diff --git a/.changeset/uppercase-tag-filters.md b/.changeset/uppercase-tag-filters.md
new file mode 100644
index 00000000..2d024281
--- /dev/null
+++ b/.changeset/uppercase-tag-filters.md
@@ -0,0 +1,5 @@
+---
+"nostream": patch
+---
+
+Support uppercase tag filters (#A-Z) in filter schema validation
diff --git a/CONFIGURATION.md b/CONFIGURATION.md
index 9698e4b1..df63db91 100644
--- a/CONFIGURATION.md
+++ b/CONFIGURATION.md
@@ -87,7 +87,7 @@ The schema ships with a small, query-driven set of indexes. The most important o
 | `events_active_pubkey_kind_created_at_idx`   | `REQ` with `authors`+`kinds` ordered by `created_at DESC, event_id ASC`; `hasActiveRequestToVanish`; by-pubkey deletes. Composite key `(event_pubkey, event_kind, event_created_at DESC, event_id)` so the ORDER BY tie-breaker is satisfied from the index without a sort step. |
 | `events_deleted_at_partial_idx`              | Retention purge over soft-deleted rows. Partial on `deleted_at IS NOT NULL`.                             |
 | `invoices_pending_created_at_idx`            | `findPendingInvoices` poll (`ORDER BY created_at ASC`). Partial on `status = 'pending'`.                  |
-| `event_tags (tag_name, tag_value)`           | NIP-01 generic tag filters (`#e`, `#p`, …) via the normalized `event_tags` table.                         |
+| `event_tags (tag_name, tag_value)`           | NIP-01 generic tag filters (`#e`, `#p`, `#K`, `#I`, …) via the normalized `event_tags` table. Both lowercase and uppercase single-letter tag filters are supported. |
 | `events_event_created_at_index`              | Time-range scans (`since` / `until`).                                                                    |
 | `events_event_kind_index`                    | Kind-only filters and purge kind-whitelist logic.                                                        |
 
@@ -108,6 +108,17 @@ npm run db:verify-index-impact
 
 The hot-path index migration (`20260420_120000_add_hot_path_indexes.js`) uses `CREATE INDEX CONCURRENTLY`, so it can be applied to a running relay without taking `ACCESS EXCLUSIVE` locks on the `events` or `invoices` tables.
 
+## Tag filter scope
+
+Subscription filters support single-letter tag filters using the `#<letter>` key syntax (NIP-01). Both lowercase (`#a`–`#z`) and uppercase (`#A`–`#Z`) variants are accepted.
+
+| Scope | Examples | Usage |
+|-------|---------|-------|
+| Lowercase (`#a`–`#z`) | `#e`, `#p`, `#a`, `#k` | Standard NIP-01 tag queries; parent-level references in NIP-22 comment threading |
+| Uppercase (`#A`–`#Z`) | `#E`, `#P`, `#A`, `#K`, `#I` | Root-level references in NIP-22 comment threading and other NIPs that use uppercase to distinguish root vs. parent scope |
+
+**NIP-22 comment threading (kind 1111):** NIP-22 comment events use lowercase tags (`#e`, `#a`, `#i`, `#k`) to reference the immediate parent and uppercase tags (`#E`, `#A`, `#I`, `#K`) to reference the root item. Filters must therefore accept both cases to allow clients to query the full comment thread hierarchy. For example, to find all comments on a root event: `{"kinds":[1111],"#E":["<root-event-id>"]}`, or to find comments of a specific root kind: `{"kinds":[1111],"#K":["1"]}`.
+
 # Settings
 
 Running `nostream` for the first time creates the settings file in `<project_root>/.nostr/settings.yaml`. If the file is not created and an error is thrown ensure that the `<project_root>/.nostr` folder exists. The configuration directory can be changed by setting the `NOSTR_CONFIG_DIR` environment variable. `nostream` will pick up any changes to this settings file without needing to restart.
diff --git a/src/schemas/filter-schema.ts b/src/schemas/filter-schema.ts
index dc1baa27..e27817c4 100644
--- a/src/schemas/filter-schema.ts
+++ b/src/schemas/filter-schema.ts
@@ -1,6 +1,7 @@
 import { z } from 'zod'
 
 import { createdAtSchema, kindSchema, prefixSchema } from './base-schema'
+import { isGenericTagQuery } from '../utils/filter'
 
 const knownFilterKeys = new Set(['ids', 'authors', 'kinds', 'since', 'until', 'limit'])
 
@@ -16,7 +17,7 @@ export const filterSchema = z
   .catchall(z.array(z.string().min(1).max(1024)))
   .superRefine((data, ctx) => {
     for (const key of Object.keys(data)) {
-      if (!knownFilterKeys.has(key) && !/^#[a-z]$/.test(key)) {
+      if (!knownFilterKeys.has(key) && !isGenericTagQuery(key)) {
         ctx.addIssue({
           code: z.ZodIssueCode.custom,
           message: `Unknown key: ${key}`,
diff --git a/test/unit/schemas/filter-schema.spec.ts b/test/unit/schemas/filter-schema.spec.ts
index 4840e102..92683751 100644
--- a/test/unit/schemas/filter-schema.spec.ts
+++ b/test/unit/schemas/filter-schema.spec.ts
@@ -22,6 +22,32 @@ describe('NIP-01', () => {
       }
     })
 
+    it('accepts NIP-22 comment threading filters for kind 1111', () => {
+      const nip22Filter = {
+        kinds: [1111],
+        '#E': ['aaaa'],
+        '#K': ['1'],
+        '#I': ['identifier1'],
+        '#A': ['10000:pubkey:dtag'],
+      }
+      const result = validateSchema(filterSchema)(nip22Filter)
+      expect(result.error).to.be.undefined
+      expect(result.value).to.deep.equal(nip22Filter)
+    })
+
+    it('accepts uppercase tag filters (#A-Z)', () => {
+      const filterWithUppercase = {
+        ...filter,
+        '#I': ['identifier1', 'identifier2'],
+        '#K': ['1111'],
+        '#E': ['aa', 'bb'],
+        '#A': ['10000:pubkey:dtag'],
+      }
+      const result = validateSchema(filterSchema)(filterWithUppercase)
+      expect(result.error).to.be.undefined
+      expect(result.value).to.deep.equal(filterWithUppercase)
+    })
+
     it('returns same filter if filter is valid', () => {
       const result = validateSchema(filterSchema)(filter)