diff --git a/.github/workflows/ci-host.yaml b/.github/workflows/ci-host.yaml
index 3b6073f6cc..5a0fb629dc 100644
--- a/.github/workflows/ci-host.yaml
+++ b/.github/workflows/ci-host.yaml
@@ -21,7 +21,7 @@ on:
 
 permissions:
   checks: write
-  contents: read
+  contents: write
   id-token: write
   pull-requests: write
 
@@ -219,6 +219,17 @@ jobs:
           name: host-test-report-${{ matrix.shardIndex }}
           path: junit/host-${{ matrix.shardIndex }}.xml
           retention-days: 30
+      - name: Extract memory report
+        if: ${{ !cancelled() }}
+        run: node scripts/extract-memory-report.mjs /tmp/test-output.log /tmp/memory-report-${{ matrix.shardIndex }}.json
+        working-directory: packages/host
+      - name: Upload memory report
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        if: ${{ !cancelled() }}
+        with:
+          name: memory-report-${{ matrix.shardIndex }}
+          path: /tmp/memory-report-${{ matrix.shardIndex }}.json
+          retention-days: 30
       - name: Print realm server logs
         if: ${{ !cancelled() }}
         run: cat /tmp/server.log
@@ -360,3 +371,29 @@ jobs:
         with:
           junit_files: host.xml
           check_name: Host Test Results
+
+      - name: Download memory reports
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        if: ${{ !cancelled() }}
+        with:
+          path: all-memory-reports
+          pattern: memory-report-*
+          merge-multiple: true
+
+      - name: Check memory baseline
+        if: ${{ !cancelled() }}
+        run: node packages/host/scripts/check-memory-baseline.mjs all-memory-reports packages/host/memory-baseline.json
+
+      - name: Update memory baseline
+        if: ${{ !cancelled() && github.ref == 'refs/heads/main' && needs.host-test.result == 'success' }}
+        run: |
+          node packages/host/scripts/update-memory-baseline.mjs all-memory-reports packages/host/memory-baseline.json
+          if git diff --quiet packages/host/memory-baseline.json; then
+            echo "Baseline unchanged — nothing to commit."
+          else
+            git config user.name "github-actions[bot]"
+            git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+            git add packages/host/memory-baseline.json
+            git commit -m "Update host test memory baseline [skip ci]"
+            git push
+          fi
diff --git a/packages/host/memory-baseline.json b/packages/host/memory-baseline.json
new file mode 100644
index 0000000000..f1357e5661
--- /dev/null
+++ b/packages/host/memory-baseline.json
@@ -0,0 +1,520 @@
+{
+  "version": 1,
+  "generated": "2026-04-15",
+  "threshold": {
+    "relative": 0.1,
+    "absolute_mb": 5
+  },
+  "modules": {
+    "Acceptance | AI Assistant tests": {
+      "delta_mb": 1540.4
+    },
+    "Acceptance | Code patches tests": {
+      "delta_mb": 402.9
+    },
+    "Acceptance | Commands tests": {
+      "delta_mb": 418.8
+    },
+    "Acceptance | Freestyle": {
+      "delta_mb": 8.6
+    },
+    "Acceptance | Query Fields | host respects server-populated results": {
+      "delta_mb": 19.7
+    },
+    "Acceptance | Spec preview": {
+      "delta_mb": 614.0
+    },
+    "Acceptance | avif image def": {
+      "delta_mb": 39.4
+    },
+    "Acceptance | basic tests": {
+      "delta_mb": 60.7
+    },
+    "Acceptance | code submode tests": {
+      "delta_mb": 752.4
+    },
+    "Acceptance | code submode | create-file tests": {
+      "delta_mb": 875.6
+    },
+    "Acceptance | code submode | editor tests": {
+      "delta_mb": 308.5
+    },
+    "Acceptance | code submode | file def live reload": {
+      "delta_mb": 42.0
+    },
+    "Acceptance | code submode | file def navigation": {
+      "delta_mb": 57.4
+    },
+    "Acceptance | code submode | file-tree tests": {
+      "delta_mb": 441.8
+    },
+    "Acceptance | code submode | head format preview": {
+      "delta_mb": 87.9
+    },
+    "Acceptance | code submode | inspector tests": {
+      "delta_mb": 1002.4
+    },
+    "Acceptance | code submode | recent files tests": {
+      "delta_mb": 119.2
+    },
