test(session): cover isAnthropicReasoningModel and tighten provider match

Address review on #30046:
- Extract MessageV2.isAnthropicReasoningModel and replace the broad
  providerID.includes("anthropic") with exact provider IDs (anthropic,
  google-vertex-anthropic) plus ai-sdk npm checks, consistent with how
  provider.ts/transform.ts detect Anthropic.
- Add focused unit tests: first-party, google-vertex-anthropic, the
  @ai-sdk/* npm packages, negative cases (openai/google/openrouter), and a
  regression test that a provider merely containing "anthropic" is not matched.

test/session/message-v2.test.ts: 41 pass / 0 fail.

Author: EQCommunication

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

@@ -377,10 +377,7 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
           // lives in part.metadata, so unlike text/tool metadata it must be preserved
           // even when differentModel is true (fallback model switches, variant changes,
           // or proxy setups where the persisted providerID differs from the live one).
-          const isAnthropicReasoning =
-            model.providerID === "anthropic" ||
-            model.providerID.includes("anthropic") ||
-            model.api?.npm === "@ai-sdk/anthropic"
+          const isAnthropicReasoning = isAnthropicReasoningModel(model)
           if (differentModel && !isAnthropicReasoning) {
             if (part.text.trim().length > 0)
               assistantMessage.parts.push({

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

@@ -1658,3 +1658,38 @@ describe("session.message-v2.latest", () => {
     expect(state.tasks[0]).toMatchObject({ type: "compaction", auto: true })
   })
 })
+
+describe("session.message-v2.isAnthropicReasoningModel", () => {
+  test("matches first-party Anthropic provider", () => {
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "anthropic" })).toBe(true)
+  })
+
+  test("matches Google Vertex Anthropic provider", () => {
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "google-vertex-anthropic" })).toBe(true)
+  })
+
+  test("matches by ai-sdk npm package", () => {
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "custom", api: { npm: "@ai-sdk/anthropic" } })).toBe(true)
+    expect(
+      MessageV2.isAnthropicReasoningModel({ providerID: "custom", api: { npm: "@ai-sdk/google-vertex/anthropic" } }),
+    ).toBe(true)
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "custom", api: { npm: "@ai-sdk/amazon-bedrock" } })).toBe(
+      true,
+    )
+  })
+
+  test("does not match non-Anthropic providers", () => {
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "openai" })).toBe(false)
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "google" })).toBe(false)
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "openrouter", api: { npm: "@openrouter/ai-sdk-provider" } })).toBe(
+      false,
+    )
+  })
+
+  test("does not loosely match providers that merely contain 'anthropic'", () => {
+    // regression: a previous version used providerID.includes("anthropic"),
+    // which would wrongly match a hypothetical "not-anthropic" provider.
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "not-anthropic-router" })).toBe(false)
+    expect(MessageV2.isAnthropicReasoningModel({ providerID: "anthropic-compatible-proxy" })).toBe(false)
+  })
+})