diff --git a/src/dashboard.ts b/src/dashboard.ts
index a412298..3506095 100644
--- a/src/dashboard.ts
+++ b/src/dashboard.ts
@@ -150,21 +150,16 @@ export interface RecentRow {
  */
 interface SessionTotals {
   sessionId: string;
-  firstSeen: number; // unix seconds
-  lastSeen: number; // unix seconds
-  requests: number;
   // Dollar-weighted accumulators. baseline/actual are the SAME math as the
-  // global `Totals.allBaselineEquivalentWeighted` / `allActualInputWeighted`
-  // + `allOutputWeighted`, but scoped to a single session.
+  // global `Totals.allBaselineEquivalentWeighted` / `allActualInputWeighted`,
+  // but scoped to a single session. Output is excluded because the proxy
+  // doesn't touch it.
   baselineInputWeighted: number;
   actualInputWeighted: number;
-  outputWeighted: number;
-  // Per-bucket char attribution from ev.info.bucketChars.
-  bucketChars: { [bucket: string]: number };
-  // Passthrough reason histogram from ev.info.passthroughReasons.
-  passthroughReasons: Record<string, number>;
-  passthroughRequests: number; // count of requests where !ev.info.compressed
-  compressedRequests: number;
+  // Honest denominator for the headline: count of requests that contributed
+  // to `baselineInputWeighted` (events that carried a baseline measurement,
+  // matching the global `Totals.baselineMeasuredCount`).
+  baselineMeasuredCount: number;
 }
 
 interface Totals {
@@ -512,59 +507,30 @@ export class DashboardState {
       if (!s) {
         s = {
           sessionId: sid,
-          firstSeen: Date.now() / 1000,
-          lastSeen: Date.now() / 1000,
-          requests: 0,
           baselineInputWeighted: 0,
           actualInputWeighted: 0,
-          outputWeighted: 0,
-          bucketChars: {},
-          passthroughReasons: {},
-          passthroughRequests: 0,
-          compressedRequests: 0,
+          baselineMeasuredCount: 0,
         };
         this.sessions.set(sid, s);
-        // Cap memory — drop the oldest session by lastSeen when over budget.
+        // Cap memory — drop the first (oldest by insertion order) session
+        // when over budget. We no longer track lastSeen privately on the
+        // class — insertion order is a fine proxy because `currentSessionId`
+        // (set above on every update) is what the serve path uses to pick
+        // the most-recent session, not a scan of `this.sessions`.
         if (this.sessions.size > DashboardState.SESSION_CAP) {
-          const oldest = [...this.sessions.values()].sort(
-            (a, b) => a.lastSeen - b.lastSeen,
-          )[0];
-          if (oldest) this.sessions.delete(oldest.sessionId);
+          const firstKey = this.sessions.keys().next().value;
+          if (firstKey !== undefined) this.sessions.delete(firstKey);
         }
       }
-      s.lastSeen = Date.now() / 1000;
-      s.requests += 1;
       // Reuse the same haveUsage / haveBaseline guards + the
-      // baselineInputEff / actualInputEff / outputEquiv locals computed
-      // earlier in update() so the lifetime totals block (above) and the
-      // per-session block (here) read the same values. Re-deriving them
-      // here would duplicate the cache-aware-baseline math and invite drift.
+      // baselineInputEff / actualInputEff locals computed earlier in
+      // update() so the lifetime totals block (above) and the per-session
+      // block (here) read the same values. Re-deriving them here would
+      // duplicate the cache-aware-baseline math and invite drift.
       if (haveBaseline && haveUsage) {
         s.baselineInputWeighted += baselineInputEff;
         s.actualInputWeighted += actualInputEff;
-        s.outputWeighted += outputEquiv;
-      }
-      if (compressed) s.compressedRequests += 1;
-      else s.passthroughRequests += 1;
-      // Per-bucket char attribution from `info.bucketChars` (flat
-      // `Partial<Record<BucketName, number>>` — see src/core/tracker.ts).
-      // Matches the shape the Svelte panel iterates over.
-      const bc = info?.bucketChars;
-      if (bc && typeof bc === 'object') {
-        for (const [key, val] of Object.entries(bc) as [string, unknown][]) {
-          if (typeof val === 'number' && Number.isFinite(val)) {
-            s.bucketChars[key] = (s.bucketChars[key] ?? 0) + val;
-          }
-        }
-      }
-      // Passthrough reason histogram from `info.passthroughReasons`.
-      const pr = info?.passthroughReasons;
-      if (pr && typeof pr === 'object') {
-        for (const [reason, count] of Object.entries(pr as Record<string, unknown>)) {
-          if (typeof count === 'number' && Number.isFinite(count) && count > 0) {
-            s.passthroughReasons[reason] = (s.passthroughReasons[reason] ?? 0) + count;
-          }
-        }
+        s.baselineMeasuredCount += 1;
       }
     }
 