+    "Acceptance | code submode | schema editor tests": {
+      "delta_mb": 389.8
+    },
+    "Acceptance | code-submode | card playground": {
+      "delta_mb": 859.5
+    },
+    "Acceptance | code-submode | field playground": {
+      "delta_mb": 411.6
+    },
+    "Acceptance | csv file def": {
+      "delta_mb": 56.4
+    },
+    "Acceptance | file chooser keyboard tests": {
+      "delta_mb": 1.8
+    },
+    "Acceptance | file chooser tests": {
+      "delta_mb": 82.7
+    },
+    "Acceptance | file chooser tests | upload size limit": {
+      "delta_mb": 0.1
+    },
+    "Acceptance | file def": {
+      "delta_mb": 68.1
+    },
+    "Acceptance | gif image def": {
+      "delta_mb": 38.7
+    },
+    "Acceptance | gts file def": {
+      "delta_mb": 37.8
+    },
+    "Acceptance | host mode tests": {
+      "delta_mb": 106.5
+    },
+    "Acceptance | host submode": {
+      "delta_mb": 187.9
+    },
+    "Acceptance | interact submode creation & permissions": {
+      "delta_mb": 266.5
+    },
+    "Acceptance | interact submode tests": {
+      "delta_mb": 469.1
+    },
+    "Acceptance | interact submode | create-file tests": {
+      "delta_mb": 170.0
+    },
+    "Acceptance | jpg image def": {
+      "delta_mb": 39.4
+    },
+    "Acceptance | json file def": {
+      "delta_mb": 37.3
+    },
+    "Acceptance | markdown BFM card references": {
+      "delta_mb": 95.8
+    },
+    "Acceptance | markdown file def": {
+      "delta_mb": 37.5
+    },
+    "Acceptance | operator mode tests": {
+      "delta_mb": 595.3
+    },
+    "Acceptance | png image def": {
+      "delta_mb": 39.6
+    },
+    "Acceptance | prerender | file-extract": {
+      "delta_mb": 37.3
+    },
+    "Acceptance | prerender | html": {
+      "delta_mb": 771.4
+    },
+    "Acceptance | prerender | meta": {
+      "delta_mb": 23.8
+    },
+    "Acceptance | prerender | module": {
+      "delta_mb": 26.0
+    },
+    "Acceptance | svg image def": {
+      "delta_mb": 41.6
+    },
+    "Acceptance | text file def": {
+      "delta_mb": 13.5
+    },
+    "Acceptance | theme-card-test": {
+      "delta_mb": 45.9
+    },
+    "Acceptance | ts file def": {
+      "delta_mb": 36.5
+    },
+    "Acceptance | webp image def": {
+      "delta_mb": 40.5
+    },
+    "Acceptance | workspace-chooser": {
+      "delta_mb": 13.7
+    },
+    "Acceptance | workspace-chooser-delete": {
+      "delta_mb": 32.8
+    },
+    "Acceptance | workspace-delete-multiple": {
+      "delta_mb": 11.8
+    },
+    "Integration | CardDef-FieldDef relationships test": {
+      "delta_mb": 50.4
+    },
+    "Integration | Command | create-specs": {
+      "delta_mb": 12.3
+    },
+    "Integration | Command | generate-example-cards (one-shot)": {
+      "delta_mb": -0.3
+    },
+    "Integration | Command | host command schema generation test": {
+      "delta_mb": 8.0
+    },
+    "Integration | Command | patch-fields": {
+      "delta_mb": 36.8
+    },
+    "Integration | Command | preview-format": {
+      "delta_mb": 25.2
+    },
+    "Integration | Command | show-card": {
+      "delta_mb": 0.0
+    },
+    "Integration | Command | update-room-skills": {
+      "delta_mb": 17.2
+    },
+    "Integration | Component | FormattedAiBotMessage": {
+      "delta_mb": 86.7
+    },
+    "Integration | Component | FormattedUserMessage": {
+      "delta_mb": 4.0
+    },
+    "Integration | Component | RoomMessage": {
+      "delta_mb": 6.8
+    },
+    "Integration | EmailField": {
+      "delta_mb": -5.4
+    },
+    "Integration | PhoneNumberField": {
+      "delta_mb": 4.7
+    },
+    "Integration | RichMarkdownField": {
+      "delta_mb": 178.1
+    },
+    "Integration | add-workspace": {
+      "delta_mb": 4.3
+    },
+    "Integration | ai-assistant-panel | binary upload dedupe": {
+      "delta_mb": 65.7
+    },
+    "Integration | ai-assistant-panel | codeblocks": {
+      "delta_mb": 108.6
+    },
+    "Integration | ai-assistant-panel | commands": {
+      "delta_mb": 487.2
+    },
+    "Integration | ai-assistant-panel | debug-message": {
+      "delta_mb": -8.4
+    },
