diff --git a/README.md b/README.md
index acfe7b2..8c29c23 100644
--- a/README.md
+++ b/README.md
@@ -102,7 +102,7 @@ Using these rules, everyone can build their own copy of Bantam from scratch in l
 1. **Initialization**: Read the config file (`.bantam.cfg` if present, else `model.cfg`). Discover context window size from the `/models` endpoint (or fallback to `context_window` from `model.cfg` or 200,000 tokens). Prepare an array of messages starting with the built-in system prompt `{"role": "system", "content": system_prompt}`.
 2. **Input Processing**: Take user prompt (via command-line file parameter or interactive stdin). If prefixed with `!`, execute the command directly via `shell_exec` without appending to conversation context. Otherwise, append `{"role": "user", "content": prompt}`, and invoke `AL(cfg, messages)`.
 3. **Agentic Loop (`AL`)**:
-   - Send `messages` and tool definitions (`shell_exec`, `write_file`) to the OpenAI-compatible `/chat/completions` API endpoint with custom `User-Agent` headers and `stream_options: {"include_usage": true}`.
+   - Send `messages` and tool definitions (`shell_exec`, `write_file`) to the OpenAI-compatible `/chat/completions` API endpoint with custom request headers (see below) and `stream_options: {"include_usage": true}`.
    - Support context cancellation (e.g. on `SIGINT` / Ctrl+C) to cleanly abort in-flight requests without appending incomplete messages.
    - On network or HTTP failure, retry using Fibonacci backoff delays (`1s, 1s, 2s, 3s, 5s, 8s, 13s, 21s, 34s`).
    - If `stream=true`, parse SSE data chunks (`data: {...}`) in real-time to stream reasoning content (`reasoning_content`) and response text directly to stdout, bracketing the reasoning block with `--- reasoning start ---` / `--- reasoning end ---` markers, rendering Markdown and tables constrained to terminal width.
@@ -125,7 +125,7 @@ Using these rules, everyone can build their own copy of Bantam from scratch in l
 ### Agentic loop (`AL(cfg, messages)`) function
 
 1. Call OpenAI-compatible Completions API (`POST {endpoint}/chat/completions`) forwarding relevant parameters from `cfg` (`model`, `temperature`, `stream`, `reasoning_effort`, etc., excluding internal agent configs like `endpoint`, `api_key`, `timeout`, `shell_timeout`, `max_al_iterations`, `color`, `context_window`) and optional `api_key` bearer header.
-   - Set custom `User-Agent` header (`Mozilla/5.0 (compatible; Bantam/1.0)`) to avoid gateway 403 blocks.
+   - Set custom request headers. If the endpoint host is `opencode.ai` (or a subdomain), send the OpenCode Zen header set: `User-Agent: opencode/1.18.31 ai-sdk/provider-utils/4.0.23 runtime/bun/1.3.14`, `x-opencode-client: cli`, `x-opencode-project: <project-md5>`, `x-opencode-session: <ses_...>`, and a fresh `x-opencode-request: msg_<26 base32 chars>` per call. For every other endpoint, keep the legacy `User-Agent: opencode/1.18.31` plus `X-Session-Id` and `x-session-affinity`. The `Authorization: Bearer <api_key>` header is added whenever `api_key` is set and not `-`.
    - Retry network/HTTP errors with Fibonacci backoff delays (`1s, 1s, 2s, 3s, 5s, 8s, 13s, 21s, 34s`).
    - If `stream=true`, parse SSE stream (`data: {...}`) for real-time reasoning and text output, bracketing reasoning with `--- reasoning start ---` / `--- reasoning end ---` markers.
 2. Append the assistant's response message object to `messages`. If non-streaming and response has reasoning tokens (`reasoning_content` or `reasoning`), output them wrapped in `--- reasoning start ---` / `--- reasoning end ---` markers.
