diff --git a/index.bs b/index.bs
index 7538ea9b28..d1b8c6fdf7 100644
--- a/index.bs
+++ b/index.bs
@@ -849,6 +849,8 @@ An attribution trigger is a [=struct=] with the following items:
 :: An [=aggregation coordinator=].
 : <dfn>aggregatable source registration time configuration</dfn>
 :: An [=aggregatable source registration time configuration=].
+: <dfn>trigger context ID</dfn>
+:: Null or a [=string=].
 
 </dl>
 
@@ -925,6 +927,8 @@ An aggregatable report is an [=attribution report=] with the following additiona
 :: An [=aggregatable source registration time configuration=].
 : <dfn>is null report</dfn> (default false)
 :: A [=boolean=].
+: <dfn>trigger context ID</dfn>
+:: Null or a [=string=].
 
 </dl>
 
@@ -1172,6 +1176,10 @@ Its value is «[=source type/navigation=] → 8, [=source type/event=] → 2».
 controls the maximum [=map/size=] of a [=trigger spec map=] for an
 [=attribution source=]. Its value is 32.
 
+<dfn>Max length per trigger context ID</dfn> is a positive integer that controls
+the maximum [=string/length=] of an [=attribution trigger=]'s [=attribution trigger/trigger context ID=].
+Its value is 64.
+
 # Vendor-Specific Values # {#vendor-specific-values}
 
 <dfn>Max pending sources per source origin</dfn> is a positive integer that
@@ -2134,6 +2142,8 @@ a [=trigger state=] |triggerState|:
     :: «»
     : [=attribution trigger/aggregatable source registration time configuration=]
     :: "<code>[=aggregatable source registration time configuration/exclude=]</code>"
+    : [=attribution trigger/trigger context ID=]
+    :: null
 1. Let |fakeReport| be the result of running [=obtain an event-level report=] with |source|,
     |fakeTrigger|, and |fakeConfig|.
 1. [=Assert=]: |fakeReport| is not null.
@@ -2425,6 +2435,13 @@ and a [=moment=] |triggerTime|:
     1. If |value|["`aggregatable_source_registration_time`"] is not an [=aggregatable source registration time configuration=],
         return null.
     1. Set |aggregatableSourceRegTimeConfig| to |value|["`aggregatable_source_registration_time`"].
+1. Let |triggerContextID| be null.
+1. If |value|["`trigger_context_id`"] [=map/exists=]:
+    1. If |value|["`trigger_context_id`"] is not a [=string=], return null.
+    1. If |value|["`trigger_context_id`"]'s [=string/length=] is 0 or is greater than the [=max length per trigger context ID=],
+        return null.
+    1. If |aggregatableSourceRegTimeConfig| is not "<code>[=aggregatable source registration time configuration/exclude=]</code>", return null.
+    1. Set |triggerContextID| to |value|["`trigger_context_id`"].
 1. Let |trigger| be a new [=attribution trigger=] with the items:
     : [=attribution trigger/attribution destination=]
     :: |destination|
@@ -2454,6 +2471,8 @@ and a [=moment=] |triggerTime|:
     :: |aggregationCoordinator|
     : [=attribution trigger/aggregatable source registration time configuration=]
     :: |aggregatableSourceRegTimeConfig|
+    : [=attribution trigger/trigger context ID=]
+    :: |triggerContextID|
 1. Return |trigger|.
 
 Issue: Determine proper charset-handling for the JSON header value.
@@ -3036,9 +3055,11 @@ To <dfn>obtain an event-level report delivery time</dfn> given an [=attribution
         |window|'s [=report window/end=].
 1. [=Assert=]: not reached.
 
-To <dfn>obtain an aggregatable report delivery time</dfn> given a [=moment=]
-|triggerTime|, perform the following steps. They return a [=moment=].
+To <dfn>obtain an aggregatable report delivery time</dfn> given an [=attribution trigger=]
+|trigger|, perform the following steps. They return a [=moment=].
 