+    "Integration | ai-assistant-panel | file-attachment": {
+      "delta_mb": 261.4
+    },
+    "Integration | ai-assistant-panel | general": {
+      "delta_mb": 405.0
+    },
+    "Integration | ai-assistant-panel | past sessions": {
+      "delta_mb": 72.5
+    },
+    "Integration | ai-assistant-panel | reasoning": {
+      "delta_mb": 45.2
+    },
+    "Integration | ai-assistant-panel | scrolling": {
+      "delta_mb": 80.6
+    },
+    "Integration | ai-assistant-panel | sending": {
+      "delta_mb": 100.8
+    },
+    "Integration | ai-assistant-panel | skills": {
+      "delta_mb": 235.3
+    },
+    "Integration | ask-ai": {
+      "delta_mb": 43.3
+    },
+    "Integration | card api (Usage of publicAPI actions)": {
+      "delta_mb": 17.6
+    },
+    "Integration | card-basics": {
+      "delta_mb": 54.9
+    },
+    "Integration | card-catalog": {
+      "delta_mb": 1.8
+    },
+    "Integration | card-copy": {
+      "delta_mb": 98.9
+    },
+    "Integration | card-delete": {
+      "delta_mb": 27.7
+    },
+    "Integration | codemirror-context": {
+      "delta_mb": 1.2
+    },
+    "Integration | commands | add-field-to-card-definition": {
+      "delta_mb": 1.1
+    },
+    "Integration | commands | ai-assistant": {
+      "delta_mb": 15.3
+    },
+    "Integration | commands | apply-markdown-edit": {
+      "delta_mb": 7.3
+    },
+    "Integration | commands | apply-search-replace-block": {
+      "delta_mb": 9.8
+    },
+    "Integration | commands | bot-registration": {
+      "delta_mb": 15.6
+    },
+    "Integration | commands | cancel-indexing-job": {
+      "delta_mb": 15.2
+    },
+    "Integration | commands | check-correctness": {
+      "delta_mb": 65.9
+    },
+    "Integration | commands | commands-calling": {
+      "delta_mb": -4.3
+    },
+    "Integration | commands | copy-and-edit": {
+      "delta_mb": 28.2
+    },
+    "Integration | commands | copy-card": {
+      "delta_mb": 42.0
+    },
+    "Integration | commands | copy-file-to-realm": {
+      "delta_mb": 6.1
+    },
+    "Integration | commands | copy-source": {
+      "delta_mb": 15.4
+    },
+    "Integration | commands | create-listing-pr-request": {
+      "delta_mb": -58.8
+    },
+    "Integration | commands | full-reindex-realm": {
+      "delta_mb": 7.4
+    },
+    "Integration | commands | get-user-system-card": {
+      "delta_mb": -22.8
+    },
+    "Integration | commands | invalidate-realm-urls": {
+      "delta_mb": -10.6
+    },
+    "Integration | commands | invite-user-to-room": {
+      "delta_mb": 12.7
+    },
+    "Integration | commands | open-create-listing-modal": {
+      "delta_mb": -8.8
+    },
+    "Integration | commands | open-workspace": {
+      "delta_mb": 14.0
+    },
+    "Integration | commands | patch-code": {
+      "delta_mb": 6.2
+    },
+    "Integration | commands | patch-instance": {
+      "delta_mb": 2.8
+    },
+    "Integration | commands | read-card-for-ai-assistant": {
+      "delta_mb": 13.9
+    },
+    "Integration | commands | read-file-for-ai-assistant": {
+      "delta_mb": 9.7
+    },
+    "Integration | commands | read-source": {
+      "delta_mb": 4.5
+    },
+    "Integration | commands | read-text-file": {
+      "delta_mb": -14.1
+    },
+    "Integration | commands | reindex-realm": {
+      "delta_mb": -12.9
+    },
+    "Integration | commands | search": {
+      "delta_mb": 17.4
+    },
+    "Integration | commands | search-and-choose": {
+      "delta_mb": 6.7
+    },
+    "Integration | commands | search-google-images": {
+      "delta_mb": -8.8
+    },
+    "Integration | commands | send-ai-assistant-message": {
+      "delta_mb": -1.2
+    },
+    "Integration | commands | send-bot-trigger-event": {
+      "delta_mb": 12.8
+    },
+    "Integration | commands | send-request-via-proxy": {
+      "delta_mb": 56.2
+    },
+    "Integration | commands | set-user-system-card": {
+      "delta_mb": 12.7
+    },
+    "Integration | commands | summarize-session": {
+      "delta_mb": 28.2
+    },
+    "Integration | commands | switch-submode": {
+      "delta_mb": 4.1
+    },
