fix(ai): reject stateful empty text

packages/ai/src/protocols/anthropic-messages.ts

@@ -359,6 +359,8 @@ const redactedDataFromMetadata = (metadata: ProviderMetadata | undefined): strin
   return typeof anthropic.redactedData === "string" ? anthropic.redactedData : undefined
 }
 
+const hasText = (part: { readonly text: string }) => part.text.trim().length > 0
+
 const lowerTool = (breakpoints: Cache.Breakpoints, tool: ToolDefinition, inputSchema: JsonSchema): AnthropicTool => ({
   name: tool.name,
   description: tool.description,
@@ -534,6 +536,7 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
       const content: AnthropicUserBlock[] = []
       for (const part of message.content) {
         if (part.type === "text") {
+          if (!hasText(part)) continue
           content.push({ type: "text", text: part.text, cache_control: cacheControl(breakpoints, part.cache) })
           continue
         }
@@ -543,7 +546,7 @@ 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
     }
 
@@ -551,6 +554,11 @@ const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (
       const content: AnthropicAssistantBlock[] = []
       for (const part of message.content) {
         if (part.type === "text") {
+          if (!hasText(part)) {
+            if (part.providerMetadata !== undefined && Object.keys(part.providerMetadata).length > 0)
+              return yield* invalid("Anthropic Messages cannot discard provider state attached to empty assistant text")
+            continue
+          }
           content.push({ type: "text", text: part.text, cache_control: cacheControl(breakpoints, part.cache) })
           continue
         }
@@ -579,7 +587,7 @@ 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
     }
 

packages/ai/test/provider/anthropic-messages.test.ts

@@ -58,6 +58,79 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("filters empty user and assistant text while preserving replay state", () =>
+    Effect.gen(function* () {
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [
+            Message.user(" \n\t"),
+            Message.user([
+              { type: "text", text: "" },
+              { type: "text", text: "  Use the tool.  " },
+              { type: "text", text: " \n\t" },
+            ]),
+            Message.assistant([
+              { type: "text", text: "" },
+              { type: "reasoning", text: "", providerMetadata: { anthropic: { signature: "sig_1" } } },
+              ToolCallPart.make({ id: "call_1", name: "lookup", input: {} }),
+            ]),
+            Message.tool({
+              id: "call_1",
+              name: "lookup",
+              resultType: "text",
+              result: "Tool result.",
+            }),
+            Message.assistant(" \n\t"),
+            Message.user("Continue."),
+          ],
+          cache: "none",
+        }),
+      )
+
+      expect(prepared.body).toMatchObject({
+        messages: [
+          { role: "user", content: [{ type: "text", text: "  Use the tool.  " }] },
+          {
+            role: "assistant",
+            content: [
+              { type: "thinking", thinking: "", signature: "sig_1" },
+              { type: "tool_use", id: "call_1", name: "lookup", input: {} },
+            ],
+          },
+          {
+            role: "user",
+            content: [
+              {
+                type: "tool_result",
+                tool_use_id: "call_1",
+                content: "Tool result.",
+              },
+            ],
+          },
+          { role: "user", content: [{ type: "text", text: "Continue." }] },
+        ],
+      })
+    }),
+  )
+
+  it.effect("rejects empty assistant text carrying provider state", () =>
+    Effect.gen(function* () {
+      const error = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [
+            Message.assistant([
+              { type: "text", text: "", providerMetadata: { anthropic: { encryptedContent: "opaque" } } },
+            ]),
+          ],
+        }),
+      ).pipe(Effect.flip)
+
+      expect(error.message).toContain("cannot discard provider state attached to empty assistant text")
+    }),
+  )
+
   it.effect("lowers adaptive thinking settings with effort", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(

packages/core/src/session/runner/to-llm-message.ts

@@ -7,6 +7,9 @@ import type { FileAttachment } from "@opencode-ai/schema/prompt"
 
 const imageMimes = new Set(["image/png", "image/jpeg", "image/gif", "image/webp"])
 
+const hasProviderMetadata = (metadata: ProviderMetadata | undefined) =>
+  metadata !== undefined && Object.keys(metadata).length > 0
+
 const media = (file: FileAttachment): ContentPart => ({
   type: "media",
   mediaType: file.mime,
@@ -188,9 +191,9 @@ const assistant = (message: SessionMessage.Assistant, model: Model.Ref, provider
     return result ? [call, result] : [call]
   })
   const meaningful = content.filter((part) => {
-    if (part.type === "text") return part.text !== ""
+    if (part.type === "text") return part.text !== "" || hasProviderMetadata(part.providerMetadata)
     if (part.type !== "reasoning") return true
-    return part.text !== "" || (part.providerMetadata !== undefined && Object.keys(part.providerMetadata).length > 0)
+    return part.text !== "" || hasProviderMetadata(part.providerMetadata)
   })
   const results = message.content
     .filter((item): item is SessionMessage.AssistantTool => item.type === "tool" && item.executed !== true)

packages/core/test/session-runner-message.test.ts

@@ -1031,7 +1031,7 @@ Recent work
           content: [
             SessionMessage.AssistantText.make({
               type: "text",
-              text: "Checking.",
+              text: "",
               state: { phase: "commentary" },
             }),
           ],
@@ -1045,7 +1045,7 @@ Recent work
     expect(messages[0]?.content).toEqual([
       {
         type: "text",
-        text: "Checking.",
+        text: "",
         providerMetadata: { provider: { phase: "commentary" } },
       },
     ])