diff --git a/src/proxy/middleware/common.ts b/src/proxy/middleware/common.ts
index f5c8bd1..a50728e 100644
--- a/src/proxy/middleware/common.ts
+++ b/src/proxy/middleware/common.ts
@@ -42,7 +42,7 @@ export function writeErrorResponse(
   // the stream. Otherwise just send a normal error response.
   if (
     res.headersSent ||
-    res.getHeader("content-type") === "text/event-stream"
+    String(res.getHeader("content-type")).startsWith("text/event-stream")
   ) {
     const errorContent =
       statusCode === 403
diff --git a/src/proxy/middleware/request/transform-outbound-payload.ts b/src/proxy/middleware/request/transform-outbound-payload.ts
index 7659ab6..7c1a3c0 100644
--- a/src/proxy/middleware/request/transform-outbound-payload.ts
+++ b/src/proxy/middleware/request/transform-outbound-payload.ts
@@ -166,12 +166,7 @@ function openaiToAnthropic(req: Request) {
     throw result.error;
   }
 
-  // Anthropic has started versioning their API, indicated by an HTTP header
-  // `anthropic-version`. The new June 2023 version is not backwards compatible
-  // with our OpenAI-to-Anthropic transformations so we need to explicitly
-  // request the older version for now. 2023-01-01 will be removed in September.
-  // https://docs.anthropic.com/claude/reference/versioning
-  req.headers["anthropic-version"] = "2023-01-01";
+  req.headers["anthropic-version"] = "2023-06-01";
 
   const { messages, ...rest } = result.data;
   const prompt = openAIMessagesToClaudePrompt(messages);
diff --git a/src/proxy/middleware/response/handle-streamed-response.ts b/src/proxy/middleware/response/handle-streamed-response.ts
index 605a5c5..7696ca0 100644
--- a/src/proxy/middleware/response/handle-streamed-response.ts
+++ b/src/proxy/middleware/response/handle-streamed-response.ts
@@ -1,44 +1,16 @@
-import { Request, Response } from "express";
-import * as http from "http";
+import { pipeline } from "stream";
+import { promisify } from "util";
 import { buildFakeSseMessage } from "../common";
-import { RawResponseBodyHandler, decodeResponseBody } from ".";
-import { assertNever } from "../../../shared/utils";
-import { ServerSentEventStreamAdapter } from "./sse-stream-adapter";
+import { decodeResponseBody, RawResponseBodyHandler } from ".";
+import { SSEStreamAdapter } from "./streaming/sse-stream-adapter";
+import { SSEMessageTransformer } from "./streaming/sse-message-transformer";
+import { EventAggregator } from "./streaming/event-aggregator";
+import {
+  copySseResponseHeaders,
+  initializeSseStream,
+} from "../../../shared/streaming";
 
-type OpenAiChatCompletionResponse = {
-  id: string;
-  object: string;
-  created: number;
-  model: string;
-  choices: {
-    message: { role: string; content: string };
-    finish_reason: string | null;
-    index: number;
-  }[];
-};
-
-type OpenAiTextCompletionResponse = {
-  id: string;
-  object: string;
-  created: number;
-  model: string;
-  choices: {
-    text: string;
-    finish_reason: string | null;
-    index: number;
-    logprobs: null;
-  }[];
-};
-
-type AnthropicCompletionResponse = {
-  completion: string;
-  stop_reason: string;
-  truncated: boolean;
-  stop: any;
-  model: string;
-  log_id: string;
-  exception: null;
-};
+const pipelineAsync = promisify(pipeline);
 
 /**
  * Consume the SSE stream and forward events to the client. Once the stream is
@@ -49,370 +21,67 @@ type AnthropicCompletionResponse = {
  * in the event a streamed request results in a non-200 response, we need to
  * fall back to the non-streaming response handler so that the error handler
  * can inspect the error response.
- *
- * Currently most frontends don't support Anthropic streaming, so users can opt
- * to send requests for Claude models via an endpoint that accepts OpenAI-
- * compatible requests and translates the received Anthropic SSE events into
- * OpenAI ones, essentially pretending to be an OpenAI streaming API.
  */
 export const handleStreamedResponse: RawResponseBodyHandler = async (
   proxyRes,
   req,
   res
 ) => {
-  // If these differ, the user is using the OpenAI-compatibile endpoint, so
-  // we need to translate the SSE events into OpenAI completion events for their
-  // frontend.
+  const { hash } = req.key!;
   if (!req.isStreaming) {
-    const err = new Error(
-      "handleStreamedResponse called for non-streaming request."
-    );
-    req.log.error({ stack: err.stack, api: req.inboundApi }, err.message);
-    throw err;
+    throw new Error("handleStreamedResponse called for non-streaming request.");
   }
 
