Add Workload Identity Federation (OIDC) authentication support (#1338)

* Add workload identity federation auth support

Adds anthropic_federation_rule_id, anthropic_organization_id,
anthropic_service_account_id, anthropic_workspace_id, and
anthropic_oidc_audience inputs. When the federation rule and organization
are set, the action fetches the workflow's GitHub Actions OIDC token,
writes it to a file in RUNNER_TEMP, keeps it refreshed during execution,
and points the Claude Code CLI at it via ANTHROPIC_IDENTITY_TOKEN_FILE so
the CLI can exchange it for a short-lived access token instead of using a
static API key.

* Add WIF example workflow and base-action federation docs

* Default workload identity OIDC audience to https://api.anthropic.com

src/auth/workload-identity.ts

@@ -0,0 +1,120 @@
+#!/usr/bin/env bun
+
+/**
+ * Workload Identity Federation support.
+ *
+ * When the federation inputs are configured, the action fetches a GitHub
+ * Actions OIDC token (JWT), writes it to a file, and points the Claude Code
+ * CLI at it via ANTHROPIC_IDENTITY_TOKEN_FILE. The CLI exchanges the JWT for
+ * a short-lived Anthropic access token using the federation rule, so no
+ * static ANTHROPIC_API_KEY is needed.
+ *
+ * GitHub's OIDC tokens are short-lived and the CLI re-reads the token file
+ * every time it refreshes its Anthropic access token, so the action keeps the
+ * file fresh in the background for long-running executions.
+ */
+
+import * as core from "@actions/core";
+import { mkdirSync, writeFileSync } from "fs";
+import { join } from "path";
+import { retryWithBackoff } from "../utils/retry";
+
+/** How often the GitHub OIDC identity token file is rewritten. */
+const REFRESH_INTERVAL_MS = 4 * 60 * 1000;
+
+/**
+ * Default audience requested on the GitHub OIDC token. Scopes the JWT to the
+ * Claude API token exchange; override with the anthropic_oidc_audience input
+ * if your federation rule expects a different audience.
+ */
+const DEFAULT_OIDC_AUDIENCE = "https://api.anthropic.com";
+
+export type WorkloadIdentityHandle = {
+  tokenFile: string;
+  stop: () => void;
+};
+
+/**
+ * Whether the workload identity federation inputs are configured.
+ * Mirrors the Claude Code CLI's env detection, which requires the federation
+ * rule ID and organization ID.
+ */
+export function isWorkloadIdentityConfigured(): boolean {
+  return Boolean(
+    process.env.ANTHROPIC_FEDERATION_RULE_ID?.trim() &&
+      process.env.ANTHROPIC_ORGANIZATION_ID?.trim(),
+  );
+}
+
+async function fetchIdentityToken(audience: string) {
+  return retryWithBackoff(() => core.getIDToken(audience));
+}
+
+/**
+ * Fetches a GitHub Actions OIDC token, writes it to a file in RUNNER_TEMP,
+ * exports ANTHROPIC_IDENTITY_TOKEN_FILE, and starts a background refresh so
+ * the file stays valid for long executions.
+ *
+ * Returns undefined when federation is not configured or is shadowed by a
+ * higher-precedence credential. Callers must invoke stop() when execution
+ * finishes.
+ */
+export async function setupWorkloadIdentity(): Promise<
+  WorkloadIdentityHandle | undefined
+> {
+  if (!isWorkloadIdentityConfigured()) {
+    return undefined;
+  }
+
+  if (
+    process.env.ANTHROPIC_API_KEY?.trim() ||
+    process.env.CLAUDE_CODE_OAUTH_TOKEN?.trim()
+  ) {
+    core.warning(
+      "Workload identity federation inputs are set alongside anthropic_api_key or claude_code_oauth_token. The API key/OAuth token takes precedence, so federation will not be used.",
+    );
+    return undefined;
+  }
+
+  const audience =
+    process.env.ANTHROPIC_OIDC_AUDIENCE?.trim() || DEFAULT_OIDC_AUDIENCE;
+  const tokenDir = join(
+    process.env.RUNNER_TEMP || "/tmp",
+    "claude-workload-identity",
+  );
+  const tokenFile = join(tokenDir, "identity-token");
+
+  const writeIdentityToken = async () => {
+    const identityToken = await fetchIdentityToken(audience);
+    core.setSecret(identityToken);
+    mkdirSync(tokenDir, { recursive: true, mode: 0o700 });
+    writeFileSync(tokenFile, identityToken, { mode: 0o600 });
+  };
+
+  try {
+    await writeIdentityToken();
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    throw new Error(
+      `Failed to fetch a GitHub Actions OIDC token for workload identity federation: ${message}. Did you remember to add \`id-token: write\` to your workflow permissions?`,
+    );
+  }
+
+  process.env.ANTHROPIC_IDENTITY_TOKEN_FILE = tokenFile;
+  console.log(
+    `Workload identity federation configured (rule: ${process.env.ANTHROPIC_FEDERATION_RULE_ID}, identity token file: ${tokenFile})`,
+  );
+
+  const refreshInterval = setInterval(() => {
+    writeIdentityToken().catch((error) => {
+      core.warning(
+        `Failed to refresh the GitHub Actions OIDC identity token: ${error instanceof Error ? error.message : String(error)}`,
+      );
+    });
+  }, REFRESH_INTERVAL_MS);
+
+  return {
+    tokenFile,
+    stop: () => clearInterval(refreshInterval),
+  };
+}

