From c5ad7c1b0f7623bbfdf64dd4967fa6e99c15c01a Mon Sep 17 00:00:00 2001
From: Cristina Poncela Cubeiro
 <140309543+cristinaponcela@users.noreply.github.com>
Date: Tue, 25 Aug 2026 11:19:31 +0200
Subject: [PATCH] fix(ai): concatenate openai completions reasoning deltas
 (#8605)

---
 packages/ai/src/api/openai-completions.ts     | 32 ++++++++-
 ...enai-completions-reasoning-details.test.ts | 67 +++++++++++++++++++
 2 files changed, 96 insertions(+), 3 deletions(-)

diff --git a/packages/ai/src/api/openai-completions.ts b/packages/ai/src/api/openai-completions.ts
index 08b181f23..04326ca2f 100644
--- a/packages/ai/src/api/openai-completions.ts
+++ b/packages/ai/src/api/openai-completions.ts
@@ -247,6 +247,31 @@ function parseLegacyEncryptedReasoningDetail(
 	}
 }
 
+function fillMissingCommonReasoningDetailFields(
+	target: OpenAIReasoningDetailBase,
+	source: OpenAIReasoningDetail,
+): void {
+	target.id ??= source.id;
+	target.format ||= source.format;
+	target.index ??= source.index;
+}
+
+function appendOpenAIReasoningDetail(details: OpenAIReasoningDetail[], detail: OpenAIReasoningDetail): void {
+	const lastDetail = details[details.length - 1];
+	if (detail.type === "reasoning.text" && lastDetail?.type === "reasoning.text") {
+		lastDetail.text += detail.text;
+		lastDetail.signature ||= detail.signature;
+		fillMissingCommonReasoningDetailFields(lastDetail, detail);
+		return;
+	}
+	if (detail.type === "reasoning.summary" && lastDetail?.type === "reasoning.summary") {
+		lastDetail.summary += detail.summary;
+		fillMissingCommonReasoningDetailFields(lastDetail, detail);
+		return;
+	}
+	details.push({ ...detail });
+}
+
 const OPENAI_COMPLETIONS_REASONING_FIELDS = ["reasoning", "reasoning_content", "reasoning_text"] as const;
 
 type OpenAICompletionsReasoningField = (typeof OPENAI_COMPLETIONS_REASONING_FIELDS)[number];
@@ -623,9 +648,10 @@ export const stream: StreamFunction<"openai-completions", OpenAICompletionsOptio
 							if (!isOpenAIReasoningDetail(detail)) continue;
 							const block = ensureThinkingBlock("");
 							const preservedDetails = parseOpenAIReasoningDetails(block.thinkingSignature) ?? [];
-							preservedDetails.push(detail);
-							// Keep provider replay data in the existing signature slot. OpenRouter
-							// requires the complete reasoning_details sequence in its original order.
+							appendOpenAIReasoningDetail(preservedDetails, detail);
+							// Keep provider replay data in the existing signature slot. OpenRouter streams
+							// reasoning_details as deltas: consecutive text/summary deltas are merged into
+							// logical entries, while encrypted entries remain opaque and discrete.
 							block.thinkingSignature = JSON.stringify(preservedDetails);
 						}
 					}
diff --git a/packages/ai/test/openai-completions-reasoning-details.test.ts b/packages/ai/test/openai-completions-reasoning-details.test.ts
index 723a51b79..a76c3b1dd 100644
--- a/packages/ai/test/openai-completions-reasoning-details.test.ts
+++ b/packages/ai/test/openai-completions-reasoning-details.test.ts
@@ -181,4 +181,71 @@ describe("openai-completions reasoning_details streaming", () => {
 		expect(payload?.reasoning_details).toEqual(expectedReasoningDetails);
 		expect(payload?.reasoning).toBeUndefined();
 	});
+
+	it("merges consecutive text and summary reasoning_details deltas before replay", async () => {
+		const textDelta = { type: "reasoning.text", text: "The", index: 0 };
+		const textDeltaWithSignature = {
+			type: "reasoning.text",
+			text: " user wants the time.",
+			signature: "sha256:text-signature",
+			format: "openai-responses-v1",
+			index: 0,
+		};
+		const summaryDelta = { type: "reasoning.summary", summary: "Looked", index: 0 };
+		const summaryDeltaWithFormat = {
+			type: "reasoning.summary",
+			summary: " up time.",
+			format: "openai-responses-v1",
+			index: 0,
+		};
+		const laterSummaryDelta = {
+			type: "reasoning.summary",
+			summary: "After encrypted block.",
+			format: "openai-responses-v1",
+			index: 0,
+		};
+		const expectedReasoningDetails = [
+			{
+				type: "reasoning.text",
+				text: "The user wants the time.",
+				index: 0,
+				signature: "sha256:text-signature",
+				format: "openai-responses-v1",
+			},
+			{
+				type: "reasoning.summary",
+				summary: "Looked up time.",
+				index: 0,
+				format: "openai-responses-v1",
+			},
+			reasoningDetail,
+			laterSummaryDelta,
+		];
+
+		mockState.chunkSets = [
+			[
+				chunk({ reasoning_details: [textDelta] }),
+				chunk({ reasoning_details: [textDeltaWithSignature] }),
+				chunk({ reasoning_details: [summaryDelta] }),
+				chunk({ reasoning_details: [summaryDeltaWithFormat] }),
+				chunk({ reasoning_details: [reasoningDetail] }),
+				chunk({ reasoning_details: [laterSummaryDelta] }),
+				toolCallChunk(),
+				chunk({}, "tool_calls"),
+			],
+			[chunk({ content: "ok" }), chunk({}, "stop")],
+		];
+
+		const assistantMessage = await runOpenAICompletionsStream();
+		const thinking = assistantMessage.content.find((block) => block.type === "thinking");
+		expect(thinking).toEqual({
+			type: "thinking",
+			thinking: "",
+			thinkingSignature: JSON.stringify(expectedReasoningDetails),
+		});
+
+		await runOpenAICompletionsStream([assistantMessage]);
+
+		expect(getAssistantPayload(mockState.payloads[1])?.reasoning_details).toEqual(expectedReasoningDetails);
+	});
 });