-  const key = req.key!;
-  if (proxyRes.statusCode !== 200) {
-    // Ensure we use the non-streaming middleware stack since we won't be
-    // getting any events.
-    req.isStreaming = false;
+  if (proxyRes.statusCode! > 201) {
+    req.isStreaming = false; // Forces non-streaming response handler to execute
     req.log.warn(
-      { statusCode: proxyRes.statusCode, key: key.hash },
+      { statusCode: proxyRes.statusCode, key: hash },
       `Streaming request returned error status code. Falling back to non-streaming response handler.`
     );
     return decodeResponseBody(proxyRes, req, res);
   }
 
   req.log.debug(
-    { headers: proxyRes.headers, key: key.hash },
-    `Received SSE headers.`
+    { headers: proxyRes.headers, key: hash },
+    `Starting to proxy SSE stream.`
   );
 
-  return new Promise((resolve, reject) => {
-    req.log.info({ key: key.hash }, `Starting to proxy SSE stream.`);
+  // Users waiting in the queue already have a SSE connection open for the
+  // heartbeat, so we can't always send the stream headers.
+  if (!res.headersSent) {
+    copySseResponseHeaders(proxyRes, res);
+    initializeSseStream(res);
+  }
 
-    // Queued streaming requests will already have a connection open and headers
-    // sent due to the heartbeat handler.  In that case we can just start
-    // streaming the response without sending headers.
-    if (!res.headersSent) {
-      res.setHeader("Content-Type", "text/event-stream");
-      res.setHeader("Cache-Control", "no-cache");
-      res.setHeader("Connection", "keep-alive");
-      res.setHeader("X-Accel-Buffering", "no");
-      copyHeaders(proxyRes, res);
-      res.flushHeaders();
-    }
+  const prefersNativeEvents = req.inboundApi === req.outboundApi;
+  const contentType = proxyRes.headers["content-type"];
 
-    const adapter = new ServerSentEventStreamAdapter({
-      isAwsStream:
-        proxyRes.headers["content-type"] ===
-        "application/vnd.amazon.eventstream",
+  const adapter = new SSEStreamAdapter({ contentType });
+  const aggregator = new EventAggregator({ format: req.outboundApi });
+  const transformer = new SSEMessageTransformer({
+    inputFormat: req.outboundApi, // outbound from the request's perspective
+    inputApiVersion: String(req.headers["anthropic-version"]),
+    logger: req.log,
+    requestId: String(req.id),
+    requestedModel: req.body.model,
+  })
+    .on("originalMessage", (msg: string) => {
+      if (prefersNativeEvents) res.write(msg);
+    })
+    .on("data", (msg) => {
+      if (!prefersNativeEvents) res.write(`data: ${JSON.stringify(msg)}\n\n`);
+      aggregator.addEvent(msg);
     });
 
