diff --git a/packages/core/src/session/model-transport.ts b/packages/core/src/session/model-transport.ts
index 2a052f41e86..398910c5ebc 100644
--- a/packages/core/src/session/model-transport.ts
+++ b/packages/core/src/session/model-transport.ts
@@ -50,6 +50,7 @@ interface Channel {
 interface State {
   readonly lock: Semaphore.Semaphore
   closed: boolean
+  httpFallback: boolean
   channel?: Channel
 }
 
@@ -119,7 +120,7 @@ export const makeLayer = (connector: WebSocketConnector) =>
       const state = (sessionID: SessionSchema.ID) => {
         const current = states.get(sessionID)
         if (current) return current
-        const created = { lock: Semaphore.makeUnsafe(1), closed: false }
+        const created = { lock: Semaphore.makeUnsafe(1), closed: false, httpFallback: false }
         states.set(sessionID, created)
         return created
       }
@@ -241,7 +242,9 @@ export const makeLayer = (connector: WebSocketConnector) =>
                           channel.active?.lifecycle.delivery === "terminal" ||
                           (error.reason._tag === "Transport" && error.reason.code === "queue-overflow")
                             ? "accepted"
-                            : "ambiguous",
+                            : error.reason._tag === "Transport" && error.reason.code === "1009"
+                              ? "rejected"
+                              : "ambiguous",
                       }),
                     ),
               ),
@@ -274,6 +277,7 @@ export const makeLayer = (connector: WebSocketConnector) =>
             phase: "queue",
             delivery: "not-sent",
           })
+        if (owner.httpFallback) return fallback(exchange)
         const key = affinity(exchange)
         const now = yield* Clock.currentTimeMillis
         const current = owner.channel
@@ -420,6 +424,26 @@ export const makeLayer = (connector: WebSocketConnector) =>
               yield* poison(owner, channel, error)
             }),
           ),
+          Stream.catch((error) => {
+            if (
+              error.reason._tag !== "Transport" ||
+              error.reason.code !== "1009" ||
+              error.reason.delivery !== "rejected"
+            )
+              return Stream.fail(error)
+            owner.httpFallback = true
+            return Stream.unwrap(
+              Effect.logWarning("session websocket request too large; using http", {
+                sessionTransport: "websocket",
+                phase: "close",
+                delivery: "rejected",
+                code: error.reason.code,
+              }).pipe(
+                Effect.andThen(metric("fallback", { reason: "message_too_large" })),
+                Effect.as(exchange.fallback()),
+              ),
+            )
+          }),
         )
         const complete = Effect.sync(() => {
           if (owner.channel !== channel || channel.pending?.token !== token) return
diff --git a/packages/core/test/session-model-transport.test.ts b/packages/core/test/session-model-transport.test.ts
index 2e2bc23ebf0..6e6b1fd3727 100644
--- a/packages/core/test/session-model-transport.test.ts
+++ b/packages/core/test/session-model-transport.test.ts
@@ -8,7 +8,7 @@ import type {
 } from "@opencode-ai/ai/route"
 import { SessionModelTransport } from "@opencode-ai/core/session/model-transport"
 import { Session } from "@opencode-ai/schema/session"
-import { Deferred, Effect, Fiber, Metric, Queue, Stream } from "effect"
+import { Cause, Deferred, Effect, Fiber, Metric, Queue, Stream } from "effect"
 import { TestClock } from "effect/testing"
 import { Headers } from "effect/unstable/http"
 
@@ -546,6 +546,63 @@ describe("SessionModelTransport", () => {
     )
   })
 
+  test("falls back to HTTP after close code 1009 and keeps the Session on HTTP", async () => {
+    const messages = queue<string | Uint8Array, AIError>()
+    let opened = 0
+    let fallbacks = 0
+    let closed = 0
+    const connector: WebSocketConnector = {
+      open: () =>
+        Effect.sync(() => {
+          opened++
+          return {
+            sendText: () =>
+              Effect.sync(() => {
+                Queue.failCauseUnsafe(
+                  messages,
+                  Cause.fail(
+                    new AIError({
+                      module: "test",
+                      method: "websocket",
+                      reason: new TransportReason({
+                        message: "message too big",
+                        transport: "websocket",
+                        operation: "read",
+                        code: "1009",
+                        phase: "close",
+                      }),
+                    }),
+                  ),
+                )
+              }),
+            messages: Stream.fromQueue(messages),
+            close: Effect.sync(() => closed++).pipe(Effect.andThen(Queue.shutdown(messages)), Effect.asVoid),
+          }
+        }),
+    }
+    const item = (id: string) =>
+      exchange(id, {
+        fallback: () => {
+          fallbacks++
+          return Stream.make(`http:${id}`)
+        },
+      })
+
+    await run(
+      connector,
+      Effect.gen(function* () {
+        const transport = yield* SessionModelTransport.Service
+        const executor = transport.bind(session)
+
+        expect(yield* collect(executor, item("first"))).toEqual(["http:first"])
+        expect(yield* collect(executor, item("second"))).toEqual(["http:second"])
+        expect(opened).toBe(1)
+        expect(fallbacks).toBe(2)
+        expect(closed).toBe(1)
+      }),
+    )
+  })
+
   test("does not fall back after an ambiguous send failure", async () => {
     const messages = queue<string | Uint8Array, AIError>()
     let fallbacks = 0