diff --git a/src/providers/messages-to-responses.test.ts b/src/providers/messages-to-responses.test.ts
index 8a25243..15326f6 100644
--- a/src/providers/messages-to-responses.test.ts
+++ b/src/providers/messages-to-responses.test.ts
@@ -146,4 +146,59 @@ describe("messages-to-responses", () => {
       },
     ]);
   });
+
+  it("normalizes a plaintext reasoning item onto the assistant message", () => {
+    const output = responsesTurnToModelOutput({
+      text: "Answer: B",
+      outputItems: [
+        {
+          type: "reasoning",
+          id: "rs_1",
+          content: [{ type: "reasoning_text", text: "1. Analyze" }],
+          summary: [],
+        },
+        {
+          type: "message",
+          content: [{ type: "output_text", text: "Answer: B" }],
+        },
+      ],
+      functionCalls: [],
+      generationTimeMs: 7,
+    });
+    expect(output.message.reasoning).toBe("1. Analyze");
+    expect(output.message.reasoningDetails).toEqual([
+      { type: "reasoning.text", text: "1. Analyze", id: "rs_1" },
+    ]);
+  });
+
+  it("keeps an encrypted reasoning item as details without inventing readable text", () => {
+    const output = responsesTurnToModelOutput({
+      text: "Answer: B",
+      outputItems: [
+        { type: "reasoning", id: "rs_1", encrypted_content: "opaque" },
+      ],
+      functionCalls: [],
+      generationTimeMs: 7,
+    });
+    expect(output.message).not.toHaveProperty("reasoning");
+    expect(output.message.reasoningDetails).toEqual([
+      { type: "reasoning.encrypted", data: "opaque", id: "rs_1" },
+    ]);
+  });
+
+  it("leaves both reasoning fields absent when the turn carries no reasoning", () => {
+    const output = responsesTurnToModelOutput({
+      text: "Answer: B",
+      outputItems: [
+        {
+          type: "message",
+          content: [{ type: "output_text", text: "Answer: B" }],
+        },
+      ],
+      functionCalls: [],
+      generationTimeMs: 7,
+    });
+    expect(output.message).not.toHaveProperty("reasoning");
+    expect(output.message).not.toHaveProperty("reasoningDetails");
+  });
 });
diff --git a/src/providers/messages-to-responses.ts b/src/providers/messages-to-responses.ts
index d99d2df..64ef412 100644
--- a/src/providers/messages-to-responses.ts
+++ b/src/providers/messages-to-responses.ts
@@ -6,6 +6,7 @@ import type {
 } from "../harness/core";
 import { MessageRole } from "../harness/core";
 import { definedValues } from "../internal/guards";
+import { extractReasoning } from "./responses-client";
 import type {
   ResponsesFunctionTool,
   ResponsesInputItem,
@@ -111,11 +112,13 @@ export function toolDefinitionToResponses(
 }
 
