diff --git a/README.md b/README.md
index caeaafd..f06f2c8 100644
--- a/README.md
+++ b/README.md
@@ -234,6 +234,23 @@ const client = new OpenAI({
 | POST   | `/v1/responses`        | Responses text + `function_call`; supports semantic SSE streaming     |
 | POST   | `/v1/messages`         | Anthropic Messages + `tool_use`; supports `stream: true`              |
 
+### Reasoning effort
+
+OpenAI clients can select a Cursor reasoning variant without hard-coding its
+full Cursor model ID:
+
+- Chat Completions: `reasoning_effort`
+- Responses: `reasoning.effort`
+
+For example, model `gpt-5.6-sol` with effort `high` resolves to
+`gpt-5.6-sol-high` when that ID is present in the live `agent --list-models`
+catalog. Explicit variants can be retargeted, and `-fast` is preserved
+(`gpt-5.6-sol-high-fast` plus `low` resolves to
+`gpt-5.6-sol-low-fast`). Supported spellings are `none`/`off`, `minimal`,
+`low`, `medium`, `high`, `xhigh`/`extra-high`, and `max`. The API returns
+`400 unsupported_reasoning_effort` instead of silently choosing another model
+when the requested family does not offer that level.
+
 Usage and token fields: responses may include `usage` token fields (`prompt_tokens`/`completion_tokens` for Chat Completions, `input_tokens`/`output_tokens` for Responses). These are heuristic estimates (character count ÷ 4), not Cursor billing meters. Do not use them for invoicing.
 
 ## Environment variables
diff --git a/src/lib/handlers/chat-completions.ts b/src/lib/handlers/chat-completions.ts
index 15e9128..7ee79d0 100644
--- a/src/lib/handlers/chat-completions.ts
+++ b/src/lib/handlers/chat-completions.ts
@@ -14,7 +14,10 @@ import {
 import type { ToolTurnEvent, ToolTurnResult } from "../acp-tool-session.js";
 import { createStreamParser } from "../cli-stream-parser.js";
 import { json, writeSseHeaders } from "../http.js";
-import { resolveModelForExecution } from "../model-map.js";
+import {
+  resolveModelForExecution,
+  UnsupportedReasoningEffortError,
+} from "../model-map.js";
 import {
   buildPromptFromMessages,
   normalizeModelId,
@@ -263,11 +266,25 @@ export async function handleChatCompletions(
   const requested = normalizeModelId(body.model);
   const model = resolveModel(requested, lastRequestedModelRef, config);
   const models = await getCachedCursorModels(config, modelCacheRef);
-  const decision = resolveModelForExecution({
-    requested: model,
-    defaultModel: config.defaultModel,
-    availableCursorIds: models.map((m) => m.id),
-  });
+  let decision;
+  try {
+    decision = resolveModelForExecution({
+      requested: model,
+      defaultModel: config.defaultModel,
+      availableCursorIds: models.map((m) => m.id),
+      reasoningEffort: body.reasoning_effort,
+    });
+  } catch (error) {
+    if (!(error instanceof UnsupportedReasoningEffortError)) throw error;
+    json(res, 400, {
+      error: {
+        message: error.message,
+        code: error.code,
+        type: "invalid_request_error",
+      },
+    });
+    return;
+  }
   const cursorModel = decision.final;
   rememberResolvedModel(cursorModel, lastRequestedModelRef);
   logModelResolution(config.verbose, decision);
diff --git a/src/lib/handlers/responses.ts b/src/lib/handlers/responses.ts
index c0411c6..209e3fb 100644
--- a/src/lib/handlers/responses.ts
+++ b/src/lib/handlers/responses.ts
@@ -25,7 +25,10 @@ import {
 } from "../agent-runner.js";
 import type { ToolTurnEvent, ToolTurnResult } from "../acp-tool-session.js";
 import { createStreamParser } from "../cli-stream-parser.js";
-import { resolveModelForExecution } from "../model-map.js";
+import {
+  resolveModelForExecution,
+  UnsupportedReasoningEffortError,
+} from "../model-map.js";
 import {
   buildPromptFromMessages,
   normalizeModelId,
@@ -510,11 +513,28 @@ export async function handleResponses(
   const requested = normalizeModelId(body.model);
   const model = resolveModel(requested, lastRequestedModelRef, config);
   const models = await getCachedCursorModels(config, modelCacheRef);
-  const decision = resolveModelForExecution({
-    requested: model,
-    defaultModel: config.defaultModel,
-    availableCursorIds: models.map((m) => m.id),
-  });
+  let decision;
+  try {
+    decision = resolveModelForExecution({
+      requested: model,
+      defaultModel: config.defaultModel,
+      availableCursorIds: models.map((m) => m.id),
+      reasoningEffort:
+        typeof body.reasoning?.effort === "string"
+          ? body.reasoning.effort
+          : undefined,
+    });
+  } catch (error) {
+    if (!(error instanceof UnsupportedReasoningEffortError)) throw error;
+    json(res, 400, {
+      error: {
+        message: error.message,
+        code: error.code,
+        type: "invalid_request_error",
+      },
+    });
+    return;
+  }
   const cursorModel = decision.final;
   rememberResolvedModel(cursorModel, lastRequestedModelRef);
   logModelResolution(config.verbose, decision);
diff --git a/src/lib/model-map.test.ts b/src/lib/model-map.test.ts
index b89aede..45f9c6e 100644
--- a/src/lib/model-map.test.ts
+++ b/src/lib/model-map.test.ts
@@ -1,6 +1,10 @@
 import { describe, expect, it } from "vitest";
 
