From 2ddf257c6f2c54386c7290609596acd851d9df95 Mon Sep 17 00:00:00 2001
From: Aiden Cline <63023139+rekram1-node@users.noreply.github.com>
Date: Sun, 30 Aug 2026 14:39:46 -0500
Subject: [PATCH] fix(ai): filter empty Anthropic messages (#46291)

---
 .../ai/src/protocols/anthropic-messages.ts    |  6 ++--
 .../test/provider/anthropic-messages.test.ts  | 32 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/packages/ai/src/protocols/anthropic-messages.ts b/packages/ai/src/protocols/anthropic-messages.ts
index e312d0f2224..6388d4184f4 100644
--- a/packages/ai/src/protocols/anthropic-messages.ts
+++ b/packages/ai/src/protocols/anthropic-messages.ts
@@ -831,6 +831,7 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
       const content: AnthropicUserBlock[] = []
       for (const part of message.content) {
         if (part.type === "text") {
+          if (part.text.trim().length === 0) continue
           content.push({ type: "text", text: part.text, cache_control: cacheControl(breakpoints, part.cache) })
           continue
         }
@@ -840,7 +841,7 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
         }
         return yield* ProviderShared.unsupportedContent("Anthropic Messages", "user", ["text", "media"])
       }
-      messages.push({ role: "user", content })
+      if (content.length > 0) messages.push({ role: "user", content })
       continue
     }
 
@@ -848,6 +849,7 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
       const content: AnthropicAssistantBlock[] = []
       for (const part of message.content) {
         if (part.type === "text") {
+          if (part.text.trim().length === 0) continue
           content.push({ type: "text", text: part.text, cache_control: cacheControl(breakpoints, part.cache) })
           continue
         }
@@ -891,7 +893,7 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
           `Anthropic Messages assistant messages only support text, reasoning, and tool-call content for now`,
         )
       }
-      messages.push({ role: "assistant", content })
+      if (content.length > 0) messages.push({ role: "assistant", content })
       continue
     }
 
diff --git a/packages/ai/test/provider/anthropic-messages.test.ts b/packages/ai/test/provider/anthropic-messages.test.ts
index f5fa21ca68d..b5696462f81 100644
--- a/packages/ai/test/provider/anthropic-messages.test.ts
+++ b/packages/ai/test/provider/anthropic-messages.test.ts
@@ -84,6 +84,38 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("filters whitespace-only text and removes empty messages", () =>
+    Effect.gen(function* () {
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [
+            Message.user(" \n\t"),
+            Message.user([]),
+            Message.user([
+              { type: "text", text: "" },
+              { type: "text", text: "  Keep this spacing.  " },
+              { type: "text", text: " \n\t" },
+            ]),
+            Message.assistant(" \n\t"),
+            Message.assistant([]),
+            Message.assistant([{ type: "reasoning", text: "" }]),
+            Message.assistant([
+              { type: "text", text: "" },
+              { type: "reasoning", text: "", providerMetadata: { anthropic: { signature: "sig_1" } } },
+            ]),
+          ],
+          cache: "none",
+        }),
+      )
+
+      expect(prepared.body.messages).toEqual([
+        { role: "user", content: [{ type: "text", text: "  Keep this spacing.  " }] },
+        { role: "assistant", content: [{ type: "thinking", thinking: "", signature: "sig_1" }] },
+      ])
+    }),
+  )
+
   it.effect("lowers adaptive thinking settings with effort", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(