diff --git a/packages/client/src/experiment/RunnerContext.test.ts b/packages/client/src/experiment/RunnerContext.test.ts
new file mode 100644
index 00000000..ee0fa605
--- /dev/null
+++ b/packages/client/src/experiment/RunnerContext.test.ts
@@ -0,0 +1,245 @@
+import { describe, expect, it, vi } from "vitest";
+
+import type { LangfuseClient } from "../LangfuseClient.js";
+
+import { RegressionError, RunnerContext } from "./RunnerContext.js";
+import type { ExperimentResult } from "./types.js";
+
+function createResult(): ExperimentResult {
+  return {
+    experimentId: "exp-1",
+    runName: "run-1",
+    itemResults: [],
+    runEvaluations: [],
+    format: vi.fn(async () => "formatted"),
+  };
+}
+
+function createContext(params?: {
+  data?: { input?: string; expectedOutput?: string }[];
+  datasetVersion?: string;
+  metadata?: Record<string, any>;
+}) {
+  const run = vi.fn();
+  const client = {
+    experiment: { run },
+  } as unknown as LangfuseClient;
+
+  return {
+    run,
+    ctx: new RunnerContext({
+      client,
+      data: params?.data,
+      datasetVersion: params?.datasetVersion,
+      metadata: params?.metadata,
+    }),
+  };
+}
+
+describe("RunnerContext", () => {
+  it("uses context defaults when call-time values are omitted", async () => {
+    const result = createResult();
+    const { ctx, run } = createContext({
+      data: [{ input: "ctx" }],
+      datasetVersion: "2026-01-01T00:00:00.000Z",
+      metadata: { sha: "abc" },
+    });
+    run.mockResolvedValue(result);
+
+    await expect(
+      ctx.runExperiment({
+        name: "exp",
+        task: async () => "output",
+      }),
+    ).resolves.toBe(result);
+
+    expect(run).toHaveBeenCalledWith({
+      name: "exp",
+      task: expect.any(Function),
+      data: [{ input: "ctx" }],
+      datasetVersion: "2026-01-01T00:00:00.000Z",
+      metadata: { sha: "abc" },
+    });
+  });
+
+  it("lets call-time overrides win", async () => {
+    const result = createResult();
+    const { ctx, run } = createContext({
+      data: [{ input: "ctx" }],
+      datasetVersion: "2026-01-01T00:00:00.000Z",
+      metadata: { sha: "abc" },
+    });
+    const overrideData = [{ input: "override" }];
+    run.mockResolvedValue(result);
+
+    await ctx.runExperiment({
+      name: "exp",
+      runName: "call-run",
+      data: overrideData,
+      datasetVersion: "2026-06-06T00:00:00.000Z",
+      metadata: { sha: "def", pr: 42 },
+      task: async () => "output",
+    });
+
+    expect(run).toHaveBeenCalledWith({
+      name: "exp",
+      runName: "call-run",
+      data: overrideData,
+      datasetVersion: "2026-06-06T00:00:00.000Z",
+      metadata: { sha: "def", pr: 42 },
+      task: expect.any(Function),
+    });
+  });
+
+  it("merges metadata with call-time keys winning on collision", async () => {
+    const { ctx, run } = createContext({
+      data: [{ input: "ctx" }],
+      metadata: { sha: "abc", branch: "main" },
+    });
+    run.mockResolvedValue(createResult());
+
+    await ctx.runExperiment({
+      name: "exp",
+      metadata: { sha: "def", pr: 42 },
+      task: async () => "output",
+    });
+
+    expect(run).toHaveBeenCalledWith({
+      name: "exp",
+      data: [{ input: "ctx" }],
+      datasetVersion: undefined,
+      metadata: { sha: "def", branch: "main", pr: 42 },
+      task: expect.any(Function),
+    });
+  });
+
+  it("keeps metadata undefined when neither side provides it", async () => {
+    const { ctx, run } = createContext({
+      data: [{ input: "ctx" }],
+    });
+    run.mockResolvedValue(createResult());
+
+    await ctx.runExperiment({
+      name: "exp",
+      task: async () => "output",
+    });
+
+    expect(run).toHaveBeenCalledWith({
+      name: "exp",
+      data: [{ input: "ctx" }],
+      datasetVersion: undefined,
+      metadata: undefined,
+      task: expect.any(Function),
+    });
+  });
+
+  it("throws when data is missing on both the context and the call", async () => {
+    const { ctx } = createContext();
+
+    await expect(
+      ctx.runExperiment({
+        name: "exp",
+        task: async () => "output",
+      }),
+    ).rejects.toThrow(
+      "`data` must be provided either on the RunnerContext or the runExperiment call",
