diff --git a/CHANGELOG.md b/CHANGELOG.md
index b7b4987..e2c619f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,16 @@ and this project uses [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 
 ## [Unreleased]
 
+## [v0.2.1] - 2026-06-13
+
+A module can now read its own **trusted app id** on every guarded surface — including Public routes, which previously had no identity at all.
+
+### Added
+- **`ms.AppID(ctx) string`** — the inbound twin of `ms.WithAppID`. Returns the app id from the request context's auth identity (`""` when none is set). This is the single **unspoofable** way a handler reads its own app: the SDK promotes the platform's trusted, dispatch-injected app id into the identity before the handler runs. Read this instead of pulling an app id off request data (query/body/path), which the caller controls and can forge.
+
+### Changed
+- **The proxy guard (`auth.RequireProxy`) now promotes trusted app identity on its success path.** After the platform token validates — which proves the `X-MS-*` headers were injected by dispatch, not client-forged — the guard sets `auth.Identity` (`AppID`/`UserID`/`AppRole`) from those headers before the handler runs. This closes a gap on **Public** routes: they mount only the proxy guard (not `PlatformAuth`), so `auth.Get(ctx).AppID` was always empty there and a module could not read its own app. Promotion never happens on a path that has not validated the token (standalone/inert and rejected requests don't promote), and never clobbers an identity already set (e.g. Lambda's `InjectResources`). Mirrors the prod-Lambda asymmetry that `runtime.InjectResources` already closed.
+
 ## [v0.2.0] - 2026-05-06
 
 Phase 2 — module identity, prefix-aware schema resolution, and the cross-module data-routing contract. **Trust model: app owner is the trust root** for cross-module reads. The contributor declares nothing about who can read; the consumer declares what it wants from each dep; the catalog surfaces the pairing to the app owner at install time. Read-only by design — `GRANT SELECT` only, never write. Cross-module *writes* go through events or internal HTTP.
diff --git a/VERSION b/VERSION
index 0ea3a94..0c62199 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.2.0
+0.2.1
diff --git a/auth/middleware.go b/auth/middleware.go
index c387c2c..cee493d 100644
--- a/auth/middleware.go
+++ b/auth/middleware.go
@@ -268,7 +268,10 @@ func requireProxy(inLambda bool) func(http.Handler) http.Handler {
 
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			// Lambda: headers already stripped + identity injected from payload.
+			// Lambda: headers already stripped + identity injected from payload
+			// (runtime.InjectResources). The payload is the trust boundary, so
+			// pass through WITHOUT promoting from headers — there are none to
+			// read, and the preset identity must not be touched.
 			if inLambda {
 				next.ServeHTTP(w, r)
 				return
@@ -276,7 +279,10 @@ func requireProxy(inLambda bool) func(http.Handler) http.Handler {
 
 			expected, header, configured := readSecret()
 
-			// No token SOURCE configured: inert (standalone unit tests).
+			// No token SOURCE configured: inert (standalone unit tests). DO NOT
+			// promote identity here — nothing validated that the X-MS-* headers
+			// came from dispatch, so trusting them would let a direct caller
+			// forge an app id. The surface is simply open (local dev / tests).
 			if !configured {
 				next.ServeHTTP(w, r)
 				return
@@ -288,27 +294,54 @@ func requireProxy(inLambda bool) func(http.Handler) http.Handler {
 			// reject rather than silently pass every request through.
 			if expected == "" {
 				log.Printf("mirrorstack: proxy guard rejected (token source configured but unreadable) from %s %s", r.RemoteAddr, r.URL.Path)
-				httputil.JSON(w, http.StatusForbidden, httputil.ErrorResponse{
-					Error: "request did not come through the platform proxy",
-					Code:  CodeNotProxied,
-				})
+				rejectNotProxied(w)
 				return
 			}
 
