diff --git a/packages/ai/src/providers/google-vertex-shared.ts b/packages/ai/src/providers/google-vertex-shared.ts
index 8c2bf89759f..7588058687b 100644
--- a/packages/ai/src/providers/google-vertex-shared.ts
+++ b/packages/ai/src/providers/google-vertex-shared.ts
@@ -1,4 +1,4 @@
-import type { AnyAuthClient } from "google-auth-library"
+import type { AnyAuthClient, GoogleAuthOptions } from "google-auth-library"
 import { Effect, Redacted } from "effect"
 import { Auth, MissingCredentialError } from "../route/auth.js"
 import { ProviderConfigurationError, ProviderID } from "../schema/index.js"
@@ -6,6 +6,31 @@ import { ProviderConfigurationError, ProviderID } from "../schema/index.js"
 const SCOPE = "https://www.googleapis.com/auth/cloud-platform"
 const id = ProviderID.make("google-vertex")
 
+export const loadADCClient = async (project?: string) => {
+  const { GoogleAuth } = await import("google-auth-library")
+  // google-auth-library 10.5.0 ignores CLOUDSDK_CONFIG during ADC file discovery.
+  // Fix this in shipped AI code: a repository-level Bun dependency patch would not
+  // reach npm or Bun consumers of the published @opencode/ai package.
+  // Override only the well-known-file lookup: keyFilename bypasses ADC's quota-project
+  // preparation. Keep explicit credentials and metadata fallback in Google's ADC chain.
+  class CloudSDKAuth extends GoogleAuth {
+    override async _tryGetApplicationCredentialsFromWellKnownFile(options?: GoogleAuthOptions["clientOptions"]) {
+      const config = process.env.CLOUDSDK_CONFIG
+      if (!config) return super._tryGetApplicationCredentialsFromWellKnownFile(options)
+      // Keep local filesystem helpers lazy for explicitly authenticated Worker callers;
+      // this ADC-file lookup itself requires a Node-compatible filesystem.
+      const { existsSync } = await import("node:fs")
+      const { homedir } = await import("node:os")
+      const { join } = await import("node:path")
+      const directory = config === "~" ? homedir() : config.startsWith("~/") ? join(homedir(), config.slice(2)) : config
+      const file = join(directory, "application_default_credentials.json")
+      if (!existsSync(file)) return null
+      return this._getApplicationCredentialsFromFilePath(file, options)
+    }
+  }
+  return new CloudSDKAuth({ projectId: project, scopes: [SCOPE] }).getClient()
+}
+
 export type OAuthOptions =
   | { readonly accessToken?: string; readonly auth?: never }
   | { readonly accessToken?: never; readonly auth?: Auth.Definition }
@@ -61,9 +86,7 @@ const adc = (project?: string) => {
   let client: Promise<AnyAuthClient> | undefined
   const loadClient = () => {
     if (client) return client
-    client = import("google-auth-library").then(({ GoogleAuth }) =>
-      new GoogleAuth({ projectId: project, scopes: [SCOPE] }).getClient(),
-    )
+    client = loadADCClient(project)
     return client
   }
   return Auth.effect(
diff --git a/packages/ai/test/provider/google-vertex.test.ts b/packages/ai/test/provider/google-vertex.test.ts
index e09aeac3d67..2f3da7b3a8a 100644
--- a/packages/ai/test/provider/google-vertex.test.ts
+++ b/packages/ai/test/provider/google-vertex.test.ts
@@ -1,6 +1,9 @@
 import { describe, expect, test } from "bun:test"
 import { Effect } from "effect"
 import { HttpClientRequest } from "effect/http"
+import { mkdtemp, rm } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
 import { LanguageModel, LLM, Message, ToolCallPart } from "../../src/index.js"
 import { GoogleVertex, GoogleVertexChat, GoogleVertexMessages, GoogleVertexResponses } from "../../src/providers.js"
 import { LLMClient } from "../../src/route.js"
@@ -11,6 +14,57 @@ import { deltaChunk, finishChunk } from "../lib/openai-chunks.js"
 import { sseEvents } from "../lib/sse.js"
 
 describe("Google Vertex providers", () => {
+  test("loads CLOUDSDK_CONFIG credentials and preserves quota-project overrides", async () => {
+    const directory = await mkdtemp(join(tmpdir(), "vertex-adc-"))
+    try {
+      await Bun.write(
+        join(directory, "application_default_credentials.json"),
+        JSON.stringify({
+          type: "authorized_user",
+          client_id: "fixture-client",
+          client_secret: "fixture-secret",
+          refresh_token: "fixture-refresh-token",
+          quota_project_id: "file-quota",
+        }),
+      )
+      // Isolate Google's environment and credential caches; no token refresh or model call.
+      const child = Bun.spawn(
+        [
+          process.execPath,
+          "-e",
+          `import { GoogleVertexShared } from ${JSON.stringify(join(import.meta.dir, "../../src/providers/google-vertex-shared.ts"))};
+           const client = await GoogleVertexShared.loadADCClient("fixture-project");
+           console.log(JSON.stringify({
+             custom: client.credentials.refresh_token === "fixture-refresh-token",
+             quota: client.quotaProjectId,
+           }));`,
+        ],
+        {
+          env: {
+            ...process.env,
+            HOME: directory,
+            APPDATA: directory,
+            CLOUDSDK_CONFIG: directory,
+            GOOGLE_APPLICATION_CREDENTIALS: undefined,
+            google_application_credentials: undefined,
+            GOOGLE_CLOUD_QUOTA_PROJECT: "environment-quota",
+            METADATA_SERVER_DETECTION: "none",
+          },
+          stdout: "pipe",
+          stderr: "pipe",
+        },
+      )
+      const result = JSON.parse(await new Response(child.stdout).text())
+      expect({ exitCode: await child.exited, stderr: await new Response(child.stderr).text() }).toEqual({
+        exitCode: 0,
+        stderr: "",
+      })
+      expect(result).toEqual({ custom: true, quota: "environment-quota" })
+    } finally {
+      await rm(directory, { recursive: true, force: true })
+    }
+  })
+
   it.effect("sends Gemini requests to the global Vertex endpoint", () =>
     Effect.gen(function* () {
       const response = yield* LLMClient.generate(
diff --git a/packages/core/src/plugin/provider/google-vertex.ts b/packages/core/src/plugin/provider/google-vertex.ts
index 12f56735c10..964f15fd7e4 100644
--- a/packages/core/src/plugin/provider/google-vertex.ts
+++ b/packages/core/src/plugin/provider/google-vertex.ts
@@ -58,7 +58,13 @@ export const GoogleVertexPlugin = define({
     const read = (file: string) => fs.readFileStringSafe(file).pipe(Effect.orElseSucceed(() => undefined))
     // Same lookup as gcloud itself. Only project IDs are read; nothing here contacts Google.
     const gcloud =
-      process.env.CLOUDSDK_CONFIG ??
+      (process.env.CLOUDSDK_CONFIG
+        ? process.env.CLOUDSDK_CONFIG === "~"
+          ? Global.Path.home
+          : process.env.CLOUDSDK_CONFIG.startsWith("~/")
+            ? path.join(Global.Path.home, process.env.CLOUDSDK_CONFIG.slice(2))
+            : process.env.CLOUDSDK_CONFIG
+        : undefined) ??
       (process.platform === "win32" && process.env.APPDATA
         ? path.join(process.env.APPDATA, "gcloud")
         : path.join(Global.Path.home, ".config", "gcloud"))