diff --git a/cloud/apps/relay/src/relay-server.ts b/cloud/apps/relay/src/relay-server.ts
index a14240cfa6a..32a83962d81 100644
--- a/cloud/apps/relay/src/relay-server.ts
+++ b/cloud/apps/relay/src/relay-server.ts
@@ -31,6 +31,16 @@ import { createRelayTokenVerifier, readBearer } from './relay-token-verifier.js'
 import { closeRelayWebSocket } from './relay-websocket-close.js'
 import { ProcessQueuedByteBudget } from './splice-forwarder.js'
 
+// A malformed percent-escape in the request target must be a client error, never a URIError
+// thrown out of the `upgrade` listener (which is uncaught and kills the process).
+function decodePathSegment(value: string): string | null {
+  try {
+    return decodeURIComponent(value)
+  } catch {
+    return null
+  }
+}
+
 function rejectUpgrade(socket: NodeJS.WritableStream, status: number, message: string): void {
   socket.write(`HTTP/1.1 ${status} ${message}\r\nConnection: close\r\nContent-Length: 0\r\n\r\n`)
   if ('destroy' in socket && typeof socket.destroy === 'function') socket.destroy()
@@ -278,8 +288,8 @@ export function createRelayServer(
       return
     }
     if (url.pathname.startsWith('/v1/connect/')) {
-      const hostId = decodeURIComponent(url.pathname.slice('/v1/connect/'.length))
-      if (!/^[A-Za-z0-9_-]{16}$/.test(hostId)) {
+      const hostId = decodePathSegment(url.pathname.slice('/v1/connect/'.length))
+      if (hostId === null || !/^[A-Za-z0-9_-]{16}$/.test(hostId)) {
         rejectUpgrade(socket, 429, 'Too Many Requests')
         return
       }
@@ -373,7 +383,7 @@ export function createRelayServer(
         rejectUpgrade(socket, 404, 'Not Found')
         return
       }
-      const connId = decodeURIComponent(url.pathname.slice('/v1/host/data/'.length))
+      const connId = decodePathSegment(url.pathname.slice('/v1/host/data/'.length))
       if (!connId || connId.length > 128) {
         rejectUpgrade(socket, 429, 'Too Many Requests')
         return
diff --git a/cloud/apps/relay/src/relay-upgrade-malformed-uri.blackbox.test.ts b/cloud/apps/relay/src/relay-upgrade-malformed-uri.blackbox.test.ts
new file mode 100644
index 00000000000..24635a68e6f
--- /dev/null
+++ b/cloud/apps/relay/src/relay-upgrade-malformed-uri.blackbox.test.ts
@@ -0,0 +1,122 @@
+import { connect, createServer as createNetServer } from 'node:net'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import type { RelayConfig } from './config.js'
+import type { RelayDatabase } from './database.js'
+import { createRelayServer } from './relay-server.js'
+
+async function unusedPort(): Promise<number> {
+  const server = createNetServer()
+  await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+  const address = server.address()
+  if (!address || typeof address === 'string') throw new Error('missing test port')
+  await new Promise<void>((resolve) => server.close(() => resolve()))
+  return address.port
+}
+
+function rawUpgrade(port: number, target: string): Promise<{ status: string; closed: boolean }> {
+  return new Promise((resolve, reject) => {
+    const socket = connect(port, '127.0.0.1')
+    let data = ''
+    socket.once('connect', () => {
+      socket.write(
+        `GET ${target} HTTP/1.1\r\nHost: 127.0.0.1\r\nConnection: Upgrade\r\n` +
+          'Upgrade: websocket\r\nSec-WebSocket-Version: 13\r\n' +
+          // RFC 6455 §1.3 example nonce; allowlisted in cloud/.gitleaks.toml.