+1. Let |triggerTime| be |trigger|'s [=attribution trigger/trigger time=].
+1. If |trigger|'s [=attribution trigger/trigger context ID=] is not null, return |triggerTime|.
 1. Let |r| be a random double between 0 (inclusive) and 1 (exclusive) with uniform probability.
 1. Return |triggerTime| + |r| * [=randomized aggregatable report delay=].
 
@@ -3097,7 +3118,7 @@ required aggregatable budget</dfn> is the total [=aggregatable contribution/valu
 To <dfn>obtain an aggregatable report</dfn> given an [=attribution source=] |source| and
 an [=attribution trigger=] |trigger|:
 
-1. Let |reportTime| be the result of running [=obtain an aggregatable report delivery time=] with |trigger|'s [=attribution trigger/trigger time=].
+1. Let |reportTime| be the result of running [=obtain an aggregatable report delivery time=] with |trigger|.
 1. Let |report| be a new [=aggregatable report=] struct whose items are:
 
     : [=aggregatable report/reporting origin=]
@@ -3122,14 +3143,15 @@ an [=attribution trigger=] |trigger|:
     :: |trigger|'s [=attribution trigger/aggregation coordinator=].
     : [=aggregatable report/source registration time configuration=]
     :: |trigger|'s [=attribution trigger/aggregatable source registration time configuration=].
+    : [=aggregatable report/trigger context ID=]
+    :: |trigger|'s [=attribution trigger/trigger context ID=]
 1. Return |report|.
 
 <h3 id="generating-randomized-null-reports">Generating randomized null reports</h3>
 
 To <dfn>obtain a null report</dfn> given an [=attribution trigger=] |trigger| and a [=moment=] |sourceTime|:
 
-1. Let |reportTime| be the result of running [=obtain an aggregatable report delivery time=] with |trigger|'s
-    [=attribution trigger/trigger time=].
+1. Let |reportTime| be the result of running [=obtain an aggregatable report delivery time=] with |trigger|.
 1. Let |report| be a new [=aggregatable report=] struct whose items are:
 
     : [=aggregatable report/reporting origin=]
@@ -3156,6 +3178,8 @@ To <dfn>obtain a null report</dfn> given an [=attribution trigger=] |trigger| an
     :: |trigger|'s [=attribution trigger/aggregatable source registration time configuration=]
     : [=aggregatable report/is null report=]
     :: true
+    : [=aggregatable report/trigger context ID=]
+    :: |trigger|'s [=attribution trigger/trigger context ID=]
 1. Return |report|.
 
 To <dfn>obtain rounded source time</dfn> given a [=moment=] |sourceTime|, return |sourceTime| in seconds
@@ -3172,13 +3196,17 @@ To <dfn>generate null reports</dfn> given an [=attribution trigger=] |trigger| a
 
 1. Let |nullReports| be a new [=list/is empty|empty=] [=list=].
 1. If |trigger|'s [=attribution trigger/aggregatable source registration time configuration=] is "<code>[=aggregatable source registration time configuration/exclude=]</code>":
+    1. Let |randomizedNullReportRate| be [=randomized null report rate excluding source registration time=].
+    1. If |trigger|'s [=attribution trigger/trigger context ID=] is not null, set
+        |randomizedNullReportRate| to 1.
     1. If |report| is null and the result of [=determining if a randomized null report is generated=] with
-        [=randomized null report rate excluding source registration time=] is true:
+        |randomizedNullReportRate| is true:
         1. Let |nullReport| be the result of [=obtaining a null report=] with |trigger| and |trigger|'s
             [=attribution trigger/trigger time=].
         1. [=set/Append=] |nullReport| to the [=aggregatable report cache=].
         1. [=list/Append=] |nullReport| to |nullReports|.
 1. Otherwise:
+    1. [=Assert=]: |trigger|'s [=attribution trigger/trigger context ID=] is null.
     1. Let |maxSourceExpiry| be [=valid source expiry range=][1].
     1. Round |maxSourceExpiry| away from zero to the nearest day (86400 seconds).
     1. Let |roundedAttributedSourceTime| be null.
@@ -3448,6 +3476,8 @@ To <dfn>serialize an [=aggregatable report=] </dfn> |report|, run the following
 1. If |report|'s [=aggregatable report/trigger debug key=] is not null, [=map/set=]
     |data|["`trigger_debug_key`"] to |report|'s [=aggregatable report/trigger debug key=],
     [=serialize an integer|serialized=].
+1. If |report|'s [=aggregatable report/trigger context ID=] is not null, [=map/set=]
+    |data|["`trigger_context_id`"] to |report|'s [=aggregatable report/trigger context ID=].
 1. Return the [=byte sequence=] resulting from executing [=serialize an infra value to JSON bytes=] on |data|.
 