diff --git a/main.go b/main.go
index 17e1a2b..b53cd4d 100644
--- a/main.go
+++ b/main.go
@@ -8,6 +8,7 @@ import (
   "bytes"
   "context"
   "crypto/md5"
+  "crypto/rand"
   "encoding/hex"
   "encoding/json"
   "errors"
@@ -87,6 +88,8 @@ var llmTransport = &http.Transport{
 
 var defCfg = Cfg{"https://api.kilo.ai/api/openrouter", "openrouter/free", "-", 0.7, 300, 120, 1000, true, "auto", 262144, 15000, map[string]string{"reasoning_effort": "high"}}
 const opencodeAgentVersion = "opencode/1.18.31"
+const opencodeProviderUA = "ai-sdk/provider-utils/4.0.23 runtime/bun/1.3.14"
+const opencodeIDAlphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"
 
 func atoiD(s string, d int) int {
   if v, e := strconv.Atoi(strings.TrimSpace(s)); e == nil {
@@ -105,12 +108,7 @@ func queryModelsContextWindow(cfg *Cfg) int {
   }
   req, err := http.NewRequest("GET", strings.TrimRight(cfg.Endpoint, "/")+"/models", nil)
   if err != nil { return 0 }
-  req.Header.Set("User-Agent", opencodeAgentVersion)
-  req.Header.Set("X-Session-Id", projectID())
-  req.Header.Set("x-opencode-project", projectID())
-  req.Header.Set("x-opencode-client", "cli")
-  req.Header.Set("x-opencode-session", opencodeSessionID())
-  if cfg.APIKey != "" && cfg.APIKey != "-" { req.Header.Set("Authorization", "Bearer "+cfg.APIKey) }
+  applyLLMHeaders(req, cfg, ocRequestID())
   resp, err := client.Do(req)
   if err != nil || resp.StatusCode >= 400 { return 0 }
   defer resp.Body.Close()
@@ -181,12 +179,7 @@ func listModels(cfg *Cfg) (string, error) {
   }
   req, err := http.NewRequest("GET", strings.TrimRight(cfg.Endpoint, "/")+"/models", nil)
   if err != nil { return "", err }
-  req.Header.Set("User-Agent", opencodeAgentVersion)
-  req.Header.Set("X-Session-Id", projectID())
-  req.Header.Set("x-opencode-project", projectID())
-  req.Header.Set("x-opencode-client", "cli")
-  req.Header.Set("x-opencode-session", opencodeSessionID())
-  if cfg.APIKey != "" && cfg.APIKey != "-" { req.Header.Set("Authorization", "Bearer "+cfg.APIKey) }
+  applyLLMHeaders(req, cfg, ocRequestID())
   resp, err := client.Do(req)
   if err != nil { return "", err }
   defer resp.Body.Close()
@@ -1052,6 +1045,7 @@ func llm(ctx context.Context, cfg *Cfg, msgs []Message, tools []map[string]any)
   pend := c("...requesting...", 1, 2)
   var resp *http.Response
   var err error
+  reqID := ocRequestID()
   for i := 0; i <= len(fib); i++ {
     if err := ctx.Err(); err != nil {
       if COL { fmt.Print("\r\033[K") }
@@ -1060,12 +1054,7 @@ func llm(ctx context.Context, cfg *Cfg, msgs []Message, tools []map[string]any)
     if COL { fmt.Print("\r" + pend) } else { fmt.Println(pend) }
     req, _ := http.NewRequestWithContext(ctx, "POST", strings.TrimRight(cfg.Endpoint, "/")+"/chat/completions", bytes.NewReader(body))
     req.Header.Set("Content-Type", "application/json")
-    req.Header.Set("User-Agent", opencodeAgentVersion)
-    req.Header.Set("X-Session-Id", projectID())
-    req.Header.Set("x-opencode-project", projectID())
-    req.Header.Set("x-opencode-client", "cli")
-    req.Header.Set("x-opencode-session", opencodeSessionID())
-    if cfg.APIKey != "" && cfg.APIKey != "-" { req.Header.Set("Authorization", "Bearer "+cfg.APIKey) }
+    applyLLMHeaders(req, cfg, reqID)
     resp, err = client.Do(req)
     var is4xxClientErr bool
     if err == nil && resp.StatusCode >= 400 {
@@ -1694,6 +1683,57 @@ func opencodeSessionID() string {
   return "ses_" + pid
 }
 
+// isOpencodeEndpoint reports whether the endpoint belongs to OpenCode Zen and
+// therefore expects the x-opencode-* request headers.
+func isOpencodeEndpoint(endpoint string) bool {
+  u, err := url.Parse(strings.TrimSpace(endpoint))
+  if err != nil || u.Hostname() == "" {
+    return strings.Contains(strings.ToLower(endpoint), "opencode.ai")
+  }
+  h := strings.ToLower(u.Hostname())
+  return h == "opencode.ai" || strings.HasSuffix(h, ".opencode.ai")
+}
+
+// ocRequestID returns a fresh OpenCode-style message id (msg_ followed by 26
+// base32 characters) matching the ids sent in the x-opencode-request header.
