diff --git a/bun.lock b/bun.lock
index 657424dfef19..53a2ff1516f1 100644
--- a/bun.lock
+++ b/bun.lock
@@ -303,6 +303,7 @@
         "npm-package-arg": "13.0.2",
         "semver": "^7.6.3",
         "turndown": "7.2.0",
+        "undici": "8.3.0",
         "venice-ai-sdk-provider": "2.0.2",
         "which": "6.0.1",
         "xdg-basedir": "5.1.0",
@@ -590,6 +591,7 @@
         "tree-sitter-powershell": "0.25.10",
         "turndown": "7.2.0",
         "ulid": "catalog:",
+        "undici": "8.3.0",
         "venice-ai-sdk-provider": "2.0.2",
         "vscode-jsonrpc": "8.2.1",
         "web-tree-sitter": "0.25.10",
@@ -666,6 +668,7 @@
       "version": "1.17.9",
       "dependencies": {
         "cross-spawn": "catalog:",
+        "undici": "8.3.0",
       },
       "devDependencies": {
         "@hey-api/openapi-ts": "0.90.10",
diff --git a/packages/core/package.json b/packages/core/package.json
index 2aec5ecc7e50..a3a4dc8b5d2b 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -118,6 +118,7 @@
     "npm-package-arg": "13.0.2",
     "semver": "^7.6.3",
     "turndown": "7.2.0",
+    "undici": "8.3.0",
     "venice-ai-sdk-provider": "2.0.2",
     "which": "6.0.1",
     "xdg-basedir": "5.1.0",
diff --git a/packages/core/src/aisdk.ts b/packages/core/src/aisdk.ts
index 9ea79394f34a..6639d0502d4e 100644
--- a/packages/core/src/aisdk.ts
+++ b/packages/core/src/aisdk.ts
@@ -5,6 +5,7 @@ import { Cause, Context, Effect, Layer, Schema, Scope } from "effect"
 import { ModelV2 } from "./model"
 import { ProviderV2 } from "./provider"
 import { State } from "./state"
+import { createUndiciDispatcher } from "./util/undici-dispatcher"
 
 type SDK = any
 
@@ -81,6 +82,7 @@ function prepareOptions(model: ModelV2.Info, pkg: string) {
   const customFetch = options.fetch
   const chunkTimeout = options.chunkTimeout
   delete options.chunkTimeout
+  const dispatcher = typeof customFetch === "function" ? undefined : createUndiciDispatcher(options.timeout)
   options.fetch = async (input: Parameters<typeof fetch>[0], init?: RequestInit) => {
     const opts = { ...(init ?? {}) }
     const signals = [
@@ -111,8 +113,9 @@ function prepareOptions(model: ModelV2.Info, pkg: string) {
 
