diff --git a/open-sse/executors/vertex.ts b/open-sse/executors/vertex.ts
index 7412fa24b9..02994026d0 100644
--- a/open-sse/executors/vertex.ts
+++ b/open-sse/executors/vertex.ts
@@ -21,6 +21,28 @@ export function parseSAFromApiKey(apiKey: string): ServiceAccount {
   }
 }
 
+/**
+ * A Service Account credential is a JSON object (type/client_email/private_key). A Vertex AI
+ * Express-mode API key is an opaque non-JSON string. Distinguishing them lets the executor
+ * support BOTH: Service Account JSON (JWT → OAuth → project-scoped endpoint + Bearer auth) and
+ * Express keys (project-less publisher endpoint + x-goog-api-key auth), instead of failing every
+ * Express key with "requires a valid Service Account JSON".
+ */
+export function looksLikeServiceAccountJson(apiKey: string): boolean {
+  if (!apiKey || typeof apiKey !== "string") return false;
+  try {
+    const parsed = JSON.parse(apiKey);
+    return !!parsed && typeof parsed === "object" && !Array.isArray(parsed);
+  } catch {
+    return false;
+  }
+}
+
+/** True for a Vertex AI Express-mode API key (a non-empty, non-JSON, non-OAuth credential). */
+export function isExpressApiKey(apiKey?: string | null): boolean {
+  return typeof apiKey === "string" && apiKey.trim().length > 0 && !looksLikeServiceAccountJson(apiKey);
+}
+
 export async function getAccessToken(sa: ServiceAccount): Promise<string> {
   if (!sa.client_email || !sa.private_key) {
     throw new Error(
@@ -110,7 +132,13 @@ export class VertexExecutor extends BaseExecutor {
 
   async execute(input: ExecuteInput) {
     const { credentials, log } = input;
-    if (credentials.apiKey && !credentials.accessToken) {
+    // Defensive: trim stray surrounding whitespace from a pasted credential.
+    if (typeof credentials.apiKey === "string") {
+      credentials.apiKey = credentials.apiKey.trim();
+    }
+    // Service Account JSON → mint a short-lived OAuth token (Bearer). An Express-mode API key is
+    // sent as-is via x-goog-api-key (see buildHeaders), so no token exchange is needed for it.
+    if (credentials.apiKey && !credentials.accessToken && looksLikeServiceAccountJson(credentials.apiKey)) {
       try {
         const sa = parseSAFromApiKey(credentials.apiKey);
         credentials.accessToken = await getAccessToken(sa);
@@ -123,6 +151,19 @@ export class VertexExecutor extends BaseExecutor {
   }
 
   buildUrl(model: string, stream: boolean, urlIndex = 0, credentials: any = null) {
+    // Vertex AI Express mode: project-less v1 publisher endpoint with the API key passed as a
+    // ?key= query parameter (verified working contract — same as the CaptionAI GeminiClient). The
+    // Express key is NOT accepted as a Bearer/OAuth credential or via x-goog-api-key on this API.
+    if (isExpressApiKey(credentials?.apiKey) && !credentials?.accessToken) {
+      const expressKey = encodeURIComponent(String(credentials.apiKey).trim());
+      if (isPartnerModel(model)) {
+        // Partner (Anthropic/etc.) models are not available via Express keys; best-effort.
+        return `https://aiplatform.googleapis.com/v1/publishers/openapi/chat/completions?key=${expressKey}`;
+      }
+      const op = stream ? "streamGenerateContent?alt=sse&" : "generateContent?";
+      return `https://aiplatform.googleapis.com/v1/publishers/google/models/${model}:${op}key=${expressKey}`;
+    }
+
     const region = credentials?.providerSpecificData?.region || "us-central1";
     let project = "unknown-project";
 