+    "Integration | commands | transform-cards": {
+      "delta_mb": 23.1
+    },
+    "Integration | commands | write-text-file": {
+      "delta_mb": -12.7
+    },
+    "Integration | components | create-listing-modal": {
+      "delta_mb": 293.5
+    },
+    "Integration | components | realm field": {
+      "delta_mb": -7.2
+    },
+    "Integration | computeds": {
+      "delta_mb": 13.9
+    },
+    "Integration | create app module via ai-assistant": {
+      "delta_mb": -0.0
+    },
+    "Integration | enumField": {
+      "delta_mb": 62.5
+    },
+    "Integration | field configuration": {
+      "delta_mb": 9.1
+    },
+    "Integration | loading": {
+      "delta_mb": 14.5
+    },
+    "Integration | markdown highlighting error scenarios": {
+      "delta_mb": 16.9
+    },
+    "Integration | message service subscription": {
+      "delta_mb": 11.8
+    },
+    "Integration | operator-mode | basics": {
+      "delta_mb": 356.1
+    },
+    "Integration | operator-mode | card catalog": {
+      "delta_mb": 208.6
+    },
+    "Integration | operator-mode | links": {
+      "delta_mb": 129.2
+    },
+    "Integration | operator-mode | ui": {
+      "delta_mb": 343.4
+    },
+    "Integration | overlay-menu-items": {
+      "delta_mb": 16.9
+    },
+    "Integration | prerendered-card-search": {
+      "delta_mb": 22.0
+    },
+    "Integration | preview": {
+      "delta_mb": 6.8
+    },
+    "Integration | realm": {
+      "delta_mb": 32.1
+    },
+    "Integration | realm indexing": {
+      "delta_mb": 3.7
+    },
+    "Integration | realm querying": {
+      "delta_mb": 39.6
+    },
+    "Integration | search data resource": {
+      "delta_mb": 18.1
+    },
+    "Integration | search resource": {
+      "delta_mb": 24.1
+    },
+    "Integration | serialization": {
+      "delta_mb": 29.2
+    },
+    "Integration | serializeFileDef": {
+      "delta_mb": 8.3
+    },
+    "Integration | text-input-validator": {
+      "delta_mb": 15.8
+    },
+    "Integration | text-suggestion | card-chooser-title": {
+      "delta_mb": 7.9
+    },
+    "Unit | CardDef menu items": {
+      "delta_mb": 4.1
+    },
+    "Unit | FileDef menu items": {
+      "delta_mb": 6.6
+    },
+    "Unit | Lib | command-definitions": {
+      "delta_mb": 0.0
+    },
+    "Unit | Lib | limited-set": {
+      "delta_mb": 0.0
+    },
+    "Unit | Utility | field-path-parser": {
+      "delta_mb": 0.1
+    },
+    "Unit | ai-function-generation-test": {
+      "delta_mb": 9.5
+    },
+    "Unit | auth-error-guard": {
+      "delta_mb": -0.0
+    },
+    "Unit | auth-service-worker": {
+      "delta_mb": 0.0
+    },
+    "Unit | bfm-card-references": {
+      "delta_mb": 0.1
+    },
+    "Unit | box": {
+      "delta_mb": 6.6
+    },
+    "Unit | cached-fetch": {
+      "delta_mb": -0.0
+    },
+    "Unit | code patching | parse search replace blocks": {
+      "delta_mb": 0.0
+    },
+    "Unit | convertToClassName": {
+      "delta_mb": 0.1
+    },
+    "Unit | diffing": {
+      "delta_mb": 0.0
+    },
+    "Unit | extract-css-variables": {
+      "delta_mb": 0.0
+    },
+    "Unit | fetcher": {
+      "delta_mb": -0.0
+    },
+    "Unit | file-def-manager canonicalize": {
+      "delta_mb": 0.0
+    },
+    "Unit | generate-css-variables": {
+      "delta_mb": 0.0
+    },
+    "Unit | identity-context garbage collection": {
+      "delta_mb": 8.1
+    },
+    "Unit | index-writer": {
+      "delta_mb": 68.5
+    },
+    "Unit | isolated-render": {
+      "delta_mb": 4.0
+    },
+    "Unit | last-modified-date": {
+      "delta_mb": 0.0
+    },
+    "Unit | query": {
+      "delta_mb": 3.2
+    },
+    "Unit | runtime-common | card directory names": {
+      "delta_mb": 0.0
+    },
+    "code-ref": {
+      "delta_mb": 9.2
+    }
+  }
+}
diff --git a/packages/host/scripts/check-memory-baseline.mjs b/packages/host/scripts/check-memory-baseline.mjs
new file mode 100644
index 0000000000..e2c348a6f0
--- /dev/null
+++ b/packages/host/scripts/check-memory-baseline.mjs
@@ -0,0 +1,158 @@
+#!/usr/bin/env node
+
+// Compares per-module memory deltas from a CI run against a committed baseline.
