fix(provider): skip empty-text filtering for assistant messages in normalizeMessages (#16748)

packages/opencode/src/provider/transform.ts

@@ -51,40 +51,46 @@ function normalizeMessages(
   _options: Record<string, unknown>,
 ): ModelMessage[] {
   // Anthropic rejects messages with empty content - filter out empty string messages
-  // and remove empty text/reasoning parts from array content
-  if (model.api.npm === "@ai-sdk/anthropic") {
+  // and remove empty text/reasoning parts from array content.
+  // Assistant messages with reasoning blocks are excluded: their content must be
+  // replayed verbatim because thinking block signatures encode positional context.
+  // Removing an empty text part between two reasoning blocks changes the block
+  // arrangement and invalidates the cryptographic signatures, causing the API to
+  // reject the request. Assistant messages without reasoning blocks are filtered
+  // normally since there are no signatures to preserve.
+  if (model.api.npm === "@ai-sdk/anthropic" || model.api.npm === "@ai-sdk/amazon-bedrock") {
     msgs = msgs
       .map((msg) => {
-        if (typeof msg.content === "string") {
-          if (msg.content === "") return undefined
-          return msg
-        }
-        if (!Array.isArray(msg.content)) return msg
-        const filtered = msg.content.filter((part) => {
-          if (part.type === "text" || part.type === "reasoning") {
-            return part.text !== ""
+        if (msg.role === "assistant") {
+          if (!Array.isArray(msg.content)) return msg
+          if (msg.content.some((part) => part.type === "reasoning")) {
+            // Strip trailing empty text parts — only interstitial ones
+            // (between reasoning blocks) affect thinking-block signature
+            // positions. A trailing empty text can receive cache_control
+            // from applyCaching, which Anthropic rejects with:
+            // "cache_control cannot be set for empty text blocks"
+            let end = msg.content.length
+            while (end > 0) {
+              const part = msg.content[end - 1]
+              if (part.type === "text" && part.text === "") {
+                end--
+                continue
+              }
+              break
+            }
+            if (end === 0) return undefined
+            if (end < msg.content.length) return { ...msg, content: msg.content.slice(0, end) }
+            return msg
           }
-          return true
-        })
-        if (filtered.length === 0) return undefined
-        return { ...msg, content: filtered }
-      })
-      .filter((msg): msg is ModelMessage => msg !== undefined && msg.content !== "")
-  }
-
-  // Bedrock specific transforms
-  if (model.api.npm === "@ai-sdk/amazon-bedrock") {
-    msgs = msgs
-      .map((msg) => {
+          // No reasoning blocks — safe to filter empty text
+        }
         if (typeof msg.content === "string") {
           if (msg.content === "") return undefined
           return msg
         }
         if (!Array.isArray(msg.content)) return msg
         const filtered = msg.content.filter((part) => {
-          if (part.type === "text" || part.type === "reasoning") {
-            return part.text !== ""
-          }
+          if (part.type === "text") return part.text !== ""
           return true
         })
         if (filtered.length === 0) return undefined
@@ -283,15 +289,27 @@ function applyCaching(msgs: ModelMessage[], model: Provider.Model): ModelMessage
       model.api.npm === "@ai-sdk/amazon-bedrock"
     const shouldUseContentOptions = !useMessageLevelOptions && Array.isArray(msg.content) && msg.content.length > 0
 
-    if (shouldUseContentOptions) {
-      const lastContent = msg.content[msg.content.length - 1]
+    if (shouldUseContentOptions && Array.isArray(msg.content)) {
+      // Walk backwards to skip empty text parts — Anthropic rejects
+      // cache_control on empty text blocks.
+      const target = msg.content.findLast(
+        (part: unknown) =>
+          !(
+            typeof part === "object" &&
+            part &&
+            "type" in part &&
+            part.type === "text" &&
+            "text" in part &&
+            part.text === ""
+          ),
+      )
       if (
-        lastContent &&
-        typeof lastContent === "object" &&
-        lastContent.type !== "tool-approval-request" &&
-        lastContent.type !== "tool-approval-response"
+        target &&
+        typeof target === "object" &&
+        target.type !== "tool-approval-request" &&
+        target.type !== "tool-approval-response"
       ) {
-        lastContent.providerOptions = mergeDeep(lastContent.providerOptions ?? {}, providerOptions)
+        target.providerOptions = mergeDeep(target.providerOptions ?? {}, providerOptions)
         continue
       }
     }

packages/opencode/src/session/message-v2.ts

@@ -844,7 +844,10 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
         msg.info.error &&
         !(
           AbortedError.isInstance(msg.info.error) &&
-          msg.parts.some((part) => part.type !== "step-start" && part.type !== "reasoning")
+          msg.parts.some(
+            (part) =>
+              part.type !== "step-start" && part.type !== "reasoning" && !(part.type === "text" && part.text === ""),
+          )
         )
       ) {
         continue
@@ -859,7 +862,13 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
           assistantMessage.parts.push({
             type: "text",
             text: part.text,
-            ...(differentModel ? {} : { providerMetadata: part.metadata }),
+            // Empty text parts between reasoning blocks must survive the AI SDK's
+            // internal convertToLanguageModelPrompt filter, which strips text parts
+            // where text==="" and providerOptions==null. Fall back to {} so the
+            // downstream filter sees a non-null value and preserves the part.
+            // Removing an empty text part shifts thinking block positions and
+            // invalidates cryptographic signatures.
+            ...(differentModel ? {} : { providerMetadata: part.text === "" && !part.metadata ? {} : part.metadata }),
           })
         if (part.type === "step-start")
           assistantMessage.parts.push({