-import { resolveModelForExecution, resolveToCursorModel } from "./model-map.js";
+import {
+  resolveModelForExecution,
+  resolveToCursorModel,
+  UnsupportedReasoningEffortError,
+} from "./model-map.js";
 
 describe("resolveToCursorModel", () => {
   it("maps dated sonnet id to cursor sonnet-4.5", () => {
@@ -48,4 +52,73 @@ describe("resolveModelForExecution", () => {
     expect(decision.final).toBe("default");
     expect(decision.requestedWasDefault).toBe(true);
   });
+
+  it("maps a logical model and reasoning effort to a Cursor variant", () => {
+    const decision = resolveModelForExecution({
+      requested: "gpt-5.6-sol",
+      reasoningEffort: "high",
+      defaultModel: "auto",
+      availableCursorIds: ["auto", "gpt-5.6-sol-low", "gpt-5.6-sol-high"],
+    });
+    expect(decision.final).toBe("gpt-5.6-sol-high");
+    expect(decision.reasoningEffort).toBe("high");
+    expect(decision.fallbackUsed).toBe(false);
+  });
+
+  it("replaces an explicit effort while preserving the fast variant", () => {
+    const decision = resolveModelForExecution({
+      requested: "gpt-5.6-sol-high-fast",
+      reasoningEffort: "low",
+      defaultModel: "auto",
+      availableCursorIds: [
+        "auto",
+        "gpt-5.6-sol-high-fast",
+        "gpt-5.6-sol-low-fast",
+      ],
+    });
+    expect(decision.final).toBe("gpt-5.6-sol-low-fast");
+  });
+
+  it("maps off to Cursor's none suffix", () => {
+    const decision = resolveModelForExecution({
+      requested: "gpt-5.6-sol",
+      reasoningEffort: "off",
+      defaultModel: "auto",
+      availableCursorIds: ["auto", "gpt-5.6-sol-none"],
+    });
+    expect(decision.final).toBe("gpt-5.6-sol-none");
+    expect(decision.reasoningEffort).toBe("none");
+  });
+
+  it("accepts extra-high when the catalog spells it xhigh", () => {
+    const decision = resolveModelForExecution({
+      requested: "gpt-5.6-sol",
+      reasoningEffort: "extra-high",
+      defaultModel: "auto",
+      availableCursorIds: ["auto", "gpt-5.6-sol-xhigh"],
+    });
+    expect(decision.final).toBe("gpt-5.6-sol-xhigh");
+  });
+
+  it("rejects a reasoning effort unavailable for the requested family", () => {
+    expect(() =>
+      resolveModelForExecution({
+        requested: "composer-2.5",
+        reasoningEffort: "high",
+        defaultModel: "auto",
+        availableCursorIds: ["auto", "composer-2.5"],
+      }),
+    ).toThrow(UnsupportedReasoningEffortError);
+  });
+
+  it("rejects an unknown reasoning effort", () => {
+    expect(() =>
+      resolveModelForExecution({
+        requested: "gpt-5.6-sol",
+        reasoningEffort: "ultra",
+        defaultModel: "auto",
+        availableCursorIds: ["auto", "gpt-5.6-sol-high"],
+      }),
+    ).toThrow(/ultra/);
+  });
 });
diff --git a/src/lib/model-map.ts b/src/lib/model-map.ts
index 93d9e2e..bbe210e 100644
--- a/src/lib/model-map.ts
+++ b/src/lib/model-map.ts
@@ -7,12 +7,34 @@ export type ModelResolutionDecision = {
   requested?: string;
   mapped?: string;
   final: string;
+  reasoningEffort?: CursorReasoningEffort;
   requestedWasDefault: boolean;
   validated: boolean;
   fallbackUsed: boolean;
   fallbackReason?: string;
 };
 
