diff --git a/.changeset/shy-rats-lick.md b/.changeset/shy-rats-lick.md
new file mode 100644
index 000000000..9ef1a3780
--- /dev/null
+++ b/.changeset/shy-rats-lick.md
@@ -0,0 +1,5 @@
+---
+"braintrust": patch
+---
+
+Fix eval summaries to compare against the experiment’s explicit base experiment ID.
diff --git a/js/src/framework.test.ts b/js/src/framework.test.ts
index 7c77dc4e2..12bcbb8c1 100644
--- a/js/src/framework.test.ts
+++ b/js/src/framework.test.ts
@@ -759,6 +759,91 @@ test("Eval with returnResults: true collects all results", async () => {
   expect(result.summary.scores.exact_match.score).toBe(1);
 });
 
+test("runEvaluator forwards baseExperimentId to summary", async () => {
+  await _exportsForTestingOnly.simulateLoginForTests();
+  const experiment = _exportsForTestingOnly.initTestExperiment(
+    "js-base-experiment-id",
+    "proj",
+  );
+  const expectedSummary = {
+    projectName: "proj",
+    experimentName: "js-base-experiment-id",
+    projectId: "proj",
+    experimentId: "js-base-experiment-id",
+    scores: {},
+    metrics: {},
+  };
+  const summarize = vi
+    .spyOn(experiment, "summarize")
+    .mockResolvedValue(expectedSummary);
+
+  const result = await runEvaluator(
+    experiment,
+    {
+      projectName: "proj",
+      evalName: "js-base-experiment-id",
+      data: [{ input: "hello", expected: "hello" }],
+      task: (input) => input,
+      scores: [],
+      baseExperimentId: "base-exp-id",
+    },
+    new NoopProgressReporter(),
+    [],
+    undefined,
+    undefined,
+    true,
+  );
+
+  expect(result.summary).toBe(expectedSummary);
+  expect(summarize).toHaveBeenCalledWith({
+    summarizeScores: undefined,
+    comparisonExperimentId: "base-exp-id",
+  });
+});
+
+test("runEvaluator forwards persisted baseExperimentName id to summary", async () => {
+  await _exportsForTestingOnly.simulateLoginForTests();
+  const experiment = _exportsForTestingOnly.initTestExperiment(
+    "js-base-experiment-name",
+    "proj",
+    { base_exp_id: "resolved-base-exp-id" },
+  );
+  const expectedSummary = {
+    projectName: "proj",
+    experimentName: "js-base-experiment-name",
+    projectId: "proj",
+    experimentId: "js-base-experiment-name",
+    scores: {},
+    metrics: {},
+  };
+  const summarize = vi
+    .spyOn(experiment, "summarize")
+    .mockResolvedValue(expectedSummary);
+
+  const result = await runEvaluator(
+    experiment,
+    {
+      projectName: "proj",
+      evalName: "js-base-experiment-name",
+      data: [{ input: "hello", expected: "hello" }],
+      task: (input) => input,
+      scores: [],
+      baseExperimentName: "base-exp",
+    },
+    new NoopProgressReporter(),
+    [],
+    undefined,
+    undefined,
+    true,
+  );
+
+  expect(result.summary).toBe(expectedSummary);
+  expect(summarize).toHaveBeenCalledWith({
+    summarizeScores: undefined,
+    comparisonExperimentId: "resolved-base-exp-id",
+  });
+});
+
 test("tags can be appended and logged to root span", async () => {
   await _exportsForTestingOnly.simulateLoginForTests();
   const memoryLogger = _exportsForTestingOnly.useTestBackgroundLogger();
diff --git a/js/src/framework.ts b/js/src/framework.ts
index 73629e7ac..3183a575a 100644
--- a/js/src/framework.ts
+++ b/js/src/framework.ts
@@ -410,6 +410,16 @@ export type {
   ReporterDef,
 } from "./reporters/types";
 
+async function getPersistedBaseExperimentId(
+  experiment: Experiment,
+): Promise<string | undefined> {
+  try {
+    return await experiment._getBaseExperimentId();
+  } catch {
+    return undefined;
+  }
+}
+
 function makeEvalName(projectName: string, experimentName?: string) {
   let out = projectName;
   if (experimentName) {
@@ -1636,9 +1646,17 @@ async function runEvaluatorInternal(
       }
     }
 