-    const events: string[] = [];
-    let lastPosition = 0;
-    let eventCount = 0;
-
-    proxyRes.pipe(adapter);
-
-    adapter.on("data", (chunk: any) => {
-      try {
-        const { event, position } = transformEvent({
-          data: chunk.toString(),
-          requestApi: req.inboundApi,
-          responseApi: req.outboundApi,
-          lastPosition,
-          index: eventCount++,
-        });
-        events.push(event);
-        lastPosition = position;
-        res.write(event + "\n\n");
-      } catch (err) {
-        adapter.emit("error", err);
-      }
-    });
-
-    adapter.on("end", () => {
-      try {
-        req.log.info({ key: key.hash }, `Finished proxying SSE stream.`);
-        const finalBody = convertEventsToFinalResponse(events, req);
-        res.end();
-        resolve(finalBody);
-      } catch (err) {
-        adapter.emit("error", err);
-      }
-    });
-
-    adapter.on("error", (err) => {
-      req.log.error({ error: err, key: key.hash }, `Mid-stream error.`);
-      const errorEvent = buildFakeSseMessage("stream-error", err.message, req);
-      res.write(`data: ${JSON.stringify(errorEvent)}\n\ndata: [DONE]\n\n`);
-      res.end();
-      reject(err);
-    });
-  });
+  try {
+    await pipelineAsync(proxyRes, adapter, transformer);
+    req.log.debug({ key: hash }, `Finished proxying SSE stream.`);
+    res.end();
+    return aggregator.getFinalResponse();
+  } catch (err) {
+    const errorEvent = buildFakeSseMessage("stream-error", err.message, req);
+    res.write(`${errorEvent}data: [DONE]\n\n`);
+    res.end();
+    throw err;
+  }
 };
-
-type SSETransformationArgs = {
-  data: string;
-  requestApi: string;
-  responseApi: string;
-  lastPosition: number;
-  index: number;
-};
-
-/**
- * Transforms SSE events from the given response API into events compatible with
- * the API requested by the client.
- */
-function transformEvent(params: SSETransformationArgs) {
-  const { data, requestApi, responseApi } = params;
-  if (requestApi === responseApi) {
-    return { position: -1, event: data };
-  }
-
-  const trans = `${requestApi}->${responseApi}`;
-  switch (trans) {
-    case "openai->openai-text":
-      return transformOpenAITextEventToOpenAIChat(params);
-    case "openai->anthropic":
-      // TODO: handle new anthropic streaming format
-      return transformV1AnthropicEventToOpenAI(params);
-    default:
-      throw new Error(`Unsupported streaming API transformation. ${trans}`);
-  }
-}
-
-function transformOpenAITextEventToOpenAIChat(params: SSETransformationArgs) {
-  const { data, index } = params;
-
-  if (!data.startsWith("data:")) return { position: -1, event: data };
-  if (data.startsWith("data: [DONE]")) return { position: -1, event: data };
-
-  const event = JSON.parse(data.slice("data: ".length));
-
-  // The very first event must be a role assignment with no content.
-
-  const createEvent = () => ({
-    id: event.id,
-    object: "chat.completion.chunk",
-    created: event.created,
-    model: event.model,
-    choices: [
-      {
-        message: { role: "", content: "" } as {
-          role?: string;
-          content: string;
-        },
-        index: 0,
-        finish_reason: null,
-      },
-    ],
-  });
-
-  let buffer = "";
-
-  if (index === 0) {
-    const initialEvent = createEvent();
-    initialEvent.choices[0].message.role = "assistant";
-    buffer = `data: ${JSON.stringify(initialEvent)}\n\n`;
-  }
-
-  const newEvent = {
-    ...event,
-    choices: [
-      {
-        ...event.choices[0],
-        delta: { content: event.choices[0].text },
-        text: undefined,
-      },
-    ],
-  };
-
-  buffer += `data: ${JSON.stringify(newEvent)}`;
-
-  return { position: -1, event: buffer };
-}
-
-function transformV1AnthropicEventToOpenAI(params: SSETransformationArgs) {
-  const { data, lastPosition } = params;
-  // Anthropic sends the full completion so far with each event whereas OpenAI
-  // only sends the delta. To make the SSE events compatible, we remove
-  // everything before `lastPosition` from the completion.
-  if (!data.startsWith("data:")) {
-    return { position: lastPosition, event: data };
-  }
-
-  if (data.startsWith("data: [DONE]")) {
-    return { position: lastPosition, event: data };
-  }
-
-  const event = JSON.parse(data.slice("data: ".length));
-  const newEvent = {
-    id: "ant-" + event.log_id,
-    object: "chat.completion.chunk",
-    created: Date.now(),
-    model: event.model,
-    choices: [
-      {
-        index: 0,
-        delta: { content: event.completion?.slice(lastPosition) },
-        finish_reason: event.stop_reason,
-      },
-    ],
-  };
-  return {
-    position: event.completion.length,
-    event: `data: ${JSON.stringify(newEvent)}`,
-  };
-}
-
-/** Copy headers, excluding ones we're already setting for the SSE response. */
-function copyHeaders(proxyRes: http.IncomingMessage, res: Response) {
-  const toOmit = [
-    "content-length",
-    "content-encoding",
-    "transfer-encoding",
-    "content-type",
-    "connection",
-    "cache-control",
-  ];
-  for (const [key, value] of Object.entries(proxyRes.headers)) {
-    if (!toOmit.includes(key) && value) {
