diff --git a/mcpjam-inspector/client/src/components/connection/share-usage/ShareUsageThreadDetail.tsx b/mcpjam-inspector/client/src/components/connection/share-usage/ShareUsageThreadDetail.tsx
index 4b26d5ddc6..36833f72cd 100644
--- a/mcpjam-inspector/client/src/components/connection/share-usage/ShareUsageThreadDetail.tsx
+++ b/mcpjam-inspector/client/src/components/connection/share-usage/ShareUsageThreadDetail.tsx
@@ -19,6 +19,7 @@ import {
 import { TraceViewer } from "@/components/evals/trace-viewer";
 import { BrowserArtifactsView } from "@/components/evals/browser-artifacts-view";
 import { hasReplayArtifacts } from "@/components/evals/browser-step-replay";
+import { hydrateTurnTraceSpans } from "@/components/evals/turn-trace-spans";
 import {
   ChatTraceViewModeHeaderBar,
   type TraceViewMode,
@@ -216,28 +217,6 @@ interface ShareUsageThreadDetailProps {
  */
 const PROMOTABLE_SOURCE_TYPES = new Set(["swarm", "scenario"]);
 
-/**
- * Fetch span blobs from turn trace URLs and flatten into a single span array.
- */
-async function hydrateSpans(
-  traces: SharedChatTurnTrace[]
-): Promise<EvalTraceSpan[]> {
-  const results = await Promise.all(
-    traces.map(async (trace) => {
-      if (!trace.spansBlobUrl) return [];
-      try {
-        const response = await fetch(trace.spansBlobUrl);
-        if (!response.ok) return [];
-        const parsed = await response.json();
-        return Array.isArray(parsed) ? (parsed as EvalTraceSpan[]) : [];
-      } catch {
-        return [];
-      }
-    })
-  );
-  return results.flat();
-}
-
 export function ShareUsageThreadDetail({
   threadId,
   sessionLink,
@@ -312,7 +291,7 @@ export function ShareUsageThreadDetail({
     }
 
