diff --git a/README.md b/README.md
index 873afe4..c27a954 100644
--- a/README.md
+++ b/README.md
@@ -140,7 +140,7 @@ Have a look under [Advanced Configuration](#advanced-configuration) for detailed
 - `python-version`: The Python version that was set.
 - `python-runtime-id`: An opaque identifier for the activated venv's Python runtime, including
   implementation, full version, and free-threaded build information. Useful as a cache-key component.
-  Empty when `activate-environment` is false or the runtime cannot be determined.
+  Empty when `activate-environment` is false. The action fails if the activated runtime cannot be determined.
 - `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
index 328a73f..3e882fd 100644
--- a/__tests__/utils/python-runtime.test.ts
+++ b/__tests__/utils/python-runtime.test.ts
@@ -7,14 +7,12 @@ const mockExecFile =
   jest.fn<
     (...args: unknown[]) => Promise<{ stdout: string; stderr: string }>
   >();
-const mockDebug = jest.fn();
 const originalPlatform = process.platform;
 const inputs = createSetupInputs({
   activateEnvironment: true,
   pythonVersion: "3.15t",
 });
 
-jest.unstable_mockModule("@actions/core", () => ({ debug: mockDebug }));
 jest.unstable_mockModule("node:child_process", () => ({
   // execFile's custom promisifier returns both stdout and stderr.
   execFile: Object.assign(mockExecFile, { [promisify.custom]: mockExecFile }),
@@ -70,16 +68,13 @@ it.each([
   ["pypy", [7, 3, 24, "beta", 2], "pypy-7.3.24b2"],
   ["pypy", [7, 3, 24, "candidate", 3], "pypy-7.3.24rc3"],
   ["graalpy", [25, 0, 0, "final", 0], "graalpy-25.0.0"],
-  ["pypy", [7, 3, 24, "unknown", 0], ""],
 ])("formats %s implementation version %j", async (name, version, expected) => {
   mockRuntime({
     implementation: name,
     implementationVersion: version,
     pythonVersion: "3.11.15",
   });
-  expect(await getPythonRuntimeId(inputs)).toBe(
-    expected ? `${expected}-python-3.11.15` : "",
-  );
+  expect(await getPythonRuntimeId(inputs)).toBe(`${expected}-python-3.11.15`);
 });
 
 it.each([
@@ -97,15 +92,25 @@ it.each([
   );
 });
 
-it.each([new Error("interpreter missing"), "", "not JSON", "null", "{}"])(
-  "returns an empty ID for interpreter failure: %s",
-  async (result) => {
-    if (result instanceof Error) {
-      mockExecFile.mockRejectedValue(result);
-    } else {
-      mockExecFile.mockResolvedValue({ stderr: "", stdout: result });
-    }
-    expect(await getPythonRuntimeId(inputs)).toBe("");
-    expect(mockDebug).toHaveBeenCalled();
-  },
-);
+it.each([
+  new Error("interpreter missing"),
+  "",
+  "not JSON",
+  "null",
+  "{}",
+  JSON.stringify({
+    freethreaded: false,
+    implementation: "pypy",
+    implementationVersion: [7, 3, 24, "unknown", 0],
+    pythonVersion: "3.11.15",
+  }),
+])("rejects interpreter failure or invalid metadata: %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.yml b/action.yml
index 905ddd5..22729dc 100644
--- a/action.yml
+++ b/action.yml
@@ -108,7 +108,7 @@ outputs:
   python-version:
     description: "The Python version that was set."
   python-runtime-id:
-    description: "An opaque identifier for the activated venv's Python runtime, including implementation, full version, and free-threaded build information. Empty when activate-environment is false or the runtime cannot be determined."
+    description: "An opaque identifier for the activated venv's Python runtime, including implementation, full version, and free-threaded build information. Empty when activate-environment is false. The action fails if the activated runtime cannot be determined."
   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 9c633ce..73b6d57 100644
--- a/dist/setup/index.cjs
+++ b/dist/setup/index.cjs
@@ -102116,10 +102116,10 @@ async function getPythonRuntimeId(inputs) {
     );
     return formatRuntimeId(JSON.parse(stdout));
   } catch (error2) {
-    debug(
-      `Failed to identify the activated environment's Python runtime. Error: ${error2 instanceof Error ? error2.message : String(error2)}`
+    throw new Error(
+      `Failed to identify the activated environment's Python runtime: ${error2 instanceof Error ? error2.message : String(error2)}`,
+      { cause: error2 }
     );
-    return "";
   }
 }
 
diff --git a/docs/environment-and-tools.md b/docs/environment-and-tools.md
index 9fdb139..da9c112 100644
--- a/docs/environment-and-tools.md
+++ b/docs/environment-and-tools.md
@@ -30,9 +30,9 @@ key: build-${{ runner.os }}-${{ runner.arch }}-${{ steps.setup-uv.outputs.python
 ```
 
 The free-threaded marker describes the interpreter's build even when the GIL is
-enabled at runtime. The output is empty when `activate-environment` is false or the
-runtime cannot be determined. The existing `python-version` output and setup-uv's
-cache keys are unaffected.
+enabled at runtime. The output is empty when `activate-environment` is false. If
+the activated runtime cannot be determined, the action fails. The existing
+`python-version` output and setup-uv's cache keys are unaffected.
 
 You can customize the venv location with `venv-path`, for example to place it in the runner temp directory:
 
diff --git a/src/utils/python-runtime.ts b/src/utils/python-runtime.ts
index 0b5651c..c10153e 100644
--- a/src/utils/python-runtime.ts
+++ b/src/utils/python-runtime.ts
@@ -1,7 +1,6 @@
 import { execFile } from "node:child_process";
 import { join } from "node:path";
 import { promisify } from "node:util";
-import * as core from "@actions/core";
 import type { SetupInputs } from "./inputs";
 
 const execFileAsync = promisify(execFile);
@@ -79,9 +78,9 @@ export async function getPythonRuntimeId(inputs: SetupInputs): Promise<string> {
     );
     return formatRuntimeId(JSON.parse(stdout));
   } catch (error) {
-    core.debug(
-      `Failed to identify the activated environment's Python runtime. Error: ${error instanceof Error ? error.message : String(error)}`,
+    throw new Error(
+      `Failed to identify the activated environment's Python runtime: ${error instanceof Error ? error.message : String(error)}`,
+      { cause: error },
     );
-    return "";
   }
 }