diff --git a/.ci/tinygo-net-roundtrip.patch b/.ci/tinygo-net-roundtrip.patch
new file mode 100644
index 0000000..65db49f
--- /dev/null
+++ b/.ci/tinygo-net-roundtrip.patch
@@ -0,0 +1,36 @@
+Temporary CI patch: fix the TinyGo net/http js/wasm build on Go 1.25/1.26.
+
+TinyGo <= 0.41.1 vendors github.com/tinygo-org/net at commit e54965e, whose
+http/roundtrip_js.go calls the private t.roundTrip fallback that no longer
+exists in Go 1.25+/1.26 net/http, so any wasm build importing net/http fails:
+
+    net/http/roundtrip_js.go: t.roundTrip undefined (type *Transport has no
+    field or method roundTrip, but does have method RoundTrip)
+
+This is the exact upstream fix tinygo-org/net@1026408a ("http: fix t.roundTrip
+undefined on js/wasm builds", whose parent is e54965e -- the commit TinyGo
+0.41.1 ships), applied to the installed TinyGo source in CI before the wasm
+build. The .patch is applied by the "Patch TinyGo net/http (temporary)" step
+in .github/workflows/ci.yml.
+
+TODO: remove this file AND that CI step once a TinyGo release > 0.41.1 ships
+the fix. It is already on TinyGo's dev branch (which pins tinygo-org/net@
+1026408a). Tracking: issue #26; upstream tinygo-org/tinygo#5467.
+
+diff --git a/src/net/http/roundtrip_js.go b/src/net/http/roundtrip_js.go
+--- a/src/net/http/roundtrip_js.go
++++ b/src/net/http/roundtrip_js.go
+@@ -68,9 +68,11 @@
+ 	// to fall back on the Fetch API, unless it's not available.
+ 
+ 	// TINYGO: Dial/DialTLS & DialContext/DialTLSContext are not present in tinygo Transport struct, therefore the
+-	// corresponding if statements were removed
++	// corresponding if statements were removed.
++	// TINYGO: t.roundTrip (the private fallback used by upstream Go) is not present in the TinyGo stub
++	// Transport, so return an error when the Fetch API is unavailable instead of calling it.
+ 	if jsFetchMissing || jsFetchDisabled {
+-		return t.roundTrip(req)
++		return nil, errors.New("net/http: Fetch API is not available and no fallback transport is implemented for js/wasm")
+ 	}
+ 
+ 	ac := js.Global().Get("AbortController")
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..a518a5f
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,4 @@
+# Keep CI-critical text files LF on every platform. .ci/*.patch is applied with
+# `git apply` against TinyGo's Unix (LF) source in CI, so it must never be
+# checked out with CRLF (which would make the patch fail to apply).
+*.patch text eol=lf
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6dbab4c..d43f1c5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -38,9 +38,10 @@ jobs:
         with:
           go-version: '1.26'
 
-      # TinyGo is needed only for the Wasm Worker build (pnpm run build). The
-      # 0.41.x line is the first with Go 1.26 support; install-binaryen (default
-      # true) provides wasm-opt, which TinyGo invokes for the -target wasm build.
+      # TinyGo is needed only for the Wasm Worker build (pnpm run build).
+      # install-binaryen (default true) provides wasm-opt, which TinyGo invokes
+      # for the -target wasm build. 0.41.1's vendored net/http js overlay is
+      # patched just before the build (see "Patch TinyGo net/http" below).
       - name: Set up TinyGo
         uses: acifani/setup-tinygo@dd8a7075d951a7595b2ef2123ed0ab1af0c13e56 # v3.0.0
         with:
@@ -79,6 +80,25 @@ jobs:
             echo "infra/go.mod not present; skipping (no-op until ticket #2)"
           fi
 
+      # TEMPORARY (tracking #26; tinygo-org/tinygo#5467): TinyGo 0.41.1 and
+      # earlier vendor tinygo-org/net@e54965e, whose net/http js/wasm overlay
+      # (roundtrip_js.go) calls the private t.roundTrip fallback that no longer
+      # exists in Go 1.25+/1.26 net/http, so the wasm build fails to compile.
+      # Apply the exact upstream fix (tinygo-org/net@1026408a) to the installed
+      # TinyGo source. git apply exits non-zero (failing the job loudly) if the
+      # source has drifted, so we notice when TinyGo changes upstream.
+      # TODO: delete this step and .ci/tinygo-net-roundtrip.patch once a TinyGo
+      # release later than 0.41.1 ships the fix (already on TinyGo's dev branch).
+      - name: Patch TinyGo net/http (temporary)
+        run: |
+          patch_file="$PWD/.ci/tinygo-net-roundtrip.patch"
+          tinygoroot="$(tinygo env TINYGOROOT)"
+          echo "Applying $patch_file to $tinygoroot/src/net/http/roundtrip_js.go"
+          git -C "$tinygoroot" apply --verbose "$patch_file" || {
+            echo "::error::TinyGo net/http patch did not apply cleanly; TinyGo source may have changed. Update or remove .ci/tinygo-net-roundtrip.patch (see #26)."
+            exit 1
+          }
+
       # Confirms the Wasm Worker builds end to end: workers-assets-gen emits the
       # JS shim and TinyGo compiles ./worker into build/app.wasm.
       - name: Build Wasm Worker
diff --git a/README.md b/README.md
index e953552..699570d 100644
--- a/README.md
+++ b/README.md
@@ -64,7 +64,17 @@ This runs the exact handler the deployed Worker uses, minus the Workers runtime.
 ### Build & run the real Worker (requires TinyGo)
 
 Node tooling is managed with **pnpm**; wrangler is a dev dependency. The Wasm
-build uses [TinyGo](https://tinygo.org) 0.35.0+.
+build uses [TinyGo](https://tinygo.org) 0.41.1 on the Go 1.26 toolchain.
+
+> **Temporary toolchain patch.** TinyGo 0.41.1 and earlier vendor a `net/http`
+> js/wasm overlay (`tinygo-org/net@e54965e`) that fails to compile against Go
+> 1.25+/1.26 with `t.roundTrip undefined` (see
+> [tinygo-org/tinygo#5467](https://github.com/tinygo-org/tinygo/issues/5467)).
+> CI applies the exact upstream fix (`tinygo-org/net@1026408a`, checked in as
+> `.ci/tinygo-net-roundtrip.patch`) to the installed TinyGo before building.
+> Building locally on Go 1.26 needs the same one-file patch until a TinyGo
+> release later than 0.41.1 ships it, at which point the patch and the CI step
+> are removed (tracked in #26).
 
 ```console
 pnpm install                 # install wrangler