+export type CursorReasoningEffort =
+  | "none"
+  | "minimal"
+  | "low"
+  | "medium"
+  | "high"
+  | "xhigh"
+  | "max";
+
+export class UnsupportedReasoningEffortError extends Error {
+  readonly code = "unsupported_reasoning_effort";
+
+  constructor(
+    readonly model: string,
+    readonly effort: string,
+  ) {
+    super(`Cursor model "${model}" does not offer reasoning effort "${effort}"`);
+    this.name = "UnsupportedReasoningEffortError";
+  }
+}
+
 /** Anthropic-style model name (any case) -> Cursor CLI model id */
 const ANTHROPIC_TO_CURSOR: Record<string, string> = {
   // Claude 4.6
@@ -93,16 +115,112 @@ function matchAvailableModel(
   return byLower.get(candidate.toLowerCase());
 }
 
+const EFFORT_ALIASES: Record<string, CursorReasoningEffort> = {
+  off: "none",
+  none: "none",
+  minimal: "minimal",
+  low: "low",
+  medium: "medium",
+  high: "high",
+  xhigh: "xhigh",
+  "extra-high": "xhigh",
+  extra_high: "xhigh",
+  max: "max",
+};
+
+const EFFORT_SUFFIXES = [
+  "extra-high",
+  "minimal",
+  "medium",
+  "xhigh",
+  "high",
+  "none",
+  "low",
+  "max",
+] as const;
+
+function normalizeReasoningEffort(
+  effort: string | undefined,
+): CursorReasoningEffort | undefined {
+  if (!effort?.trim()) return undefined;
+  return EFFORT_ALIASES[effort.trim().toLowerCase()];
+}
+
+function splitFastSuffix(model: string): { base: string; fast: boolean } {
+  return model.toLowerCase().endsWith("-fast")
+    ? { base: model.slice(0, -5), fast: true }
+    : { base: model, fast: false };
+}
+
+function stripEffortSuffix(model: string): string {
+  const lower = model.toLowerCase();
+  const suffix = EFFORT_SUFFIXES.find((value) =>
+    lower.endsWith(`-${value}`),
+  );
+  return suffix ? model.slice(0, -(suffix.length + 1)) : model;
+}
+
+function effortSuffixCandidates(effort: CursorReasoningEffort): string[] {
+  if (effort === "xhigh") return ["xhigh", "extra-high"];
+  return [effort];
+}
+
+function resolveReasoningModel(args: {
+  model: string;
+  effort: CursorReasoningEffort;
+  availableCursorIds: string[];
+}): string | undefined {
+  if (args.model === "default" || args.model === "auto") return undefined;
+
+  const { base: withoutFast, fast } = splitFastSuffix(args.model);
+  const base = stripEffortSuffix(withoutFast);
+  for (const suffix of effortSuffixCandidates(args.effort)) {
+    const candidate = `${base}-${suffix}${fast ? "-fast" : ""}`;
+    const matched = matchAvailableModel(candidate, args.availableCursorIds);
+    if (matched) return matched;
+  }
+  return undefined;
+}
+
 export function resolveModelForExecution(args: {
   requested: string | undefined;
   defaultModel: string;
   availableCursorIds: string[];
+  reasoningEffort?: string;
 }): ModelResolutionDecision {
   const requested = args.requested?.trim();
   const requestedWasDefault = requested === "default";
   const mapped = requestedWasDefault
     ? "default"
     : resolveToCursorModel(requested) ?? args.defaultModel;
+  const reasoningEffort = normalizeReasoningEffort(args.reasoningEffort);
+
+  if (args.reasoningEffort && !reasoningEffort) {
+    throw new UnsupportedReasoningEffortError(
+      mapped,
+      args.reasoningEffort,
+    );
+  }
+
+  if (reasoningEffort) {
+    const reasoningModel = resolveReasoningModel({
+      model: mapped,
+      effort: reasoningEffort,
+      availableCursorIds: args.availableCursorIds,
+    });
+    if (!reasoningModel) {
+      throw new UnsupportedReasoningEffortError(mapped, reasoningEffort);
+    }
+    return {
+      requested,
+      mapped,
+      final: reasoningModel,
+      reasoningEffort,
+      requestedWasDefault,
+      validated: true,
+      fallbackUsed: false,
+    };
+  }
 
