, apiToken, body)
 
 	// Add Copilot-Vision-Request header if the request contains vision content
 	if detectVisionContent(body) {
@@ -414,7 +417,7 @@ func (e *GitHubCopilotExecutor) ensureAPIToken(ctx context.Context, auth *clipro
 }
 
 // applyHeaders sets the required headers for GitHub Copilot API requests.
-func (e *GitHubCopilotExecutor) applyHeaders(r *http.Request, apiToken string) {
+func (e *GitHubCopilotExecutor) applyHeaders(r *http.Request, apiToken string, body []byte) {
 	r.Header.Set("Content-Type", "application/json")
 	r.Header.Set("Authorization", "Bearer "+apiToken)
 	r.Header.Set("Accept", "application/json")
@@ -424,6 +427,20 @@ func (e *GitHubCopilotExecutor) applyHeaders(r *http.Request, apiToken string) {
 	r.Header.Set("Openai-Intent", copilotOpenAIIntent)
 	r.Header.Set("Copilot-Integration-Id", copilotIntegrationID)
 	r.Header.Set("X-Request-Id", uuid.NewString())
+
+	initiator := "user"
+	if len(body) > 0 {
+		if messages := gjson.GetBytes(body, "messages"); messages.Exists() && messages.IsArray() {
+			arr := messages.Array()
+			if len(arr) > 0 {
+				lastRole := arr[len(arr)-1].Get("role").String()
+				if lastRole != "" && lastRole != "user" {
+					initiator = "agent"
+				}
+			}
+		}
+	}
+	r.Header.Set("X-Initiator", initiator)
 }
 
 // detectVisionContent checks if the request body contains vision/image content.
@@ -464,6 +481,38 @@ func useGitHubCopilotResponsesEndpoint(sourceFormat sdktranslator.Format) bool {
 	return sourceFormat.String() == "openai-response"
 }
 
+// flattenAssistantContent converts assistant message content from array format
+// to a joined string. GitHub Copilot requires assistant content as a string;
+// sending it as an array causes Claude models to re-answer all previous prompts.
+func flattenAssistantContent(body []byte) []byte {
+	messages := gjson.GetBytes(body, "messages")
+	if !messages.Exists() || !messages.IsArray() {
+		return body
+	}
+	result := body
+	for i, msg := range messages.Array() {
+		if msg.Get("role").String() != "assistant" {
+			continue
+		}
+		content := msg.Get("content")
+		if !content.Exists() || !content.IsArray() {
+			continue
+		}
+		var textParts []string
+		for _, part := range content.Array() {
+			if part.Get("type").String() == "text" {
+				if t := part.Get("text").String(); t != "" {
+					textParts = append(textParts, t)
+				}
+			}
+		}
+		joined := strings.Join(textParts, "")
+		path := fmt.Sprintf("messages.%d.content", i)
+		result, _ = sjson.SetBytes(result, path, joined)
+	}
+	return result
+}
+
 // isHTTPSuccess checks if the status code indicates success (2xx).
 func isHTTPSuccess(statusCode int) bool {
 	return statusCode >= 200 && statusCode < 300