diff --git a/packages/core/src/api.js b/packages/core/src/api.js
index 21652835b..8fae6d516 100644
--- a/packages/core/src/api.js
+++ b/packages/core/src/api.js
@@ -422,13 +422,34 @@ export function createPercyServer(percy, port) {
   // post a comparison by reading a Maestro screenshot from disk
     .route('post', '/percy/maestro-screenshot', async (req, res) => {
       /* istanbul ignore next — req.body falsy guard; tests always pass a body. */
-      let { name, sessionId } = req.body || {};
+      let { name, sessionId, runtime } = req.body || {};
 
       if (!name) throw new ServerError(400, 'Missing required field: name');
-      // `sessionId` is host-injected on BrowserStack; its absence signals
-      // self-hosted mode (gated separately on PERCY_MAESTRO_SCREENSHOT_DIR
-      // below). When present, the BS path runs byte-identical.
-      let selfHosted = !sessionId;
+
+      // Validate `runtime` if present: must be a string, but unknown values
+      // are NOT rejected — we treat anything other than "selfhosted" /
+      // "browserstack" as "fall back to sessionId-absent detection". This
+      // is defensive against future runtime values (maestro-cloud,
+      // saucelabs, etc.) the SDK might introduce ahead of relay support.
+      if (runtime !== undefined && typeof runtime !== 'string') {
+        throw new ServerError(400, 'Invalid runtime: must be a string');
+      }
+
+      // Self-hosted detection — prefer the SDK's explicit declaration when
+      // present, fall back to sessionId absence (the implicit signal used
+      // by SDKs that predate the runtime field). The fallback is what
+      // ships in cli#2261 today; the explicit runtime field arrives with
+      // @percy/maestro-app >= v1.0.0-Beta.1 (cf. percy-maestro-app#7+R2).
+      let selfHosted = (runtime === 'selfhosted') || (!runtime && !sessionId);
+
+      // Diagnostic surface for any inconsistency between the two signals
+      // (debug-only — never breaks the request).
+      if (runtime === 'browserstack' && !sessionId) {
+        percy.log.debug('maestro-screenshot: runtime=browserstack but sessionId missing — trusting runtime field');
+      }
+      if (runtime === 'selfhosted' && sessionId) {
+        percy.log.debug('maestro-screenshot: runtime=selfhosted but sessionId present — trusting runtime field');
+      }
 
       // Strict character-class validation — rejects path separators, shell metacharacters,
       // NUL, newlines, and anything else that could confuse the glob or the filesystem.
diff --git a/packages/core/test/api.test.js b/packages/core/test/api.test.js
index e432f9d2c..9dadae5ad 100644
--- a/packages/core/test/api.test.js
+++ b/packages/core/test/api.test.js
@@ -1255,6 +1255,103 @@ describe('API Server', () => {
         .toBeRejectedWithError(/Invalid platform: must be a string/);
     });
 
+    describe('runtime field gating', () => {
+      // The relay's self-hosted detection prefers an explicit `runtime` field
+      // from the SDK when present, falling back to sessionId-absence for older
+      // SDKs. These tests cover the eight-way matrix of (runtime ∈
+      // {undefined, "selfhosted", "browserstack", "unknown"}) × (sessionId ∈
+      // {present, absent}) — plus the type-validation 400.
+
+      it('rejects non-string runtime type with 400', async () => {
+        await percy.start();
+        await expectAsync(postMaestro({ name: SS_NAME, sessionId: SID, runtime: 123 }))
+          .toBeRejectedWithError(/Invalid runtime: must be a string/);
+      });
+
+      it('runtime=selfhosted + no sessionId → self-hosted branch (400s on missing env)', async () => {
+        let prior = process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        try {
+          await percy.start();
+          await expectAsync(postMaestro({ name: SS_NAME, runtime: 'selfhosted' }))
+            .toBeRejectedWithError(/Missing required env: PERCY_MAESTRO_SCREENSHOT_DIR/);
+        } finally {
+          if (prior === undefined) delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+          else process.env.PERCY_MAESTRO_SCREENSHOT_DIR = prior;
+        }
+      });
+
+      it('runtime=selfhosted + sessionId PRESENT → still self-hosted branch (trust runtime field)', async () => {
+        // Bidirectional-consistency case: SDK explicitly says self-hosted but
+        // a sessionId leaked into the payload. Trust the SDK's declaration.
+        let prior = process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        try {
+          await percy.start();
+          await expectAsync(postMaestro({ name: SS_NAME, sessionId: SID, runtime: 'selfhosted' }))
+            .toBeRejectedWithError(/Missing required env: PERCY_MAESTRO_SCREENSHOT_DIR/);
+        } finally {
+          if (prior === undefined) delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+          else process.env.PERCY_MAESTRO_SCREENSHOT_DIR = prior;
+        }
+      });
+
+      it('runtime=browserstack + sessionId present → BS branch finds file under /tmp/<sid>/...', async () => {
+        await percy.start();
+        const res = await postMaestro({ name: SS_NAME, sessionId: SID, runtime: 'browserstack' });
+        expect(res.status).toBe(200);
+      });
+
+      it('runtime=browserstack + sessionId ABSENT → BS branch (trust runtime), but file-find fails → 404', async () => {
+        // Bidirectional-consistency case: SDK explicitly says browserstack
+        // but sessionId is missing. BS branch runs, can't locate the file
+        // because `/tmp/<undefined>/...` doesn't resolve.
+        await percy.start();
+        await expectAsync(postMaestro({ name: SS_NAME, runtime: 'browserstack' }))
+          .toBeRejectedWithError(/Screenshot not found|sessionId/i);
+      });
+
+      it('runtime=unknown-value + no sessionId → falls back to sessionId-absent → self-hosted branch', async () => {
+        // Defensive: unknown runtime values are NOT rejected. The relay falls
+        // back to sessionId-absence detection, so this lands in self-hosted.
+        let prior = process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        try {
+          await percy.start();
+          await expectAsync(postMaestro({ name: SS_NAME, runtime: 'maestro-cloud' }))
+            .toBeRejectedWithError(/Missing required env: PERCY_MAESTRO_SCREENSHOT_DIR/);
+        } finally {
+          if (prior === undefined) delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+          else process.env.PERCY_MAESTRO_SCREENSHOT_DIR = prior;
+        }
+      });
+
+      it('runtime=unknown-value + sessionId present → falls back to sessionId-present → BS branch', async () => {
+        await percy.start();
+        const res = await postMaestro({ name: SS_NAME, sessionId: SID, runtime: 'maestro-cloud' });
+        expect(res.status).toBe(200);
+      });
+
+      it('no runtime + sessionId present → BS branch (backward-compat with pre-runtime SDKs)', async () => {
+        await percy.start();
+        const res = await postMaestro({ name: SS_NAME, sessionId: SID });
+        expect(res.status).toBe(200);
+      });
+
+      it('no runtime + no sessionId → self-hosted branch (backward-compat with pre-runtime SDKs)', async () => {
+        let prior = process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+        try {
+          await percy.start();
+          await expectAsync(postMaestro({ name: SS_NAME }))
+            .toBeRejectedWithError(/Missing required env: PERCY_MAESTRO_SCREENSHOT_DIR/);
+        } finally {
+          if (prior === undefined) delete process.env.PERCY_MAESTRO_SCREENSHOT_DIR;
+          else process.env.PERCY_MAESTRO_SCREENSHOT_DIR = prior;
+        }
+      });
+    });
+
     it('rejects non-object element selector with 400', async () => {
       await percy.start();
       await expectAsync(postMaestro({