fix: preserve reasoning metadata across model switches

The differentModel guard strips providerMetadata from all parts when the
current model differs from the stored message model. For reasoning parts
this removes thinking block signatures, causing Anthropic to reject the
history. Reasoning parts now always preserve their metadata since it is
provider-scoped and harmless to other providers.

Author: chan1103

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

@@ -795,7 +795,7 @@ export namespace MessageV2 {
             assistantMessage.parts.push({
               type: "reasoning",
               text: part.text,
-              ...(differentModel ? {} : { providerMetadata: part.metadata }),
+              providerMetadata: part.metadata,
             })
           }
         }

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

@@ -863,6 +863,55 @@ describe("session.message-v2.toModelMessage", () => {
       },
     ])
   })
+
+  test("reasoning parts preserve metadata even when model differs", async () => {
+    const userID = "m-user"
+    const assistantID = "m-assistant"
+
+    const differentModel: Provider.Model = {
+      ...model,
+      id: ModelID.make("different-model"),
+      providerID: ProviderID.make("different-provider"),
+    }
+
+    const input: MessageV2.WithParts[] = [
+      {
+        info: userInfo(userID),
+        parts: [
+          {
+            ...basePart(userID, "p1"),
+            type: "text",
+            text: "hello",
+          },
+        ] as MessageV2.Part[],
+      },
+      {
+        info: assistantInfo(assistantID, userID),
+        parts: [
+          {
+            ...basePart(assistantID, "p2"),
+            type: "reasoning",
+            text: "thinking about it",
+            time: { start: 0 },
+            metadata: { anthropic: { signature: "sig_test123" } },
+          },
+          {
+            ...basePart(assistantID, "p3"),
+            type: "text",
+            text: "here is my answer",
+          },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    const result = await MessageV2.toModelMessages(input, differentModel)
+
+    const assistantMsg = result.find((m) => m.role === "assistant")
+    expect(assistantMsg).toBeDefined()
+    const reasoning = (assistantMsg!.content as any[]).find((p: any) => p.type === "reasoning")
+    expect(reasoning).toBeDefined()
+    expect(reasoning.providerOptions).toEqual({ anthropic: { signature: "sig_test123" } })
+  })
 })
 
 describe("session.message-v2.fromError", () => {