diff --git a/packages/host/apiproxy/src/api-proxy.ts b/packages/host/apiproxy/src/api-proxy.ts
index e55fc29f58..7eec1d8bb6 100644
--- a/packages/host/apiproxy/src/api-proxy.ts
+++ b/packages/host/apiproxy/src/api-proxy.ts
@@ -419,6 +419,12 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
       : { provider: logged.provider, model: logged.model }
   }
 
+  /** Pair a registry agent only with the exact Session lifecycle it owns. */
+  function metricsAgentFor(session: Session): Agent | undefined {
+    const agent = ctx.agents.get(session.id)
+    return agent?.session === session ? agent : undefined
+  }
+
   /** Pre-publication setup used by both fresh and resumed Web agents. */
   function installTarget(agentCtx: Context): void {
     const agent = agentCtx.agent
@@ -464,7 +470,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
         broadcast({
           type: 'session/metrics',
           sessionId: current.id,
-          metrics: metricsProjector.snapshot(current, ctx.agents.get(current.id)),
+          metrics: metricsProjector.snapshot(current, metricsAgentFor(current)),
         })
       }
     })
@@ -1186,7 +1192,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
           queue.push(frame({
             type: 'session/metrics',
             sessionId: session.id,
-            metrics: metricsProjector.snapshot(session, ctx.agents.get(session.id)),
+            metrics: metricsProjector.snapshot(session, metricsAgentFor(session)),
           }))
         }
         for (const pending of pendingQuestions.values()) {
@@ -1243,7 +1249,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
             queue.push(frame({
               type: 'session/metrics',
               sessionId: session.id,
-              metrics: metricsProjector.snapshot(session, ctx.agents.get(session.id)),
+              metrics: metricsProjector.snapshot(session, metricsAgentFor(session)),
             }))
           }),
           ctx.on('session/disposed', (session: Session) => {
diff --git a/packages/host/apiproxy/tests/api-proxy-models.spec.ts b/packages/host/apiproxy/tests/api-proxy-models.spec.ts
index 366c3cd597..722c2e7412 100644
--- a/packages/host/apiproxy/tests/api-proxy-models.spec.ts
+++ b/packages/host/apiproxy/tests/api-proxy-models.spec.ts
@@ -454,4 +454,59 @@ describe('Web session model selection', () => {
     liveSession.detach()
     await ctx.fiber.dispose()
   })
+
+  it('does not project retired agent capacity into replacement session snapshots', async () => {
+    const ctx = await hostContext()
+    const deferred = new DeferredCatalogAdapter()
+    ctx.llm.registerAdapter(['deferred'], deferred)
+    const sessionId = SessionId('capacity-snapshot-lifecycle')
+    const retiredSession = attachLifecycleSession(ctx, sessionId)
+    const retireAgent = attachLifecycleAgent(ctx, retiredSession.session)
+    const api = createApiProxy(ctx, { provider: 'deepseek', model: 'deepseek-chat', cwd: '/tmp', workspaceRoot: '/tmp' })
+    const primaryController = new AbortController()
+    const primary = api.events.mux(request({}), primaryController.signal)[Symbol.asyncIterator]()
+
+    expect((await nextMetrics(primary)).contextWindow).toBeUndefined()
+    await vi.waitFor(() => { expect(deferred.pending).toHaveLength(1) })
+    deferred.resolve(0, 64_000)
+    await settleCapacityCompletion()
+    expect((await nextMetrics(primary)).contextWindow).toBe(64_000)
+
+    retiredSession.detach()
+    const replacement = attachLifecycleSession(ctx, sessionId)
+    const createdBaseline = await nextMetrics(primary)
+    replacement.session.append('user/message', {
+      content: [{ type: 'text', text: 'replacement marker' }],
+      source: { kind: 'plugin', plugin: 'test' },
+    }, { surfaceOp: 'append' })
+    const scheduledFlush = await nextMetrics(primary)
+    const reconnectController = new AbortController()
+    const reconnect = api.events.mux(request({}), reconnectController.signal)[Symbol.asyncIterator]()
+    const reconnectBaseline = await nextMetrics(reconnect)
+    expect(createdBaseline.logRevision).toBe(1)
+    for (const metrics of [scheduledFlush, reconnectBaseline]) {
+      expect(metrics.logRevision).toBe(2)
+    }
+    expect({
+      created: createdBaseline.contextWindow,
+      scheduled: scheduledFlush.contextWindow,
+      reconnect: reconnectBaseline.contextWindow,
+    }).toEqual({ created: undefined, scheduled: undefined, reconnect: undefined })
+
+    retireAgent()
+    const detachReplacementAgent = attachLifecycleAgent(ctx, replacement.session)
+    expect((await nextMetrics(primary)).contextWindow).toBeUndefined()
+    await vi.waitFor(() => { expect(deferred.pending).toHaveLength(2) })
+    deferred.resolve(1, 128_000)
+    await settleCapacityCompletion()
+    expect((await nextMetrics(primary)).contextWindow).toBe(128_000)
+
+    primaryController.abort()
+    reconnectController.abort()
+    await primary.return?.()
+    await reconnect.return?.()
+    detachReplacementAgent()
+    replacement.detach()
+    await ctx.fiber.dispose()
+  })
 })