@@ -695,28 +661,16 @@ export class DashboardState {
     if (!s) {
       return jsonResponse({ sessionId: null, message: 'no active session yet' });
     }
-    // Dollar-weighted savings ratio. Same math as the global `saved_pct`
-    // (compressed AND passthrough requests — passthrough has baseline=actual,
-    // so it correctly drags the ratio down when we leak). Honest denominator:
-    // includes both compressed and uncompressed flows that the proxy saw.
-    const baselineUsd = (s.baselineInputWeighted * ASSUMED_INPUT_USD_PER_MTOK) / 1e6;
-    const actualUsd = (s.actualInputWeighted * ASSUMED_INPUT_USD_PER_MTOK) / 1e6;
-    const savedUsd = baselineUsd - actualUsd;
-    const savedPct = baselineUsd > 0 ? (savedUsd / baselineUsd) * 100 : 0;
+    // Minimal headline payload: the same dollar-weighted baseline/actual
+    // input accumulators the global Totals block exposes, plus the honest
+    // denominator (`baselineMeasuredCount` — only requests that carried a
+    // baseline measurement). The Svelte panel does the savings math itself
+    // so we don't round-trip dollar values through the wire.
     return jsonResponse({
       sessionId: s.sessionId,
-      firstSeen: s.firstSeen,
-      lastSeen: s.lastSeen,
-      uptimeSec: Math.max(0, s.lastSeen - s.firstSeen),
-      requests: s.requests,
-      compressedRequests: s.compressedRequests,
-      passthroughRequests: s.passthroughRequests,
-      baselineUsd: round4(baselineUsd),
-      actualUsd: round4(actualUsd),
-      savedUsd: round4(savedUsd),
-      savedPct: round1(savedPct),
-      bucketChars: s.bucketChars,
-      passthroughReasons: s.passthroughReasons,
+      baselineInputWeighted: s.baselineInputWeighted,
+      actualInputWeighted: s.actualInputWeighted,
+      baselineMeasuredCount: s.baselineMeasuredCount,
     });
   }
 
diff --git a/src/dashboard/components/SessionSummary.svelte b/src/dashboard/components/SessionSummary.svelte
index 25713c0..1ff29d3 100644
--- a/src/dashboard/components/SessionSummary.svelte
+++ b/src/dashboard/components/SessionSummary.svelte
@@ -1,263 +1,66 @@
 <script lang="ts">
-  // Top-of-dashboard headline. Dollar-weighted savings ratio scoped to the
-  // most-recently-active session, with per-bucket breakdown of where the
-  // savings came from and a list of passthrough reasons for requests we
-  // didn't compress. Polls 2s via the `currentSession` store.
+  // One-line headline for the current session: dollar-weighted savings ratio.
+  //
+  // Math (no cherry-pick):
+  //   saved_$ = Σ baseline_$ − Σ actual_$         (over requests where we measured baseline)
+  //   saved_% = saved_$ / Σ baseline_$
+  //
+  // `baseline_$` is the cache-aware bill Anthropic would have charged for the
+  // uncompressed body with the SAME cache_control markers Claude Code sent —
+  // measured by the counterfactual baseline probe on a sampled subset of
+  // requests. Caching savings stay credited to Claude Code; what remains is
+  // proxy-attributable savings only.
+  //
+  // Unmeasured requests (probe skipped, passthroughs) don't enter either sum.
+  // We only report what we measured.
   import { currentSession } from '../stores/index.js';
 
   $: data = $currentSession.data;
   $: err = $currentSession.error;
-  $: hasSession = data && data.sessionId != null;
+
+  $: baselineUsd = data?.baselineInputWeighted ?? 0;
+  $: actualUsd = data?.actualInputWeighted ?? 0;
+  $: savedUsd = Math.max(0, baselineUsd - actualUsd);
+  $: savedPct = baselineUsd > 0 ? (savedUsd / baselineUsd) * 100 : 0;
+  $: measuredReqs = data?.baselineMeasuredCount ?? 0;
 