-      res.setHeader(key, value);
-    }
-  }
-}
-
-/**
- * Converts the list of incremental SSE events into an object that resembles a
- * full, non-streamed response from the API so that subsequent middleware can
- * operate on it as if it were a normal response.
- * Events are expected to be in the format they were received from the API.
- */
-function convertEventsToFinalResponse(events: string[], req: Request) {
-  switch (req.outboundApi) {
-    case "openai": {
-      let merged: OpenAiChatCompletionResponse = {
-        id: "",
-        object: "",
-        created: 0,
-        model: "",
-        choices: [],
-      };
-      merged = events.reduce((acc, event, i) => {
-        if (!event.startsWith("data: ")) return acc;
-        if (event === "data: [DONE]") return acc;
-
-        const data = JSON.parse(event.slice("data: ".length));
-
-        // The first chat chunk only contains the role assignment and metadata
-        if (i === 0) {
-          return {
-            id: data.id,
-            object: data.object,
-            created: data.created,
-            model: data.model,
-            choices: [
-              {
-                message: { role: data.choices[0].delta.role, content: "" },
-                index: 0,
-                finish_reason: null,
-              },
-            ],
-          };
-        }
-
-        if (data.choices[0].delta.content) {
-          acc.choices[0].message.content += data.choices[0].delta.content;
-        }
-        acc.choices[0].finish_reason = data.choices[0].finish_reason;
-        return acc;
-      }, merged);
-      return merged;
-    }
-    case "openai-text": {
-      let merged: OpenAiTextCompletionResponse = {
-        id: "",
-        object: "",
-        created: 0,
-        model: "",
-        choices: [],
-        // TODO: merge logprobs
-      };
-      merged = events.reduce((acc, event) => {
-        if (!event.startsWith("data: ")) return acc;
-        if (event === "data: [DONE]") return acc;
-
-        const data = JSON.parse(event.slice("data: ".length));
-
-        return {
-          id: data.id,
-          object: data.object,
-          created: data.created,
-          model: data.model,
-          choices: [
-            {
-              text: acc.choices[0]?.text + data.choices[0].text,
-              index: 0,
-              finish_reason: data.choices[0].finish_reason,
-              logprobs: null,
-            },
-          ],
-        };
-      }, merged);
-      return merged;
-    }
-    case "anthropic": {
-      if (req.headers["anthropic-version"] === "2023-01-01") {
-        return convertAnthropicV1(events, req);
-      }
-
-      let merged: AnthropicCompletionResponse = {
-        completion: "",
-        stop_reason: "",
-        truncated: false,
-        stop: null,
-        model: req.body.model,
-        log_id: "",
-        exception: null,
-      }
-
-      merged = events.reduce((acc, event) => {
-        if (!event.startsWith("data: ")) return acc;
-        if (event === "data: [DONE]") return acc;
-
-        const data = JSON.parse(event.slice("data: ".length));
-
-        return {
-          completion: acc.completion + data.completion,
-          stop_reason: data.stop_reason,
-          truncated: data.truncated,
-          stop: data.stop,
-          log_id: data.log_id,
-          exception: data.exception,
-          model: acc.model,
-        };
-      }, merged);
-      return merged;
-    }
-    case "google-palm": {
-      throw new Error("PaLM streaming not yet supported.");
-    }
-    default:
-      assertNever(req.outboundApi);
-  }
-}
-
-/** Older Anthropic streaming format which sent full completion each time. */
-function convertAnthropicV1(
-  events: string[],
-  req: Request
-) {
-  const lastEvent = events[events.length - 2].toString();
-  const data = JSON.parse(
-    lastEvent.slice(lastEvent.indexOf("data: ") + "data: ".length)
-  );
-  const final: AnthropicCompletionResponse = { ...data, log_id: req.id };
-  return final;
-}
diff --git a/src/proxy/middleware/response/index.ts b/src/proxy/middleware/response/index.ts
index 530a6b5..c79f3df 100644
--- a/src/proxy/middleware/response/index.ts
+++ b/src/proxy/middleware/response/index.ts
@@ -4,13 +4,16 @@ import * as http from "http";
 import util from "util";
 import zlib from "zlib";
 import { logger } from "../../../logger";
+import { enqueue, trackWaitTime } from "../../queue";
+import { HttpError } from "../../../shared/errors";
 import { keyPool } from "../../../shared/key-management";
 import { getOpenAIModelFamily } from "../../../shared/models";
-import { enqueue, trackWaitTime } from "../../queue";
+import { countTokens } from "../../../shared/tokenization";
 import {
   incrementPromptCount,
   incrementTokenCount,
 } from "../../../shared/users/user-store";
+import { assertNever } from "../../../shared/utils";
 import {
   getCompletionFromBody,
   isCompletionRequest,
@@ -18,8 +21,6 @@ import {
 } from "../common";
 import { handleStreamedResponse } from "./handle-streamed-response";
 import { logPrompt } from "./log-prompt";
-import { countTokens } from "../../../shared/tokenization";
-import { assertNever } from "../../../shared/utils";
 