     let isActive = true;
-    void hydrateSpans(turnTraces).then((spans) => {
+    void hydrateTurnTraceSpans(turnTraces).then((spans) => {
       if (isActive) setHydratedSpans(spans);
     });
     return () => {
diff --git a/mcpjam-inspector/client/src/components/evals/__tests__/turn-trace-spans.test.ts b/mcpjam-inspector/client/src/components/evals/__tests__/turn-trace-spans.test.ts
new file mode 100644
index 0000000000..801ffa59b1
--- /dev/null
+++ b/mcpjam-inspector/client/src/components/evals/__tests__/turn-trace-spans.test.ts
@@ -0,0 +1,186 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { EvalTraceSpan } from "@/shared/eval-trace";
+import { hydrateTurnTraceSpans } from "../turn-trace-spans";
+
+/** A turn-relative span blob, exactly as a producer writes it: offsets from 0. */
+function turnBlob(durationMs: number): EvalTraceSpan[] {
+  return [
+    {
+      id: `step-${durationMs}`,
+      name: "step",
+      category: "step",
+      startMs: 0,
+      endMs: durationMs,
+    },
+    {
+      id: `llm-${durationMs}`,
+      name: "llm",
+      category: "llm",
+      startMs: 10,
+      endMs: durationMs - 10,
+    },
+  ];
+}
+
+const originalFetch = global.fetch;
+
+function mockBlobs(byUrl: Record<string, unknown>) {
+  global.fetch = vi.fn(async (url: unknown) => {
+    const body = byUrl[String(url)];
+    if (body === undefined) {
+      return { ok: false, status: 404, json: async () => null } as Response;
+    }
+    return { ok: true, status: 200, json: async () => body } as Response;
+  }) as unknown as typeof fetch;
+}
+
+describe("hydrateTurnTraceSpans", () => {
+  beforeEach(() => {
+    global.fetch = originalFetch;
+  });
+
+  afterEach(() => {
+    global.fetch = originalFetch;
+    vi.restoreAllMocks();
+  });
+
+  it("shifts each turn by its wall-clock distance from the session start", async () => {
+    // Turn 1 at t=0 runs 5s; turn 2 starts 8s in — a 3s idle gap between them.
+    mockBlobs({
+      "blob://t1": turnBlob(5_000),
+      "blob://t2": turnBlob(4_000),
+    });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 1_000_000, spansBlobUrl: "blob://t1" },
+      { startedAt: 1_008_000, spansBlobUrl: "blob://t2" },
+    ]);
+
+    const steps = spans.filter((span) => span.category === "step");
+    expect(steps.map((span) => [span.startMs, span.endMs])).toEqual([
+      [0, 5_000],
+      [8_000, 12_000],
+    ]);
+    // The 3s the session spent idle between turns stays visible.
+    expect(steps[1].startMs - steps[0].endMs).toBe(3_000);
+  });
+
+  it("no longer collapses every turn onto 0ms (BB-153)", async () => {
+    mockBlobs({
+      "blob://t1": turnBlob(33_000),
+      "blob://t2": turnBlob(11_600),
+      "blob://t3": turnBlob(14_600),
+    });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 500, spansBlobUrl: "blob://t1" },
+      { startedAt: 33_500, spansBlobUrl: "blob://t2" },
+      { startedAt: 45_100, spansBlobUrl: "blob://t3" },
+    ]);
+
+    const startsAtZero = spans.filter((span) => span.startMs === 0);
+    expect(startsAtZero).toHaveLength(1);
+    expect(spans.every((span) => span.endMs >= span.startMs)).toBe(true);
+  });
+
+  it("anchors offset 0 at the earliest turn regardless of input order", async () => {
+    mockBlobs({
+      "blob://late": turnBlob(1_000),
+      "blob://early": turnBlob(1_000),
+    });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 2_000, spansBlobUrl: "blob://late" },
+      { startedAt: 0, spansBlobUrl: "blob://early" },
+    ]);
+
+    expect(Math.min(...spans.map((span) => span.startMs))).toBe(0);
+    expect(Math.max(...spans.map((span) => span.endMs))).toBe(3_000);
+  });
+
+  it("preserves every non-timing field on the span", async () => {
+    mockBlobs({
+      "blob://t1": [
+        {
+          id: "tool-1",
+          name: "tools/call",
+          category: "tool",
+          startMs: 100,
+          endMs: 900,
+          toolName: "create_workflow",
+          status: "error",
+          mcpErrorCode: -32602,
+        } satisfies EvalTraceSpan,
+      ],
+    });
+
+    const [span] = await hydrateTurnTraceSpans([
+      { startedAt: 5_000, spansBlobUrl: "blob://t1" },
+    ]);
+
+    expect(span).toMatchObject({
+      id: "tool-1",
+      toolName: "create_workflow",
+      status: "error",
+      mcpErrorCode: -32602,
+      startMs: 100,
+      endMs: 900,
+    });
+  });
+
+  it("skips turns with no blob url, and keeps the rest correctly placed", async () => {
+    mockBlobs({ "blob://t2": turnBlob(2_000) });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 0, spansBlobUrl: null },
+      { startedAt: 6_000, spansBlobUrl: "blob://t2" },
+    ]);
+
+    // The missing turn still sets the session start, so turn 2 keeps its real
+    // 6s offset instead of being pulled back to zero.
+    const step = spans.find((span) => span.category === "step");
+    expect(step).toMatchObject({ startMs: 6_000, endMs: 8_000 });
+  });
+
+  it("drops turns whose blob fails to load without shifting the others", async () => {
+    mockBlobs({ "blob://ok": turnBlob(1_000) });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 0, spansBlobUrl: "blob://ok" },
+      { startedAt: 4_000, spansBlobUrl: "blob://gone" },
+    ]);
+
+    expect(spans).toHaveLength(2);
+    expect(Math.max(...spans.map((span) => span.endMs))).toBe(1_000);
+  });
+
+  it("tolerates a non-array blob body", async () => {
+    mockBlobs({ "blob://weird": { spans: [] } });
+
+    await expect(
+      hydrateTurnTraceSpans([{ startedAt: 0, spansBlobUrl: "blob://weird" }]),
+    ).resolves.toEqual([]);
+  });
+
+  it("returns an empty array for no turns and never calls fetch", async () => {
+    mockBlobs({});
+    await expect(hydrateTurnTraceSpans([])).resolves.toEqual([]);
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it("falls back to offset 0 for a turn with an unusable startedAt", async () => {
+    mockBlobs({
+      "blob://t1": turnBlob(1_000),
+      "blob://bad": turnBlob(1_000),
+    });
+
+    const spans = await hydrateTurnTraceSpans([
+      { startedAt: 1_000, spansBlobUrl: "blob://t1" },
+      { startedAt: Number.NaN, spansBlobUrl: "blob://bad" },
+    ]);
+
+    // The NaN row must not poison the base for the valid turn.
+    expect(spans.every((span) => Number.isFinite(span.startMs))).toBe(true);
+    expect(Math.min(...spans.map((span) => span.startMs))).toBe(0);
+  });
+});
diff --git a/mcpjam-inspector/client/src/components/evals/turn-trace-spans.ts b/mcpjam-inspector/client/src/components/evals/turn-trace-spans.ts
new file mode 100644
index 0000000000..fe8957fa2e
--- /dev/null
+++ b/mcpjam-inspector/client/src/components/evals/turn-trace-spans.ts
@@ -0,0 +1,71 @@
+import type { EvalTraceSpan } from "@/shared/eval-trace";
+import { rebaseTraceSpans } from "@/shared/live-chat-trace";
+
+/**
+ * The fields of a persisted turn trace this module needs. Structural on
+ * purpose — `SharedChatTurnTrace` satisfies it, and tests don't have to build
+ * a full row to exercise the rebasing.
+ */
+export type TurnTraceSpanSource = {
+  /** Absolute epoch ms the turn started — the epoch its span offsets are measured from. */
+  startedAt: number;
+  spansBlobUrl?: string | null;
+};
+
+/**
+ * Fetch every turn's span blob and flatten them into ONE session timeline.
+ *
+ * Each turn is traced with its own `createAiSdkEvalTraceContext(turnStartedAt)`
+ * (see `runDirectChatTurn`), so the offsets inside a blob are relative to THAT
+ * turn — every turn's first span sits at `startMs: 0`. Flattening the blobs
+ * without re-anchoring them stacks all turns on top of each other and the
+ * timeline renders every span starting at 0.0s (BB-153).
+ *
+ * So each turn is shifted by its own distance from the session start
+ * (`startedAt - sessionStart`), which preserves real wall-clock gaps between
+ * turns — the model's think time and any tool latency BETWEEN turns stays
+ * visible as empty space, rather than being packed away.
+ *
+ * The base is the earliest `startedAt` of the turns passed in, so offset 0 is
+ * the session start and matches the `traceStartedAtMs` anchor callers compute
+ * the same way. Blobs that fail to load contribute nothing and never shift the
+ * turns that did load.
+ *
+ * NOTE: this assumes blobs are turn-relative, which holds for every chat
+ * producer (only the eval runner passes an explicit `traceStartedAt`, and eval
+ * traces are read through `use-eval-trace-blob`, not here). A chat producer
+ * that starts anchoring spans at the session start would double-shift them.
+ */
+export async function hydrateTurnTraceSpans(
+  traces: readonly TurnTraceSpanSource[],
+): Promise<EvalTraceSpan[]> {
+  if (traces.length === 0) return [];
+
+  // A row with a missing/garbage `startedAt` can't be placed on the timeline;
+  // it falls back to offset 0 (today's behaviour) instead of poisoning the base
+  // for every other turn with a NaN.
+  const startTimes = traces
+    .map((trace) => trace.startedAt)
+    .filter((startedAt) => Number.isFinite(startedAt));
+  const sessionStartedAt = startTimes.length > 0 ? Math.min(...startTimes) : 0;
+
+  const perTurn = await Promise.all(
+    traces.map(async (trace) => {
+      if (!trace.spansBlobUrl) return [];
+      try {
+        const response = await fetch(trace.spansBlobUrl);
+        if (!response.ok) return [];
+        const parsed = await response.json();
+        if (!Array.isArray(parsed)) return [];
+        const offsetMs = Number.isFinite(trace.startedAt)
+          ? trace.startedAt - sessionStartedAt
+          : 0;
+        return rebaseTraceSpans(parsed as EvalTraceSpan[], offsetMs);
+      } catch {
+        return [];
+      }
+    }),
+  );
+
+  return perTurn.flat();
+}
diff --git a/mcpjam-inspector/client/src/components/swarms/use-persisted-session-trace.ts b/mcpjam-inspector/client/src/components/swarms/use-persisted-session-trace.ts
index 600b914e97..6c34943a80 100644
--- a/mcpjam-inspector/client/src/components/swarms/use-persisted-session-trace.ts
+++ b/mcpjam-inspector/client/src/components/swarms/use-persisted-session-trace.ts
@@ -4,12 +4,12 @@ import {
   useSharedChatThread,
   useSharedChatTurnTraces,
   useSharedChatWidgetSnapshots,
-  type SharedChatTurnTrace,
 } from "@/hooks/useSharedChatThreads";
 import {
   snapshotsToTraceWidgetSnapshots,
   type TraceEnvelope,
 } from "@/components/evals/trace-viewer-adapter";
+import { hydrateTurnTraceSpans } from "@/components/evals/turn-trace-spans";
 import type { EvalTraceSpan } from "@/shared/eval-trace";
 
