fix(core): accept interleaved Copilot reasoning_opaque values

packages/core/src/github-copilot/chat/convert-to-openai-compatible-chat-messages.ts

@@ -82,10 +82,12 @@ export function convertToOpenAICompatibleChatMessages(prompt: LanguageModelV3Pro
 
         for (const part of content) {
           const partMetadata = getOpenAIMetadata(part)
-          // Check for reasoningOpaque on any part (may be attached to text/tool-call)
+          // reasoningOpaque may be attached to any part. Interleaved thinking yields one per
+          // reasoning segment; like the Copilot VS Code client, replay the latest opaque value
+          // with the concatenated reasoning text.
           const partOpaque = (part.providerOptions as { copilot?: { reasoningOpaque?: string } })?.copilot
             ?.reasoningOpaque
-          if (partOpaque && !reasoningOpaque) {
+          if (partOpaque) {
             reasoningOpaque = partOpaque
           }
 
@@ -95,7 +97,7 @@ export function convertToOpenAICompatibleChatMessages(prompt: LanguageModelV3Pro
               break
             }
             case "reasoning": {
-              if (part.text) reasoningText = part.text
+              if (part.text) reasoningText = (reasoningText ?? "") + part.text
               break
             }
             case "tool-call": {

packages/core/src/github-copilot/chat/openai-compatible-chat-language-model.ts

@@ -465,17 +465,9 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
 
             const delta = choice.delta
 
-            // Capture reasoning_opaque for Copilot multi-turn reasoning
-            if (delta.reasoning_opaque) {
-              if (reasoningOpaque != null) {
-                throw new InvalidResponseDataError({
-                  data: delta,
-                  message:
-                    "Multiple reasoning_opaque values received in a single response. Only one thinking part per response is supported.",
-                })
-              }
-              reasoningOpaque = delta.reasoning_opaque
-            }
+            // Interleaved thinking (Claude) sends a new reasoning_opaque before each tool call.
+            // Keep the latest, matching the Copilot VS Code client, which replays only that one.
+            if (delta.reasoning_opaque) reasoningOpaque = delta.reasoning_opaque
 
             // enqueue reasoning before text deltas (Copilot uses reasoning_text):
             const reasoningContent = delta.reasoning_text

packages/core/test/github-copilot/convert-to-copilot-messages.test.ts

@@ -475,6 +475,48 @@ describe("reasoning (copilot-specific)", () => {
       },
     ])
   })
+
+  test("should replay interleaved reasoning as concatenated text with the latest reasoning_opaque", () => {
+    const result = convertToCopilotMessages([
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "reasoning",
+            text: "Read the readme first.",
+            providerOptions: { copilot: { reasoningOpaque: "opaque-first" } },
+          },
+          {
+            type: "tool-call",
+            toolCallId: "call_first",
+            toolName: "read_file",
+            input: { filePath: "/README.md" },
+            providerOptions: { copilot: { reasoningOpaque: "opaque-first" } },
+          },
+          {
+            type: "reasoning",
+            text: "Then the manifest.",
+            providerOptions: { copilot: { reasoningOpaque: "opaque-second" } },
+          },
+          {
+            type: "tool-call",
+            toolCallId: "call_second",
+            toolName: "read_file",
+            input: { filePath: "/package.json" },
+            providerOptions: { copilot: { reasoningOpaque: "opaque-second" } },
+          },
+        ],
+      },
+    ])
+
+    expect(result).toMatchObject([
+      {
+        role: "assistant",
+        reasoning_text: "Read the readme first.Then the manifest.",
+        reasoning_opaque: "opaque-second",
+      },
+    ])
+  })
 })
 