     const res = await (typeof customFetch === "function" ? customFetch : fetch)(input, {
       ...opts,
+      ...(dispatcher ? { dispatcher } : {}),
       timeout: false,
-    })
+    } as RequestInit)
     if (!chunkAbortCtl || typeof chunkTimeout !== "number") return res
     return wrapSSE(res, chunkTimeout, chunkAbortCtl)
   }
diff --git a/packages/core/src/util/undici-dispatcher.ts b/packages/core/src/util/undici-dispatcher.ts
new file mode 100644
index 000000000000..9b6eb219fe95
--- /dev/null
+++ b/packages/core/src/util/undici-dispatcher.ts
@@ -0,0 +1,30 @@
+import { Agent } from "undici"
+
+// Node's undici fetch defaults both headersTimeout and bodyTimeout to 300s.
+// This kills long-running provider requests — including SSE streams with gaps
+// >300s between chunks — at the ~303s mark regardless of provider.timeout config.
+//
+// resolveTimeoutMs maps the provider `timeout` option to a millisecond value:
+//   false      → 0 (disabled)
+//   <number> >0 → that value (ms)
+//   anything else → undefined (no dispatcher, undici defaults apply)
+//
+// createUndiciDispatcher wraps resolveTimeoutMs with a Node-only guard and
+// constructs the Agent. Bun's fetch has no such defaults, so the guard via
+// process.versions.bun keeps Bun behavior unchanged.
+//
+// This does not conflict with the existing chunkTimeout / headerTimeout abort
+// signals: those use AbortSignal/AbortController which are independent of the
+// undici dispatcher's socket-level timeouts.
+export function resolveTimeoutMs(timeout: unknown): number | undefined {
+  if (timeout === false) return 0
+  if (typeof timeout === "number" && timeout > 0) return timeout
+  return undefined
+}
+
+export function createUndiciDispatcher(timeout: unknown) {
+  if (typeof process !== "object" || (process.versions as Record<string, string | undefined>).bun) return undefined
+  const ms = resolveTimeoutMs(timeout)
+  if (ms === undefined) return undefined
+  return new Agent({ headersTimeout: ms, bodyTimeout: ms })
+}
diff --git a/packages/core/test/util/undici-dispatcher.test.ts b/packages/core/test/util/undici-dispatcher.test.ts
new file mode 100644
index 000000000000..1efaa2d45ab8
--- /dev/null
+++ b/packages/core/test/util/undici-dispatcher.test.ts
@@ -0,0 +1,75 @@
+import { describe, expect, test } from "bun:test"
+import { Agent } from "undici"
+import { resolveTimeoutMs, createUndiciDispatcher } from "@opencode-ai/core/util/undici-dispatcher"
+
+describe("resolveTimeoutMs", () => {
+  test("false → 0 (disabled)", () => {
+    expect(resolveTimeoutMs(false)).toBe(0)
+  })
+
+  test("positive number → that value in ms", () => {
+    expect(resolveTimeoutMs(600_000)).toBe(600_000)
+  })
+
+  test("undefined → undefined (undici defaults apply)", () => {
+    expect(resolveTimeoutMs(undefined)).toBeUndefined()
+  })
+
+  test("0 → undefined (not a positive number)", () => {
+    expect(resolveTimeoutMs(0)).toBeUndefined()
+  })
+
+  test("negative number → undefined", () => {
+    expect(resolveTimeoutMs(-1)).toBeUndefined()
+  })
+
+  test("null → undefined", () => {
+    expect(resolveTimeoutMs(null)).toBeUndefined()
+  })
+
+  test("string → undefined", () => {
+    expect(resolveTimeoutMs("300000")).toBeUndefined()
+  })
+})
+
+describe("createUndiciDispatcher", () => {
+  test("Bun guard: returns undefined when process.versions.bun is set", () => {
+    const original = (process.versions as Record<string, string | undefined>).bun
+    ;(process.versions as Record<string, string | undefined>).bun = "1.0.0"
+    try {
+      expect(createUndiciDispatcher(600_000)).toBeUndefined()
+      expect(createUndiciDispatcher(false)).toBeUndefined()
+    } finally {
+      if (original === undefined) delete (process.versions as Record<string, string | undefined>).bun
+      else (process.versions as Record<string, string | undefined>).bun = original
+    }
+  })
+
+  test("returns undefined when no bun version and timeout resolves to undefined", () => {
+    const original = (process.versions as Record<string, string | undefined>).bun
+    ;(process.versions as Record<string, string | undefined>).bun = undefined
+    try {
+      expect(createUndiciDispatcher(undefined)).toBeUndefined()
+      expect(createUndiciDispatcher(0)).toBeUndefined()
+    } finally {
+      if (original === undefined) delete (process.versions as Record<string, string | undefined>).bun
+      else (process.versions as Record<string, string | undefined>).bun = original
+    }
+  })
+
+  // Under Bun, createUndiciDispatcher always returns undefined because of the
+  // guard. The Agent construction path only runs under Node. We verify it
+  // returns an Agent instance (not private fields) when the bun guard is
+  // removed and a valid timeout is provided.
+  test("returns an Agent instance under Node (bun guard bypassed)", () => {
+    const original = (process.versions as Record<string, string | undefined>).bun
+    ;(process.versions as Record<string, string | undefined>).bun = undefined
+    try {
+      const agent = createUndiciDispatcher(600_000)
+      expect(agent).toBeInstanceOf(Agent)
+    } finally {
+      if (original === undefined) delete (process.versions as Record<string, string | undefined>).bun
+      else (process.versions as Record<string, string | undefined>).bun = original
+    }
+  })
+})
diff --git a/packages/opencode/package.json b/packages/opencode/package.json
index 139217e348a4..4354042337ff 100644
--- a/packages/opencode/package.json
+++ b/packages/opencode/package.json
@@ -141,6 +141,7 @@
     "tree-sitter-powershell": "0.25.10",
     "turndown": "7.2.0",
     "ulid": "catalog:",
+    "undici": "8.3.0",
     "venice-ai-sdk-provider": "2.0.2",
     "vscode-jsonrpc": "8.2.1",
     "web-tree-sitter": "0.25.10",
diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index d7c85f2ee6ae..bfb604abc30b 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -7,6 +7,7 @@ import { mapValues, mergeDeep, omit, pickBy, sortBy } from "remeda"
 import { NoSuchModelError, type Provider as SDK } from "ai"
 import { Npm } from "@opencode-ai/core/npm"
 import { Hash } from "@opencode-ai/core/util/hash"
+import { createUndiciDispatcher } from "@opencode-ai/core/util/undici-dispatcher"
 import { Plugin } from "../plugin"
 import { serviceUse } from "@opencode-ai/core/effect/service-use"
 import { type LanguageModelV3 } from "@ai-sdk/provider"
@@ -1688,6 +1689,7 @@ export const layer = Layer.effect(
         const headerTimeout = options["headerTimeout"]
         delete options["chunkTimeout"]
         delete options["headerTimeout"]
+        const dispatcher = customFetch ? undefined : createUndiciDispatcher(options["timeout"])
 
         options["fetch"] = async (input: any, init?: BunFetchRequestInit) => {
           const fetchFn = customFetch ?? fetch
@@ -1708,9 +1710,10 @@ export const layer = Layer.effect(
 
