diff --git a/src/relay/client-request-aborts.ts b/src/relay/client-request-aborts.ts
index 558dd9e4fb7..c896fb446b4 100644
--- a/src/relay/client-request-aborts.ts
+++ b/src/relay/client-request-aborts.ts
@@ -11,7 +11,12 @@ export class ClientRequestAborts {
   // keys are scattered through the map, so any correct loop still visits every entry, which made a
   // full churn of N clients cost K*N*(N+1)/2 visits. Only an index makes a teardown proportional to
   // what that client actually owns.
-  private readonly byClient = new Map<number, Map<number, AbortController>>()
+  //
+  // Why the inner key is a string: the codec only checks `jsonrpc === '2.0'`, so a request `id` can
+  // arrive as `"7"` while `rpc.cancel` coerces its `id` through `Number(...)` and looks up `7`. The
+  // flat map's template key folded both onto `"7"`; keying the raw value would file them in
+  // different buckets and silently drop the cancel. `String(...)` is the template literal's coercion.
+  private readonly byClient = new Map<number, Map<string, AbortController>>()
 
   create(
     clientId: number,
@@ -20,15 +25,15 @@ export class ClientRequestAborts {
     const controller = new AbortController()
     let requests = this.byClient.get(clientId)
     if (!requests) {
-      requests = new Map<number, AbortController>()
+      requests = new Map<string, AbortController>()
       this.byClient.set(clientId, requests)
     }
-    requests.set(requestId, controller)
+    requests.set(String(requestId), controller)
     return { key: { clientId, requestId }, controller }
   }
 
   get(clientId: number, requestId: number): AbortController | undefined {
-    return this.byClient.get(clientId)?.get(requestId)
+    return this.byClient.get(clientId)?.get(String(requestId))
   }
 
   delete(key: ClientRequestAbortHandle): void {
@@ -36,7 +41,7 @@ export class ClientRequestAborts {
     if (!requests) {
       return
     }
-    requests.delete(key.requestId)
+    requests.delete(String(key.requestId))
     // Why drop the empty bucket: otherwise a churned client leaves an entry behind for the life of
     // the relay, which is the retention the index exists to avoid.
     if (requests.size === 0) {
diff --git a/src/relay/dispatcher-per-connection-state-baseline.test.ts b/src/relay/dispatcher-per-connection-state-baseline.test.ts
index 9eb7b19bdef..70d5b325b71 100644
--- a/src/relay/dispatcher-per-connection-state-baseline.test.ts
+++ b/src/relay/dispatcher-per-connection-state-baseline.test.ts
@@ -13,7 +13,7 @@ type Probed = {
   requestHandlers: Map<string, unknown>
   notificationHandlers: Map<string, unknown>
   requestAborts: {
-    byClient: Map<number, Map<number, AbortController>>
+    byClient: Map<number, Map<string, AbortController>>
     create: (clientId: number, requestId: number) => unknown
   }
   publicationLedger: { clientBytes: Map<string, number>; aggregateBytes: number }
diff --git a/src/relay/dispatcher-rpc-cancel-id-coercion.test.ts b/src/relay/dispatcher-rpc-cancel-id-coercion.test.ts
new file mode 100644
index 00000000000..9a015ea10c5
--- /dev/null
+++ b/src/relay/dispatcher-rpc-cancel-id-coercion.test.ts
@@ -0,0 +1,43 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { RelayDispatcher } from './dispatcher'
+import { encodeFrame, MessageType } from './protocol'
+
+// Why both id shapes: parseJsonRpcMessage only checks the version, so a request id may arrive as a
+// string, while rpc.cancel coerces its id through Number(...). The abort index must file both
+// under one key or the cancel for a string-id request is silently dropped.
+describe('rpc.cancel request-id coercion', () => {
+  let dispatcher: RelayDispatcher
+
+  beforeEach(() => {
+    vi.useFakeTimers()
+    dispatcher = new RelayDispatcher(() => {})
+  })
+
+  afterEach(() => {
+    dispatcher.dispose()
+    vi.useRealTimers()
+  })
+
+  it.each([
+    { label: 'numeric', requestId: 7, cancelId: 7 },
+    { label: 'string', requestId: '7', cancelId: '7' },
+    { label: 'string request, numeric cancel', requestId: '7', cancelId: 7 }
+  ])('aborts an in-flight request with a $label id', async ({ requestId, cancelId }) => {
+    let signal: AbortSignal | undefined
+    dispatcher.onRequest('test.slow', (_params, ctx) => {
+      signal = ctx.signal
+      return new Promise(() => {})
+    })
+    // Raw frames: the typed encoder would not admit a string id, and that is the point.
+    const rawFrame = (msg: Record<string, unknown>, seq: number): Buffer =>
+      encodeFrame(MessageType.Regular, seq, 0, Buffer.from(JSON.stringify(msg), 'utf-8'))
+
+    dispatcher.feed(rawFrame({ jsonrpc: '2.0', id: requestId, method: 'test.slow' }, 1))
+    await vi.advanceTimersByTimeAsync(0)
+    expect(signal?.aborted).toBe(false)
+
+    dispatcher.feed(rawFrame({ jsonrpc: '2.0', method: 'rpc.cancel', params: { id: cancelId } }, 2))
+
+    expect(signal?.aborted).toBe(true)
+  })
+})
diff --git a/src/relay/relay-hot-path-operation-counts.test.ts b/src/relay/relay-hot-path-operation-counts.test.ts
index 77a3875bda1..c864f058cc5 100644
--- a/src/relay/relay-hot-path-operation-counts.test.ts
+++ b/src/relay/relay-hot-path-operation-counts.test.ts
@@ -114,7 +114,7 @@ describe('relay hot-path operation counts', () => {
       }
     }
     // oxlint-disable-next-line typescript/consistent-type-assertions -- SAFETY: byClient is the private index this test exists to measure; the shape mirrors its declaration in client-request-aborts.ts.
-    const byClient = (aborts as unknown as { byClient: Map<number, Map<number, AbortController>> })
+    const byClient = (aborts as unknown as { byClient: Map<number, Map<string, AbortController>> })
       .byClient
 
     // The census must be able to find things: prove the maps really hold 160 controllers across 40
@@ -126,11 +126,11 @@ describe('relay hot-path operation counts', () => {
     }
     expect(totalControllers).toBe(clientCount * inFlightPerClient)
 
-    const index = new CountingMap<number, Map<number, AbortController>>()
+    const index = new CountingMap<number, Map<string, AbortController>>()
     for (const [k, v] of byClient) {
       index.set(k, v)
     }
-    const targetBucket = new CountingMap<number, AbortController>()
+    const targetBucket = new CountingMap<string, AbortController>()
     for (const [k, v] of byClient.get(1)!) {
       targetBucket.set(k, v)
     }