fix(ai): support newer Anthropic system updates (#43180)

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

@@ -437,10 +437,19 @@ const lowerToolResultContent = Effect.fnUntraced(function* (part: ToolResultPart
   return yield* Effect.forEach(content, lowerToolResultContentItem)
 })
 
-// Mid-conversation system messages are a native Claude API feature only for
-// Opus 4.8. Other Anthropic models intentionally use the same visible wrapped-
-// user fallback as non-Anthropic routes rather than sending a role they reject.
-const supportsNativeSystemUpdates = (request: LLMRequest) => String(request.model.id) === "claude-opus-4-8"
+// Mid-conversation system messages became available with Opus 4.8 and version
+// 5 of the other supported Claude families. Treat later family versions as
+// compatible without assuming that every Anthropic Messages model is Claude.
+const supportsNativeSystemUpdates = (request: LLMRequest) => {
+  const match = /(?:^|[./])claude-(fable|haiku|mythos|opus|sonnet)-(\d+)(?:[.-](\d+))?/.exec(
+    String(request.model.id).toLowerCase(),
+  )
+  if (!match) return false
+  const major = Number(match[2])
+  if (match[1] !== "opus") return major >= 5
+  if (major !== 4) return major >= 5
+  return match[3] !== undefined && match[3].length <= 2 && Number(match[3]) >= 8
+}
 
 const endsInServerToolUse = (message: LLMRequest["messages"][number]) => {
   const last = message.content.at(-1)

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

@@ -136,6 +136,33 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("supports native chronological system updates on documented and later Claude family versions", () =>
+    Effect.gen(function* () {
+      const ids = [
+        "claude-opus-4-8",
+        "claude-opus-5-1",
+        "claude-sonnet-5",
+        "claude-haiku-5-1",
+        "claude-fable-6",
+        "anthropic/claude-mythos-7.2",
+      ]
+
+      const prepared = yield* Effect.forEach(ids, (id) =>
+        compileRequest(
+          LLM.request({
+            model: AnthropicMessages.route
+              .with({ endpoint: { baseURL: "https://api.anthropic.test/v1/" }, auth: Auth.header("x-api-key", "test") })
+              .model({ id }),
+            messages: [Message.user("Before."), Message.system("Update."), Message.assistant("After.")],
+            cache: "none",
+          }),
+        ),
+      )
+
+      expect(prepared.map((item) => item.body.messages[1]?.role)).toEqual(ids.map(() => "system"))
+    }),
+  )
+
   it.effect("lowers chronological system updates to wrapped user text for unsupported Anthropic models", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(
@@ -163,6 +190,34 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("does not infer native system update support for older or undocumented Claude families", () =>
+    Effect.gen(function* () {
+      const ids = [
+        "claude-opus-4-7",
+        "claude-opus-4-20250514",
+        "claude-sonnet-4-9",
+        "claude-haiku-4-9",
+        "custom-model-7",
+      ]
+
+      const prepared = yield* Effect.forEach(ids, (id) =>
+        compileRequest(
+          LLM.request({
+            model: AnthropicMessages.route
+              .with({ endpoint: { baseURL: "https://api.anthropic.test/v1/" }, auth: Auth.header("x-api-key", "test") })
+              .model({ id }),
+            messages: [Message.user("Before."), Message.system("Update."), Message.assistant("After.")],
+            cache: "none",
+          }),
+        ),
+      )
+
+      expect(prepared.map((item) => item.body.messages.some((message) => message.role === "system"))).toEqual(
+        ids.map(() => false),
+      )
+    }),
+  )
+
   it.effect("rejects non-text chronological system update content before send", () =>
     Effect.gen(function* () {
       const error = yield* compileRequest(