diff --git a/matrix/mreq.js b/matrix/mreq.js
index 0bd7505..70d9ac7 100644
--- a/matrix/mreq.js
+++ b/matrix/mreq.js
@@ -2,6 +2,8 @@
 
 const fetch = require("node-fetch").default
 const mixin = require("mixin-deep")
+const stream = require("stream")
+const getStream = require("get-stream")
 
 const passthrough = require("../passthrough")
 const { sync } = passthrough
@@ -27,9 +29,15 @@ class MatrixServerError extends Error {
  * @param {any} [extra]
  */
 async function mreq(method, url, body, extra = {}) {
+	if (body == undefined || Object.is(body.constructor, Object)) {
+		body = JSON.stringify(body)
+	} else if (body instanceof stream.Readable && reg.ooye.content_length_workaround) {
+		body = await getStream.buffer(body)
+	}
+
 	const opts = mixin({
 		method,
-		body: (body == undefined || Object.is(body.constructor, Object)) ? JSON.stringify(body) : body,
+		body,
 		headers: {
 			Authorization: `Bearer ${reg.as_token}`
 		}
@@ -39,7 +47,10 @@ async function mreq(method, url, body, extra = {}) {
 	const res = await fetch(baseUrl + url, opts)
 	const root = await res.json()
 
-	if (!res.ok || root.errcode) throw new MatrixServerError(root, {baseUrl, url, ...opts})
+	if (!res.ok || root.errcode) {
+		delete opts.headers.Authorization
+		throw new MatrixServerError(root, {baseUrl, url, ...opts})
+	}
 	return root
 }
 
diff --git a/package-lock.json b/package-lock.json
index 054e701..4fb1269 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -16,6 +16,7 @@
         "deep-equal": "^2.2.3",
         "discord-markdown": "git+https://git.sr.ht/~cadence/nodejs-discord-markdown#de519353668c87ecc8c543e9749093481bc72ff8",
         "entities": "^4.5.0",
+        "get-stream": "^6.0.1",
         "giframe": "github:cloudrac3r/giframe#v0.4.1",
         "heatsync": "^2.4.1",
         "html-template-tag": "github:cloudrac3r/html-template-tag#v5.0",
@@ -1456,6 +1457,17 @@
         "source-map": "^0.6.1"
       }
     },
+    "node_modules/get-stream": {
+      "version": "6.0.1",
+      "resolved": "https://registry.npmjs.org/get-stream/-/get-stream-6.0.1.tgz",
+      "integrity": "sha512-ts6Wi+2j3jQjqi70w5AlN8DFnkSwC+MqmxEzdEALB2qXZYV3X/b1CTfgPLGJNMeAWxdPfU8FO1ms3NUfaHCPYg==",
+      "engines": {
+        "node": ">=10"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/giframe": {
       "version": "0.3.0",
       "resolved": "git+ssh://git@github.com/cloudrac3r/giframe.git#1630f4d3b2bf5acd197409c85edd11e0da72d0a1",
diff --git a/package.json b/package.json
index 58a8674..4aba845 100644
--- a/package.json
+++ b/package.json
@@ -22,6 +22,7 @@
     "deep-equal": "^2.2.3",
     "discord-markdown": "git+https://git.sr.ht/~cadence/nodejs-discord-markdown#de519353668c87ecc8c543e9749093481bc72ff8",
     "entities": "^4.5.0",
+    "get-stream": "^6.0.1",
     "giframe": "github:cloudrac3r/giframe#v0.4.1",
     "heatsync": "^2.4.1",
     "html-template-tag": "github:cloudrac3r/html-template-tag#v5.0",
diff --git a/readme.md b/readme.md
index e7baf18..df42234 100644
--- a/readme.md
+++ b/readme.md
@@ -164,6 +164,7 @@ Follow these steps:
 * (8) snowtransfer: Discord API library with bring-your-own-caching that I trust.
 * (0) deep-equal: It's already pulled in by supertape.
 * (1) discord-markdown: This is my fork!
+* (0) get-stream: Only needed if content_length_workaround is true.
 * (0) giframe: This is my fork!
 * (1) heatsync: Module hot-reloader that I trust.
 * (0) entities: Looks fine. No dependencies.
diff --git a/registration.example.yaml b/registration.example.yaml
index 9e7cd2c..9a562cd 100644
--- a/registration.example.yaml
+++ b/registration.example.yaml
@@ -18,6 +18,7 @@ ooye:
   max_file_size: 5000000
   server_name: [the part after the colon in your matrix id, like cadence.moe]
   server_origin: [the full protocol and domain of your actual matrix server's location, with no trailing slash, like https://matrix.cadence.moe]
+  content_length_workaround: false
   invite:
     # uncomment this to auto-invite the named user to newly created spaces and mark them as admin (PL 100) everywhere
     # - @cadence:cadence.moe
diff --git a/scripts/seed.js b/scripts/seed.js
index 4dc08e0..20cb5dd 100644
--- a/scripts/seed.js
+++ b/scripts/seed.js
@@ -71,7 +71,17 @@ async function uploadAutoEmoji(guild, name, filename) {
 	try {
 		await api.register(reg.sender_localpart)
 	} catch (e) {
-		if (e.errcode === "M_USER_IN_USE" || e.data?.error !== "Internal server error") throw e // "Internal server error" is the only OK error because Synapse says this if you try to register the same username twice.
+		if (e.errcode === "M_USER_IN_USE" || e.data?.error === "Internal server error") {
+			// "Internal server error" is the only OK error because older versions of Synapse say this if you try to register the same username twice.
+		} else if (e.data?.error?.includes("Content-Length")) {
+			die(`OOYE cannot stream uploads to Synapse. Please choose one of these workarounds:`
+				+ `\n  * Run an nginx reverse proxy to Synapse, and point registration.yaml's`
+				+ `\n    \`server_origin\` to nginx`
+				+ `\n  * Set \`content_length_workaround: true\` in registration.yaml (this will`
+				+ `\n    halve the speed of bridging d->m files)`)
+		} else {
+			throw e
+		}
 	}
 
 	// upload initial images...
diff --git a/types.d.ts b/types.d.ts
index 9e9d72b..477b41d 100644
--- a/types.d.ts
+++ b/types.d.ts
@@ -21,6 +21,7 @@ export type AppServiceRegistrationConfig = {
 		max_file_size: number
 		server_name: string
 		server_origin: string
+		content_length_workaround: boolean
 		invite: string[]
 	}
 	old_bridge?: {