diff --git a/test/providers.test.ts b/test/providers.test.ts
new file mode 100644
index 0000000..d4a8a72
--- /dev/null
+++ b/test/providers.test.ts
@@ -0,0 +1,166 @@
+import { expect, it, describe, vi, beforeEach, afterEach } from "vitest";
+import { detectProvider } from "../src/providers.ts";
+
+// All env vars that provider detection may check
+const providerEnvKeys = [
+  "APPVEYOR",
+  "AWS_APP_ID",
+  "SYSTEM_TEAMFOUNDATIONCOLLECTIONURI",
+  "INPUT_AZURE_STATIC_WEB_APPS_API_TOKEN",
+  "AC_APPCIRCLE",
+  "bamboo_planKey",
+  "BITBUCKET_COMMIT",
+  "BITRISE_IO",
+  "BUDDY_WORKSPACE_ID",
+  "BUILDKITE",
+  "CIRCLECI",
+  "CIRRUS_CI",
+  "CF_PAGES",
+  "WORKERS_CI",
+  "K_SERVICE",
+  "CLOUD_RUN_JOB",
+  "CODEBUILD_BUILD_ARN",
+  "CF_BUILD_ID",
+  "DRONE",
+  "DRONE_BUILD_EVENT",
+  "DSARI",
+  "GITHUB_ACTIONS",
+  "GITLAB_CI",
+  "CI_MERGE_REQUEST_ID",
+  "GO_PIPELINE_LABEL",
+  "LAYERCI",
+  "JENKINS_URL",
+  "HUDSON_URL",
+  "MAGNUM",
+  "NETLIFY",
+  "NETLIFY_LOCAL",
+  "NEVERCODE",
+  "RENDER",
+  "SAILCI",
+  "SEMAPHORE",
+  "SCREWDRIVER",
+  "SHIPPABLE",
+  "TDDIUM",
+  "STRIDER",
+  "TEAMCITY_VERSION",
+  "TRAVIS",
+  "NOW_BUILDER",
+  "VERCEL",
+  "VERCEL_ENV",
+  "APPCENTER_BUILD_ID",
+  "CODESANDBOX_SSE",
+  "CODESANDBOX_HOST",
+  "STACKBLITZ",
+  "STORMKIT",
+  "CLEAVR",
+  "ZEABUR",
+  "CODESPHERE_APP_ID",
+  "RAILWAY_PROJECT_ID",
+  "RAILWAY_SERVICE_ID",
+  "DENO_DEPLOY",
+  "DENO_DEPLOYMENT_ID",
+  "FIREBASE_APP_HOSTING",
+  "SHELL",
+];
+
+describe("detectProvider", () => {
+  beforeEach(() => {
+    for (const key of providerEnvKeys) {
+      vi.stubEnv(key, "");
+    }
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it("returns empty name with ci: false when no provider is detected", () => {
+    expect(detectProvider()).toEqual({ name: "", ci: false });
+  });
+
+  describe("env var detection", () => {
+    // [expectedName, envKey, envValue, optionalMeta]
+    const cases: [string, string, string, Record<string, any>?][] = [
+      ["appveyor", "APPVEYOR", "true"],
+      ["aws_amplify", "AWS_APP_ID", "d1234", { ci: true }],
+      ["azure_pipelines", "SYSTEM_TEAMFOUNDATIONCOLLECTIONURI", "https://dev.azure.com/org"],
+      ["azure_static", "INPUT_AZURE_STATIC_WEB_APPS_API_TOKEN", "token-123"],
+      ["appcircle", "AC_APPCIRCLE", "true"],
+      ["bamboo", "bamboo_planKey", "PROJ-PLAN"],
+      ["bitbucket", "BITBUCKET_COMMIT", "abc123"],
+      ["bitrise", "BITRISE_IO", "true"],
+      ["buddy", "BUDDY_WORKSPACE_ID", "123"],
+      ["buildkite", "BUILDKITE", "true"],
+      ["circle", "CIRCLECI", "true"],
+      ["cirrus", "CIRRUS_CI", "true"],
+      ["cloudflare_pages", "CF_PAGES", "1", { ci: true }],
+      ["cloudflare_workers", "WORKERS_CI", "true", { ci: true }],
+      ["google_cloudrun", "K_SERVICE", "my-service"],
+      ["google_cloudrun_job", "CLOUD_RUN_JOB", "my-job"],