+func ocRequestID() string {
+  buf := make([]byte, 26)
+  if _, err := rand.Read(buf); err != nil {
+    h := md5.Sum([]byte(fmt.Sprintf("%d-%d", time.Now().UnixNano(), os.Getpid())))
+    for i := range buf {
+      buf[i] = h[i%len(h)]
+    }
+  }
+  var b strings.Builder
+  b.WriteString("msg_")
+  for _, x := range buf {
+    b.WriteByte(opencodeIDAlphabet[int(x)&31])
+  }
+  return b.String()
+}
+
+// applyLLMHeaders sets the request headers appropriate for the configured
+// endpoint. OpenCode (Zen) endpoints receive the x-opencode-* family plus the
+// full provider User-Agent; all other endpoints keep the legacy
+// session-affinity headers. msgID is used for the x-opencode-request header.
+func applyLLMHeaders(req *http.Request, cfg *Cfg, msgID string) {
+  sid := opencodeSessionID()
+  if isOpencodeEndpoint(cfg.Endpoint) {
+    req.Header.Set("User-Agent", opencodeAgentVersion+" "+opencodeProviderUA)
+    req.Header.Set("x-opencode-client", "cli")
+    req.Header.Set("x-opencode-project", projectID())
+    req.Header.Set("x-opencode-session", sid)
+    req.Header.Set("x-opencode-request", msgID)
+  } else {
+    req.Header.Set("User-Agent", opencodeAgentVersion)
+    req.Header.Set("x-session-affinity", sid)
+    req.Header.Set("X-Session-Id", sid)
+  }
+  if cfg.APIKey != "" && cfg.APIKey != "-" {
+    req.Header.Set("Authorization", "Bearer "+cfg.APIKey)
+  }
+}
+
 func saveSession(msgs []Message, sid string) (string, string) {
   sid = strings.TrimSpace(sid)
   if sid == "" {
diff --git a/main_test.go b/main_test.go
index c49718d..91cc7d6 100644
--- a/main_test.go
+++ b/main_test.go
@@ -1147,13 +1147,15 @@ func TestCol(t *testing.T) {
 // ---------- llm / AL / summarize / compact via httptest (no real network) ----------
 
 func TestLLMNonStreamingAndHeaders(t *testing.T) {
-  var gotPath, gotAuth, gotUA, gotOCClient, gotOCSession string
+  var gotPath, gotAuth, gotUA, gotLegacy, gotAffinity, gotOCClient, gotOCReq string
   srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
     gotPath = r.URL.Path
     gotAuth = r.Header.Get("Authorization")
     gotUA = r.Header.Get("User-Agent")
+    gotLegacy = r.Header.Get("X-Session-Id")
+    gotAffinity = r.Header.Get("x-session-affinity")
     gotOCClient = r.Header.Get("x-opencode-client")
-    gotOCSession = r.Header.Get("x-opencode-session")
+    gotOCReq = r.Header.Get("x-opencode-request")
     w.Write([]byte(`{"choices":[{"message":{"role":"assistant","content":"hi","reasoning_content":"think"}}]}`))
   }))
   defer srv.Close()
@@ -1175,17 +1177,111 @@ func TestLLMNonStreamingAndHeaders(t *testing.T) {
   if !strings.Contains(gotUA, "opencode/1.18.31") {
     t.Errorf("user-agent = %q", gotUA)
   }
-  if gotOCClient != "cli" {
-    t.Errorf("x-opencode-client = %q, want cli", gotOCClient)
+  // Non-OpenCode endpoints keep the legacy session-affinity headers and must
+  // not receive any x-opencode-* headers.
