diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 2221d44..e05836f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -347,9 +347,14 @@ jobs:
           if [ "$PYTHON_VERSION" != "3.13.1t" ]; then
             exit 1
           fi
+          if [ -n "$PYTHON_RUNTIME_ID" ]; then
+            echo "python-runtime-id should be empty without environment activation"
+            exit 1
+          fi
         shell: bash
         env:
           PYTHON_VERSION: ${{ steps.setup-uv.outputs.python-version }}
+          PYTHON_RUNTIME_ID: ${{ steps.setup-uv.outputs.python-runtime-id }}
       - run: uv sync
         working-directory: __tests__/fixtures/uv-project
 
@@ -439,6 +444,15 @@ jobs:
               raise SystemExit(f"Python is not running from custom venv: {sys.executable}")
           PY
         shell: bash
+      - name: Verify Python runtime ID from custom venv
+        run: |
+          case "$PYTHON_RUNTIME_ID" in
+            cpython-3.13.1+freethreaded-*) ;;
+            *) echo "Wrong Python runtime ID: $PYTHON_RUNTIME_ID"; exit 1 ;;
+          esac
+        shell: bash
+        env:
+          PYTHON_RUNTIME_ID: ${{ steps.setup-uv.outputs.python-runtime-id }}
 
   test-activate-environment-no-project:
     runs-on: ubuntu-latest
diff --git a/README.md b/README.md
index 260224d..1851422 100644
--- a/README.md
+++ b/README.md
@@ -138,6 +138,7 @@ Have a look under [Advanced Configuration](#advanced-configuration) for detailed
 - `cache-hit`: A boolean value to indicate a cache entry was found.
 - `venv`: Path to the activated venv if activate-environment is true.
 - `python-version`: The Python version that was set.
+- `python-runtime-id`: An opaque identifier reported by uv for the activated venv's Python runtime. Empty when `activate-environment` is false.
 - `python-cache-hit`: A boolean value to indicate the Python cache entry was found.
 