+// Exits 0 on pass/warn, exits 1 on hard failure (>2x baseline or +50MB absolute).
+//
+// Usage: node check-memory-baseline.mjs <reports-dir> <baseline-json>
+//
+// <reports-dir> contains per-shard memory-report.json files (merged into one dir).
+// <baseline-json> is the committed packages/host/memory-baseline.json.
+
+import { readdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const [reportsDir, baselinePath] = process.argv.slice(2);
+
+if (!reportsDir || !baselinePath) {
+  console.error(
+    'Usage: node check-memory-baseline.mjs <reports-dir> <baseline-json>',
+  );
+  process.exit(1);
+}
+
+// Load and merge all shard reports
+const current = {};
+for (const file of readdirSync(reportsDir)) {
+  if (!file.endsWith('.json')) continue;
+  const shard = JSON.parse(readFileSync(join(reportsDir, file), 'utf8'));
+  Object.assign(current, shard);
+}
+
+// Load baseline
+let baseline;
+try {
+  baseline = JSON.parse(readFileSync(baselinePath, 'utf8'));
+} catch (err) {
+  console.log(`No baseline found at ${baselinePath} — skipping check.`);
+  process.exit(0);
+}
+
+const SOFT_RELATIVE = baseline.threshold?.relative ?? 0.10;
+const SOFT_ABSOLUTE_MB = baseline.threshold?.absolute_mb ?? 5;
+const HARD_RELATIVE = 1.0; // 2x = 100% increase
+const HARD_ABSOLUTE_MB = 50;
+
+const warnings = [];
+const failures = [];
+const newModules = [];
+
+for (const [mod, data] of Object.entries(current)) {
+  if (mod === '__shard_warmup__') continue;
+  if (data.delta_mb == null) continue;
+
+  const base = baseline.modules?.[mod];
+  if (!base) {
+    newModules.push({ mod, delta: data.delta_mb });
+    continue;
+  }
+
+  const baseDelta = base.delta_mb;
+  if (baseDelta == null) continue;
+
+  const diff = data.delta_mb - baseDelta;
+  const absDiff = Math.abs(diff);
+  // Only flag increases
+  if (diff <= 0) continue;
+
+  const softThreshold = Math.max(SOFT_ABSOLUTE_MB, Math.abs(baseDelta) * SOFT_RELATIVE);
+  const hardThreshold = Math.max(HARD_ABSOLUTE_MB, Math.abs(baseDelta) * HARD_RELATIVE);
+
+  if (absDiff >= hardThreshold) {
+    failures.push({
+      mod,
+      baseline: baseDelta,
+      current: data.delta_mb,
+      diff,
+      pct: baseDelta !== 0 ? ((diff / Math.abs(baseDelta)) * 100).toFixed(0) : 'inf',
+    });
+  } else if (absDiff >= softThreshold) {
+    warnings.push({
+      mod,
+      baseline: baseDelta,
+      current: data.delta_mb,
+      diff,
+      pct: baseDelta !== 0 ? ((diff / Math.abs(baseDelta)) * 100).toFixed(0) : 'inf',
+    });
+  }
+}
+
+// Build summary
+const lines = [];
+lines.push('## Memory Baseline Check\n');
+
+const totalModules = Object.keys(current).filter((m) => m !== '__shard_warmup__').length;
+const baselineModules = Object.keys(baseline.modules || {}).length;
+lines.push(
+  `Checked **${totalModules}** modules against baseline (${baselineModules} baselined).\n`,
+);
+
+if (failures.length === 0 && warnings.length === 0) {
+  lines.push('All modules within threshold. No memory regressions detected.\n');
+}
+
+if (failures.length > 0) {
+  lines.push(`### Failures (>${HARD_RELATIVE * 100}% increase or +${HARD_ABSOLUTE_MB}MB)\n`);
+  lines.push('| Module | Baseline | Current | Change |');
+  lines.push('|--------|----------|---------|--------|');
+  for (const f of failures.sort((a, b) => b.diff - a.diff)) {
+    lines.push(
+      `| ${f.mod} | ${f.baseline.toFixed(1)} MB | ${f.current.toFixed(1)} MB | +${f.diff.toFixed(1)} MB (+${f.pct}%) |`,
+    );
+  }
+  lines.push('');
+}
+
+if (warnings.length > 0) {
+  lines.push(`### Warnings (>${SOFT_RELATIVE * 100}% + ${SOFT_ABSOLUTE_MB}MB increase)\n`);