 /** One pinned plugin version recorded on a synthetic session's resume config. */
@@ -20,29 +20,6 @@ export type SessionPluginVersion = {
   bundleHash: string;
 };
 
-/**
- * Fetch span blobs from turn trace URLs and flatten into a single span array.
- * Same contract as ShareUsageThreadDetail's hydrateSpans.
- */
-async function hydrateSpans(
-  traces: SharedChatTurnTrace[],
-): Promise<EvalTraceSpan[]> {
-  const results = await Promise.all(
-    traces.map(async (trace) => {
-      if (!trace.spansBlobUrl) return [];
-      try {
-        const response = await fetch(trace.spansBlobUrl);
-        if (!response.ok) return [];
-        const parsed = await response.json();
-        return Array.isArray(parsed) ? (parsed as EvalTraceSpan[]) : [];
-      } catch {
-        return [];
-      }
-    }),
-  );
-  return results.flat();
-}
-
 function extractMessages(data: unknown): unknown[] | null {
   if (Array.isArray(data)) return data;
   if (
@@ -171,7 +148,7 @@ export function usePersistedSessionTrace(threadId: string | null): {
 
     let active = true;
     setLoadingSpans(true);
-    void hydrateSpans(turnTraces).then((hydrated) => {
+    void hydrateTurnTraceSpans(turnTraces).then((hydrated) => {
       if (!active) return;
       setSpans(hydrated);
       setLoadingSpans(false);