+    );
+  });
+});
+
+describe("RegressionError", () => {
+  it("is an error and stores the result", () => {
+    const result = createResult();
+    const error = new RegressionError({ result });
+
+    expect(error).toBeInstanceOf(Error);
+    expect(error.name).toBe("RegressionError");
+    expect(error.result).toBe(result);
+  });
+
+  it("uses the default message when no details are provided", () => {
+    const error = new RegressionError({ result: createResult() });
+
+    expect(error.message).toBe("Experiment regression detected");
+    expect(error.metric).toBeUndefined();
+    expect(error.value).toBeUndefined();
+    expect(error.threshold).toBeUndefined();
+  });
+
+  it("renders a structured message when metric and value are provided", () => {
+    const error = new RegressionError({
+      result: createResult(),
+      metric: "avg_accuracy",
+      value: 0.78,
+      threshold: 0.9,
+    });
+
+    expect(error.metric).toBe("avg_accuracy");
+    expect(error.value).toBe(0.78);
+    expect(error.threshold).toBe(0.9);
+    expect(error.message).toBe(
+      "Regression on `avg_accuracy`: 0.78 (threshold 0.9)",
+    );
+  });
+
+  it("omits the threshold suffix when no threshold is provided", () => {
+    const error = new RegressionError({
+      result: createResult(),
+      metric: "avg_accuracy",
+      value: 0.78,
+    });
+
+    expect(error.metric).toBe("avg_accuracy");
+    expect(error.value).toBe(0.78);
+    expect(error.threshold).toBeUndefined();
+    expect(error.message).toBe("Regression on `avg_accuracy`: 0.78");
+  });
+
+  it("lets an explicit message win over the structured format", () => {
+    const error = new RegressionError({
+      result: createResult(),
+      metric: "avg_accuracy",
+      value: 0.5,
+      threshold: 0.9,
+      message: "custom explanation",
+    });
+
+    expect(error.message).toBe("custom explanation");
+    expect(error.metric).toBe("avg_accuracy");
+    expect(error.value).toBe(0.5);
+    expect(error.threshold).toBe(0.9);
+  });
+
+  it("falls back to the default message for partial structured input", () => {
+    // @ts-expect-error Testing the runtime fallback when a caller bypasses the type system.
+    const error = new RegressionError({
+      result: createResult(),
+      metric: "avg_accuracy",
+    });
+
+    expect(error.message).toBe("Experiment regression detected");
+  });
+
+  it("only accepts the intended constructor shapes at type level", () => {
+    const result = createResult();
+
+    expect(
+      new RegressionError({ result, message: "custom explanation" }),
+    ).toBeInstanceOf(RegressionError);
+    expect(
+      new RegressionError({
+        result,
+        metric: "avg_accuracy",
+        value: 0.5,
+        threshold: 0.9,
+        message: "custom explanation",
+      }),
+    ).toBeInstanceOf(RegressionError);
+
+    // @ts-expect-error metric requires value.
+    new RegressionError({ result, metric: "avg_accuracy" });
+
+    // @ts-expect-error threshold is only valid in the structured shape.
+    new RegressionError({ result, threshold: 0.9 });
+  });
+});
diff --git a/packages/client/src/experiment/RunnerContext.ts b/packages/client/src/experiment/RunnerContext.ts
new file mode 100644
index 00000000..b0f9011c
--- /dev/null
+++ b/packages/client/src/experiment/RunnerContext.ts
@@ -0,0 +1,166 @@
+import type { LangfuseClient } from "../LangfuseClient.js";
+
+import type {
+  ExperimentItem,
+  ExperimentParams,
+  ExperimentResult,
+} from "./types.js";
+
+export type RunnerContextOptions<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> = {
+  client: LangfuseClient;
+  data?: ExperimentItem<Input, ExpectedOutput, Metadata>[];
+  datasetVersion?: string;
+  metadata?: Record<string, any>;
+};
+
+export type RunnerContextExperimentParams<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> = Omit<
+  ExperimentParams<Input, ExpectedOutput, Metadata>,
+  "data" | "datasetVersion" | "metadata"
+> & {
+  data?: ExperimentItem<Input, ExpectedOutput, Metadata>[];
+  datasetVersion?: string;
+  metadata?: Record<string, any>;
+};
+
+/**
+ * Wraps `langfuse.experiment.run` with CI-injected defaults.
