fix: add response.status extraction, extract inner message from JSON errors

- Add response.status to status code extraction chain
- Extract human-readable message from JSON-encoded error messages
- Add tests for response.status and plain object with response.status
- Verify inner message extraction from JSON error bodies

Author: okuyam2y

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

@@ -990,24 +990,29 @@ export namespace MessageV2 {
           { cause: e },
         ).toObject()
       case e instanceof Error: {
+        // Non-APICallError with HTTP status — treat 5xx as retryable.
         const code =
-          (e as any).status ?? (e as any).statusCode ?? (e as any).response?.statusCode ?? (() => {
+          (e as any).status ?? (e as any).statusCode ??
+          (e as any).response?.status ?? (e as any).response?.statusCode ??
+          (() => {
             try {
               const obj = JSON.parse(e.message)
-              return obj?.status ?? obj?.statusCode ?? obj?.response?.statusCode
+              return obj?.status ?? obj?.statusCode
             } catch {
               return undefined
             }
           })()
-        if (typeof code === "number" && code >= 500)
+        if (typeof code === "number" && code >= 500) {
+          let msg = e.message
+          try {
+            const obj = JSON.parse(msg)
+            if (typeof obj?.message === "string") msg = obj.message
+          } catch {}
           return new MessageV2.APIError(
-            {
-              message: e.message,
-              statusCode: code,
-              isRetryable: true,
-            },
+            { message: msg, statusCode: code, isRetryable: true },
             { cause: e },
           ).toObject()
+        }
         return new NamedError.Unknown({ message: e.message }, { cause: e }).toObject()
       }
       default:
@@ -1035,12 +1040,15 @@ export namespace MessageV2 {
             ).toObject()
           }
         } catch {}
+        // Plain-object 5xx — treat as retryable.
         if (typeof e === "object" && e !== null) {
-          const code = (e as any).status ?? (e as any).statusCode ?? (e as any).response?.statusCode
+          const code =
+            (e as any).status ?? (e as any).statusCode ??
+            (e as any).response?.status ?? (e as any).response?.statusCode
           if (typeof code === "number" && code >= 500)
             return new MessageV2.APIError(
               {
-                message: JSON.stringify(e),
+                message: typeof (e as any).message === "string" ? (e as any).message : JSON.stringify(e),
                 statusCode: code,
                 isRetryable: true,
               },

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

@@ -950,14 +950,26 @@ describe("session.message-v2.fromError", () => {
     expect((result as MessageV2.APIError).data.statusCode).toBe(503)
   })
 
-  test("retries 5xx from JSON-encoded Error message", () => {
+  test("retries 5xx Error with response.status", () => {
+    const err = new Error("Gateway Timeout")
+    ;(err as any).response = { status: 504 }
+
+    const result = MessageV2.fromError(err, { providerID })
+
+    expect(MessageV2.APIError.isInstance(result)).toBe(true)
+    expect((result as MessageV2.APIError).data.isRetryable).toBe(true)
+    expect((result as MessageV2.APIError).data.statusCode).toBe(504)
+  })
+
+  test("retries 5xx from JSON-encoded Error message and extracts inner message", () => {
     const err = new Error(JSON.stringify({ statusCode: 500, message: "server error" }))
 
     const result = MessageV2.fromError(err, { providerID })
 
     expect(MessageV2.APIError.isInstance(result)).toBe(true)
     expect((result as MessageV2.APIError).data.isRetryable).toBe(true)
     expect((result as MessageV2.APIError).data.statusCode).toBe(500)
+    expect((result as MessageV2.APIError).data.message).toBe("server error")
   })
 
   test("retries 5xx from plain object with statusCode", () => {
@@ -970,6 +982,15 @@ describe("session.message-v2.fromError", () => {
     expect((result as MessageV2.APIError).data.statusCode).toBe(502)
   })
 
+  test("retries 5xx from plain object with response.status", () => {
+    const err = { response: { status: 500 }, message: "fail" }
+
+    const result = MessageV2.fromError(err, { providerID })
+
+    expect(MessageV2.APIError.isInstance(result)).toBe(true)
+    expect((result as MessageV2.APIError).data.isRetryable).toBe(true)
+  })
+
   test("does not retry 4xx Error", () => {
     const err = new Error("Not Found")
     ;(err as any).statusCode = 404