diff --git a/actions/setup/js/load_experiment_state_from_repo.cjs b/actions/setup/js/load_experiment_state_from_repo.cjs
index 2b55ab778c8..1a301f459e2 100644
--- a/actions/setup/js/load_experiment_state_from_repo.cjs
+++ b/actions/setup/js/load_experiment_state_from_repo.cjs
@@ -23,6 +23,9 @@ const fs = require("fs");
 const path = require("path");
 
 const MAX_STATE_FILE_BYTES = 102400;
+// Keep this allowlist aligned with actions/setup/js/normalize_branch_name.cjs valid characters.
+const BRANCH_NAME_PATTERN = /^[A-Za-z0-9._/-]+$/;
+const REPOSITORY_PATTERN = /^[A-Za-z0-9_.-]+\/[A-Za-z0-9_.-]+$/;
 
 /**
  * Returns true when decoded state content exceeds allowed byte length.
@@ -35,6 +38,35 @@ function checkLimit(content, maxBytes) {
   return content.length > maxBytes;
 }
 
+/**
+ * Validate required input values before any API calls.
+ *
+ * @param {string} branch
+ * @param {string} owner
+ * @param {string} repo
+ * @param {string} repository
+ * @returns {{valid: boolean, error?: string}}
+ */
+function validateInputs(branch, owner, repo, repository) {
+  if (!branch) {
+    return { valid: false, error: "GH_AW_EXPERIMENT_BRANCH is not set" };
+  }
+
+  if (!BRANCH_NAME_PATTERN.test(branch)) {
+    return { valid: false, error: "GH_AW_EXPERIMENT_BRANCH contains invalid characters" };
+  }
+
+  if (branch.includes("..")) {
+    return { valid: false, error: "GH_AW_EXPERIMENT_BRANCH contains invalid characters" };
+  }
+
+  if (!REPOSITORY_PATTERN.test(repository)) {
+    return { valid: false, error: "GITHUB_REPOSITORY is not set or invalid" };
+  }
+
+  return { valid: true };
+}
+
 /**
  * Fetch experiment state from the git branch via the GitHub API.
  * Returns the raw file content as a string, or null when the branch/file is absent.
@@ -76,16 +108,12 @@ async function main() {
   const stateFile = process.env.GH_AW_EXPERIMENT_STATE_FILE || "/tmp/gh-aw/experiments/state.json";
   const stateDir = process.env.GH_AW_EXPERIMENT_STATE_DIR || "/tmp/gh-aw/experiments";
   const branch = process.env.GH_AW_EXPERIMENT_BRANCH || "";
+  const repository = process.env.GITHUB_REPOSITORY || "";
+  const [owner, repo] = repository.split("/");
 
-  if (!branch) {
-    core.warning("GH_AW_EXPERIMENT_BRANCH is not set – starting with empty experiment state");
-    fs.mkdirSync(stateDir, { recursive: true });
-    return;
-  }
-
-  const [owner, repo] = (process.env.GITHUB_REPOSITORY || "/").split("/");
-  if (!owner || !repo) {
-    core.warning("GITHUB_REPOSITORY is not set – starting with empty experiment state");
+  const validationResult = validateInputs(branch, owner, repo, repository);
+  if (!validationResult.valid) {
+    core.warning(`${validationResult.error} – starting with empty experiment state`);
     fs.mkdirSync(stateDir, { recursive: true });
     return;
   }
@@ -133,4 +161,4 @@ async function main() {
   core.info(`Experiment state written to ${stateFile}`);
 }
 
-module.exports = { main, fetchFileFromBranch };
+module.exports = { main, fetchFileFromBranch, validateInputs };
diff --git a/actions/setup/js/load_experiment_state_from_repo.test.cjs b/actions/setup/js/load_experiment_state_from_repo.test.cjs
index 4013c562451..4c9ea475345 100644
--- a/actions/setup/js/load_experiment_state_from_repo.test.cjs
+++ b/actions/setup/js/load_experiment_state_from_repo.test.cjs
@@ -16,7 +16,7 @@ const mockGetOctokit = vi.fn();
 global.core = mockCore;
 global.getOctokit = mockGetOctokit;
 
-const { fetchFileFromBranch, main } = await import("./load_experiment_state_from_repo.cjs");
+const { fetchFileFromBranch, main, validateInputs } = await import("./load_experiment_state_from_repo.cjs");
 const ENV_KEYS = ["GH_AW_EXPERIMENT_STATE_FILE", "GH_AW_EXPERIMENT_STATE_DIR", "GH_AW_EXPERIMENT_BRANCH", "GITHUB_REPOSITORY"];
 const MAX_STATE_FILE_BYTES = 102400;
 