 const DECODER_MAP = {
   gzip: util.promisify(zlib.gunzip),
@@ -83,7 +84,7 @@ export const createOnProxyResHandler = (apiMiddleware: ProxyResMiddleware) => {
       ? handleStreamedResponse
       : decodeResponseBody;
 
-    let lastMiddlewareName = initialHandler.name;
+    let lastMiddleware = initialHandler.name;
 
     try {
       const body = await initialHandler(proxyRes, req, res);
@@ -112,37 +113,38 @@ export const createOnProxyResHandler = (apiMiddleware: ProxyResMiddleware) => {
       }
 
       for (const middleware of middlewareStack) {
-        lastMiddlewareName = middleware.name;
+        lastMiddleware = middleware.name;
         await middleware(proxyRes, req, res, body);
       }
 
       trackWaitTime(req);
-    } catch (error: any) {
+    } catch (error) {
       // Hack: if the error is a retryable rate-limit error, the request has
       // been re-enqueued and we can just return without doing anything else.
       if (error instanceof RetryableError) {
         return;
       }
 
-      const errorData = {
-        error: error.stack,
-        thrownBy: lastMiddlewareName,
-        key: req.key?.hash,
-      };
-      const message = `Error while executing proxy response middleware: ${lastMiddlewareName} (${error.message})`;
-      if (res.headersSent) {
-        req.log.error(errorData, message);
-        // This should have already been handled by the error handler, but
-        // just in case...
-        if (!res.writableEnded) {
-          res.end();
-        }
+      // Already logged and responded to the client by handleUpstreamErrors
+      if (error instanceof HttpError) {
+        if (!res.writableEnded) res.end();
         return;
       }
-      logger.error(errorData, message);
-      res
-        .status(500)
-        .json({ error: "Internal server error", proxy_note: message });
+
+      const { stack, message } = error;
+      const info = { stack, lastMiddleware, key: req.key?.hash };
+      const description = `Error while executing proxy response middleware: ${lastMiddleware} (${message})`;
+
+      if (res.headersSent) {
+        req.log.error(info, description);
+        if (!res.writableEnded) res.end();
+        return;
+      } else {
+        req.log.error(info, description);
+        res
+          .status(500)
+          .json({ error: "Internal server error", proxy_note: description });
+      }
     }
   };
 };
@@ -203,7 +205,7 @@ export const decodeResponseBody: RawResponseBodyHandler = async (
         return resolve(body.toString());
       } catch (error: any) {
         const errorMessage = `Proxy received response with invalid JSON: ${error.message}`;
-        logger.warn({ error, key: req.key?.hash }, errorMessage);
+        logger.warn({ error: error.stack, key: req.key?.hash }, errorMessage);
         writeErrorResponse(req, res, 500, { error: errorMessage });
         return reject(errorMessage);
       }
@@ -223,7 +225,7 @@ type ProxiedErrorPayload = {
  * an error to stop the middleware stack.
  * On 429 errors, if request queueing is enabled, the request will be silently
  * re-enqueued.  Otherwise, the request will be rejected with an error payload.
- * @throws {Error} On HTTP error status code from upstream service
+ * @throws {HttpError} On HTTP error status code from upstream service
  */
 const handleUpstreamErrors: ProxyResHandlerWithBody = async (
   proxyRes,
@@ -258,7 +260,7 @@ const handleUpstreamErrors: ProxyResHandlerWithBody = async (
       proxy_note: `This is likely a temporary error with the upstream service.`,
     };
     writeErrorResponse(req, res, statusCode, errorObject);
-    throw new Error(parseError.message);
+    throw new HttpError(statusCode, parseError.message);
   }
 
   const errorType =
@@ -371,7 +373,7 @@ const handleUpstreamErrors: ProxyResHandlerWithBody = async (
   }
 
   writeErrorResponse(req, res, statusCode, errorPayload);
-  throw new Error(errorPayload.error?.message);
+  throw new HttpError(statusCode, errorPayload.error?.message);
 };
 