+    const comparisonExperimentId = experiment
+      ? (evaluator.baseExperimentId ??
+        (await getPersistedBaseExperimentId(experiment)))
+      : undefined;
+
     const summary = experiment
       ? await experiment.summarize({
           summarizeScores: evaluator.summarizeScores,
+          ...(comparisonExperimentId !== undefined
+            ? { comparisonExperimentId }
+            : {}),
         })
       : buildLocalSummary(
           evaluator,
diff --git a/js/src/logger.test.ts b/js/src/logger.test.ts
index 352b71f52..7a5e12b70 100644
--- a/js/src/logger.test.ts
+++ b/js/src/logger.test.ts
@@ -825,6 +825,53 @@ test("dataset.toEvalData preserves dataset_snapshot_name", async () => {
   vi.restoreAllMocks();
 });
 
+test("experiment.summarize resolves explicit comparison experiment name", async () => {
+  const state = await _exportsForTestingOnly.simulateLoginForTests();
+  const memoryLogger = _exportsForTestingOnly.useTestBackgroundLogger();
+  const experiment = _exportsForTestingOnly.initTestExperiment(
+    "test-evaluator",
+    "test-project",
+  );
+
+  try {
+    const getJson = vi
+      .spyOn(state.apiConn(), "get_json")
+      .mockImplementation(async (path, args) => {
+        if (path === "v1/experiment/base-exp-id") {
+          return { name: "base-exp" };
+        }
+        if (path === "/experiment-comparison2") {
+          expect(args).toEqual({
+            experiment_id: "test-evaluator",
+            base_experiment_id: "base-exp-id",
+          });
+          return { scores: {}, metrics: {} };
+        }
+        throw new Error(`Unexpected get_json call: ${path}`);
+      });
+
+    const summary = await experiment.summarize({
+      comparisonExperimentId: "base-exp-id",
+    });
+
+    expect(summary.comparisonExperimentName).toBe("base-exp");
+    expect(getJson).toHaveBeenCalledWith("v1/experiment/base-exp-id");
+    expect(getJson).toHaveBeenCalledWith(
+      "/experiment-comparison2",
+      {
+        experiment_id: "test-evaluator",
+        base_experiment_id: "base-exp-id",
+      },
+      3,
+    );
+  } finally {
+    await memoryLogger.flush();
+    _exportsForTestingOnly.clearTestBackgroundLogger();
+    _exportsForTestingOnly.simulateLogoutForTests();
+    vi.restoreAllMocks();
+  }
+});
+
 test("dataset.version preserves pinned-version fast path", async () => {
   const state = await _exportsForTestingOnly.simulateLoginForTests();
   const login = vi.spyOn(state, "login").mockResolvedValue(state);
diff --git a/js/src/logger.ts b/js/src/logger.ts
index ccbe22ef6..f81200f66 100644
--- a/js/src/logger.ts
+++ b/js/src/logger.ts
@@ -1063,6 +1063,7 @@ function clearTestBackgroundLogger() {
 function initTestExperiment(
   experimentName: string,
   projectName?: string,
+  experimentFullInfo: Record<string, unknown> = {},
 ): Experiment {
   setInitialTestState();
   const state = _internalGetGlobalState();
@@ -1071,7 +1072,11 @@ function initTestExperiment(
   const lazyMetadata: LazyValue<ProjectExperimentMetadata> = new LazyValue(
     async () => ({
       project: { id: project, name: project, fullInfo: {} },
-      experiment: { id: experimentName, name: experimentName, fullInfo: {} },
+      experiment: {
+        id: experimentName,
+        name: experimentName,
+        fullInfo: experimentFullInfo,
+      },
     }),
   );
 
@@ -6317,6 +6322,14 @@ export class Experiment
     })();
   }
 
+  public async _getBaseExperimentId(): Promise<string | undefined> {
+    const baseExperimentId = (await this.lazyMetadata.get()).experiment
+      .fullInfo["base_exp_id"];
+    return typeof baseExperimentId === "string" && baseExperimentId
+      ? baseExperimentId
+      : undefined;
+  }
+
   private parentObjectType() {
     return SpanObjectTypeV3.EXPERIMENT;
   }
@@ -6484,6 +6497,17 @@ export class Experiment
           comparisonExperimentId = baseExperiment.id;
           comparisonExperimentName = baseExperiment.name;
         }
+      } else {
+        try {
+          const comparisonExperiment = await state
+            .apiConn()
+            .get_json(`v1/experiment/${comparisonExperimentId}`);
+          if (typeof comparisonExperiment["name"] === "string") {
+            comparisonExperimentName = comparisonExperiment["name"];
+          }
+        } catch {
+          // If the explicit comparison name lookup fails, still summarize by ID.
+        }
       }
 
       try {