   function fmtUsd(n: number): string {
     return '$' + n.toFixed(2);
   }
-  function fmtPct(n: number): string {
-    return n.toFixed(1) + '%';
-  }
-  function fmtDuration(sec: number): string {
-    if (sec < 60) return Math.round(sec) + 's';
-    const m = Math.floor(sec / 60);
-    if (m < 60) return m + 'm';
-    const h = Math.floor(m / 60);
-    return h + 'h ' + (m % 60) + 'm';
-  }
-  function fmtChars(n: number): string {
-    if (n < 1000) return String(n);
-    if (n < 1_000_000) return (n / 1000).toFixed(1) + 'k';
-    return (n / 1_000_000).toFixed(1) + 'M';
-  }
-
-  $: bucketTotal = data?.bucketChars
-    ? Object.values(data.bucketChars).reduce((a, b) => a + b, 0)
-    : 0;
-  $: bucketEntries = data?.bucketChars
-    ? (Object.entries(data.bucketChars) as [string, number][])
-        .filter(([, v]) => v > 0)
-        .sort((a, b) => b[1] - a[1])
-    : [];
-  $: passthroughEntries = data?.passthroughReasons
-    ? (Object.entries(data.passthroughReasons) as [string, number][])
-        .sort((a, b) => b[1] - a[1])
-    : [];
-
-  // Static key→label / key→color maps. The Record type matches the spec's
-  // `bucketChars` shape so any new bucket added in tracker.ts will show up
-  // in legend order. Hex codes chosen to match the legacy panel palette.
-  const BUCKET_LABELS: Record<string, string> = {
-    static_slab: 'slab',
-    reminder: 'reminder',
-    tool_result: 'tool_result',
-    history: 'history',
-    billing: 'billing',
-    dynamic: 'dynamic',
-  };
-  const BUCKET_COLORS: Record<string, string> = {
-    static_slab: '#b9be0',
-    reminder: '#85c5f6',
-    tool_result: '#10b981',
-    history: '#f59e0b',
-    billing: '#8e7681',
-    dynamic: '#ef4444',
-  };
 </script>
 
-<div class="panel session-summary">
-  {#if err}
-    <div class="error">error: {err}</div>
-  {:else if !data}
-    <div class="loading">loading…</div>
-  {:else if !hasSession}
-    <h2>This session</h2>
-    <div class="empty">{data.message ?? 'no active session yet'}</div>
-  {:else}
-    <div class="header">
-      <h2>This session</h2>
-      <div class="sub">
-        started {fmtDuration(data.uptimeSec ?? 0)} ago · {data.requests} requests
-      </div>
-    </div>
-
-    <div class="headline">
-      <div class="big">
-        Saved <span class="pct">{fmtPct(data.savedPct ?? 0)}</span>
-      </div>
-      <div class="usd">
-        {fmtUsd(data.savedUsd ?? 0)} of {fmtUsd(data.baselineUsd ?? 0)} baseline
-      </div>
-    </div>
-
-    <div class="bar">
-      <div class="bar-fill" style="width: {Math.max(0, Math.min(100, data.savedPct ?? 0))}%"></div>
-    </div>
-
-    {#if bucketEntries.length > 0}
-      <div class="section">
-        <div class="section-label">Where it came from</div>
-        <div class="bucket-bar">
-          {#each bucketEntries as [key, val]}
-            {@const w = bucketTotal > 0 ? (val / bucketTotal) * 100 : 0}
-            <div
-              class="bucket-seg"
-              style="width: {w}%; background: {BUCKET_COLORS[key] ?? '#8e7681'}"
-              title="{BUCKET_LABELS[key] ?? key}: {fmtChars(val)} chars ({w.toFixed(1)}%)"
-            ></div>
-          {/each}
-        </div>
-        <div class="bucket-legend">
-          {#each bucketEntries as [key, val]}
-            {@const w = bucketTotal > 0 ? (val / bucketTotal) * 100 : 0}
-            <span class="legend-item">
-              <span class="swatch" style="background: {BUCKET_COLORS[key] ?? '#8e7681'}"></span>
-              {BUCKET_LABELS[key] ?? key} {w.toFixed(0)}%
-            </span>
-          {/each}
-        </div>
-      </div>
-    {/if}
-
-    {#if passthroughEntries.length > 0}
-      <div class="section">
-        <div class="section-label">
-          Where it didn't · {data.passthroughRequests ?? 0} of {data.requests} requests passed through
-        </div>
-        <ul class="passthrough-list">
-          {#each passthroughEntries as [reason, count]}
-            <li><span class="reason">{reason}</span> <span class="count">{count}</span></li>
-          {/each}
-        </ul>
-      </div>
-    {:else if (data.passthroughRequests ?? 0) === 0}
-      <div class="section">
-        <div class="section-label-good">✓ Every request compressed</div>
-      </div>
-    {/if}
-  {/if}
-</div>
+{#if err}
+  <div class="line muted">session: {err}</div>
+{:else if measuredReqs > 0}
+  <div class="line">
+    <span class="label">THIS SESSION</span>
+    — saved <span class="num">{fmtUsd(savedUsd)}</span>
+    (<span class="num">{savedPct.toFixed(1)}%</span>)
+    · <span class="muted">{measuredReqs} requests</span>
+  </div>
+{/if}
 
