diff --git a/bun.lock b/bun.lock
index 51ebb3f1069..1fe141c9046 100644
--- a/bun.lock
+++ b/bun.lock
@@ -249,6 +249,7 @@
         "jose": "6.0.11",
         "jsonc-parser": "3.3.1",
         "mime-types": "3.0.2",
+        "undici": "8.12.3",
         "which": "6.0.1",
         "zod": "catalog:",
       },
diff --git a/packages/core/package.json b/packages/core/package.json
index 07ab612afba..939756c8873 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -64,6 +64,12 @@
       "node": "./src/image/photon-wasm.node.ts",
       "default": "./src/image/photon-wasm.bun.ts"
     },
+    "#client-tls": {
+      "workerd": "./src/effect/client-tls/client-tls.workerd.ts",
+      "bun": "./src/effect/client-tls/client-tls.bun.ts",
+      "node": "./src/effect/client-tls/client-tls.node.ts",
+      "default": "./src/effect/client-tls/client-tls.bun.ts"
+    },
     "#v1-migration": {
       "types": "./src/database/v1-migration.bun.ts",
       "bun": "./src/database/v1-migration.bun.ts",
@@ -129,6 +135,7 @@
     "jose": "6.0.11",
     "jsonc-parser": "3.3.1",
     "mime-types": "3.0.2",
+    "undici": "8.12.3",
     "which": "6.0.1",
     "zod": "catalog:"
   },
diff --git a/packages/core/src/aisdk.ts b/packages/core/src/aisdk.ts
index 4058f513f13..ba9299f689c 100644
--- a/packages/core/src/aisdk.ts
+++ b/packages/core/src/aisdk.ts
@@ -3,6 +3,7 @@ export * as AISDK from "./aisdk.js"
 import { makeLocationNode } from "@opencode/util/effect/app-node"
 import { httpFetch } from "@opencode/util/effect/app-node-platform"
 import { APICallError } from "@ai-sdk/provider"
+import { ClientTls } from "#client-tls"
 import type {
   JSONSchema7,
   JSONValue,
@@ -161,9 +162,12 @@ function prepareOptions(model: RuntimeInfo, pkg: string, outbound: Fetch) {
       }
     }
 
+    const https = String(input instanceof Request ? input.url : input).startsWith("https:")
+    const certified = tls && https ? ClientTls.sender(tls) : undefined
+    if (tls && https && !certified) throw new Error("Client certificates aren't supported on this runtime")
     // A followed redirect keeps the TLS options, so it would present the client certificate to another host.
-    if (tls) Object.assign(opts, { tls, redirect: "error" })
-    const send: Fetch = isFetch(customFetch) ? customFetch : outbound
+    if (certified) Object.assign(opts, certified.init, { redirect: "error" })
+    const send: Fetch = certified ? certified.fetch : isFetch(customFetch) ? customFetch : outbound
     const middleware = httpMiddleware.getStore()
     const res = await (
       middleware
diff --git a/packages/core/src/effect/app-node-platform.ts b/packages/core/src/effect/app-node-platform.ts
index ae4afdf5288..d4de27864ef 100644
--- a/packages/core/src/effect/app-node-platform.ts
+++ b/packages/core/src/effect/app-node-platform.ts
@@ -1,9 +1,10 @@
 import { LLMClient, RequestExecutor } from "@opencode/ai/route"
 import { Effect, Layer, Option } from "effect"
-import { FetchHttpClient, HttpClient } from "effect/http"
+import { FetchHttpClient, HttpClient, HttpClientError } from "effect/http"
 import { Socket } from "effect/socket"
 import { makeGlobalNode } from "@opencode/util/effect/app-node"
 import { httpClient } from "@opencode/util/effect/app-node-platform"
+import { ClientTls } from "#client-tls"
 import { WebSocketConstructor } from "./websocket-constructor.js"
 
 // Bun's fetch ends a request after five quiet minutes, before provider routes' own header and chunk timeouts.
@@ -18,8 +19,8 @@ const withoutBunTimeout = Layer.effect(
   }),
 )
 