+  lines.push('| Module | Baseline | Current | Change |');
+  lines.push('|--------|----------|---------|--------|');
+  for (const w of warnings.sort((a, b) => b.diff - a.diff)) {
+    lines.push(
+      `| ${w.mod} | ${w.baseline.toFixed(1)} MB | ${w.current.toFixed(1)} MB | +${w.diff.toFixed(1)} MB (+${w.pct}%) |`,
+    );
+  }
+  lines.push('');
+}
+
+if (newModules.length > 0) {
+  lines.push(
+    `<details><summary>${newModules.length} new module(s) not in baseline</summary>\n`,
+  );
+  for (const n of newModules.sort((a, b) => b.delta - a.delta)) {
+    lines.push(`- **${n.mod}**: ${n.delta.toFixed(1)} MB`);
+  }
+  lines.push('</details>\n');
+}
+
+const summary = lines.join('\n');
+console.log(summary);
+
+// Write to GITHUB_STEP_SUMMARY if available
+if (process.env.GITHUB_STEP_SUMMARY) {
+  writeFileSync(process.env.GITHUB_STEP_SUMMARY, summary + '\n', {
+    flag: 'a',
+  });
+}
+
+if (failures.length > 0) {
+  console.error(
+    `\nFAILED: ${failures.length} module(s) exceeded hard memory threshold.`,
+  );
+  process.exit(1);
+}
+
+if (warnings.length > 0) {
+  console.log(
+    `\nWARN: ${warnings.length} module(s) exceeded soft memory threshold.`,
+  );
+}
diff --git a/packages/host/scripts/extract-memory-report.mjs b/packages/host/scripts/extract-memory-report.mjs
new file mode 100644
index 0000000000..2fe3e6c906
--- /dev/null
+++ b/packages/host/scripts/extract-memory-report.mjs
@@ -0,0 +1,60 @@
+#!/usr/bin/env node
+
+// Extracts MEMPROBE_FILE lines from a host test log and writes a JSON report.
+// Handles both raw console.log format and testem's JSON-wrapped format.
+//
+// Usage: node extract-memory-report.mjs <input-log> <output-json>
+
+import { readFileSync, writeFileSync } from 'node:fs';
+
+const [inputPath, outputPath] = process.argv.slice(2);
+
+if (!inputPath || !outputPath) {
+  console.error(
+    'Usage: node extract-memory-report.mjs <input-log> <output-json>',
+  );
+  process.exit(1);
+}
+
+const PROBE_RE =
+  /MEMPROBE_FILE module=("(?:[^"\\]|\\.)*"|\S+) tests=(\d+) used=([\d.]+)MB total=([\d.]+)MB delta=([\d.\-]+|na)MB/;
+const JSON_ENVELOPE_RE = /\{"type":"log","text":"(.*?)"\}\s*$/;
+
+const log = readFileSync(inputPath, 'utf8');
+const report = {};
+
+for (const rawLine of log.split('\n')) {
+  if (!rawLine.includes('MEMPROBE_FILE')) continue;
+
+  let line = rawLine;
+
+  // Unwrap testem JSON envelope if present
+  const envMatch = line.match(JSON_ENVELOPE_RE);
+  if (envMatch) {
+    try {
+      line = JSON.parse(`"${envMatch[1]}"`);
+    } catch {
+      // fall through to raw parse
+    }
+  }
+
+  const m = line.match(PROBE_RE);
+  if (!m) continue;
+
+  const mod = m[1].startsWith('"') ? JSON.parse(m[1]) : m[1];
+  const deltaMb = m[5] === 'na' ? null : parseFloat(m[5]);
+  const usedMb = parseFloat(m[3]);
+  const totalMb = parseFloat(m[4]);
+  const tests = parseInt(m[2], 10);
+
+  report[mod] = { delta_mb: deltaMb, used_mb: usedMb, total_mb: totalMb, tests };
+}
+
+writeFileSync(outputPath, JSON.stringify(report, null, 2) + '\n');
+
+const count = Object.keys(report).length;
+if (count === 0) {
+  console.log('extract-memory-report: no MEMPROBE_FILE lines found');
+} else {
+  console.log(`extract-memory-report: wrote ${count} modules to ${outputPath}`);
+}
diff --git a/packages/host/scripts/update-memory-baseline.mjs b/packages/host/scripts/update-memory-baseline.mjs
new file mode 100644
index 0000000000..482afde762
--- /dev/null
+++ b/packages/host/scripts/update-memory-baseline.mjs
@@ -0,0 +1,48 @@
+#!/usr/bin/env node
+
+// Regenerates memory-baseline.json from per-shard memory reports.