 To <dfn>serialize an [=attribution report=]</dfn> |report|, run the following steps:
diff --git a/ts/src/constants.ts b/ts/src/constants.ts
index c38935c009..868324c078 100644
--- a/ts/src/constants.ts
+++ b/ts/src/constants.ts
@@ -17,6 +17,8 @@ export const maxAggregationKeysPerSource: number = 20
 
 export const maxLengthPerAggregationKeyIdentifier: number = 25
 
+export const maxLengthPerTriggerContextID: number = 64
+
 export const minReportWindow: number = 1 * SECONDS_PER_HOUR
 
 export const validSourceExpiryRange: Readonly<[min: number, max: number]> = [
diff --git a/ts/src/header-validator/trigger.test.ts b/ts/src/header-validator/trigger.test.ts
index c8e8c8f6ca..2ddaa253f9 100644
--- a/ts/src/header-validator/trigger.test.ts
+++ b/ts/src/header-validator/trigger.test.ts
@@ -62,6 +62,7 @@ const testCases: jsontest.TestCase<Trigger>[] = [
       aggregatableSourceRegistrationTime:
         AggregatableSourceRegistrationTime.include,
       aggregationCoordinatorOrigin: null,
+      triggerContextID: null,
       aggregatableTriggerData: [
         {
           positive: [
@@ -966,6 +967,61 @@ const testCases: jsontest.TestCase<Trigger>[] = [
     ],
   },
 
+  {
+    name: 'trigger-context-id-wrong-type',
+    json: `{"trigger_context_id": 1}`,
+    expectedErrors: [
+      {
+        path: ['trigger_context_id'],
+        msg: 'must be a string',
+      },
+    ],
+  },
+  {
+    name: 'trigger-context-id-empty',
+    json: `{"trigger_context_id": ""}`,
+    expectedErrors: [
+      {
+        path: ['trigger_context_id'],
+        msg: 'cannot be empty',
+      },
+    ],
+  },
+  {
+    name: 'trigger-context-id-too-long',
+    json: `{"trigger_context_id": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}`,
+    expectedErrors: [
+      {
+        path: ['trigger_context_id'],
+        msg: 'exceeds max length per trigger context ID (65 > 64)',
+      },
+    ],
+  },
+  {
+    name: 'trigger-context-id-invalid-aggregatable-source-registration-time',
+    json: `{"trigger_context_id": "a", "aggregatable_source_registration_time": 1}`,
+    expectedErrors: [
+      {
+        path: ['aggregatable_source_registration_time'],
+        msg: 'must be a string',
+      },
+      {
+        path: ['trigger_context_id'],
+        msg: 'cannot be fully validated without a valid aggregatable_source_registration_time',
+      },
+    ],
+  },
+  {
+    name: 'trigger-context-id-prohibited-aggregatable-source-registration-time-include',
+    json: `{"aggregatable_source_registration_time": "include", "trigger_context_id": "123"}`,
+    expectedErrors: [
+      {
+        path: ['trigger_context_id'],
+        msg: 'is prohibited for aggregatable_source_registration_time include',
+      },
+    ],
+  },
+
   // Full Flex
 
   {
diff --git a/ts/src/header-validator/validate-json.ts b/ts/src/header-validator/validate-json.ts
index e3d22ec4de..83d97550f6 100644
--- a/ts/src/header-validator/validate-json.ts
+++ b/ts/src/header-validator/validate-json.ts
@@ -1252,6 +1252,41 @@ function warnInconsistentAggregatableKeys(ctx: Context, t: Trigger): void {
   })
 }
 