 describe("full conversation", () => {

packages/core/test/github-copilot/copilot-chat-model.test.ts

@@ -71,6 +71,15 @@ const FIXTURES = {
     `data: {"choices":[{"finish_reason":"tool_calls","index":0,"delta":{"content":null,"role":"assistant","tool_calls":[{"function":{"arguments":"{}","name":"read_file"},"id":"call_reasoning_only_2","index":1,"type":"function"}]}}],"created":1769917420,"id":"opaque-only","usage":{"completion_tokens":12,"prompt_tokens":123,"prompt_tokens_details":{"cached_tokens":0},"total_tokens":135,"reasoning_tokens":0},"model":"gemini-3-flash-preview"}`,
     `data: [DONE]`,
   ],
+
+  // Interleaved thinking (Claude) sends a new reasoning_opaque before each tool call
+  interleavedReasoningOpaque: [
+    `data: {"choices":[{"index":0,"delta":{"content":null,"role":"assistant","reasoning_text":"Read the readme first."}}],"created":1791590400,"id":"interleaved","model":"claude-opus-5.5"}`,
+    `data: {"choices":[{"index":0,"delta":{"content":null,"role":"assistant","tool_calls":[{"function":{"arguments":"{\\"filePath\\":\\"/README.md\\"}","name":"read_file"},"id":"call_first","index":0,"type":"function"}],"reasoning_opaque":"opaque-first"}}],"created":1791590400,"id":"interleaved","model":"claude-opus-5.5"}`,
+    `data: {"choices":[{"index":0,"delta":{"content":null,"role":"assistant","reasoning_text":"Then the manifest."}}],"created":1791590401,"id":"interleaved","model":"claude-opus-5.5"}`,
+    `data: {"choices":[{"finish_reason":"tool_calls","index":0,"delta":{"content":null,"role":"assistant","tool_calls":[{"function":{"arguments":"{\\"filePath\\":\\"/package.json\\"}","name":"read_file"},"id":"call_second","index":1,"type":"function"}],"reasoning_opaque":"opaque-second"}}],"created":1791590401,"id":"interleaved","usage":{"completion_tokens":42,"prompt_tokens":1200,"total_tokens":1242},"model":"claude-opus-5.5"}`,
+    `data: [DONE]`,
+  ],
 }
 
 function createMockFetch(chunks: string[]) {
@@ -482,6 +491,43 @@ describe("doStream", () => {
     })
   })
 
+  test("should pair each interleaved reasoning segment with its own reasoning_opaque", async () => {
+    const mockFetch = createMockFetch(FIXTURES.interleavedReasoningOpaque)
+    const model = createModel(mockFetch)
+
+    const { stream } = await model.doStream({
+      prompt: TEST_PROMPT,
+      includeRawChunks: false,
+    })
+
+    const parts = await convertReadableStreamToArray(stream)
+
+    expect(parts.filter((p) => p.type === "error")).toEqual([])
+    expect(
+      parts.filter((p) => p.type === "reasoning-delta" || p.type === "reasoning-end" || p.type === "tool-call"),
+    ).toMatchObject([
+      { type: "reasoning-delta", delta: "Read the readme first." },
+      { type: "reasoning-end", providerMetadata: { copilot: { reasoningOpaque: "opaque-first" } } },
+      {
+        type: "tool-call",
+        toolCallId: "call_first",
+        providerMetadata: { copilot: { reasoningOpaque: "opaque-first" } },
+      },
+      { type: "reasoning-delta", delta: "Then the manifest." },
+      { type: "reasoning-end", providerMetadata: { copilot: { reasoningOpaque: "opaque-second" } } },
+      {
+        type: "tool-call",
+        toolCallId: "call_second",
+        providerMetadata: { copilot: { reasoningOpaque: "opaque-second" } },
+      },
+    ])
+    expect(parts.at(-1)).toMatchObject({
+      type: "finish",
+      finishReason: { unified: "tool-calls" },
+      providerMetadata: { copilot: { reasoningOpaque: "opaque-second" } },
+    })
+  })
+
   test("should include response metadata from first chunk", async () => {
     const mockFetch = createMockFetch(FIXTURES.basicText)
     const model = createModel(mockFetch)