+// Used by CI on main merge (auto-update) or locally by developers.
+//
+// Usage: node update-memory-baseline.mjs <reports-dir> <baseline-json>
+
+import { readdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const [reportsDir, baselinePath] = process.argv.slice(2);
+
+if (!reportsDir || !baselinePath) {
+  console.error(
+    'Usage: node update-memory-baseline.mjs <reports-dir> <baseline-json>',
+  );
+  process.exit(1);
+}
+
+// Merge all shard reports
+const merged = {};
+for (const file of readdirSync(reportsDir)) {
+  if (!file.endsWith('.json')) continue;
+  const shard = JSON.parse(readFileSync(join(reportsDir, file), 'utf8'));
+  Object.assign(merged, shard);
+}
+
+// Build baseline — exclude warmup since it varies by environment
+const modules = {};
+for (const [mod, data] of Object.entries(merged).sort(([a], [b]) =>
+  a.localeCompare(b),
+)) {
+  if (mod === '__shard_warmup__') continue;
+  if (data.delta_mb == null) continue;
+  modules[mod] = { delta_mb: Math.round(data.delta_mb * 10) / 10 };
+}
+
+const baseline = {
+  version: 1,
+  generated: new Date().toISOString().slice(0, 10),
+  threshold: { relative: 0.1, absolute_mb: 5 },
+  modules,
+};
+
+writeFileSync(baselinePath, JSON.stringify(baseline, null, 2) + '\n');
+console.log(
+  `update-memory-baseline: wrote ${Object.keys(modules).length} modules to ${baselinePath}`,
+);
diff --git a/packages/host/tests/helpers/setup-qunit.js b/packages/host/tests/helpers/setup-qunit.js
index 90308008f1..e38e49b918 100644
--- a/packages/host/tests/helpers/setup-qunit.js
+++ b/packages/host/tests/helpers/setup-qunit.js
@@ -13,6 +13,57 @@ export function setupQUnit() {
   setup(QUnit.assert);
   QUnit.config.autostart = false;
 
+  // Per-module memory delta probe — log each test file's contribution to
+  // retained memory, independent of where it falls in shard order. We GC
+  // at module boundaries so the start/end snapshots compare like-for-like.
+  // QUnit module name is normally 1:1 with the test file. We only probe
+  // top-level modules (fullName.length === 1) so nested modules don't
+  // create overlapping start/end pairs.