+  if gotLegacy != opencodeSessionID() {
+    t.Errorf("X-Session-Id = %q, want %q", gotLegacy, opencodeSessionID())
   }
-  if gotOCSession != opencodeSessionID() {
-    t.Errorf("x-opencode-session = %q, want %q", gotOCSession, opencodeSessionID())
+  if gotAffinity != opencodeSessionID() {
+    t.Errorf("x-session-affinity = %q, want %q", gotAffinity, opencodeSessionID())
+  }
+  if gotOCClient != "" || gotOCReq != "" {
+    t.Errorf("unexpected x-opencode headers: client=%q request=%q", gotOCClient, gotOCReq)
   }
   if m.Content == nil || *m.Content != "hi" || m.ReasoningContent != "think" {
     t.Errorf("message = %+v", m)
   }
 }
 
+func TestIsOpencodeEndpoint(t *testing.T) {
+  yes := []string{
+    "https://opencode.ai/zen/v1",
+    "https://opencode.ai/zen/go/v1",
+    "https://opencode.ai/zen/v1/chat/completions",
+    "https://api.opencode.ai/v1",
+    "https://OPENCODE.AI/zen/v1",
+    "opencode.ai",
+  }
+  for _, e := range yes {
+    if !isOpencodeEndpoint(e) {
+      t.Errorf("isOpencodeEndpoint(%q) = false, want true", e)
+    }
+  }
+  no := []string{
+    "https://api.kilo.ai/api/openrouter",
+    "http://127.0.0.1:4321/v1",
+    "https://api.openai.com/v1",
+    "https://notopencode.ai.example.com/v1",
+    "https://example.com/opencode.ai",
+  }
+  for _, e := range no {
+    if isOpencodeEndpoint(e) {
+      t.Errorf("isOpencodeEndpoint(%q) = true, want false", e)
+    }
+  }
+}
+
+func TestOcRequestID(t *testing.T) {
+  id := ocRequestID()
+  if !strings.HasPrefix(id, "msg_") {
+    t.Fatalf("request id %q does not start with msg_", id)
+  }
+  if len(id) != 30 {
+    t.Fatalf("request id length = %d, want 30: %q", len(id), id)
+  }
+  for _, c := range id[4:] {
+    if !strings.ContainsRune(opencodeIDAlphabet, c) {
+      t.Fatalf("invalid character %c in request id %q", c, id)
+    }
+  }
+  if ocRequestID() == id {
+    t.Fatalf("consecutive request ids should differ: %q", id)
+  }
+}
+
+func TestApplyLLMHeadersOpencode(t *testing.T) {
+  cfg := defCfg
+  cfg.Endpoint = "https://opencode.ai/zen/v1"
+  cfg.APIKey = "-"
+  req, err := http.NewRequest("POST", "https://opencode.ai/zen/v1/chat/completions", nil)
+  if err != nil {
+    t.Fatalf("NewRequest: %v", err)
+  }
+  applyLLMHeaders(req, &cfg, "msg_test")
+  if got := req.Header.Get("User-Agent"); !strings.Contains(got, "opencode/1.18.31") || !strings.Contains(got, "runtime/bun") {
+    t.Errorf("user-agent = %q", got)
+  }
+  if got := req.Header.Get("x-opencode-client"); got != "cli" {
+    t.Errorf("x-opencode-client = %q, want cli", got)
+  }
+  if got := req.Header.Get("x-opencode-project"); got != projectID() {
+    t.Errorf("x-opencode-project = %q, want %q", got, projectID())
+  }
+  if got := req.Header.Get("x-opencode-session"); got != opencodeSessionID() {
+    t.Errorf("x-opencode-session = %q, want %q", got, opencodeSessionID())
+  }
+  if got := req.Header.Get("x-opencode-request"); got != "msg_test" {
+    t.Errorf("x-opencode-request = %q, want msg_test", got)
+  }
+  if got := req.Header.Get("X-Session-Id"); got != "" {
+    t.Errorf("legacy X-Session-Id should be absent, got %q", got)
+  }
+  if got := req.Header.Get("Authorization"); got != "" {
+    t.Errorf("Authorization should be absent for '-' key, got %q", got)
+  }
+}
+
+func TestApplyLLMHeadersOpencodeAuth(t *testing.T) {
+  cfg := defCfg
+  cfg.Endpoint = "https://opencode.ai/zen/v1"
+  cfg.APIKey = "public"
+  req, _ := http.NewRequest("POST", "https://opencode.ai/zen/v1/chat/completions", nil)
+  applyLLMHeaders(req, &cfg, "msg_test")
+  if got := req.Header.Get("Authorization"); got != "Bearer public" {
+    t.Errorf("Authorization = %q, want Bearer public", got)
+  }
+}
+
 func TestOpencodeSessionID(t *testing.T) {
   sid := opencodeSessionID()
   if !strings.HasPrefix(sid, "ses_") {