+ *
+ * Intended for use with the `langfuse/experiment-action` GitHub Action.
+ * Defaults set here are applied when the caller omits them on the
+ * `runExperiment` call, while explicit call-time values still win.
+ *
+ * @public
+ */
+export class RunnerContext<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> {
+  public readonly client: LangfuseClient;
+  public readonly data?: ExperimentItem<Input, ExpectedOutput, Metadata>[];
+  public readonly datasetVersion?: string;
+  public readonly metadata?: Record<string, any>;
+
+  constructor({
+    client,
+    data,
+    datasetVersion,
+    metadata,
+  }: RunnerContextOptions<Input, ExpectedOutput, Metadata>) {
+    this.client = client;
+    this.data = data;
+    this.datasetVersion = datasetVersion;
+    this.metadata = metadata;
+  }
+
+  async runExperiment(
+    params: RunnerContextExperimentParams<Input, ExpectedOutput, Metadata>,
+  ): Promise<ExperimentResult<Input, ExpectedOutput, Metadata>> {
+    const resolvedData = params.data ?? this.data;
+
+    if (resolvedData === undefined) {
+      throw new Error(
+        "`data` must be provided either on the RunnerContext or the runExperiment call",
+      );
+    }
+
+    const mergedMetadata =
+      this.metadata === undefined && params.metadata === undefined
+        ? undefined
+        : {
+            ...(this.metadata ?? {}),
+            ...(params.metadata ?? {}),
+          };
+
+    return this.client.experiment.run({
+      ...params,
+      data: resolvedData,
+      datasetVersion: params.datasetVersion ?? this.datasetVersion,
+      metadata: mergedMetadata,
+    });
+  }
+}
+
+export type RegressionErrorMessageOptions<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> = {
+  result: ExperimentResult<Input, ExpectedOutput, Metadata>;
+  message?: string;
+  metric?: never;
+  value?: never;
+  threshold?: never;
+};
+
+export type RegressionErrorMetricOptions<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> = {
+  result: ExperimentResult<Input, ExpectedOutput, Metadata>;
+  metric: string;
+  value: number;
+  threshold?: number;
+  message?: string;
+};
+
+export type RegressionErrorOptions<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> =
+  | RegressionErrorMessageOptions<Input, ExpectedOutput, Metadata>
+  | RegressionErrorMetricOptions<Input, ExpectedOutput, Metadata>;
+
+/**
+ * Raised by experiment runners to signal a CI gate failure.
+ *
+ * Intended for use with the `langfuse/experiment-action` GitHub Action.
+ *
+ * @public
+ */
+export class RegressionError<
+  Input = any,
+  ExpectedOutput = any,
+  Metadata extends Record<string, any> = Record<string, any>,
+> extends Error {
+  public readonly result: ExperimentResult<Input, ExpectedOutput, Metadata>;
+  public readonly metric?: string;
+  public readonly value?: number;
+  public readonly threshold?: number;
+
+  constructor(
+    params: RegressionErrorMessageOptions<Input, ExpectedOutput, Metadata>,
+  );
+  constructor(
+    params: RegressionErrorMetricOptions<Input, ExpectedOutput, Metadata>,
+  );
+  constructor(params: RegressionErrorOptions<Input, ExpectedOutput, Metadata>) {
+    const { result, message } = params;
+    const metric = "metric" in params ? params.metric : undefined;
+    const value = "value" in params ? params.value : undefined;
+    const threshold = "threshold" in params ? params.threshold : undefined;
+
+    super(
+      message ??
+        (metric !== undefined && value !== undefined
+          ? `Regression on \`${metric}\`: ${value}${threshold !== undefined ? ` (threshold ${threshold})` : ""}`
+          : "Experiment regression detected"),
+    );
+
+    this.name = "RegressionError";
+    this.result = result;
+    this.metric = metric;
+    this.value = value;
+    this.threshold = threshold;
+  }
+}
diff --git a/packages/client/src/index.ts b/packages/client/src/index.ts
index d55bc403..d9dcc83e 100644
--- a/packages/client/src/index.ts
+++ b/packages/client/src/index.ts
@@ -4,5 +4,6 @@ export * from "./score/index.js";
 export * from "./dataset/index.js";
 export * from "./media/index.js";
 export * from "./experiment/ExperimentManager.js";
+export * from "./experiment/RunnerContext.js";
 export * from "./experiment/adapters.js";
 export * from "./experiment/types.js";