fix: skip reasoning parts from different models to prevent Anthropic signature errors

When switching models mid-conversation, Anthropic reasoning (thinking) blocks
carry a cryptographic signature bound to the model that generated them. Replaying
those parts to a different model causes a 400 error: 'thinking.signature: Field required'.

- message-v2.ts: skip reasoning parts entirely when the historical message came
  from a different model (differentModel flag already computed upstream)
- transform.ts: defence-in-depth guard that strips unsigned reasoning parts
  before they reach the Anthropic/Bedrock SDK, preventing the 400 even if
  message-v2 reconstruction is bypassed

Author: lgarceau768

packages/opencode/src/provider/transform.ts

@@ -81,6 +81,25 @@ function normalizeMessages(
       .filter((msg): msg is ModelMessage => msg !== undefined && msg.content !== "")
   }
 
+  // Strip reasoning parts that have no valid provider signature — they cannot be sent
+  // as thinking blocks to Anthropic without a signature and would cause a 400 error.
+  // This is a defence-in-depth guard; the primary prevention is in message-v2.ts where
+  // reasoning parts from a different model are skipped before reaching this point.
+  if (model.api.npm === "@ai-sdk/anthropic" || model.api.npm === "@ai-sdk/amazon-bedrock") {
+    msgs = msgs
+      .map((msg) => {
+        if (msg.role !== "assistant" || !Array.isArray(msg.content)) return msg
+        const filtered = msg.content.filter((part) => {
+          if ((part as any).type !== "reasoning") return true
+          const opts = (part as any).providerOptions?.anthropic
+          return opts?.signature != null || opts?.redactedData != null
+        })
+        if (filtered.length === 0) return undefined
+        return { ...msg, content: filtered }
+      })
+      .filter((msg): msg is ModelMessage => msg !== undefined)
+  }
+
   if (model.api.id.includes("claude")) {
     const scrub = (id: string) => id.replace(/[^a-zA-Z0-9_-]/g, "_")
     msgs = msgs.map((msg) => {

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

@@ -786,11 +786,15 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
               ...(differentModel ? {} : { callProviderMetadata: providerMeta(part.metadata) }),
             })
         }
-        if (part.type === "reasoning") {
+        // Reasoning parts carry a provider-specific signature that is cryptographically
+        // bound to the model that generated them. They cannot be replayed to a different
+        // model — doing so causes Anthropic to return "thinking.signature: Field required".
+        // Skip them entirely when the historical message came from a different model.
+        if (part.type === "reasoning" && !differentModel) {
           assistantMessage.parts.push({
             type: "reasoning",
             text: part.text,
-            ...(differentModel ? {} : { providerMetadata: part.metadata }),
+            providerMetadata: part.metadata,
           })
         }
       }