 export function responsesTurnToModelOutput(turn: ResponsesTurn): ModelOutput {
+  const reasoning = extractReasoning(turn.outputItems);
   return definedValues({
     completion: turn.text,
     message: {
       role: MessageRole.Assistant,
       content: turn.text,
+      ...reasoning,
       ...definedValues({
         toolCalls:
           turn.functionCalls.length > 0
diff --git a/src/providers/responses-client.test.ts b/src/providers/responses-client.test.ts
index 10fba65..faa103b 100644
--- a/src/providers/responses-client.test.ts
+++ b/src/providers/responses-client.test.ts
@@ -28,6 +28,7 @@ import type { ModelErrorIdentifiers } from "./request-identifiers";
 import {
   consumeStream,
   extractMessageText,
+  extractReasoning,
   findOutputItems,
   makeResponsesLayer,
   Responses,
@@ -115,6 +116,176 @@ describe("extractMessageText", () => {
     expect(extractMessageText([])).toBe("");
   });
 });
+describe("extractReasoning", () => {
+  it("normalizes plaintext reasoning content into readable reasoning and text details", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          id: "rs_1",
+          content: [
+            { type: "reasoning_text", text: "1. Analyze the request" },
+            { type: "reasoning_text", text: "2. Answer" },
+          ],
+          summary: [],
+        },
+        { type: "message", content: [{ type: "output_text", text: "B" }] },
+      ])
+    ).toEqual({
+      reasoning: "1. Analyze the request\n\n2. Answer",
+      reasoningDetails: [
+        {
+          type: "reasoning.text",
+          text: "1. Analyze the request",
+          id: "rs_1",
+        },
+        { type: "reasoning.text", text: "2. Answer", id: "rs_1" },
+      ],
+    });
+  });
+
+  it("carries the signature and format of a plaintext item", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          id: "rs_1",
+          format: "anthropic-claude-v1",
+          signature: "sig",
+          content: [{ type: "reasoning_text", text: "thought" }],
+        },
+      ])
+    ).toEqual({
+      reasoning: "thought",
+      reasoningDetails: [
+        {
+          type: "reasoning.text",
+          text: "thought",
+          id: "rs_1",
+          format: "anthropic-claude-v1",
+          signature: "sig",
+        },
+      ],
+    });
+  });
+
+  it("keeps an encrypted blob as a detail and emits no readable reasoning", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          id: "rs_2",
+          encrypted_content: "gAAAAAopaque",
+          summary: [],
+        },
+      ])
+    ).toEqual({
+      reasoningDetails: [
+        { type: "reasoning.encrypted", data: "gAAAAAopaque", id: "rs_2" },
+      ],
+    });
+  });
+
+  it("falls back to provider summaries when no plaintext is exposed", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          id: "rs_3",
+          summary: [
+            { type: "summary_text", text: "Considered two options" },
+            { type: "summary_text", text: "Picked the second" },
+          ],
+          encrypted_content: "blob",
+        },
+      ])
+    ).toEqual({
+      reasoning: "Considered two options\n\nPicked the second",
+      reasoningDetails: [
+        {
+          type: "reasoning.summary",
+          summary: "Considered two options",
+          id: "rs_3",
+        },
+        {
+          type: "reasoning.summary",
+          summary: "Picked the second",
+          id: "rs_3",
+        },
+        { type: "reasoning.encrypted", data: "blob", id: "rs_3" },
+      ],
+    });
+  });
+
+  it("prefers plaintext over summaries for readable reasoning while keeping both details", () => {
+    const result = extractReasoning([
+      {
+        type: "reasoning",
+        content: [{ type: "reasoning_text", text: "raw thought" }],
+        summary: [{ type: "summary_text", text: "short summary" }],
+      },
+    ]);
+    expect(result.reasoning).toBe("raw thought");
+    expect(result.reasoningDetails).toEqual([
+      { type: "reasoning.text", text: "raw thought" },
+      { type: "reasoning.summary", summary: "short summary" },
+    ]);
+  });
+
+  it("joins plaintext across multiple reasoning items in wire order", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          content: [{ type: "reasoning_text", text: "first" }],
+        },
+        { type: "function_call", call_id: "c1", name: "t", arguments: "{}" },
+        {
+          type: "reasoning",
+          content: [{ type: "reasoning_text", text: "second" }],
+        },
+      ]).reasoning
+    ).toBe("first\n\nsecond");
+  });
+
+  it("returns nothing when no reasoning item is present", () => {
+    expect(
+      extractReasoning([
+        { type: "message", content: [{ type: "output_text", text: "B" }] },
+      ])
+    ).toEqual({});
+  });
+
+  it("returns nothing for a reasoning item carrying no reasoning at all", () => {
+    expect(
+      extractReasoning([
+        { type: "reasoning", id: "rs_4", summary: [], content: [] },
+      ])
+    ).toEqual({});
+  });
+
+  it("skips malformed parts rather than repairing them", () => {
+    expect(
+      extractReasoning([
+        {
+          type: "reasoning",
+          content: [
+            "not-an-object",
+            { type: "reasoning_text" },
+            { type: "reasoning_text", text: "" },
+            { type: "reasoning_text", text: 42 },
+            { type: "reasoning_text", text: "kept" },
+          ],
+          summary: "not-an-array",
+          encrypted_content: 7,
+        },
+      ])
+    ).toEqual({
+      reasoning: "kept",
+      reasoningDetails: [{ type: "reasoning.text", text: "kept" }],
+    });
+  });
+});
 describe("findOutputItems", () => {
   it("returns all items matching the type", () => {
     const output = [
diff --git a/src/providers/responses-client.ts b/src/providers/responses-client.ts
index 248a6e2..db83b5e 100644
--- a/src/providers/responses-client.ts
+++ b/src/providers/responses-client.ts
@@ -24,6 +24,7 @@ import { succeed as layerSucceed } from "effect/Layer";
 
 import type { Citation, ModelUsage } from "../harness/core";
 import { ModelError } from "../harness/core";
+import type { ReasoningDetails } from "../harness/reasoning-details";
 import { Either } from "../internal/either";
 import { definedValues, isRecord } from "../internal/guards";
 import { parseSchema, z } from "../internal/zod";
@@ -70,6 +71,11 @@ export const ResponsesResultSchema = z.object({
 
 export type ResponsesResult = z.infer<typeof ResponsesResultSchema>;
 
