diff --git a/conformance/grammar/kp-pack.schema.json b/conformance/grammar/kp-pack.schema.json
index 85e3181..ad9b147 100644
--- a/conformance/grammar/kp-pack.schema.json
+++ b/conformance/grammar/kp-pack.schema.json
@@ -386,6 +386,11 @@
       "type": "object",
       "additionalProperties": true,
       "description": "AI note-taking metadata."
+    },
+    "extensions": {
+      "type": "object",
+      "additionalProperties": true,
+      "description": "Extension lane for experimental or implementation-specific manifest metadata. Consumers MUST ignore unrecognized extension content."
     }
   },
   "allOf": [
diff --git a/spec/CHANGELOG.md b/spec/CHANGELOG.md
index f6a3eef..9567fd4 100644
--- a/spec/CHANGELOG.md
+++ b/spec/CHANGELOG.md
@@ -5,6 +5,20 @@
 
 ---
 
+## v0.7.4 — 2026-04-16
+
+**Manifest extension lane — standardizes where experiments belong without widening the core schema.**
+
+### Added
+- **`extensions`** (PACK.yaml, optional object) — Explicit lane for experimental or implementation-specific manifest metadata. Root-level unknown fields remain invalid; experiments now belong under `extensions`.
+
+### Changed
+- **CORE.md** — Documents the manifest root as closed and defines the `extensions` object as the sanctioned compatibility lane.
+- **SPEC.md** — Adds example manifest usage and guidance that experimental fields such as `ai_brief` belong under `extensions`, not at the manifest root.
+- **Schema** — Adds `extensions` as an allowed top-level object while preserving `additionalProperties: false` at the manifest root.
+
+---
+
 ## v0.7.3 — 2026-04-12
 
 **Archive format — sealed, hashed, versioned single-file transport for Knowledge Packs.**
diff --git a/spec/CORE.md b/spec/CORE.md
index e31269a..6d9ba39 100644
--- a/spec/CORE.md
+++ b/spec/CORE.md
@@ -94,9 +94,28 @@ PACK.yaml is a YAML file declaring pack identity and configuration. The normativ
 | `tags` | array | Topical tags for discovery and classification. Strict kebab-case (`^[a-z0-9]+(-[a-z0-9]+)*$`), unique |
 | `views` | array | View declarations with `name`, `file`, `purpose`, `display_as` (all required), `hint` (optional). Voice views add `voice` (boolean), `duration` (string, e.g. `~90 seconds`), `pace` (enum: `brisk`, `measured`, `deliberate`). When `voice` is `true`, `duration` and `pace` are REQUIRED. |
 | `tier` | enum | `hub`, `detail`, `standalone` |
+| `extensions` | object | Extension lane for experimental or implementation-specific manifest metadata. Consumers ignore unknown extension content. |
 
 The full set of optional fields and conditional constraints is defined in the JSON Schema. Key conditionals: when `tier` is `hub`, `sub_packs` is REQUIRED; when `sensitivity` is `confidential` or `restricted`, `channels` MUST NOT contain `public` or `org`; when `sensitivity` is `internal`, `channels` MUST NOT contain `public`; when a view declares `voice: true`, `duration` and `pace` are REQUIRED on that view entry.
 
+### Manifest Extensions
+
+KP:1 keeps the manifest root closed: fields not defined by the schema are invalid. Experimental or implementation-specific metadata MUST live under the optional `extensions` object instead of appearing as new top-level keys.
+
+Consumers MUST ignore extension content they do not understand. Extension content MUST NOT redefine the semantics of core KP fields or relax core validation rules.
+
+Extension names and shapes are defined by their producers, not by KP:1. The `ai_brief` payload below is one such producer-defined example.
+
+Example:
+
+```yaml
+extensions:
+  ai_brief:
+    version: 1
+    verdict: acceptable
+    headline: "Strong base attribution, but provenance gap remains"
+```
+
 ### Example
 
 ```yaml
@@ -107,6 +126,11 @@ author: Jane Chen
 confidence:
   scale: sherman_kent
   normalize: true
+extensions:
+  ai_brief:
+    version: 1
+    verdict: acceptable
+    headline: "Strong base attribution, but provenance gap remains"
 ```
 
 ---
diff --git a/spec/SPEC.md b/spec/SPEC.md
index dc1f6be..e112904 100644
--- a/spec/SPEC.md
+++ b/spec/SPEC.md
@@ -317,6 +317,12 @@ notes:                                 # AI note-taking metadata (see spec/NOTES
   participants: []                     #   Meeting participants
   disclosed: false                     #   Was AI note-taking disclosed to participants?
   consent: null                        #   Required for mode: passive — obtained | declined | pending
+
+extensions:                            # Experimental / implementation-specific manifest metadata
+  ai_brief:                            #   Example extension payload (not core KP:1)
+    version: 1
+    verdict: acceptable
+    headline: "Strong base attribution, but provenance gap remains"
 ```
 
 ### Version Semantics
@@ -398,6 +404,14 @@ channels: [org, public]
 The spec defines the `channels` field and its vocabulary. How channels map to
 infrastructure is a deployment concern.
 
+### 3.2 Manifest Extensions
+
+The PACK.yaml root is intentionally closed. New or experimental manifest keys MUST NOT be added at the top level unless and until they are standardized by a future KP revision.
+
+Implementation-specific or experimental metadata MUST live under the optional `extensions` object. Consumers MUST ignore extension content they do not understand. Extension content MUST NOT override or redefine the meaning of core KP fields.
+
+The example payload shown earlier (`ai_brief`) is illustrative — extension names and shapes are defined by their producers, not by KP:1. This creates a narrow compatibility lane: tools can ship experiments without forcing a schema break or prematurely promoting the payload into the core standard.
+
 ---
 
 ## 4. claims.md — The Primary File