 			token := r.Header.Get(header)
 			if !constantTimeEqual(token, expected) {
 				log.Printf("mirrorstack: proxy guard rejected (token mismatch, header_present=%v) from %s %s", token != "", r.RemoteAddr, r.URL.Path)
-				httputil.JSON(w, http.StatusForbidden, httputil.ErrorResponse{
-					Error: "request did not come through the platform proxy",
-					Code:  CodeNotProxied,
-				})
+				rejectNotProxied(w)
 				return
 			}
-			next.ServeHTTP(w, r)
+
+			// SUCCESS PATH ONLY. The platform token validated, which proves the
+			// X-MS-* identity headers were injected by dispatch (the browser
+			// never holds the token, so a direct caller cannot forge a request
+			// that reaches here). Promote those now-trusted headers to
+			// auth.Identity so a Public/Platform handler can read its app via
+			// auth.Get(ctx).AppID — the single unspoofable source of app id.
+			//
+			// CRITICAL ORDERING: this MUST run only after the token check above
+			// passes. Promoting before validation would trust spoofable headers.
+			//
+			// Don't clobber an identity already on the context: if something
+			// upstream (e.g. PlatformAuth on the platform surface, or a future
+			// authorizer) already set one, that wins.
+			ctx := r.Context()
+			if Get(ctx) == nil {
+				ctx = Set(ctx, Identity{
+					UserID:  r.Header.Get(HeaderUserID),
+					AppID:   r.Header.Get(HeaderAppID),
+					AppRole: r.Header.Get(HeaderAppRole),
+				})
+			}
+			next.ServeHTTP(w, r.WithContext(ctx))
 		})
 	}
 }
 
