From 3248f267acbadf1b45045032c66fccb18adbc1c0 Mon Sep 17 00:00:00 2001
From: Ishaan Jaffer <ishaanjaffer0324@gmail.com>
Date: Wed, 21 Jan 2026 19:04:36 -0800
Subject: [PATCH] Using Anthropic Beta Features on Vertex AI

---
 .../my-website/docs/pass_through/vertex_ai.md | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/docs/my-website/docs/pass_through/vertex_ai.md b/docs/my-website/docs/pass_through/vertex_ai.md
index adbd06187d5..00df6def704 100644
--- a/docs/my-website/docs/pass_through/vertex_ai.md
+++ b/docs/my-website/docs/pass_through/vertex_ai.md
@@ -461,3 +461,48 @@ generateContent();
 
 </TabItem>
 </Tabs>
+
+### Using Anthropic Beta Features on Vertex AI
+
+When using Anthropic models via Vertex AI passthrough (e.g., Claude on Vertex), you can enable Anthropic beta features like extended context windows.
+
+The `anthropic-beta` header is automatically forwarded to Vertex AI when calling Anthropic models.
+
+```bash
+curl http://localhost:4000/vertex_ai/v1/projects/${PROJECT_ID}/locations/us-east5/publishers/anthropic/models/claude-3-5-sonnet:rawPredict \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer sk-1234" \
+  -H "anthropic-beta: context-1m-2025-08-07" \
+  -d '{
+    "anthropic_version": "vertex-2023-10-16",
+    "messages": [{"role": "user", "content": "Hello"}],
+    "max_tokens": 500
+  }'
+```
+
+### Forwarding Custom Headers with `x-pass-` Prefix
+
+You can forward any custom header to the provider by prefixing it with `x-pass-`. The prefix is stripped before the header is sent to the provider.
+
+For example:
+- `x-pass-anthropic-beta: value` becomes `anthropic-beta: value`
+- `x-pass-custom-header: value` becomes `custom-header: value`
+
+This is useful when you need to send provider-specific headers that aren't in the default allowlist.
+
+```bash
+curl http://localhost:4000/vertex_ai/v1/projects/${PROJECT_ID}/locations/us-east5/publishers/anthropic/models/claude-3-5-sonnet:rawPredict \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer sk-1234" \
+  -H "x-pass-anthropic-beta: context-1m-2025-08-07" \
+  -H "x-pass-custom-feature: enabled" \
+  -d '{
+    "anthropic_version": "vertex-2023-10-16",
+    "messages": [{"role": "user", "content": "Hello"}],
+    "max_tokens": 500
+  }'
+```
+
+:::info
+The `x-pass-` prefix works for all LLM pass-through endpoints, not just Vertex AI.
+:::