   if (mapped === "default") {
     return {
diff --git a/src/lib/openai.ts b/src/lib/openai.ts
index 903b51b..94eeec9 100644
--- a/src/lib/openai.ts
+++ b/src/lib/openai.ts
@@ -9,6 +9,7 @@ export type OpenAiChatCompletionRequest = {
   parallel_tool_calls?: boolean;
   functions?: any[];
   function_call?: any;
+  reasoning_effort?: string;
 };
 
 export type OpenAiResponsesRequest = {
diff --git a/src/lib/server.test.ts b/src/lib/server.test.ts
index 1796db4..11f814f 100644
--- a/src/lib/server.test.ts
+++ b/src/lib/server.test.ts
@@ -11,6 +11,8 @@ vi.mock("./cursor-cli.js", () => ({
   listCursorCliModels: vi.fn().mockResolvedValue([
     { id: "claude-3-opus", name: "Claude 3 Opus" },
     { id: "claude-3-sonnet", name: "Claude 3 Sonnet" },
+    { id: "gpt-5.6-sol-low", name: "GPT-5.6 Sol Low" },
+    { id: "gpt-5.6-sol-high", name: "GPT-5.6 Sol High" },
   ]),
 }));
 
@@ -160,7 +162,7 @@ describe("startBridgeServer", () => {
     expect(status).toBe(200);
     const data = JSON.parse(body);
     expect(data.object).toBe("list");
-    expect(data.data).toHaveLength(2);
+    expect(data.data).toHaveLength(4);
     expect(data.data[0].id).toBe("claude-3-opus");
   });
 
@@ -369,6 +371,79 @@ describe("startBridgeServer", () => {
     expect(data.usage.output_tokens).toBeGreaterThan(0);
   });
 
+  it("maps Chat Completions reasoning_effort to a Cursor model variant", async () => {
+    const runMock = vi.mocked(run);
+    runMock.mockClear();
+    servers = startBridgeServer({
+      version: "1.0.0",
+      config: createTestConfig(),
+    });
+    await new Promise<void>((resolve) =>
+      servers[0].on("listening", () => resolve()),
+    );
+
+    const { status } = await fetchServer(servers[0], "/v1/chat/completions", {
+      method: "POST",
+      body: JSON.stringify({
+        model: "gpt-5.6-sol",
+        reasoning_effort: "high",
+        messages: [{ role: "user", content: "Hi" }],
+      }),
+    });
+    expect(status).toBe(200);
+    const [, args] = runMock.mock.calls[0];
+    expect(args).toContain("gpt-5.6-sol-high");
+  });
+
+  it("maps Responses reasoning.effort to a Cursor model variant", async () => {
+    const runMock = vi.mocked(run);
+    runMock.mockClear();
+    servers = startBridgeServer({
+      version: "1.0.0",
+      config: createTestConfig(),
+    });
+    await new Promise<void>((resolve) =>
+      servers[0].on("listening", () => resolve()),
+    );
+
+    const { status } = await fetchServer(servers[0], "/v1/responses", {
+      method: "POST",
+      body: JSON.stringify({
+        model: "gpt-5.6-sol",
+        reasoning: { effort: "low" },
+        input: "Hi",
+      }),
+    });
+    expect(status).toBe(200);
+    const [, args] = runMock.mock.calls[0];
+    expect(args).toContain("gpt-5.6-sol-low");
+  });
+
+  it("rejects reasoning effort unavailable for the model family", async () => {
+    servers = startBridgeServer({
+      version: "1.0.0",
+      config: createTestConfig(),
+    });
+    await new Promise<void>((resolve) =>
+      servers[0].on("listening", () => resolve()),
+    );
+
+    const { status, body } = await fetchServer(
+      servers[0],
+      "/v1/chat/completions",
+      {
+        method: "POST",
+        body: JSON.stringify({
+          model: "claude-3-opus",
+          reasoning_effort: "max",
+          messages: [{ role: "user", content: "Hi" }],
+        }),
+      },
+    );
+    expect(status).toBe(400);
+    expect(JSON.parse(body).error.code).toBe("unsupported_reasoning_effort");
+  });
+
   it("streams OpenAI Responses semantic SSE events", async () => {
     servers = startBridgeServer({
       version: "1.0.0",