fix(hono): resolve "ReadableStream is locked" error in AI SDK

.changeset/salty-apes-find.md

@@ -0,0 +1,13 @@
+---
+"evlog": patch
+---
+
+fix(hono): resolve "ReadableStream is locked" error with AI SDK streaming responses
+
+Using `createUIMessageStreamResponse` or `createAgentUIStreamResponse` from the Vercel AI SDK inside a Hono route would throw `ERR_INVALID_STATE: ReadableStream is locked` when running under `@hono/node-server`.
+
+**Root cause:** The middleware called `createObservedBody(c.res.body)` (which calls `body.getReader()`, locking the stream) and then relied on Hono's `compose` to update `c.res` with the wrapped response via the middleware return value. However, Hono skips that update when `context.finalized` is already `true` — which is always the case after a route handler returns a `Response`. This left `c.res` pointing at the original response whose body was now locked, so `@hono/node-server`'s subsequent `response.body.getReader()` call threw.
+
+**Fix:** Explicitly assign `c.res = await finishResponse(c.res, ...)` instead of returning the wrapped response, so `c.res` is always updated regardless of `context.finalized`.
+
+Closes #382

packages/evlog/src/hono/index.ts

@@ -63,11 +63,11 @@ export function evlog(options: EvlogHonoOptions = {}): MiddlewareHandler {
     try {
       await next()
       if (shouldDeferEmitForResponse(c.res)) {
-        const response = new Response(c.res.body, {
-          status: c.res.status,
-          headers: c.res.headers,
-        })
-        return finishResponse(response, { status: response.status })
+        // Assign directly — Hono's compose ignores middleware return values when
+        // context.finalized is already true, so returning the wrapped response
+        // would leave c.res with a locked body stream.
+        c.res = await finishResponse(c.res, { status: c.res.status })
+        return
       }
       await finish({ status: c.res.status })
     } catch (error) {

packages/evlog/test/frameworks/hono.test.ts

@@ -13,6 +13,7 @@ import {
 } from '../helpers/framework'
 import { defined, getDrainCallArg } from '../helpers/defined'
 import { describeStandardHttpMatrix } from '../helpers/frameworkMatrix'
+import { createDeferredStream } from '../helpers/stream'
 
 describeStandardHttpMatrix({
   name: 'hono',
@@ -326,4 +327,66 @@ describe('evlog/hono', () => {
       expect(enrich).not.toHaveBeenCalled()
     })
   })
+
+  describe('streaming responses', () => {
+    it('does not lock the response body when the handler returns a streaming SSE response (#382)', async () => {
+      const { drain } = createPipelineSpies()
+      let closeStream!: () => void
+
+      const app = new Hono<EvlogVariables>()
+      app.use(evlog({ drain }))
+      app.get('/api/stream', () => {
+        const { stream, close } = createDeferredStream()
+        closeStream = close
+        return new Response(stream, {
+          headers: { 'content-type': 'text/event-stream' },
+        })
+      })
+
+      const res = await app.request('/api/stream')
+      expect(res.status).toBe(200)
+
+      // Body must not be locked — @hono/node-server calls body.getReader() to
+      // stream to the client after app.fetch() resolves.
+      expect(res.body).not.toBeNull()
+      expect(res.body?.locked).toBe(false)
+
+      closeStream()
+      await res.text()
+      await waitForDrainCalls(drain)
+      assertHttpEventEmitted(drain, { path: '/api/stream', status: 200 })
+    })
+
+    it('defers drain until the SSE stream closes and captures mid-stream context (#321)', async () => {
+      const { drain } = createPipelineSpies()
+      const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {})
+      let closeStream!: () => void
+
+      const app = new Hono<EvlogVariables>()
+      app.use(evlog({ drain }))
+      app.get('/api/chat', (c) => {
+        const log = c.get('log')
+        const { stream, close } = createDeferredStream()
+        closeStream = close
+        queueMicrotask(() => {
+          log.set({ ai: { calls: 1, totalTokens: 42 } })
+        })
+        return new Response(stream, {
+          headers: { 'content-type': 'text/event-stream' },
+        })
+      })
+
+      const res = await app.request('/api/chat')
+      expect(drain).not.toHaveBeenCalled()
+
+      closeStream()
+      await expect(res.text()).resolves.toBe('hello world')
+      await vi.waitFor(() => {
+        expect(drain).toHaveBeenCalledTimes(1)
+      })
+
+      expect(warnSpy.mock.calls.some(([message]) => String(message).includes('Keys dropped: ai'))).toBe(false)
+      expect(drain.mock.calls[0]?.[0]?.event?.ai).toEqual({ calls: 1, totalTokens: 42 })
+    })
+  })
 })