+function triggerContextID(
+  ctx: Context,
+  j: Json,
+  aggregatableSourceRegTime: Maybe<AggregatableSourceRegistrationTime>
+): Maybe<string> {
+  return string(ctx, j).filter((s) => {
+    if (s.length == 0) {
+      ctx.error(`cannot be empty`)
+      return false
+    }
+    if (s.length > constants.maxLengthPerTriggerContextID) {
+      ctx.error(
+        `exceeds max length per trigger context ID (${s.length} > ${constants.maxLengthPerTriggerContextID})`
+      )
+      return false
+    }
+    if (aggregatableSourceRegTime.value === undefined) {
+      ctx.error(
+        `cannot be fully validated without a valid aggregatable_source_registration_time`
+      )
+      return false
+    }
+    if (
+      aggregatableSourceRegTime.value !==
+      AggregatableSourceRegistrationTime.exclude
+    ) {
+      ctx.error(
+        `is prohibited for aggregatable_source_registration_time ${aggregatableSourceRegTime.value}`
+      )
+      return false
+    }
+    return true
+  })
+}
+
 export type Trigger = CommonDebug &
   FilterPair & {
     aggregatableDedupKeys: AggregatableDedupKey[]
@@ -1260,39 +1295,51 @@ export type Trigger = CommonDebug &
     aggregatableValues: Map<string, number>
     aggregationCoordinatorOrigin: string | null
     eventTriggerData: EventTriggerDatum[]
+    triggerContextID: string | null
   }
 
 function trigger(ctx: Context, j: Json): Maybe<Trigger> {
-  return struct(ctx, j, {
-    aggregatableTriggerData: field(
-      'aggregatable_trigger_data',
-      aggregatableTriggerData,
-      []
-    ),
-    aggregatableValues: field(
-      'aggregatable_values',
-      aggregatableValues,
-      new Map()
-    ),
-    aggregatableDedupKeys: field(
-      'aggregatable_deduplication_keys',
-      aggregatableDedupKeys,
-      []
-    ),
-    aggregatableSourceRegistrationTime: field(
-      'aggregatable_source_registration_time',
-      aggregatableSourceRegistrationTime,
-      AggregatableSourceRegistrationTime.exclude
-    ),
-    aggregationCoordinatorOrigin: field(
-      'aggregation_coordinator_origin',
-      suitableOrigin,
-      null
-    ),
-    eventTriggerData: field('event_trigger_data', eventTriggerData, []),
-    ...commonDebugFields,
-    ...filterFields,
-  }).peek((t) => warnInconsistentAggregatableKeys(ctx, t))
+  return object(ctx, j)
+    .map((j) => {
+      const aggregatableSourceRegTimeVal = field(
+        'aggregatable_source_registration_time',
+        aggregatableSourceRegistrationTime,
+        AggregatableSourceRegistrationTime.exclude
+      )(ctx, j)
+
+      return struct(ctx, j, {
+        aggregatableTriggerData: field(
+          'aggregatable_trigger_data',
+          aggregatableTriggerData,
+          []
+        ),
+        aggregatableValues: field(
+          'aggregatable_values',
+          aggregatableValues,
+          new Map()
+        ),
+        aggregatableDedupKeys: field(
+          'aggregatable_deduplication_keys',
+          aggregatableDedupKeys,
+          []
+        ),
+        aggregatableSourceRegistrationTime: () => aggregatableSourceRegTimeVal,
+        aggregationCoordinatorOrigin: field(
+          'aggregation_coordinator_origin',
+          suitableOrigin,
+          null
+        ),
+        eventTriggerData: field('event_trigger_data', eventTriggerData, []),
+        triggerContextID: field(
+          'trigger_context_id',
+          (ctx, j) => triggerContextID(ctx, j, aggregatableSourceRegTimeVal),
+          null
+        ),
+        ...commonDebugFields,
+        ...filterFields,
+      })
+    })
+    .peek((t) => warnInconsistentAggregatableKeys(ctx, t))
 }
 
 function validateJSON<T>(