+          'Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n\r\n'
+      )
+    })
+    socket.on('data', (chunk) => {
+      data += chunk.toString()
+    })
+    socket.once('close', () => resolve({ status: data.split('\r\n')[0] ?? '', closed: true }))
+    socket.once('error', reject)
+    setTimeout(() => {
+      socket.destroy()
+      resolve({ status: data.split('\r\n')[0] ?? '', closed: false })
+    }, 1_500).unref()
+  })
+}
+
+describe('relay upgrade with a malformed request target', () => {
+  const cleanup: Array<() => Promise<void> | void> = []
+
+  afterEach(async () => {
+    for (const close of cleanup.splice(0).reverse()) await close()
+    vi.restoreAllMocks()
+  })
+
+  it('rejects an undecodable /v1/connect path without an uncaught exception', async () => {
+    const port = await unusedPort()
+    const relayUrl = `http://127.0.0.1:${port}`
+    const database: RelayDatabase = {
+      query: vi.fn(async () => []),
+      queryLocked: vi.fn(async () => []),
+      transaction: vi.fn(async (operation) => await operation(database)),
+      close: vi.fn(async () => undefined)
+    }
+    const config = {
+      port,
+      publicUrl: relayUrl,
+      cellUrl: relayUrl,
+      authIssuer: 'https://auth.example.com',
+      authAudience: 'orca-relay',
+      jwksUrl: 'https://auth.example.com/jwks',
+      assignmentSigningKey: new Uint8Array(32),
+      role: 'cell',
+      cellId: 'production-gce-c3',
+      cells: [{ id: 'production-gce-c3', url: relayUrl, capacityRequests: 4_000 }],
+      adminAudience: `${relayUrl}/admin`,
+      deployServiceAccount: 'deploy@example.com',
+      runtimeServiceAccount: 'runtime@example.com',
+      connectionHardCap: 600,
+      connectionUnobservedBound: 60,
+      adminJwksUrl: 'https://auth.example.com/admin-jwks',
+      databasePoolMax: 10,
+      publicAssignmentsEnabled: true,
+      publicAssignmentConcurrency: 2,
+      publicAssignmentQueueMax: 128,
+      publicAssignmentWaitMs: 4_000,
+      publicResolveConcurrency: 1,
+      publicResolveWaitMs: 5_000,
+      publicAssignmentRetryAfterSeconds: 5,
+      dataDir: './test-data'
+    } satisfies RelayConfig
+    const relay = createRelayServer(config, database, {
+      connectionLedgerLimits: { hardCap: 5, controlReserve: 1 }
+    })
+    relay.server.listen(port, '127.0.0.1')
+    await new Promise<void>((resolve) => relay.server.once('listening', resolve))
+    cleanup.push(() => new Promise<void>((resolve) => relay.server.close(() => resolve())))
+    vi.spyOn(console, 'log').mockImplementation(() => undefined)
+    vi.spyOn(console, 'warn').mockImplementation(() => undefined)
+
+    // Vitest installs its own uncaughtException listener; capture ours first so the test reports
+    // the exception as a verdict instead of dying with it.
+    const uncaught: unknown[] = []
+    const onUncaught = (error: unknown): void => {
+      uncaught.push(error)
+    }
+    process.prependListener('uncaughtException', onUncaught)
+    cleanup.push(() => {
+      process.off('uncaughtException', onUncaught)
+    })
+
+    const results = []
+    for (const target of [
+      '/v1/connect/%',
+      '/v1/connect/%E0%A4%A',
+      '/v1/connect/%C0%AF',
+      '/v1/host/data/%'
+    ]) {
+      results.push(await rawUpgrade(port, target))
+    }
+    // A malformed percent-escape must be a client error, never a process-level throw.
+    expect(uncaught).toEqual([])
+    for (const result of results) {
+      expect(result.status).toMatch(/^HTTP\/1\.1 4\d\d/)
+    }
+    // The server must still serve a well-formed upgrade afterwards.
+    const after = await rawUpgrade(port, '/v1/connect/abcdefghijklmnop')
+    expect(after.status).toMatch(/^HTTP\/1\.1 101/)
+  })
+})