-// Presents the model's client certificate (provider `settings.tls`) on its HTTPS requests. The TLS handshake only
-// sends it to servers that request one.
+// Presents the model's client certificate (provider `settings.tls`) on its HTTPS requests, through this runtime's
+// certificate-capable fetch. The TLS handshake only sends it to servers that request one.
 const withClientCertificate = Layer.effect(
   HttpClient.HttpClient,
   Effect.gen(function* () {
@@ -28,18 +29,25 @@ const withClientCertificate = Layer.effect(
       Effect.gen(function* () {
         const tls = yield* RequestExecutor.CurrentTls
         if (!tls || !request.url.startsWith("https:")) return yield* effect
-        const init: BunFetchRequestInit = Option.getOrElse(
-          yield* Effect.serviceOption(FetchHttpClient.RequestInit),
-          () => ({}),
-        )
-        return yield* effect.pipe(
+        const sender = ClientTls.sender(tls)
+        if (!sender)
+          return yield* new HttpClientError.HttpClientError({
+            reason: new HttpClientError.TransportError({
+              request,
+              description: "Client certificates aren't supported on this runtime",
+            }),
+          })
+        const init: RequestInit = Option.getOrElse(yield* Effect.serviceOption(FetchHttpClient.RequestInit), () => ({}))
+        return yield* HttpClient.execute(request).pipe(
+          Effect.provide(FetchHttpClient.layer),
+          Effect.provideService(FetchHttpClient.Fetch, sender.fetch),
           Effect.provideService(FetchHttpClient.RequestInit, {
             ...init,
-            tls: { ...init.tls, ...tls },
+            ...sender.init,
             // A followed redirect keeps the TLS options, so it would present the certificate to whatever host the
             // server names.
             redirect: init.redirect === "manual" ? "manual" : "error",
-          } as BunFetchRequestInit),
+          }),
         )
       }),
     )
diff --git a/packages/core/src/effect/client-tls/client-tls.bun.ts b/packages/core/src/effect/client-tls/client-tls.bun.ts
new file mode 100644
index 00000000000..1ab10156444
--- /dev/null
+++ b/packages/core/src/effect/client-tls/client-tls.bun.ts
@@ -0,0 +1,8 @@
+import type { Sender, Tls } from "./client-tls.js"
+
+// Bun's fetch takes the client certificate per request.
+export function sender(tls: Tls): Sender | undefined {
+  return { fetch: globalThis.fetch, init: { tls } }
+}
+
+export * as ClientTls from "./client-tls.bun.js"
diff --git a/packages/core/src/effect/client-tls/client-tls.node.ts b/packages/core/src/effect/client-tls/client-tls.node.ts
new file mode 100644
index 00000000000..f4ea00d0345
--- /dev/null
+++ b/packages/core/src/effect/client-tls/client-tls.node.ts
@@ -0,0 +1,17 @@
+// The deep path keeps Vite, which runs under Bun where "undici" is a built-in, from leaving the import external.
+import { Agent, fetch } from "undici/index.js"
+import type { Sender, Tls } from "./client-tls.js"
+
+// Node's fetch only takes a client certificate through an undici dispatcher, and a dispatcher from the npm package
+// only works with that package's own fetch, not Node's bundled one. One agent per certificate keeps its connections.
+const agents = new Map<string, Agent>()
+
+export function sender(tls: Tls): Sender | undefined {
+  const id = JSON.stringify([tls.cert, tls.key, tls.passphrase])
+  const agent = agents.get(id) ?? new Agent({ connect: { cert: tls.cert, key: tls.key, passphrase: tls.passphrase } })
+  agents.set(id, agent)
+  // SAFETY: undici's fetch implements the WHATWG fetch contract that FetchHttpClient and the AI SDK call.
+  return { fetch: fetch as unknown as typeof globalThis.fetch, init: { dispatcher: agent } }
+}
+
+export * as ClientTls from "./client-tls.node.js"
diff --git a/packages/core/src/effect/client-tls/client-tls.ts b/packages/core/src/effect/client-tls/client-tls.ts
new file mode 100644
index 00000000000..ac62d4680df
--- /dev/null
+++ b/packages/core/src/effect/client-tls/client-tls.ts
@@ -0,0 +1,12 @@
+/** PEM client certificate and private key, with the passphrase of an encrypted key. */
+export interface Tls {
+  readonly cert: string
+  readonly key: string
+  readonly passphrase?: string
+}
+
+/** The `fetch` and request options that present a client certificate on this runtime. */
+export interface Sender {
+  readonly fetch: typeof globalThis.fetch
+  readonly init: Record<string, unknown>
+}
diff --git a/packages/core/src/effect/client-tls/client-tls.workerd.ts b/packages/core/src/effect/client-tls/client-tls.workerd.ts
new file mode 100644
index 00000000000..f1adea63fdb
--- /dev/null
+++ b/packages/core/src/effect/client-tls/client-tls.workerd.ts
@@ -0,0 +1,8 @@
+import type { Sender, Tls } from "./client-tls.js"
+
+// Workers' fetch can't present a client certificate. Embedders use an mTLS certificate binding's fetch instead.
+export function sender(_tls: Tls): Sender | undefined {
+  return undefined
+}
+
+export * as ClientTls from "./client-tls.workerd.js"
diff --git a/services/site/src/docs/content/network.mdx b/services/site/src/docs/content/network.mdx
index a1756a470c9..202cf616a8d 100644
--- a/services/site/src/docs/content/network.mdx
+++ b/services/site/src/docs/content/network.mdx
@@ -117,6 +117,7 @@ TLS handshake only sends it to servers that ask for one.
 
 - `cert` and `key` are PEM text. Use `{file:...}` to read them from files. A file that can't be read leaves `tls` unset.
 - `passphrase` is only needed for an encrypted key.
+- Works on the Bun and Node builds. A server on Cloudflare Workers can't present a client certificate through `fetch`.
 - Requests with a client certificate don't follow redirects, so the certificate isn't presented to another host.
 
 Every model request of that provider presents the certificate: chat, compaction, image and decision models. A provider