+// rejectNotProxied writes the standard 403 not_proxied response. Both failure
+// paths in requireProxy (unreadable token source and token mismatch) produce
+// the same JSON body; the log call before each site carries the distinct
+// diagnostic. (Supersedes the dedup in PR #118.)
+func rejectNotProxied(w http.ResponseWriter) {
+	httputil.JSON(w, http.StatusForbidden, httputil.ErrorResponse{
+		Error: "request did not come through the platform proxy",
+		Code:  CodeNotProxied,
+	})
+}
+
 // secretReader returns the current secret, which header carries it, and
 // whether a secret SOURCE is configured at all. The file-backed variant
 // re-reads on every call so a refreshed token (CLI reconnect) is picked up
diff --git a/auth/middleware_test.go b/auth/middleware_test.go
index 9601bea..b2387ca 100644
--- a/auth/middleware_test.go
+++ b/auth/middleware_test.go
@@ -663,3 +663,121 @@ func TestRequireProxy_TokenFile_ReadError_NotMistakenForUnconfigured(t *testing.
 		t.Fatal("SecretConfigured() must be true when MS_PLATFORM_TOKEN_FILE is set even if unreadable")
 	}
 }
+
+// --- RequireProxy identity promotion (Public-route trusted app id) ---
+
+func TestRequireProxy_ValidToken_PromotesIdentity(t *testing.T) {
+	// The whole point of the #236 redesign: on a Public route, after the proxy
+	// token validates, the dispatch-injected X-MS-* headers are promoted to
+	// auth.Identity so the handler can read its TRUSTED app id via auth.Get —
+	// PlatformAuth doesn't run on Public, so without this promotion AppID is
+	// empty there.
+	t.Setenv("MS_PLATFORM_TOKEN", "real-token")
+	var seen *Identity
+	handler := requireProxy(false)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		seen = Get(r.Context())
+	}))
+
+	req := httptest.NewRequest("GET", "/public/me", nil)
+	req.Header.Set(HeaderPlatformToken, "real-token")
+	req.Header.Set(HeaderUserID, "u-pub-1")
+	req.Header.Set(HeaderAppID, "a-pub-2")
+	req.Header.Set(HeaderAppRole, RoleViewer)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200 for valid proxied request, got %d", rec.Code)
+	}
+	if seen == nil {
+		t.Fatal("expected promoted Identity on the context; got nil")
+	}
+	if seen.AppID != "a-pub-2" {
+		t.Errorf("AppID = %q, want a-pub-2 (the trusted, dispatch-injected app id)", seen.AppID)
+	}
+	if seen.UserID != "u-pub-1" || seen.AppRole != RoleViewer {
+		t.Errorf("identity mismatch: got %+v", seen)
+	}
+}
+
+func TestRequireProxy_RejectedRequest_NeverPromotes(t *testing.T) {
+	// A request rejected by the guard (no/wrong token) must NEVER reach the
+	// handler, so no identity is ever promoted from its (spoofable) headers.
+	t.Setenv("MS_PLATFORM_TOKEN", "real-token")
+	reached := false
+	handler := requireProxy(false)(http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+		reached = true
+	}))
+
+	req := httptest.NewRequest("GET", "/public/me", nil)
+	// Attacker omits the token but tries to assert an app id directly.
+	req.Header.Set(HeaderAppID, "spoofed-app")
+	req.Header.Set(HeaderUserID, "spoofed-user")
+	req.Header.Set(HeaderAppRole, RoleAdmin)
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusForbidden {
+		t.Fatalf("expected 403, got %d", rec.Code)
+	}
+	if reached {
+		t.Error("handler must not run on a rejected request — promotion would trust spoofed headers")
+	}
+}
+
+func TestRequireProxy_NoSecretInert_DoesNotPromote(t *testing.T) {
+	// Standalone `go test` (no token configured): the guard is inert and passes
+	// through, but it must NOT promote spoofable headers — nothing validated
+	// that they came from dispatch. Documented behavior: surface is open, app id
+	// unset (a module's own unit test injects identity explicitly if it needs
+	// one).
+	t.Setenv("MS_PLATFORM_TOKEN_FILE", "")
+	t.Setenv("MS_PLATFORM_TOKEN", "")
+	t.Setenv("MS_INTERNAL_SECRET", "")
+	var seen *Identity
+	handler := requireProxy(false)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		seen = Get(r.Context())
+	}))
+
+	req := httptest.NewRequest("GET", "/public/me", nil)
+	req.Header.Set(HeaderAppID, "spoofed-app")
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200 (inert without configured token), got %d", rec.Code)
+	}
+	if seen != nil {
+		t.Errorf("inert guard must not promote unvalidated headers; got identity %+v", seen)
+	}
+}
+
+func TestRequireProxy_PresetIdentity_NotClobbered(t *testing.T) {
+	// Lambda's InjectResources sets identity from the typed payload BEFORE any
+	// header promotion could run. The guard must never overwrite a preset
+	// identity with header values. Simulate by presetting on the context and
+	// sending conflicting headers on the validated-token path.
+	t.Setenv("MS_PLATFORM_TOKEN", "real-token")
+	var seen *Identity
+	handler := requireProxy(false)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		seen = Get(r.Context())
+	}))
+
+	req := httptest.NewRequest("GET", "/public/me", nil)
+	req.Header.Set(HeaderPlatformToken, "real-token")
+	req.Header.Set(HeaderAppID, "header-app")
+	req = req.WithContext(Set(req.Context(), Identity{
+		UserID:  "preset-user",
+		AppID:   "preset-app",
+		AppRole: RoleAdmin,
+	}))
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", rec.Code)
+	}
+	if seen == nil || seen.AppID != "preset-app" {
+		t.Errorf("preset identity must win over headers; got %+v", seen)
+	}
+}
diff --git a/docs/concepts/scopes.md b/docs/concepts/scopes.md
index ebd3135..6d638d3 100644
--- a/docs/concepts/scopes.md
+++ b/docs/concepts/scopes.md
@@ -10,9 +10,19 @@ Every HTTP route lives under one of three **scopes**. The scope determines which
 | **Public** | `ms.Public(fn)` | None | Anonymous (webhooks, OAuth callbacks, public APIs) |
 | **Internal** | `ms.Internal(fn)` | HMAC (`auth.InternalAuth`) | Platform itself (lifecycle, events, crons) |
 