+      ["codebuild", "CODEBUILD_BUILD_ARN", "arn:aws:codebuild:us-east-1:123:build/proj"],
+      ["codefresh", "CF_BUILD_ID", "build-123"],
+      ["drone", "DRONE", "true"],
+      ["drone", "DRONE_BUILD_EVENT", "push"],
+      ["dsari", "DSARI", "true"],
+      ["github_actions", "GITHUB_ACTIONS", "true"],
+      ["gitlab", "GITLAB_CI", "true"],
+      ["gitlab", "CI_MERGE_REQUEST_ID", "42"],
+      ["gocd", "GO_PIPELINE_LABEL", "pipeline-123"],
+      ["layerci", "LAYERCI", "true"],
+      ["jenkins", "JENKINS_URL", "https://ci.example.com/"],
+      ["hudson", "HUDSON_URL", "https://ci.example.com/"],
+      ["magnum", "MAGNUM", "true"],
+      ["netlify", "NETLIFY", "true"],
+      ["netlify", "NETLIFY_LOCAL", "true", { ci: false }],
+      ["nevercode", "NEVERCODE", "true"],
+      ["render", "RENDER", "true"],
+      ["sail", "SAILCI", "true"],
+      ["semaphore", "SEMAPHORE", "true"],
+      ["screwdriver", "SCREWDRIVER", "true"],
+      ["shippable", "SHIPPABLE", "true"],
+      ["solano", "TDDIUM", "true"],
+      ["strider", "STRIDER", "true"],
+      ["teamcity", "TEAMCITY_VERSION", "2023.1"],
+      ["travis", "TRAVIS", "true"],
+      ["vercel", "NOW_BUILDER", "1"],
+      ["vercel", "VERCEL", "1", { ci: false }],
+      ["vercel", "VERCEL_ENV", "production", { ci: false }],
+      ["appcenter", "APPCENTER_BUILD_ID", "42"],
+      ["codesandbox", "CODESANDBOX_SSE", "1", { ci: false }],
+      ["codesandbox", "CODESANDBOX_HOST", "abc.csb.app", { ci: false }],
+      ["stackblitz", "STACKBLITZ", "true"],
+      ["stormkit", "STORMKIT", "true"],
+      ["cleavr", "CLEAVR", "true"],
+      ["zeabur", "ZEABUR", "true"],
+      ["codesphere", "CODESPHERE_APP_ID", "123", { ci: true }],
+      ["railway", "RAILWAY_PROJECT_ID", "proj-123"],
+      ["railway", "RAILWAY_SERVICE_ID", "svc-123"],
+      ["deno-deploy", "DENO_DEPLOY", "true"],
+      ["deno-deploy", "DENO_DEPLOYMENT_ID", "dep-123"],
+      ["firebase_app_hosting", "FIREBASE_APP_HOSTING", "true", { ci: true }],
+    ];
+
+    for (const [expectedName, envKey, envValue, meta] of cases) {
+      it(`detects ${expectedName} via ${envKey}`, () => {
+        vi.stubEnv(envKey, envValue);
+        expect(detectProvider()).toEqual({ name: expectedName, ...meta });
+      });
+    }
+  });
+
+  describe("priority order", () => {
+    it("detects jenkins before hudson", () => {
+      vi.stubEnv("JENKINS_URL", "https://jenkins.example.com/");
+      vi.stubEnv("HUDSON_URL", "https://hudson.example.com/");
+      expect(detectProvider()).toEqual({ name: "jenkins" });
+    });
+
+    it("NETLIFY takes priority over NETLIFY_LOCAL", () => {
+      vi.stubEnv("NETLIFY", "true");
+      vi.stubEnv("NETLIFY_LOCAL", "true");
+      const result = detectProvider();
+      expect(result.name).toBe("netlify");
+      expect(result.ci).toBeUndefined();
+    });
+  });
+});