           const res = await fetchFn(input, {
             ...opts,
+            ...(dispatcher ? { dispatcher } : {}),
             // @ts-ignore see here: https://github.com/oven-sh/bun/issues/16682
             timeout: false,
-          }).finally(() => headerTimeoutCtl?.clear())
+          } as BunFetchRequestInit).finally(() => headerTimeoutCtl?.clear())
 
           if (!chunkAbortCtl) return res
           return wrapSSE(res, chunkTimeout, chunkAbortCtl)
diff --git a/packages/sdk/js/package.json b/packages/sdk/js/package.json
index 78643b6a1ef1..65625ec96bbd 100644
--- a/packages/sdk/js/package.json
+++ b/packages/sdk/js/package.json
@@ -30,6 +30,7 @@
     "typescript": "catalog:"
   },
   "dependencies": {
-    "cross-spawn": "catalog:"
+    "cross-spawn": "catalog:",
+    "undici": "8.3.0"
   }
 }
diff --git a/packages/sdk/js/src/client.ts b/packages/sdk/js/src/client.ts
index 5cf071e7b7ab..928cfb4b1807 100644
--- a/packages/sdk/js/src/client.ts
+++ b/packages/sdk/js/src/client.ts
@@ -4,6 +4,7 @@ import { createClient } from "./gen/client/client.gen.js"
 import { type Config } from "./gen/client/types.gen.js"
 import { OpencodeClient } from "./gen/sdk.gen.js"
 import { wrapClientError } from "./error-interceptor.js"
+import { nodeFetchWithDispatcher } from "./node-fetch.js"
 export { type Config as OpencodeClientConfig, OpencodeClient }
 
 function pick(value: string | null, fallback?: string) {
@@ -32,11 +33,7 @@ function rewrite(request: Request, directory?: string) {
 
 export function createOpencodeClient(config?: Config & { directory?: string }) {
   if (!config?.fetch) {
-    const customFetch: any = (req: any) => {
-      // @ts-ignore
-      req.timeout = false
-      return fetch(req)
-    }
+    const customFetch: any = (req: any) => nodeFetchWithDispatcher(req)
     config = {
       ...config,
       fetch: customFetch,
diff --git a/packages/sdk/js/src/node-fetch.ts b/packages/sdk/js/src/node-fetch.ts
new file mode 100644
index 000000000000..2e2b0a848f6d
--- /dev/null
+++ b/packages/sdk/js/src/node-fetch.ts
@@ -0,0 +1,24 @@
+// undici's fetch defaults headersTimeout and bodyTimeout to 300s, which kills
+// long-running sessions at the ~303s mark. The SDK client→server connection
+// (blocking POST /session/:id/message) needs both disabled so the request can
+// survive past 5 minutes. Bun is unaffected — its fetch has no such defaults.
+//
+// undici is a hard dependency, but we still import it lazily inside the guard
+// so browser bundlers don't pull undici (and its node:* deps) into the bundle
+// when the SDK is loaded in a non-Node environment.
+//
+// The promise is cached so concurrent first calls share a single Agent —
+// without this, two parallel calls could each create and race to assign an
+// Agent, leaving the loser orphaned (never closed).
+type UndiciAgent = InstanceType<typeof import("undici").Agent>
+
+const nodeDispatcher: Promise<UndiciAgent | null> =
+  typeof process === "undefined" || !process.versions || process.versions.bun
+    ? Promise.resolve(null)
+    : import("undici").then(({ Agent }) => new Agent({ headersTimeout: 0, bodyTimeout: 0 }))
+
+export async function nodeFetchWithDispatcher(req: Request) {
+  const dispatcher = await nodeDispatcher
+  if (!dispatcher) return fetch(req)
+  return fetch(req, { dispatcher } as RequestInit)
+}
diff --git a/packages/sdk/js/src/v2/client.ts b/packages/sdk/js/src/v2/client.ts
index c1956cffe037..f8c422f6794d 100644
--- a/packages/sdk/js/src/v2/client.ts
+++ b/packages/sdk/js/src/v2/client.ts
@@ -5,6 +5,7 @@ import { createClient } from "./gen/client/client.gen.js"
 import { type Config } from "./gen/client/types.gen.js"
 import { OpencodeClient } from "./gen/sdk.gen.js"
 import { wrapClientError } from "../error-interceptor.js"
+import { nodeFetchWithDispatcher } from "../node-fetch.js"
 export { type Config as OpencodeClientConfig, OpencodeClient }
 
 function pick(value: string | null, fallback?: string, encode?: (value: string) => string) {
@@ -49,11 +50,7 @@ function rewrite(request: Request, values: { directory?: string; workspace?: str
 
 export function createOpencodeClient(config?: Config & { directory?: string; experimental_workspaceID?: string }) {
   if (!config?.fetch) {
-    const customFetch: any = (req: any) => {
-      // @ts-ignore
-      req.timeout = false
-      return fetch(req)
-    }
+    const customFetch: any = (req: any) => nodeFetchWithDispatcher(req)
     config = {
       ...config,
       fetch: customFetch,