@@ -146,6 +187,7 @@ export class VertexExecutor extends BaseExecutor {
     if (credentials.accessToken) {
       headers["Authorization"] = `Bearer ${credentials.accessToken}`;
     }
+    // Express-mode keys are carried in the ?key= query parameter (see buildUrl), not a header.
     if (stream) {
       headers["Accept"] = "text/event-stream";
     }
diff --git a/src/lib/providers/validation.ts b/src/lib/providers/validation.ts
index 61c1aa1d40..c6387ec805 100644
--- a/src/lib/providers/validation.ts
+++ b/src/lib/providers/validation.ts
@@ -3954,8 +3954,13 @@ export async function validateProviderApiKey({ provider, apiKey, providerSpecifi
     },
     vertex: async ({ apiKey }: any) => {
       try {
-        const { parseSAFromApiKey, getAccessToken } =
+        const { parseSAFromApiKey, getAccessToken, isExpressApiKey } =
           await import("@omniroute/open-sse/executors/vertex.ts");
+        // Express-mode API keys are opaque strings sent directly as the ?key= query param — there is
+        // no JWT to mint, so accept any non-empty Express key (the live chat/media call validates it).
+        if (isExpressApiKey(apiKey)) {
+          return { valid: true, error: null };
+        }
         const sa = parseSAFromApiKey(apiKey);
         // Validates credentials by successfully successfully exchanging them for a JWT from Google Identity
         await getAccessToken(sa);
@@ -3966,8 +3971,11 @@ export async function validateProviderApiKey({ provider, apiKey, providerSpecifi
     },
     "vertex-partner": async ({ apiKey }: any) => {
       try {
-        const { parseSAFromApiKey, getAccessToken } =
+        const { parseSAFromApiKey, getAccessToken, isExpressApiKey } =
           await import("@omniroute/open-sse/executors/vertex.ts");
+        if (isExpressApiKey(apiKey)) {
+          return { valid: true, error: null };
+        }
         const sa = parseSAFromApiKey(apiKey);
         await getAccessToken(sa);
         return { valid: true, error: null };
diff --git a/tests/unit/executor-vertex-extended.test.ts b/tests/unit/executor-vertex-extended.test.ts
index 02cce26a2a..6318195c40 100644
--- a/tests/unit/executor-vertex-extended.test.ts
+++ b/tests/unit/executor-vertex-extended.test.ts
@@ -51,17 +51,22 @@ test("VertexExecutor.buildUrl defaults to us-central1 and unknown-project when p
     apiKey: createServiceAccountJson({ includeProjectId: false }),
     providerSpecificData: {},
   });
-  const invalidJson = executor.buildUrl("gemini-2.5-flash", false, 0, {
-    apiKey: "not-json",
-  });
 
   assert.equal(
     missingProject,
     "https://aiplatform.googleapis.com/v1/projects/unknown-project/locations/us-central1/publishers/google/models/gemini-2.5-flash:generateContent"
   );
+});
+
+test("VertexExecutor.buildUrl routes a non-JSON Express API key to the project-less publisher endpoint", () => {
+  const executor = new VertexExecutor();
+  const expressUrl = executor.buildUrl("gemini-2.5-flash", false, 0, {
+    apiKey: "express-key-abc",
+  });
+
   assert.equal(
-    invalidJson,
-    "https://aiplatform.googleapis.com/v1/projects/unknown-project/locations/us-central1/publishers/google/models/gemini-2.5-flash:generateContent"
+    expressUrl,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-2.5-flash:generateContent?key=express-key-abc"
   );
 });
 
@@ -191,20 +196,12 @@ test("VertexExecutor.execute skips Service Account parsing when accessToken is a
   }
 });
 
-test("VertexExecutor.execute rejects invalid or incomplete Service Account JSON clearly", async () => {
+test("VertexExecutor.execute rejects incomplete Service Account JSON clearly", async () => {
   const executor = new VertexExecutor();
 
-  await assert.rejects(
-    () =>
-      executor.execute({
-        model: "gemini-2.5-flash",
-        body: { contents: [] },
-        stream: false,
-        credentials: { apiKey: "not-json" },
-      }),
-    /Service Account JSON/
-  );
-
+  // A JSON object missing client_email/private_key is treated as a Service Account (not an Express
+  // key) and must fail clearly when minting a JWT. A non-JSON string is an Express key (covered
+  // elsewhere) and is intentionally NOT rejected here.
   await assert.rejects(
     () =>
       executor.execute({
diff --git a/tests/unit/t29-vertex-sa-json-executor.test.ts b/tests/unit/t29-vertex-sa-json-executor.test.ts
index e2a2207ce7..f367718084 100644
--- a/tests/unit/t29-vertex-sa-json-executor.test.ts
+++ b/tests/unit/t29-vertex-sa-json-executor.test.ts
@@ -55,17 +55,34 @@ test("T29: Vertex executor headers include Bearer token and SSE Accept when stre
   assert.equal(headers.Accept, "text/event-stream");
 });
 
-test("T29: Vertex executor rejects invalid Service Account JSON clearly", async () => {
+test("T29: Vertex executor rejects incomplete Service Account JSON clearly", async () => {
   const executor = new VertexExecutor();
 
