diff --git a/examples/line-item-audio.yaml b/examples/line-item-audio.yaml
index 4ca2f21..28b49c5 100644
--- a/examples/line-item-audio.yaml
+++ b/examples/line-item-audio.yaml
@@ -21,6 +21,8 @@ pricing:
 
 commitment_bounds: { min_budget: 5000.00 }
 commitment:
+  proposal_id: "examplepub-us-q4-2026-omni"
+  version: 1
   basis: spend
   amount: 60000.00
   currency: USD
diff --git a/examples/line-item-ctv.yaml b/examples/line-item-ctv.yaml
index fda3994..7cc3d82 100644
--- a/examples/line-item-ctv.yaml
+++ b/examples/line-item-ctv.yaml
@@ -17,6 +17,8 @@ pricing:
 
 commitment_bounds: { min_budget: 25000.00 }
 commitment:
+  proposal_id: "examplepub-us-q4-2026-omni"
+  version: 1
   basis: spend
   amount: 160000.00
   currency: USD
diff --git a/examples/line-item-display.yaml b/examples/line-item-display.yaml
index 808af24..929a346 100644
--- a/examples/line-item-display.yaml
+++ b/examples/line-item-display.yaml
@@ -62,6 +62,8 @@ pricing:
 commitment_bounds: { min_budget: 10000.00, max_budget: 500000.00 }
 
 commitment:
+  proposal_id:     "examplepub-us-q4-2026-omni"
+  version:         1
   basis:           spend
   amount:          80000.00
   currency:        USD
diff --git a/spec/CHANGELOG.md b/spec/CHANGELOG.md
index 1dfacef..1e649d5 100644
--- a/spec/CHANGELOG.md
+++ b/spec/CHANGELOG.md
@@ -5,6 +5,11 @@ issue that caused it, so the comment record and the spec history line up.
 
 ## [Unreleased]
 
+- § 4.1, § 5.6.1 — commitments carry `proposal_id` and `version`; a new
+  attempt on a stale version, or past `valid_until` or `price_valid_until`, is
+  rejected, and a replay returns the stored result. Breaking
+  ([#24](https://github.com/IABTechLab/OpenProposal/issues/24)).
+
 ## [3.0-draft-1] — 2026-09-22
 
 Initial draft published for public comment. Ported from the working-group
diff --git a/spec/openproposal-3.0.md b/spec/openproposal-3.0.md
index 6940958..62d37d5 100644
--- a/spec/openproposal-3.0.md
+++ b/spec/openproposal-3.0.md
@@ -181,7 +181,7 @@ items.
 | Field | Type | Mutability | Description |
 | :--- | :--- | :--- | :--- |
 | `proposal_id` | String | immutable | Unique identity for the proposal object |
-| `version` | Integer | derived | Monotonically increasing revision number |
+| `version` | Integer | derived | Monotonically increasing revision number. Each revision while status is `published` increments it |
 | `seller_id` | String | immutable | IAB Tech Lab registry reference |
 | `seller_name` | String | immutable | Canonical legal seller entity name, from the registry |
 | `type` | Enum | immutable | `standard` \| `custom` |
@@ -288,6 +288,8 @@ when status becomes `agreed`.
 
 | Key | Required | Meaning |
 | :--- | :--- | :--- |
+| `proposal_id` | REQUIRED | The proposal the commitment is made against |
+| `version` | REQUIRED | The proposal `version` the commitment was priced against |
 | `basis` | REQUIRED | `spend` \| `units` |
 | `amount` | REQUIRED | The figure committed |
 | `currency` | REQUIRED when `basis = spend` | ISO-4217 |
@@ -296,8 +298,22 @@ when status becomes `agreed`.
 | `derived_units` | derived | Computed from `amount`, `agreed_rate` and `cost_method`. Informative, not binding |
 | `idempotency_key` | REQUIRED | Buyer-generated, unique per commitment attempt |
 
-A request carrying an `idempotency_key` already seen MUST return the original
-result and MUST NOT create a second commitment.
+An `idempotency_key` is opaque and scoped to one buyer and seller pair. The
+seller MUST NOT interpret it.
+
+The seller MUST reject a new commitment attempt whose `version` is not the
+current version of the proposal named by `proposal_id`, whose proposal is past
+`valid_until`, or whose `pricing[].price_valid_until` has passed. The rejection
+MUST return the current version number, so the buyer can fetch it again. These
+checks MUST be deterministic, as for `idempotency_key`.
+
+For a replay from the same buyer of a commitment attempt the seller has
+already processed, with the same `idempotency_key` and the same commitment
+terms, the seller MUST return the original result and MUST NOT create a second
+commitment. The version and expiry checks apply to a new attempt, not to
+returning a stored result. Reusing an `idempotency_key` with different
+commitment terms MUST produce an explicit conflict and MUST NOT create or
+modify a commitment.
 
 #### 5.6.2 hold_status
 
@@ -369,7 +385,9 @@ buyer commitment, and `settable` bounds. See
 
 ### 6.3 CTV line item
 
-See [`examples/line-item-ctv.yaml`](../examples/line-item-ctv.yaml).
+Shows a buyer commitment naming the `proposal_id` and `version` it was priced
+against (§ 5.6.1). See
+[`examples/line-item-ctv.yaml`](../examples/line-item-ctv.yaml).
 
 ### 6.4 Audio line item