src/entrypoints/collect-inputs.ts

@@ -20,6 +20,11 @@ export function collectActionInputsPresence(): string {
     settings: "",
     anthropic_api_key: "",
     claude_code_oauth_token: "",
+    anthropic_federation_rule_id: "",
+    anthropic_organization_id: "",
+    anthropic_service_account_id: "",
+    anthropic_workspace_id: "",
+    anthropic_oidc_audience: "",
     github_token: "",
     max_turns: "",
     use_sticky_comment: "false",

src/entrypoints/run.ts

@@ -29,6 +29,8 @@ import { prepareAgentMode } from "../modes/agent";
 import { checkContainsTrigger } from "../github/validation/trigger";
 import { restoreConfigFromBase } from "../github/operations/restore-config";
 import { validateBranchName } from "../github/operations/branch";
+import { setupWorkloadIdentity } from "../auth/workload-identity";
+import type { WorkloadIdentityHandle } from "../auth/workload-identity";
 import { collectActionInputsPresence } from "./collect-inputs";
 import { updateCommentLink } from "./update-comment-link";
 import { formatTurnsFromData } from "./format-turns";
@@ -150,6 +152,7 @@ async function run() {
   let prepareError: string | undefined;
   let context: GitHubContext | undefined;
   let octokit: Octokits | undefined;
+  let workloadIdentity: WorkloadIdentityHandle | undefined;
   // Track whether we've completed prepare phase, so we can attribute errors correctly
   let prepareCompleted = false;
   try {
@@ -231,6 +234,10 @@ async function run() {
     process.env.CLAUDE_CODE_ACTION = "1";
     process.env.DETAILED_PERMISSION_MESSAGES = "1";
 
+    // When workload identity federation is configured, fetch the GitHub OIDC
+    // identity token and expose it to the CLI before validating auth env vars.
+    workloadIdentity = await setupWorkloadIdentity();
+
     validateEnvironmentVariables();
 
     // On PRs, .claude/ and .mcp.json in the checkout are attacker-controlled.
@@ -307,6 +314,9 @@ async function run() {
   } finally {
     // Phase 4: Cleanup (always runs)
 
+    // Stop refreshing the workload identity token file
+    workloadIdentity?.stop();
+
     // Update tracking comment
     if (
       commentId &&