+## Reading your app id
+
+On **both** Platform and Public routes, read your module's app id from the trusted context identity — never from request data (query string, body, path), which the caller controls and can forge:
+
+```go
+appID := ms.AppID(r.Context())
+```
+
+The SDK promotes the platform's trusted, dispatch-injected app id into the context **before your handler runs**: on Platform via the session auth, on Public via the proxy guard's validated-token path (the guard proves the request came through dispatch, so the `X-MS-App-ID` header it forwards is trustworthy). `ms.AppID` returns it; it returns `""` only in a standalone unit test where no platform token is configured. `ms.AppID` is the inbound twin of `ms.WithAppID` (which *retargets* an outbound `ms.Call` at a different app).
+
 ## Platform
 
-Authenticated users of the host dashboard. The SDK checks a session token set by the platform's auth flow. Routes receive an `auth.Identity` via context with `AppID`, `UserID`, and `AppRole`.
+Authenticated users of the host dashboard. The SDK checks a session token set by the platform's auth flow. Routes receive an `auth.Identity` via context with `AppID`, `UserID`, and `AppRole` — read the app id with `ms.AppID(r.Context())` (see above).
 
 ```go
 import p "github.com/mirrorstack-ai/app-module-sdk/roles"
@@ -39,7 +49,7 @@ ms.Public(func(r chi.Router) {
 })
 ```
 
-The SDK does not apply any auth here, but you are responsible for verifying payloads that claim identity (signed webhooks, OAuth state nonces, etc.).
+The SDK does not run user auth here, but the proxy guard still fronts every Public route: a request that did not come through the platform proxy is rejected with `403 not_proxied`. Because the guard validated the proxy token, the app id it promotes (`ms.AppID(r.Context())`) is trusted — use it instead of reading an app id off the request. You are still responsible for verifying payloads that claim *user* identity (signed webhooks, OAuth state nonces, etc.).
 
 ## Internal
 
diff --git a/docs/zh-TW/concepts/scopes.md b/docs/zh-TW/concepts/scopes.md
index 7d6d272..0e7cd02 100644
--- a/docs/zh-TW/concepts/scopes.md
+++ b/docs/zh-TW/concepts/scopes.md
@@ -10,9 +10,19 @@
 | **Public** | `ms.Public(fn)` | 無 | 匿名(webhooks、OAuth callbacks、公開 API) |
 | **Internal** | `ms.Internal(fn)` | HMAC(`auth.InternalAuth`) | 平台本身(lifecycle、events、crons) |
 
+## 讀取自己的 app id
+
+在 Platform 與 Public route **兩者**,都從受信任的 context identity 讀取自己模組的 app id,**不要**從 request 資料(query string、body、path)讀 — 那些是呼叫者可控、可偽造的:
+
+```go
+appID := ms.AppID(r.Context())
+```
+
+SDK 會在你的 handler 執行**之前**,把平台經 dispatch 注入、受信任的 app id 提升進 context:Platform 透過 session auth,Public 透過 proxy guard 驗證過 token 的路徑(guard 證明 request 確實經過 dispatch,所以它轉發的 `X-MS-App-ID` 是可信的)。`ms.AppID` 會回傳它;只有在沒有設定 platform token 的獨立單元測試裡才回傳 `""`。`ms.AppID` 是 `ms.WithAppID` 的「入站對偶」(`ms.WithAppID` 用來把一個 *對外* 的 `ms.Call` *改指向* 另一個 app)。
+
 ## Platform
 
