diff --git a/packages/cli/src/capture/contentExtractor.test.ts b/packages/cli/src/capture/contentExtractor.test.ts
index 5d27c089f1..1a02584c5e 100644
--- a/packages/cli/src/capture/contentExtractor.test.ts
+++ b/packages/cli/src/capture/contentExtractor.test.ts
@@ -556,6 +556,83 @@ describe("captionImagesWithGemini — Vertex AI provider", () => {
     expect(clientOptions[0]).toMatchObject({ location: "europe-west4" });
   });
 
+  it.each([
+    "attacker.example/",
+    "x.attacker.example#",
+    "a@attacker.example",
+    "attacker.example:443?",
+    "us-central1.attacker.example",
+  ])("skips captioning for a location that is not a region: %s", async (location) => {
+    const dir = makeProjectWithImages();
+    dirs.push(dir);
+    vertexEnv();
+    vi.stubEnv("HYPERFRAMES_VERTEX_LOCATION", location);
+
+    const warnings: string[] = [];
+    const captions = await captionImagesWithGemini(dir, () => {}, warnings);
+
+    expect(captions).toEqual({});
+    expect(clientOptions).toHaveLength(0);
+    expect(generateContentMock).not.toHaveBeenCalled();
+    expect(warnings).toEqual([
+      "HYPERFRAMES_VERTEX_LOCATION is not a Google Cloud region; skipped vision captioning.",
+    ]);
+  });
+
+  it.each([
+    "../../v1/other",
+    "gemini?alt=x",
+    "gemini#x",
+    "https://attacker.example/m",
+    "/models/x",
+  ])("skips captioning for a model that is not a model id: %s", async (model) => {
+    const dir = makeProjectWithImages();
+    dirs.push(dir);
+    vertexEnv();
+    vi.stubEnv("HYPERFRAMES_VERTEX_MODEL", model);
+
+    const warnings: string[] = [];
+    const captions = await captionImagesWithGemini(dir, () => {}, warnings);
+
+    expect(captions).toEqual({});
+    expect(clientOptions).toHaveLength(0);
+    expect(warnings).toEqual([
+      "HYPERFRAMES_VERTEX_MODEL is not a model id; skipped vision captioning.",
+    ]);
+  });
+
+  it.each([
+    "gemini-2.5-flash",
+    "google/gemini-2.5-flash",
+    "publishers/anthropic/models/claude-sonnet-4@20250514",
+    "projects/example.com:proj/locations/us-central1/endpoints/123",
+  ])("accepts the model id %s", async (model) => {
+    const dir = makeProjectWithImages();
+    dirs.push(dir);
+    vertexEnv();
+    vi.stubEnv("HYPERFRAMES_VERTEX_MODEL", model);
+    generateContentMock.mockResolvedValue({ text: "A caption." });
+
+    const captions = await captionImagesWithGemini(dir, () => {}, []);
+
+    expect(captions).toEqual({ "hero.png": "A caption." });
+    expect(generateContentMock.mock.calls[0]?.[0]).toMatchObject({ model });
+  });
+
+  it.each(["global", "us", "eu", "europe-west4"])("accepts the region %s", async (location) => {
+    const dir = makeProjectWithImages();
+    dirs.push(dir);
+    vertexEnv();
+    vi.stubEnv("HYPERFRAMES_VERTEX_LOCATION", location);
+    vi.stubEnv("HYPERFRAMES_VERTEX_MODEL", "publishers/google/models/gemini-2.5-flash-001");
+    generateContentMock.mockResolvedValue({ text: "A caption." });
+
+    const captions = await captionImagesWithGemini(dir, () => {}, []);
+
+    expect(captions).toEqual({ "hero.png": "A caption." });
+    expect(clientOptions[0]).toMatchObject({ location });
+  });
+
   it("spends no output budget on thinking", async () => {
     // Thinking tokens come out of maxOutputTokens, so a model left free to think can consume
     // the whole budget and return empty text: a successful request that produces no caption.
diff --git a/packages/cli/src/capture/contentExtractor.ts b/packages/cli/src/capture/contentExtractor.ts
index 60b85c2de2..2239693335 100644
--- a/packages/cli/src/capture/contentExtractor.ts
+++ b/packages/cli/src/capture/contentExtractor.ts
@@ -109,6 +109,27 @@ function vertexCredentials(raw: string): Record<string, unknown> | string {
     : "HYPERFRAMES_VERTEX_SERVICE_ACCOUNT is not a service-account key; skipped vision captioning.";
 }
 
+// The SDK builds the request host from the location and the request path from the model id.
+// `:` stays legal inside a segment for endpoint paths in domain-scoped projects (`example.com:proj`).
+const VERTEX_LOCATION = /^[a-z][a-z0-9-]*$/;
+const GOOGLE_MODEL_ID = /^[A-Za-z0-9][\w.:@-]*(?:\/[A-Za-z0-9][\w.:@-]*)*$/;
+
+function googleClientSettingsProblem(
+  provider: "vertex" | "gemini",
+  model: string,
+  location: string,
+): string | undefined {
+  if (provider === "vertex" && !VERTEX_LOCATION.test(location)) {
+    return "HYPERFRAMES_VERTEX_LOCATION is not a Google Cloud region; skipped vision captioning.";
+  }
+  if (!GOOGLE_MODEL_ID.test(model)) {
+    const variable =
+      provider === "vertex" ? "HYPERFRAMES_VERTEX_MODEL" : "HYPERFRAMES_GEMINI_MODEL";
+    return `${variable} is not a model id; skipped vision captioning.`;
+  }
+  return undefined;
+}
+
 function resolveVisionRequestTimeoutMs(): number {
   const configured = Number(process.env.HYPERFRAMES_VISION_TIMEOUT_MS);
   return Number.isFinite(configured) && configured > 0
@@ -398,6 +419,14 @@ export async function captionImagesWithGemini(
         }, timeoutMs);
       };
     } else {
+      const vertexLocation = process.env.HYPERFRAMES_VERTEX_LOCATION || "us-central1";
+      const settingsProblem = googleClientSettingsProblem(provider, model, vertexLocation);
+      if (settingsProblem) {
+        warnings.push(settingsProblem);
+        internalError = true;
+        reportOutcome();
+        return geminiCaptions;
+      }
       let GoogleGenAI: OptionalPackageModules["@google/genai"]["GoogleGenAI"];
       try {
         ({ GoogleGenAI } = await loadOptionalPackage("@google/genai", "vision captioning"));
@@ -421,7 +450,7 @@ export async function captionImagesWithGemini(
         ai = new GoogleGenAI({
           vertexai: true,
           project: vertexProject,
-          location: process.env.HYPERFRAMES_VERTEX_LOCATION || "us-central1",
+          location: vertexLocation,
           googleAuthOptions: { credentials },
         });
       } else {