+  let usedAtModuleStart = null;
+  let inTopLevelModule = false;
+  QUnit.on('suiteStart', (details) => {
+    let depth = Array.isArray(details.fullName) ? details.fullName.length : 1;
+    if (depth !== 1) return;
+    inTopLevelModule = true;
+    if (typeof globalThis.gc === 'function') {
+      globalThis.gc();
+      globalThis.gc();
+    }
+    try {
+      let pm = performance && performance.memory;
+      usedAtModuleStart = pm ? pm.usedJSHeapSize : null;
+    } catch (_) {
+      usedAtModuleStart = null;
+    }
+  });
+  QUnit.on('suiteEnd', (details) => {
+    let depth = Array.isArray(details.fullName) ? details.fullName.length : 1;
+    if (depth !== 1 || !inTopLevelModule) return;
+    inTopLevelModule = false;
+    if (typeof globalThis.gc === 'function') {
+      globalThis.gc();
+      globalThis.gc();
+    }
+    try {
+      let pm = performance && performance.memory;
+      if (pm) {
+        let used = pm.usedJSHeapSize;
+        let usedMB = (used / 1048576).toFixed(1);
+        let totalMB = (pm.totalJSHeapSize / 1048576).toFixed(1);
+        let deltaMB =
+          usedAtModuleStart != null
+            ? ((used - usedAtModuleStart) / 1048576).toFixed(1)
+            : 'na';
+        let tests = details.tests ? details.tests.length : 0;
+        console.log(
+          `MEMPROBE_FILE module=${JSON.stringify(details.name)} tests=${tests} used=${usedMB}MB total=${totalMB}MB delta=${deltaMB}MB`,
+        );
+      }
+    } catch (_) {
+      /* ignore */
+    }
+  });
+
   // After each test, force GC (via --expose-gc) so V8 can release
   // per-test allocations before the next test starts. Without this, V8's
   // opportunistic GC can't keep up and the heap drifts toward the 4GB
diff --git a/packages/host/tests/helpers/shard-warmup.ts b/packages/host/tests/helpers/shard-warmup.ts
new file mode 100644
index 0000000000..86f2dae78b
--- /dev/null
+++ b/packages/host/tests/helpers/shard-warmup.ts
@@ -0,0 +1,67 @@
+// Synthetic warmup module — registered first on every shard so that the
+// per-shard "boot cost" (Ember app boot, base-realm imports, mock matrix,
+// initial test realm setup) lands on this module rather than on whichever
+// real test file ember-exam happens to schedule first. The MEMPROBE_FILE
+// line for this module then becomes the "shard boot cost" baseline, and
+// real modules report a clean per-file delta independent of position.
+//
+// Imported via test-helper.js; the static import causes the module() call
+// below to register with QUnit before ember-exam loads the partition's
+// test files.
+
+import { visit } from '@ember/test-helpers';
+
+import { getService } from '@universal-ember/test-support';
+import { module, test } from 'qunit';
+
+import { baseRealm } from '@cardstack/runtime-common';
+
+import {
+  SYSTEM_CARD_FIXTURE_CONTENTS,
+  setupAcceptanceTestRealm,
+  setupAuthEndpoints,
+  setupLocalIndexing,
+  setupUserSubscription,
+  testRealmURL,
+} from './index';
+import { setupMockMatrix } from './mock-matrix';
+import { setupApplicationTest } from './setup';
+
+module('__shard_warmup__', function (hooks) {
+  setupApplicationTest(hooks);
+  setupLocalIndexing(hooks);
+
+  let mockMatrixUtils = setupMockMatrix(hooks, {
+    loggedInAs: '@testuser:localhost',
+    activeRealms: [testRealmURL],
+  });
+
+  let { createAndJoinRoom } = mockMatrixUtils;
+
+  hooks.beforeEach(async function () {
+    createAndJoinRoom({
+      sender: '@testuser:localhost',
+      name: 'room-warmup',
+    });
+    setupUserSubscription();
+    setupAuthEndpoints();
+
+    let loaderService = getService('loader-service');
+    let loader = loaderService.loader;
+    // Prime the loader with the most commonly imported base-realm modules
+    // so subsequent real tests don't pay the import cost.
+    await loader.import(`${baseRealm.url}card-api`);
+    await loader.import(`${baseRealm.url}string`);
+    await loader.import(`${baseRealm.url}spec`);
+
+    await setupAcceptanceTestRealm({
+      mockMatrixUtils,
+      contents: { ...SYSTEM_CARD_FIXTURE_CONTENTS },
+    });
+  });
+
+  test('warm boot the test environment', async function (assert) {
+    await visit('/');
+    assert.ok(true, 'shard warmup completed');
+  });
+});
diff --git a/packages/host/tests/test-helper.js b/packages/host/tests/test-helper.js
index 84b415ad9d..c08ac052bb 100644
--- a/packages/host/tests/test-helper.js
+++ b/packages/host/tests/test-helper.js
@@ -7,6 +7,10 @@ import start from 'ember-exam/test-support/start';
 // eslint-disable-next-line ember/no-test-import-export
 import { loadRealmTests } from './live-test';
 import { setupQUnit } from './helpers/setup-qunit';
+// Side-effect import: registers a `__shard_warmup__` QUnit module so the
+// per-shard boot cost is absorbed by it rather than by whichever real
+// test module happens to be scheduled first on the partition.
+import './helpers/shard-warmup';
 
 const application = Application.create({
   ...config.APP,