-已登入的 dashboard 使用者。SDK 會檢查 platform auth flow 發出的 session token。Route 可以從 context 取得 `auth.Identity`,內含 `AppID`、`UserID`、`AppRole`。
+已登入的 dashboard 使用者。SDK 會檢查 platform auth flow 發出的 session token。Route 可以從 context 取得 `auth.Identity`,內含 `AppID`、`UserID`、`AppRole` — 用 `ms.AppID(r.Context())` 讀取 app id(見上)。
 
 ```go
 ms.Platform(func(r chi.Router) {
@@ -37,7 +47,7 @@ ms.Public(func(r chi.Router) {
 })
 ```
 
-SDK 在這層不做任何認證,但如果 payload 裡宣稱了身分(signed webhook、OAuth state nonce 等),你必須自己驗證。
+SDK 在這層不做使用者認證,但 proxy guard 仍然守在每個 Public route 前面:沒有經過平台 proxy 的 request 會被以 `403 not_proxied` 拒絕。因為 guard 驗證過 proxy token,它提升出來的 app id(`ms.AppID(r.Context())`)是可信的 — 用它,不要從 request 讀 app id。但如果 payload 裡宣稱了 *使用者* 身分(signed webhook、OAuth state nonce 等),你還是必須自己驗證。
 
 ## Internal
 
diff --git a/internal/core/call.go b/internal/core/call.go
index 9132c43..851b99e 100644
--- a/internal/core/call.go
+++ b/internal/core/call.go
@@ -134,3 +134,19 @@ func CallGet(ctx context.Context, targetModuleID, path string, out any) error {
 func CallPost(ctx context.Context, targetModuleID, path string, body, out any) error {
 	return mustDefault("CallPost").CallPost(ctx, targetModuleID, path, body, out)
 }
+
+// AppID returns the app id from the request context's auth identity, or "" if
+// no identity is set. It is the inbound twin of WithAppID and the single
+// unspoofable way a handler reads its own app: the SDK promotes the trusted,
+// dispatch-injected app id into the identity (Platform via PlatformAuth, Public
+// via the proxy guard's success path; Lambda via runtime.InjectResources).
+// Reading request data (query/body) for the app id instead is forgeable.
+//
+// Not module-bound (no *Module receiver) — identity lives on the context, so
+// this works before Init and in tests.
+func AppID(ctx context.Context) string {
+	if a := auth.Get(ctx); a != nil {
+		return a.AppID
+	}
+	return ""
+}
diff --git a/internal/core/module_test.go b/internal/core/module_test.go
index 91a0ba7..15ae2d1 100644
--- a/internal/core/module_test.go
+++ b/internal/core/module_test.go
@@ -302,6 +302,37 @@ func TestPublic_ProxyGuard_AllowsProxiedCaller(t *testing.T) {
 	}
 }
 
+func TestPublic_ProxyGuard_PromotesTrustedAppID(t *testing.T) {
+	// The #236 gap closure end-to-end: a Public route mounts ONLY the proxy
+	// guard (no PlatformAuth), so before this change AppID(ctx) was always empty
+	// on Public. Now the guard's validated-token path promotes the dispatch-
+	// injected X-MS-App-ID into the identity, so the handler reads its trusted
+	// app via core.AppID — never from request data.
+	m := newTestModuleWithSecret(t, "test") // MS_INTERNAL_SECRET = "secret"
+	var gotAppID string
+	m.Public(func(r chi.Router) {
+		r.Get("/start", func(w http.ResponseWriter, r *http.Request) {
+			gotAppID = AppID(r.Context())
+			_, _ = w.Write([]byte("started"))
+		})
+	})
+
+	req := httptest.NewRequest("GET", "/public/start", nil)
+	req.Header.Set("X-MS-Internal-Secret", "secret")
+	req.Header.Set(auth.HeaderAppID, "trusted-app-42")
+	req.Header.Set(auth.HeaderUserID, "u-9")
+	req.Header.Set(auth.HeaderAppRole, auth.RoleMember)
+	rec := httptest.NewRecorder()
+	m.Router().ServeHTTP(rec, req)
+
+	if rec.Code != 200 {
+		t.Fatalf("expected 200 for proxied caller, got %d", rec.Code)
+	}
+	if gotAppID != "trusted-app-42" {
+		t.Errorf("AppID(ctx) = %q on a Public route, want trusted-app-42 (promoted by the proxy guard)", gotAppID)
+	}
+}
+
 func TestPublic_ProxyGuard_InertWithoutToken(t *testing.T) {
 	// Standalone module `go test` (no platform token configured): the guard is
 	// inert so the public surface stays open for local unit tests.
diff --git a/mirrorstack.go b/mirrorstack.go
index 134ef27..1a9e646 100644
--- a/mirrorstack.go
+++ b/mirrorstack.go
@@ -188,15 +188,36 @@ func Emit(ctx context.Context, name string, payload any) error {
 	return core.Emit(ctx, name, payload)
 }
 