+  // A JSON object (not an opaque Express key) that is missing client_email/private_key must still
+  // fail clearly when the executor tries to mint a JWT from it.
   await assert.rejects(
     () =>
       executor.execute({
         model: "gemini-2.5-flash",
         body: { contents: [] },
         stream: false,
-        credentials: { apiKey: "not-json" },
+        credentials: { apiKey: JSON.stringify({ project_id: "p" }) },
       }),
-    /Service Account JSON/i
+    /missing required fields/i
+  );
+});
+
+test("T29: Vertex executor routes a non-JSON Express API key to the project-less publisher endpoint", () => {
+  const executor = new VertexExecutor();
+  const stream = executor.buildUrl("gemini-2.5-flash", true, 0, { apiKey: "express-key-123" });
+  const nonStream = executor.buildUrl("gemini-2.5-flash", false, 0, { apiKey: "express-key-123" });
+
+  assert.equal(
+    stream,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-2.5-flash:streamGenerateContent?alt=sse&key=express-key-123"
+  );
+  assert.equal(
+    nonStream,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-2.5-flash:generateContent?key=express-key-123"
   );
 });
diff --git a/tests/unit/vertex-express-apikey.test.ts b/tests/unit/vertex-express-apikey.test.ts
new file mode 100644
index 0000000000..555b11c1de
--- /dev/null
+++ b/tests/unit/vertex-express-apikey.test.ts
@@ -0,0 +1,102 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { VertexExecutor, isExpressApiKey, looksLikeServiceAccountJson } = await import(
+  "../../open-sse/executors/vertex.ts"
+);
+
+test("looksLikeServiceAccountJson is true only for a JSON object credential", () => {
+  assert.equal(looksLikeServiceAccountJson(JSON.stringify({ project_id: "p" })), true);
+  assert.equal(looksLikeServiceAccountJson("express-opaque-key"), false);
+  assert.equal(looksLikeServiceAccountJson(JSON.stringify([1, 2, 3])), false);
+  assert.equal(looksLikeServiceAccountJson(""), false);
+});
+
+test("isExpressApiKey is true for a non-empty, non-JSON credential", () => {
+  assert.equal(isExpressApiKey("AIzaSyExpressKey"), true);
+  assert.equal(isExpressApiKey("   "), false);
+  assert.equal(isExpressApiKey(""), false);
+  assert.equal(isExpressApiKey(null), false);
+  assert.equal(isExpressApiKey(undefined), false);
+  assert.equal(isExpressApiKey(JSON.stringify({ project_id: "p" })), false);
+});
+
+test("buildUrl Express: streaming google model uses streamGenerateContent + ?alt=sse&key=", () => {
+  const executor = new VertexExecutor();
+  const url = executor.buildUrl("gemini-3-flash-preview", true, 0, { apiKey: "k-express" });
+  assert.equal(
+    url,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-3-flash-preview:streamGenerateContent?alt=sse&key=k-express"
+  );
+});
+
+test("buildUrl Express: non-streaming google model uses generateContent?key=", () => {
+  const executor = new VertexExecutor();
+  const url = executor.buildUrl("gemini-3-flash-preview", false, 0, { apiKey: "k-express" });
+  assert.equal(
+    url,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-3-flash-preview:generateContent?key=k-express"
+  );
+});
+
+test("buildUrl Express: the API key is URL-encoded and trimmed", () => {
+  const executor = new VertexExecutor();
+  const url = executor.buildUrl("gemini-2.5-flash", false, 0, { apiKey: "  a/b+c=d  " });
+  assert.equal(
+    url,
+    "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-2.5-flash:generateContent?key=a%2Fb%2Bc%3Dd"
+  );
+});
+
+test("buildUrl Express: a present accessToken takes the Service Account path, not Express", () => {
+  const executor = new VertexExecutor();
+  const url = executor.buildUrl("gemini-2.5-flash", false, 0, {
+    apiKey: "k-express",
+    accessToken: "ya29.token",
+    providerSpecificData: { region: "us-central1" },
+  });
+  assert.equal(
+    url,
+    "https://aiplatform.googleapis.com/v1/projects/unknown-project/locations/us-central1/publishers/google/models/gemini-2.5-flash:generateContent"
+  );
+});
+
+test("buildHeaders for an Express key (no accessToken) omits the Authorization header", () => {
+  const executor = new VertexExecutor();
+  const headers = executor.buildHeaders({ apiKey: "k-express" }, false);
+  assert.equal(headers["Content-Type"], "application/json");
+  assert.equal(headers.Authorization, undefined);
+});
+
+test("execute with an Express key calls the publisher endpoint directly (no OAuth token exchange)", async () => {
+  const executor = new VertexExecutor();
+  const originalFetch = globalThis.fetch;
+  const calls: string[] = [];
+
+  globalThis.fetch = async (url: any) => {
+    calls.push(String(url));
+    return new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { "Content-Type": "application/json" },
+    });
+  };
+
+  try {
+    const result = await executor.execute({
+      model: "gemini-3-flash-preview",
+      body: { contents: [{ role: "user", parts: [{ text: "hi" }] }] },
+      stream: false,
+      credentials: { apiKey: "AIzaSyExpressKey" },
+    } as any);
+
+    assert.equal(result.response.status, 200);
+    // Exactly one call — straight to Vertex, no oauth2 token mint.
+    assert.equal(calls.length, 1);
+    assert.equal(
+      calls[0],
+      "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-3-flash-preview:generateContent?key=AIzaSyExpressKey"
+    );
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});