diff --git a/browse/src/cdp-inspector.ts b/browse/src/cdp-inspector.ts
index 293b6eb8b..fd4a938d1 100644
--- a/browse/src/cdp-inspector.ts
+++ b/browse/src/cdp-inspector.ts
@@ -139,7 +139,41 @@ async function getOrCreateSession(page: Page): Promise<any> {
 
 // ─── Modification History ───────────────────────────────────────
 
+// Bounded FIFO of style modifications. Pre-cap, this was an unbounded
+// module-scoped array that grew for every CSS edit made through $B css
+// across the whole browser session — small per-entry footprint but no
+// upper bound, the kind of slow leak that compounds over multi-day
+// inspector use. The cap is 200 because per-session undo workflows
+// rarely walk back more than a handful of edits, and a user who really
+// wants to roll a long change back can `$B css reset` to revert all of
+// them. totalPushed is monotonic across the session so undoModification
+// can tell the user when their target index has been evicted, instead
+// of just "no modification at index N".
+const MOD_HISTORY_CAP = 200;
 const modificationHistory: StyleModification[] = [];
+let modHistoryTotalPushed = 0;
+
+function pushModification(mod: StyleModification): void {
+  modificationHistory.push(mod);
+  modHistoryTotalPushed++;
+  while (modificationHistory.length > MOD_HISTORY_CAP) {
+    modificationHistory.shift();
+  }
+}
+
+// Test-only entry: exposes the history-cap mechanics (push, reset, cap value)
+// without requiring a CDP-driven Page. Production code must go through
+// modifyStyle / undoModification / resetModifications.
+export const __testInternals = {
+  pushModification,
+  MOD_HISTORY_CAP,
+  getRawHistory: () => modificationHistory.slice(),
+  getTotalPushed: () => modHistoryTotalPushed,
+  resetForTest: () => {
+    modificationHistory.length = 0;
+    modHistoryTotalPushed = 0;
+  },
+};
 
 // ─── Specificity Calculation ────────────────────────────────────
 
@@ -568,7 +602,7 @@ export async function modifyStyle(
     method,
   };
 
-  modificationHistory.push(modification);
+  pushModification(modification);
   return modification;
 }
 
@@ -578,7 +612,12 @@ export async function modifyStyle(
 export async function undoModification(page: Page, index?: number): Promise<void> {
   const idx = index ?? modificationHistory.length - 1;
   if (idx < 0 || idx >= modificationHistory.length) {
-    throw new Error(`No modification at index ${idx}. History has ${modificationHistory.length} entries.`);
+    const evictedNote = modHistoryTotalPushed > MOD_HISTORY_CAP
+      ? ` (most recent ${MOD_HISTORY_CAP} only — ${modHistoryTotalPushed - MOD_HISTORY_CAP} earlier entries evicted at the cap)`
+      : '';
+    throw new Error(
+      `No modification at index ${idx}. History has ${modificationHistory.length} entries${evictedNote}.`,
+    );
   }
 
   const mod = modificationHistory[idx];
@@ -657,6 +696,7 @@ export async function resetModifications(page: Page): Promise<void> {
     }
   }
   modificationHistory.length = 0;
+  modHistoryTotalPushed = 0;
 }
 