 <style>
-  .session-summary {
-    margin-bottom: 22px;
-  }
-  .header {
-    display: flex;
-    justify-content: space-between;
-    align-items: baseline;
+  .line {
+    font-size: 14px;
+    color: #c9d1d9;
     margin-bottom: 12px;
+    padding: 8px 12px;
+    background: #161b22;
+    border: 1px solid #30363d;
+    border-radius: 6px;
   }
-  h2 {
-    margin: 0;
-    font-size: 13px;
+  .label {
     font-weight: 600;
-    text-transform: uppercase;
-    letter-spacing: 0.08em;
-    color: #8e7681;
+    color: #8b949e;
+    letter-spacing: 0.04em;
   }
-  .sub {
-    font-size: 12px;
-    color: #8e7681;
-  }
-  .headline {
-    margin-bottom: 10px;
-  }
-  .big {
-    font-size: 18px;
+  .num {
+    font-variant-numeric: tabular-nums;
+    color: #3fb950;
     font-weight: 600;
-    color: #c9d1d9;
   }
-  .pct {
-    color: #3fb950;
-    font-variant-numeric: tabular-nums;
-  }
-  .usd {
-    font-size: 12px;
-    color: #8e7681;
-    margin-top: 2px;
-    font-variant-numeric: tabular-nums;
-  }
-  .bar {
-    margin-top: 8px;
-    height: 6px;
-    background: #21262d;
-    border-radius: 3px;
-    overflow: hidden;
-  }
-  .bar-fill {
-    height: 100%;
-    background: #3fb950;
-    transition: width 1s ease;
-  }
-  .section {
-    margin-top: 16px;
-  }
-  .section-label {
-    font-size: 11px;
-    color: #8e7681;
-    text-transform: uppercase;
-    letter-spacing: 0.08em;
-    margin-bottom: 6px;
-  }
-  .section-label-good {
-    color: #3fb950;
-    text-transform: none;
-    letter-spacing: 0;
-    font-size: 12px;
-  }
-  .bucket-bar {
-    display: flex;
-    height: 8px;
-    border-radius: 4px;
-    overflow: hidden;
-    background: #21262d;
-  }
-  .bucket-seg {
-    height: 100%;
-    transition: width 1s ease;
-  }
-  .bucket-legend {
-    margin-top: 6px;
-    display: flex;
-    flex-wrap: wrap;
-    gap: 12px;
-    font-size: 11px;
-    color: #c9d1d9;
-  }
-  .legend-item {
-    display: inline-flex;
-    align-items: center;
-    gap: 4px;
-    font-variant-numeric: tabular-nums;
-  }
-  .swatch {
-    display: inline-block;
-    width: 8px;
-    height: 8px;
-    border-radius: 2px;
-  }
-  .passthrough-list {
-    margin: 0;
-    padding: 0;
-    list-style: none;
-    font-size: 12px;
-    color: #c9d1d9;
-  }
-  .passthrough-list li {
-    display: flex;
-    justify-content: space-between;
-    padding: 2px 0;
-    font-variant-numeric: tabular-nums;
-  }
-  .reason {
-    color: #8e7681;
-  }
-  .count {
-    color: #8e7681;
-  }
-  .loading, .empty, .error {
-    color: #8e7681;
-    font-size: 12px;
-  }
-  .error {
-    color: #f85149;
+  .muted {
+    color: #6e7681;
   }
 </style>
diff --git a/src/dashboard/types.ts b/src/dashboard/types.ts
index b520b1a..0852cf4 100644
--- a/src/dashboard/types.ts
+++ b/src/dashboard/types.ts
@@ -153,27 +153,13 @@ export interface CompressionToggleResponse {
   compression_enabled: boolean;
 }
 
-/** /api/current-session.json payload — per-session aggregates for the most-recently-active Claude Code session. */
+/** /api/current-session.json payload — per-session aggregates for the most-recently-active Claude Code session.
+ *  Pruned to only the fields the minimal SessionSummary.svelte headline reads:
+ *  dollar-weighted baseline/actual input + the measured-request count. */
 export interface CurrentSessionPayload {
   sessionId: string | null;
   message?: string;
-  firstSeen?: number;
-  lastSeen?: number;
-  uptimeSec?: number;
-  requests?: number;
-  compressedRequests?: number;
-  passthroughRequests?: number;
-  baselineUsd?: number;
-  actualUsd?: number;
-  savedUsd?: number;
-  savedPct?: number;
-  bucketChars?: {
-    static_slab: number;
-    reminder: number;
-    tool_result: number;
-    history: number;
-    billing: number;
-    dynamic: number;
-  };
-  passthroughReasons?: Record<string, number>;
+  baselineInputWeighted?: number;
+  actualInputWeighted?: number;
+  baselineMeasuredCount?: number;
 }