@@ -131,7 +131,116 @@ describe("load_experiment_state_from_repo", () => {
     });
   });
 
+  describe("validateInputs", () => {
+    const cases = [
+      {
+        name: "accepts valid inputs",
+        args: ["experiments/my-workflow", "owner", "repo", "owner/repo"],
+        expected: { valid: true },
+      },
+      {
+        name: "rejects empty branch",
+        args: ["", "owner", "repo", "owner/repo"],
+        expected: { valid: false, error: "GH_AW_EXPERIMENT_BRANCH is not set" },
+      },
+      {
+        name: "rejects branch names with invalid characters",
+        args: ["experiments/my workflow", "owner", "repo", "owner/repo"],
+        expected: { valid: false, error: "GH_AW_EXPERIMENT_BRANCH contains invalid characters" },
+      },
+      {
+        name: "rejects branch names with path traversal patterns",
+        args: ["experiments/../../etc/passwd", "owner", "repo", "owner/repo"],
+        expected: { valid: false, error: "GH_AW_EXPERIMENT_BRANCH contains invalid characters" },
+      },
+      {
+        name: "rejects missing owner",
+        args: ["experiments/my-workflow", "", "repo", "/repo"],
+        expected: { valid: false, error: "GITHUB_REPOSITORY is not set or invalid" },
+      },
+      {
+        name: "rejects missing repo",
+        args: ["experiments/my-workflow", "owner", "", "owner/"],
+        expected: { valid: false, error: "GITHUB_REPOSITORY is not set or invalid" },
+      },
+      {
+        name: "rejects repository with extra segments",
+        args: ["experiments/my-workflow", "owner", "repo", "owner/repo/extra"],
+        expected: { valid: false, error: "GITHUB_REPOSITORY is not set or invalid" },
+      },
+      {
+        name: "rejects repository with whitespace",
+        args: ["experiments/my-workflow", "ow ner", "repo", "ow ner/repo"],
+        expected: { valid: false, error: "GITHUB_REPOSITORY is not set or invalid" },
+      },
+    ];
+
+    for (const testCase of cases) {
+      it(testCase.name, () => {
+        expect(validateInputs(...testCase.args)).toEqual(testCase.expected);
+      });
+    }
+  });
+
   describe("main", () => {
+    it("skips fetch when branch name is invalid", async () => {
+      const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "gh-aw-state-"));
+      try {
+        const stateFile = path.join(tmpDir, "state.json");
+        const getContent = vi.fn();
+
+        process.env.GH_AW_EXPERIMENT_STATE_FILE = stateFile;
+        process.env.GH_AW_EXPERIMENT_STATE_DIR = tmpDir;
+        process.env.GH_AW_EXPERIMENT_BRANCH = "experiments/my workflow";
+        process.env.GITHUB_REPOSITORY = "owner/repo";
+
+        global.github = {
+          rest: {
+            repos: {
+              getContent,
+            },
+          },
+        };
+
+        await main();
+
+        expect(getContent).not.toHaveBeenCalled();
+        expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("contains invalid characters"));
+        expect(fs.existsSync(stateFile)).toBe(false);
+      } finally {
+        fs.rmSync(tmpDir, { recursive: true, force: true });
+      }
+    });
+
+    it("skips fetch when GITHUB_REPOSITORY format is invalid", async () => {
+      const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "gh-aw-state-"));
+      try {
+        const stateFile = path.join(tmpDir, "state.json");
+        const getContent = vi.fn();
+
+        process.env.GH_AW_EXPERIMENT_STATE_FILE = stateFile;
+        process.env.GH_AW_EXPERIMENT_STATE_DIR = tmpDir;
+        process.env.GH_AW_EXPERIMENT_BRANCH = "experiments/myworkflow";
+        process.env.GITHUB_REPOSITORY = "owner/repo/extra";
+
+        global.github = {
+          rest: {
+            repos: {
+              getContent,
+            },
+          },
+        };
+
+        await main();
+
+        expect(getContent).not.toHaveBeenCalled();
+        expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("GITHUB_REPOSITORY is not set or invalid"));
+        expect(fs.existsSync(stateFile)).toBe(false);
+      } finally {
+        fs.rmSync(tmpDir, { recursive: true, force: true });
+      }
+    });
+
     it("skips oversized state files", async () => {
       const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "gh-aw-state-"));
       try {