+export interface ResponsesReasoning {
+  readonly reasoning?: string;
+  readonly reasoningDetails?: ReasoningDetails;
+}
+
 const RawResponsesTerminalEventSchema = z.object({
   type: z.union([
     z.literal("response.completed"),
@@ -576,6 +582,82 @@ export function extractMessageText(
   return text;
 }
 
+export function extractReasoning(
+  output: readonly Record<string, unknown>[]
+): ResponsesReasoning {
+  const details: unknown[] = [];
+  const texts: string[] = [];
+  const summaries: string[] = [];
+  for (const item of output) {
+    if (item["type"] !== "reasoning") {
+      continue;
+    }
+    const id = stringField(item, "id");
+    const format = stringField(item, "format");
+    const signature = stringField(item, "signature");
+    for (const text of partTexts(item["content"], "reasoning_text")) {
+      texts.push(text);
+      details.push(
+        definedValues({
+          type: "reasoning.text",
+          text,
+          id,
+          format,
+          signature,
+        })
+      );
+    }
+    for (const summary of partTexts(item["summary"], "summary_text")) {
+      summaries.push(summary);
+      details.push(
+        definedValues({
+          type: "reasoning.summary",
+          summary,
+          id,
+          format,
+        })
+      );
+    }
+    const encrypted = stringField(item, "encrypted_content");
+    if (encrypted !== undefined) {
+      details.push(
+        definedValues({
+          type: "reasoning.encrypted",
+          data: encrypted,
+          id,
+          format,
+        })
+      );
+    }
+  }
+  const readable = texts.length > 0 ? texts : summaries;
+  return definedValues({
+    reasoning: readable.length > 0 ? readable.join("\n\n") : undefined,
+    reasoningDetails: details.length > 0 ? details : undefined,
+  });
+}
+
+function partTexts(parts: unknown, partType: string): string[] {
+  if (!Array.isArray(parts)) {
+    return [];
+  }
+  return parts.flatMap((part) => {
+    if (!isRecord(part) || part["type"] !== partType) {
+      return [];
+    }
+    const text = stringField(part, "text");
+    return text !== undefined ? [text] : [];
+  });
+}
+
+function stringField(
+  record: Readonly<Record<string, unknown>>,
+  key: string
+): string | undefined {
+  const value = record[key];
+  return typeof value === "string" && value.length > 0 ? value : undefined;
+}
+
 export function findOutputItems(
   output: readonly Record<string, unknown>[],
   itemType: string
diff --git a/src/results/parquet.test.ts b/src/results/parquet.test.ts
index e9fe883..7b97fc8 100644
--- a/src/results/parquet.test.ts
+++ b/src/results/parquet.test.ts
@@ -9,6 +9,7 @@ import { MessageRole, ScoreValue } from "../harness/core";
 import type { SampleScore } from "../harness/metric";
 import { assertRight, assertLeft } from "../internal/testing";
 import { parseSchema } from "../internal/zod";
+import { responsesTurnToModelOutput } from "../providers/messages-to-responses";
 import {
   readResultRows,
   runResultToParquet,
@@ -454,6 +455,54 @@ describe("runResultToParquet", () => {
     );
     expect(parsed[0]?.["reasoning"]).toBe("Step 1: ...");
   });
+  it("serializes reasoning from a provider Responses turn end to end", async () => {
+    const output = responsesTurnToModelOutput({
+      text: "Answer: B",
+      outputItems: [
+        {
+          type: "reasoning",
+          id: "rs_1",
+          content: [{ type: "reasoning_text", text: "1. Analyze the request" }],
+          summary: [],
+        },
+        {
+          type: "reasoning",
+          id: "rs_2",
+          encrypted_content: "gAAAAAopaque",
+          summary: [],
+        },
+        {
+          type: "message",
+          content: [{ type: "output_text", text: "Answer: B" }],
+        },
+      ],
+      functionCalls: [],
+      generationTimeMs: 7,
+    });
+    const bufferFromTurn = runResultToParquet({
+      result: {
+        metrics: METRICS,
+        usage: USAGE,
+        sampleScores: [
+          {
+            sampleId: "s0",
+            epoch: 0,
+            score: { value: ScoreValue.Correct, answer: "B", explanation: "" },
+            messages: [output.message],
+            input: "q",
+            target: "B",
+          },
+        ],
+      },
+      meta: META,
+    });
+    const turnRows = await readRows(bufferFromTurn);
+    const parsed: Record<string, unknown>[] = JSON.parse(
+      turnRows[0]!.messages!
+    );
+    expect(parsed[0]?.["reasoning"]).toBe("1. Analyze the request");
+    expect(parsed[0]).not.toHaveProperty("reasoning_details");
+  });
   it("serializes multimodal content parts (image_url) in the messages JSON", async () => {
     const messages: readonly ModelMessage[] = [
       {