 ### Python version
diff --git a/__tests__/utils/python-runtime.test.ts b/__tests__/utils/python-runtime.test.ts
new file mode 100644
index 0000000..2f00b12
--- /dev/null
+++ b/__tests__/utils/python-runtime.test.ts
@@ -0,0 +1,86 @@
+import { promisify } from "node:util";
+import { beforeEach, expect, it, jest } from "@jest/globals";
+import { createSetupInputs } from "../helpers/setup-inputs";
+
+const mockExecFile =
+  jest.fn<
+    (...args: unknown[]) => Promise<{ stdout: string; stderr: string }>
+  >();
+const inputs = createSetupInputs({
+  activateEnvironment: true,
+  pythonVersion: "3.15t",
+});
+
+jest.unstable_mockModule("node:child_process", () => ({
+  // execFile's custom promisifier returns both stdout and stderr.
+  execFile: Object.assign(mockExecFile, { [promisify.custom]: mockExecFile }),
+}));
+
+const { getPythonRuntimeId } = await import("../../src/utils/python-runtime");
+
+beforeEach(() => {
+  mockExecFile.mockReset();
+  mockExecFile.mockResolvedValue({
+    stderr: "",
+    stdout: '[{"key":"cpython-3.13.1-linux-x86_64-gnu"}]\r\n',
+  });
+});
+
+it("does not query uv without environment activation", async () => {
+  expect(
+    await getPythonRuntimeId({ ...inputs, activateEnvironment: false }),
+  ).toBe("");
+  expect(mockExecFile).not.toHaveBeenCalled();
+});
+
+it.each([
+  "cpython-3.13.1-linux-x86_64-gnu",
+  "cpython-3.15.0rc1+freethreaded-macos-aarch64-none",
+  "cpython-3.15.0rc2+freethreaded-windows-x86_64-none",
+  "pypy-3.11.15-linux-x86_64-gnu",
+])("returns uv's opaque runtime key unchanged: %s", async (key) => {
+  mockExecFile.mockResolvedValue({
+    stderr: "",
+    stdout: `${JSON.stringify([{ key }])}\r\n`,
+  });
+  expect(await getPythonRuntimeId(inputs)).toBe(key);
+});
+
+it.each(['/runner temp/a "quoted" venv', "C:\\runner temp\\custom venv"])(
+  "queries the exact venv directory: %s",
+  async (venvPath) => {
+    await getPythonRuntimeId({ ...inputs, venvPath });
+    expect(mockExecFile).toHaveBeenCalledWith(
+      "uv",
+      [
+        "python",
+        "list",
+        venvPath,
+        "--only-installed",
+        "--output-format",
+        "json",
+      ],
+      { encoding: "utf8" },
+    );
+  },
+);
+
+it.each([
+  new Error("uv failed"),
+  "not JSON",
+  "null",
+  "{}",
+  "[]",
+  '[{"key":""}]',
+  '[{"key":123}]',
+  '[{"key":"first"},{"key":"second"}]',
+])("rejects uv failure or invalid results: %s", async (result) => {
+  if (result instanceof Error) {
+    mockExecFile.mockRejectedValue(result);
+  } else {
+    mockExecFile.mockResolvedValue({ stderr: "", stdout: result });
+  }
+  await expect(getPythonRuntimeId(inputs)).rejects.toThrow(
+    "Failed to identify the activated environment's Python runtime:",
+  );
+});
diff --git a/action-types.yml b/action-types.yml
index 20c6ec4..0be5368 100644
--- a/action-types.yml
+++ b/action-types.yml
@@ -79,5 +79,7 @@ outputs:
     type: string
   python-version:
     type: string
+  python-runtime-id:
+    type: string
   python-cache-hit:
     type: boolean
diff --git a/action.yml b/action.yml
index 50b05ec..2a51899 100644
--- a/action.yml
+++ b/action.yml
@@ -107,6 +107,8 @@ outputs:
     description: "Path to the activated venv if activate-environment is true"
   python-version:
     description: "The Python version that was set."
+  python-runtime-id:
+    description: "An opaque identifier reported by uv for the activated venv's Python runtime. Empty when activate-environment is false."
   python-cache-hit:
     description: "A boolean value to indicate the Python cache entry was found"
 runs:
diff --git a/dist/setup/index.cjs b/dist/setup/index.cjs
index 0946f4f..4022d81 100644
--- a/dist/setup/index.cjs
+++ b/dist/setup/index.cjs
@@ -10833,7 +10833,7 @@ var require_mock_interceptor = __commonJS({
 var require_mock_client = __commonJS({
   "node_modules/@actions/http-client/node_modules/undici/lib/mock/mock-client.js"(exports2, module2) {
     "use strict";
-    var { promisify: promisify5 } = require("node:util");
+    var { promisify: promisify6 } = require("node:util");
     var Client = require_client();
     var { buildMockDispatch } = require_mock_utils();
     var {
@@ -10873,7 +10873,7 @@ var require_mock_client = __commonJS({
         return new MockInterceptor(opts, this[kDispatches]);
       }
       async [kClose]() {
-        await promisify5(this[kOriginalClose])();
+        await promisify6(this[kOriginalClose])();
         this[kConnected] = 0;
         this[kMockAgent][Symbols.kClients].delete(this[kOrigin]);
       }
@@ -10886,7 +10886,7 @@ var require_mock_client = __commonJS({
 var require_mock_pool = __commonJS({
   "node_modules/@actions/http-client/node_modules/undici/lib/mock/mock-pool.js"(exports2, module2) {
     "use strict";
-    var { promisify: promisify5 } = require("node:util");
+    var { promisify: promisify6 } = require("node:util");
     var Pool = require_pool();
     var { buildMockDispatch } = require_mock_utils();
     var {
@@ -10926,7 +10926,7 @@ var require_mock_pool = __commonJS({
         return new MockInterceptor(opts, this[kDispatches]);
       }
       async [kClose]() {
-        await promisify5(this[kOriginalClose])();
+        await promisify6(this[kOriginalClose])();
         this[kConnected] = 0;
         this[kMockAgent][Symbols.kClients].delete(this[kOrigin]);
       }
@@ -43099,7 +43099,7 @@ var require_mock_interceptor2 = __commonJS({
 var require_mock_client2 = __commonJS({
   "node_modules/undici/lib/mock/mock-client.js"(exports2, module2) {
     "use strict";
-    var { promisify: promisify5 } = require("node:util");
+    var { promisify: promisify6 } = require("node:util");
     var Client = require_client2();
     var { buildMockDispatch } = require_mock_utils2();
     var {
@@ -43147,7 +43147,7 @@ var require_mock_client2 = __commonJS({
         this[kDispatches] = [];
       }
       async [kClose]() {
-        await promisify5(this[kOriginalClose])();
+        await promisify6(this[kOriginalClose])();
         this[kConnected] = 0;
         this[kMockAgent][Symbols.kClients].delete(this[kOrigin]);
       }
@@ -43360,7 +43360,7 @@ var require_mock_call_history = __commonJS({
 var require_mock_pool2 = __commonJS({
   "node_modules/undici/lib/mock/mock-pool.js"(exports2, module2) {
     "use strict";
-    var { promisify: promisify5 } = require("node:util");
+    var { promisify: promisify6 } = require("node:util");
     var Pool = require_pool2();
     var { buildMockDispatch } = require_mock_utils2();
     var {
@@ -43408,7 +43408,7 @@ var require_mock_pool2 = __commonJS({
         this[kDispatches] = [];
       }
       async [kClose]() {
-        await promisify5(this[kOriginalClose])();
+        await promisify6(this[kOriginalClose])();
         this[kConnected] = 0;
         this[kMockAgent][Symbols.kClients].delete(this[kOrigin]);
       }
@@ -102102,6 +102102,40 @@ function getResolutionStrategy() {
   );
 }
 