 /**
diff --git a/browse/test/cdp-inspector-history-cap.test.ts b/browse/test/cdp-inspector-history-cap.test.ts
new file mode 100644
index 000000000..21b2d6c22
--- /dev/null
+++ b/browse/test/cdp-inspector-history-cap.test.ts
@@ -0,0 +1,95 @@
+import { describe, test, expect, beforeEach } from 'bun:test';
+import type { Page } from 'playwright';
+import {
+  __testInternals,
+  undoModification,
+} from '../src/cdp-inspector';
+
+// Regression tests for the modificationHistory cap (D6 / smoking gun #2).
+// Pre-cap, the module-scoped array grew unbounded across the session. Cap is
+// 200 entries, oldest evicted on push past the cap. undoModification reports
+// "evicted at the cap" in the error message so a user who asks for a
+// no-longer-available index understands what happened (instead of seeing the
+// pre-cap "No modification at index 500" with no context).
+
+const { pushModification, MOD_HISTORY_CAP, getRawHistory, getTotalPushed, resetForTest } = __testInternals;
+
+function fakeMod(id: number) {
+  return {
+    selector: `#node-${id}`,
+    property: 'color',
+    oldValue: 'red',
+    newValue: 'blue',
+    source: 'inline' as const,
+    timestamp: id,
+    method: 'setProperty' as 'setProperty',
+  };
+}
+
+beforeEach(() => {
+  resetForTest();
+});
+
+describe('modificationHistory cap', () => {
+  test('1. push under cap keeps every entry', () => {
+    for (let i = 0; i < 50; i++) pushModification(fakeMod(i));
+    expect(getRawHistory().length).toBe(50);
+    expect(getTotalPushed()).toBe(50);
+    expect(getRawHistory()[0].timestamp).toBe(0);
+    expect(getRawHistory()[49].timestamp).toBe(49);
+  });
+
+  test('2. push exactly cap keeps every entry', () => {
+    for (let i = 0; i < MOD_HISTORY_CAP; i++) pushModification(fakeMod(i));
+    expect(getRawHistory().length).toBe(MOD_HISTORY_CAP);
+    expect(getTotalPushed()).toBe(MOD_HISTORY_CAP);
+    expect(getRawHistory()[0].timestamp).toBe(0);
+  });
+
+  test('3. push past cap evicts oldest, keeps length at cap', () => {
+    const total = MOD_HISTORY_CAP + 50;
+    for (let i = 0; i < total; i++) pushModification(fakeMod(i));
+    expect(getRawHistory().length).toBe(MOD_HISTORY_CAP);
+    expect(getTotalPushed()).toBe(total);
+    // Oldest 50 dropped — entry that was #0 is gone; new oldest is #50.
+    expect(getRawHistory()[0].timestamp).toBe(50);
+    expect(getRawHistory()[MOD_HISTORY_CAP - 1].timestamp).toBe(total - 1);
+  });
+
+  test('4. resetForTest clears both buffer and totalPushed', () => {
+    for (let i = 0; i < 10; i++) pushModification(fakeMod(i));
+    resetForTest();
+    expect(getRawHistory().length).toBe(0);
+    expect(getTotalPushed()).toBe(0);
+  });
+});
+
+describe('undoModification eviction-aware error', () => {
+  // Stub Page: undoModification throws before any await when idx is out of
+  // range, so the stub never actually gets called.
+  const stubPage = {} as unknown as Page;
+
+  test('5. out-of-range BEFORE any eviction → no evicted note', async () => {
+    for (let i = 0; i < 5; i++) pushModification(fakeMod(i));
+    await expect(undoModification(stubPage, 99)).rejects.toThrow(
+      'No modification at index 99. History has 5 entries.',
+    );
+  });
+
+  test('6. out-of-range AFTER eviction → message names the evicted count', async () => {
+    const total = MOD_HISTORY_CAP + 73;
+    for (let i = 0; i < total; i++) pushModification(fakeMod(i));
+    // 273 pushed, 200 in buffer, 73 evicted. Ask for idx=400 (above buffer).
+    await expect(undoModification(stubPage, 400)).rejects.toThrow(
+      `No modification at index 400. History has ${MOD_HISTORY_CAP} entries ` +
+      `(most recent ${MOD_HISTORY_CAP} only — 73 earlier entries evicted at the cap).`,
+    );
+  });
+
+  test('7. negative explicit index throws cleanly (no NaN propagation)', async () => {
+    for (let i = 0; i < 10; i++) pushModification(fakeMod(i));
+    await expect(undoModification(stubPage, -1)).rejects.toThrow(
+      'No modification at index -1.',
+    );
+  });
+});