diff --git a/crates/obscura-cdp/src/domains/page.rs b/crates/obscura-cdp/src/domains/page.rs
index dd84b5e..bd06e9a 100644
--- a/crates/obscura-cdp/src/domains/page.rs
+++ b/crates/obscura-cdp/src/domains/page.rs
@@ -73,12 +73,22 @@ pub fn emit_navigation_events(
         });
     }
 
+    // executionContextsCleared invalidates every prior context id, so a
+    // Runtime.evaluate / callFunctionOn targeting a pre-navigation context
+    // must be rejected (Chrome: "Cannot find context with specified id"). The
+    // default world (id 2) and isolated worlds are re-registered below as their
+    // executionContextCreated events are emitted. Issue #407: previously this
+    // set was insert-only, so stale ids kept validating and grew unbounded.
+    ctx.valid_context_ids.clear();
     let mut phase1 = vec![
         CdpEvent { method: "Page.lifecycleEvent".into(), params: json!({"frameId": frame_id, "loaderId": loader_id, "name": "init", "timestamp": ts}), session_id: es.clone() },
         CdpEvent { method: "Runtime.executionContextsCleared".into(), params: json!({}), session_id: es.clone() },
         CdpEvent { method: "Page.frameNavigated".into(), params: json!({"frame": {"id": frame_id, "loaderId": loader_id, "url": page_url, "domainAndRegistry": "", "securityOrigin": page_url, "mimeType": nav_mime, "adFrameStatus": {"adFrameType": "none"}}, "type": "Navigation"}), session_id: es.clone() },
         CdpEvent { method: "Runtime.executionContextCreated".into(), params: json!({"context": {"id": 2, "origin": page_url, "name": "", "uniqueId": format!("ctx-nav-{}", page_id), "auxData": {"isDefault": true, "type": "default", "frameId": frame_id}}}), session_id: es.clone() },
     ];
+    // The default world is re-created as context id 2; re-register it. Isolated
+    // worlds register themselves via next_isolated_context in the loop below.
+    ctx.valid_context_ids.insert(2);
     let world_names: Vec<String> = if ctx.isolated_worlds.is_empty() {
         vec!["__puppeteer_utility_world__24.40.0".to_string()]
     } else {
diff --git a/crates/obscura-cdp/tests/execution_context_pruned_on_navigation.rs b/crates/obscura-cdp/tests/execution_context_pruned_on_navigation.rs
new file mode 100644
index 0000000..8762112
--- /dev/null
+++ b/crates/obscura-cdp/tests/execution_context_pruned_on_navigation.rs
@@ -0,0 +1,69 @@
+// Regression for issue #407: `CdpContext.valid_context_ids` was insert-only,
+// so `Runtime.executionContextsCleared` on navigation never pruned the old
+// context ids. A `Runtime.callFunctionOn` targeting a pre-navigation context
+// still ran (Chrome rejects it with "Cannot find context with specified id"),
+// and the set grew by one id per navigation on a long-lived connection.
+
+use obscura_cdp::dispatch::CdpContext;
+use obscura_cdp::domains::page::emit_navigation_events;
+use obscura_browser::lifecycle::WaitUntil;
+
+fn navigate(ctx: &mut CdpContext, page_id: &str) {
+    emit_navigation_events(
+        ctx,
+        &Some("session-1".to_string()),
+        "frame-1",
+        "loader-1",
+        "http://127.0.0.1/page",
+        page_id,
+        &[],
+        WaitUntil::Load,
+        true,
+    );
+}
+
+#[test]
+fn navigation_prunes_stale_execution_context_ids() {
+    let mut ctx = CdpContext::new();
+    // Seed a stale id that no navigation re-creates (simulates a context from
+    // a prior page state, or simply the pre-navigation main id 1).
+    ctx.valid_context_ids.insert(999);
+
+    navigate(&mut ctx, "page-1");
+
+    // The stale id is gone after executionContextsCleared; the default world
+    // (id 2) and the first isolated world (id 100, counter starts at 100) remain.
+    assert!(
+        !ctx.valid_context_ids.contains(&999),
+        "stale execution context id must be pruned on navigation"
+    );
+    assert!(ctx.valid_context_ids.contains(&2), "default world id 2 must be re-registered");
+    assert!(
+        ctx.valid_context_ids.contains(&100),
+        "isolated world id 100 must be registered: {:?}",
+        ctx.valid_context_ids
+    );
+
+    navigate(&mut ctx, "page-1");
+
+    // Second navigation: the first nav's isolated id (100) is now stale, and a
+    // fresh id (101) takes its place. The set must not grow across navigations.
+    assert!(
+        !ctx.valid_context_ids.contains(&100),
+        "previous navigation's isolated context id must be pruned"
+    );
+    assert!(ctx.valid_context_ids.contains(&101), "fresh isolated id 101 must be registered");
+    assert!(ctx.valid_context_ids.contains(&2), "default world id 2 must survive navigation");
+
+    // Unbounded-growth check: two navigations leave exactly the default world
+    // plus the current isolated world(s), not an accumulating union.
+    let count_after_two_navs = ctx.valid_context_ids.len();
+    navigate(&mut ctx, "page-1");
+    navigate(&mut ctx, "page-1");
+    assert_eq!(
+        ctx.valid_context_ids.len(),
+        count_after_two_navs,
+        "valid_context_ids must not grow across navigations (was {count_after_two_navs}, now {})",
+        ctx.valid_context_ids.len()
+    );
+}