+// src/utils/python-runtime.ts
+var import_node_child_process = require("node:child_process");
+var import_node_util4 = require("node:util");
+var execFileAsync = (0, import_node_util4.promisify)(import_node_child_process.execFile);
+async function getPythonRuntimeId(inputs) {
+  if (!inputs.activateEnvironment) {
+    return "";
+  }
+  try {
+    const { stdout } = await execFileAsync(
+      "uv",
+      [
+        "python",
+        "list",
+        inputs.venvPath,
+        "--only-installed",
+        "--output-format",
+        "json"
+      ],
+      { encoding: "utf8" }
+    );
+    const pythons = JSON.parse(stdout);
+    if (!Array.isArray(pythons) || pythons.length !== 1 || typeof pythons[0]?.key !== "string" || pythons[0].key === "") {
+      throw new Error("Expected one installed Python with a runtime key");
+    }
+    return pythons[0].key;
+  } catch (error2) {
+    throw new Error(
+      `Failed to identify the activated environment's Python runtime: ${error2 instanceof Error ? error2.message : String(error2)}`,
+      { cause: error2 }
+    );
+  }
+}
+
 // src/setup-uv.ts
 var sourceDir = __dirname;
 function formatUnexpectedFailure(error2) {
@@ -102172,6 +102206,7 @@ async function run() {
     info2(`Successfully installed uv version ${setupResult.version}`);
     const detectedPythonVersion = await getPythonVersion2(inputs);
     setOutput("python-version", detectedPythonVersion);
+    setOutput("python-runtime-id", await getPythonRuntimeId(inputs));
     if (inputs.enableCache) {
       await restoreCache2(inputs, detectedPythonVersion);
     }
diff --git a/docs/environment-and-tools.md b/docs/environment-and-tools.md
index 869d855..c98d356 100644
--- a/docs/environment-and-tools.md
+++ b/docs/environment-and-tools.md
@@ -17,6 +17,18 @@ This allows directly using it in later steps:
 
 By default, the venv is created at `.venv` inside the `working-directory`.
 
+With `activate-environment: true`, the `python-runtime-id` output identifies the
+venv's Python runtime as reported by uv. This is an opaque identifier that users of the action
+can use as a cache key if necessary; users should not assume anything about
+the stability or structure of the identifier itself.
+
+For example, you can combine it with the platform and dependency information relevant to
+your cache with `id: setup-uv` on the setup step:
+
+```yaml
+key: build-${{ runner.os }}-${{ runner.arch }}-${{ steps.setup-uv.outputs.python-runtime-id }}-${{ hashFiles('uv.lock') }}
+```
+
 You can customize the venv location with `venv-path`, for example to place it in the runner temp directory:
 
 ```yaml
diff --git a/src/setup-uv.ts b/src/setup-uv.ts
index 836ce57..f83087f 100644
--- a/src/setup-uv.ts
+++ b/src/setup-uv.ts
@@ -16,6 +16,7 @@ import {
   getPlatform,
   type Platform,
 } from "./utils/platforms";
+import { getPythonRuntimeId } from "./utils/python-runtime";
 import { resolveUvVersion } from "./version/resolve";
 
 const sourceDir = __dirname;
@@ -101,6 +102,7 @@ async function run(): Promise<void> {
 
     const detectedPythonVersion = await getPythonVersion(inputs);
     core.setOutput("python-version", detectedPythonVersion);
+    core.setOutput("python-runtime-id", await getPythonRuntimeId(inputs));
 
     if (inputs.enableCache) {
       await restoreCache(inputs, detectedPythonVersion);
diff --git a/src/utils/python-runtime.ts b/src/utils/python-runtime.ts
new file mode 100644
index 0000000..c44b49e
--- /dev/null
+++ b/src/utils/python-runtime.ts
@@ -0,0 +1,43 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import type { SetupInputs } from "./inputs";
+
+const execFileAsync = promisify(execFile);
+
+export async function getPythonRuntimeId(inputs: SetupInputs): Promise<string> {
+  if (!inputs.activateEnvironment) {
+    return "";
+  }
+
+  try {
+    // The venv path restricts results to this invocation's runtime, even if
+    // earlier setup-uv calls installed other Python versions in the same job.
+    const { stdout } = await execFileAsync(
+      "uv",
+      [
+        "python",
+        "list",
+        inputs.venvPath,
+        "--only-installed",
+        "--output-format",
+        "json",
+      ],
+      { encoding: "utf8" },
+    );
+    const pythons = JSON.parse(stdout);
+    if (
+      !Array.isArray(pythons) ||
+      pythons.length !== 1 ||
+      typeof pythons[0]?.key !== "string" ||
+      pythons[0].key === ""
+    ) {
+      throw new Error("Expected one installed Python with a runtime key");
+    }
+    return pythons[0].key;
+  } catch (error) {
+    throw new Error(
+      `Failed to identify the activated environment's Python runtime: ${error instanceof Error ? error.message : String(error)}`,
+      { cause: error },
+    );
+  }
+}