diff --git a/packages/opencode/src/cli/cmd/providers.ts b/packages/opencode/src/cli/cmd/providers.ts
index 560781ef0e4..7271ba4a754 100644
--- a/packages/opencode/src/cli/cmd/providers.ts
+++ b/packages/opencode/src/cli/cmd/providers.ts
@@ -460,6 +460,13 @@ export const ProvidersLoginCommand = effectCmd({
       )
     }
 
+    if (provider === "google-vertex") {
+      yield* Prompt.log.info(
+        "Note: this API key cannot be used with Anthropic models.\n" +
+          "Use `gcloud auth application-default login` or set `GOOGLE_APPLICATION_CREDENTIALS`.",
+      )
+    }
+
     if (provider === "opencode") {
       yield* Prompt.log.info("Create an api key at https://opencode.ai/auth")
     }
diff --git a/packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx b/packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx
index 874afa05b4b..0a8beaccf0c 100644
--- a/packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx
+++ b/packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx
@@ -384,6 +384,15 @@ function ApiMethod(props: ApiMethodProps) {
               </text>
             </box>
           ),
+          "google-vertex": (
+            <box gap={1}>
+              <text fg={theme.warning}>Note: this API key cannot be used with Anthropic models.</text>
+              <text fg={theme.textMuted}>
+                Use <span style={{ fg: theme.text }}>gcloud auth application-default login</span> or set
+                GOOGLE_APPLICATION_CREDENTIALS.
+              </text>
+            </box>
+          ),
         }[props.providerID] ?? undefined
       }
       onConfirm={async (value) => {
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 29cc83a1eb8..bd590cf8c4e 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -1095,8 +1095,9 @@ export function latest(msgs: WithParts[]) {
 
 export function fromError(
   e: unknown,
-  ctx: { providerID: ProviderID; aborted?: boolean },
+  ctx: { model: Provider.Model; aborted?: boolean } | { providerID: ProviderID; aborted?: boolean },
 ): NonNullable<Assistant["error"]> {
+  const providerID = "model" in ctx ? ctx.model.providerID : ctx.providerID
   switch (true) {
     case e instanceof DOMException && e.name === "AbortError":
       return new AbortedError(
@@ -1110,11 +1111,23 @@ export function fromError(
     case LoadAPIKeyError.isInstance(e):
       return new AuthError(
         {
-          providerID: ctx.providerID,
+          providerID,
           message: e.message,
         },
         { cause: e },
       ).toObject()
+    case e instanceof Error &&
+      "model" in ctx &&
+      ctx.model.api.npm === "@ai-sdk/google-vertex/anthropic" &&
+      e.message.includes("Could not load the default credentials"):
+      return new AuthError(
+        {
+          providerID,
+          message:
+            "Anthropic models on Google Vertex require Google Cloud credentials. Use `gcloud auth application-default login` or set `GOOGLE_APPLICATION_CREDENTIALS`.",
+        },
+        { cause: e },
+      ).toObject()
     case (e as SystemError)?.code === "ECONNRESET":
       return new APIError(
         {
@@ -1157,7 +1170,7 @@ export function fromError(
       ).toObject()
     case APICallError.isInstance(e):
       const parsed = ProviderError.parseAPICallError({
-        providerID: ctx.providerID,
+        providerID,
         error: e,
       })
       if (parsed.type === "context_overflow") {
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index a287c3b0068..b8d3025f336 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -124,7 +124,7 @@ export const layer = Layer.effect(
 
       const parse = (e: unknown) =>
         MessageV2.fromError(e, {
-          providerID: input.model.providerID,
+          model: input.model,
           aborted,
         })
 
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index 82bed0e9cc6..7eb74ebe155 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -1509,6 +1509,37 @@ describe("session.message-v2.fromError", () => {
     })
   })
 
+  test("explains ADC requirements when Vertex Anthropic credentials are missing", () => {
+    const vertexAnthropicModel = {
+      ...model,
+      providerID: ProviderID.googleVertex,
+      api: { ...model.api, npm: "@ai-sdk/google-vertex/anthropic" },
+    }
+    const result = MessageV2.fromError(
+      new Error(
+        "Could not load the default credentials. Browse to https://cloud.google.com/docs/authentication/getting-started for more information.",
+      ),
+      { model: vertexAnthropicModel },
+    )
+
+    expect(result).toStrictEqual({
+      name: "ProviderAuthError",
+      data: {
+        providerID: "google-vertex",
+        message:
+          "Anthropic models on Google Vertex require Google Cloud credentials. Use `gcloud auth application-default login` or set `GOOGLE_APPLICATION_CREDENTIALS`.",
+      },
+    })
+  })
+
+  test("does not present Anthropic ADC guidance for Gemini Vertex credentials errors", () => {
+    const result = MessageV2.fromError(new Error("Could not load the default credentials."), {
+      model: { ...model, providerID: ProviderID.googleVertex, api: { ...model.api, npm: "@ai-sdk/google-vertex" } },
+    })
+
+    expect(result.name).toBe("UnknownError")
+  })
+
   test("serializes tagged errors with their message", () => {
     const result = MessageV2.fromError(new Question.RejectedError(), { providerID })