+// AppID returns the current app id from the request context, or "" if no
+// identity is set. It is the inbound twin of WithAppID and the single
+// unspoofable way a handler reads its OWN app.
+//
+// On every guarded surface the SDK promotes the platform's trusted, dispatch-
+// injected app id into the context identity BEFORE the handler runs — Platform
+// via PlatformAuth, Public via the proxy guard's validated-token path, and
+// Lambda via the typed invoke payload (runtime.InjectResources). So a Public or
+// Platform handler reads its app with:
+//
+//	appID := ms.AppID(r.Context())
+//
+// Do NOT read the app id from request data (query string, body, path) — those
+// are caller-controlled and forgeable. ms.AppID is the trusted source.
+func AppID(ctx context.Context) string {
+	return core.AppID(ctx)
+}
+
 // WithAppID returns a context whose inter-module Call scope is the given app,
 // overriding the ambient identity's app. ms.Call reads the app id from the
 // context (auth.Get) — for a handler that is the request's authenticated app,
-// so authenticated callers need nothing extra. PUBLIC/proxy flows have no
-// ambient identity (e.g. a sign-in proxy on ms.Public routes, where the target
-// app arrives as request data, not as the caller's identity), so they set it
-// explicitly:
+// so authenticated callers need nothing extra.
+//
+// Use it to RETARGET an outbound ms.Call at a DIFFERENT app than the ambient
+// one (the rare cross-app proxy case). To read your OWN app id, use ms.AppID —
+// the SDK already promotes the request's trusted app into the identity, so a
+// Public/Platform handler does not need WithAppID just to call within its own
+// app:
 //
-//	ctx = ms.WithAppID(ctx, appID)
+//	ctx = ms.WithAppID(ctx, otherAppID)
 //	ms.CallGet(ctx, providerModuleID, "/internal/authorize-url?"+q, &out)
 //
 // Any existing UserID/AppRole on the context is preserved; only AppID changes.
diff --git a/withappid_test.go b/withappid_test.go
index 52135e8..6066dbe 100644
--- a/withappid_test.go
+++ b/withappid_test.go
@@ -47,3 +47,29 @@ func TestWithAppID(t *testing.T) {
 		}
 	})
 }
+
+func TestAppID(t *testing.T) {
+	t.Run("returns empty string when no identity is set", func(t *testing.T) {
+		if got := ms.AppID(context.Background()); got != "" {
+			t.Errorf("AppID = %q, want empty string", got)
+		}
+	})
+
+	t.Run("reads the app id from the context identity", func(t *testing.T) {
+		ctx := auth.Set(context.Background(), auth.Identity{
+			UserID:  "u-1",
+			AppID:   "app-7",
+			AppRole: auth.RoleAdmin,
+		})
+		if got := ms.AppID(ctx); got != "app-7" {
+			t.Errorf("AppID = %q, want app-7", got)
+		}
+	})
+
+	t.Run("is the inbound twin of WithAppID", func(t *testing.T) {
+		ctx := ms.WithAppID(context.Background(), "app-rt")
+		if got := ms.AppID(ctx); got != "app-rt" {
+			t.Errorf("AppID = %q, want app-rt", got)
+		}
+	})
+}