 /**
diff --git a/src/proxy/middleware/response/streaming/aggregators/anthropic.ts b/src/proxy/middleware/response/streaming/aggregators/anthropic.ts
new file mode 100644
index 0000000..56f229e
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/aggregators/anthropic.ts
@@ -0,0 +1,48 @@
+import { OpenAIChatCompletionStreamEvent } from "../index";
+
+export type AnthropicCompletionResponse = {
+  completion: string;
+  stop_reason: string;
+  truncated: boolean;
+  stop: any;
+  model: string;
+  log_id: string;
+  exception: null;
+};
+
+/**
+ * Given a list of OpenAI chat completion events, compiles them into a single
+ * finalized Anthropic completion response so that non-streaming middleware
+ * can operate on it as if it were a blocking response.
+ */
+export function mergeEventsForAnthropic(
+  events: OpenAIChatCompletionStreamEvent[]
+): AnthropicCompletionResponse {
+  let merged: AnthropicCompletionResponse = {
+    log_id: "",
+    exception: null,
+    model: "",
+    completion: "",
+    stop_reason: "",
+    truncated: false,
+    stop: null,
+  };
+  merged = events.reduce((acc, event, i) => {
+    // The first event will only contain role assignment and response metadata
+    if (i === 0) {
+      acc.log_id = event.id;
+      acc.model = event.model;
+      acc.completion = "";
+      acc.stop_reason = "";
+      return acc;
+    }
+
+    acc.stop_reason = event.choices[0].finish_reason ?? "";
+    if (event.choices[0].delta.content) {
+      acc.completion += event.choices[0].delta.content;
+    }
+
+    return acc;
+  }, merged);
+  return merged;
+}
diff --git a/src/proxy/middleware/response/streaming/aggregators/openai-chat.ts b/src/proxy/middleware/response/streaming/aggregators/openai-chat.ts
new file mode 100644
index 0000000..f1a1bd4
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/aggregators/openai-chat.ts
@@ -0,0 +1,58 @@
+import { OpenAIChatCompletionStreamEvent } from "../index";
+
+export type OpenAiChatCompletionResponse = {
+  id: string;
+  object: string;
+  created: number;
+  model: string;
+  choices: {
+    message: { role: string; content: string };
+    finish_reason: string | null;
+    index: number;
+  }[];
+};
+
+/**
+ * Given a list of OpenAI chat completion events, compiles them into a single
+ * finalized OpenAI chat completion response so that non-streaming middleware
+ * can operate on it as if it were a blocking response.
+ */
+export function mergeEventsForOpenAIChat(
+  events: OpenAIChatCompletionStreamEvent[]
+): OpenAiChatCompletionResponse {
+  let merged: OpenAiChatCompletionResponse = {
+    id: "",
+    object: "",
+    created: 0,
+    model: "",
+    choices: [],
+  };
+  merged = events.reduce((acc, event, i) => {
+    // The first event will only contain role assignment and response metadata
+    if (i === 0) {
+      acc.id = event.id;
+      acc.object = event.object;
+      acc.created = event.created;
+      acc.model = event.model;
+      acc.choices = [
+        {
+          index: 0,
+          message: {
+            role: event.choices[0].delta.role ?? "assistant",
+            content: "",
+          },
+          finish_reason: null,
+        },
+      ];
+      return acc;
+    }
+
+    acc.choices[0].finish_reason = event.choices[0].finish_reason;
+    if (event.choices[0].delta.content) {
+      acc.choices[0].message.content += event.choices[0].delta.content;
+    }
+
+    return acc;
+  }, merged);
+  return merged;
+}
diff --git a/src/proxy/middleware/response/streaming/aggregators/openai-text.ts b/src/proxy/middleware/response/streaming/aggregators/openai-text.ts
new file mode 100644
index 0000000..f343934
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/aggregators/openai-text.ts
@@ -0,0 +1,57 @@
+import { OpenAIChatCompletionStreamEvent } from "../index";
+
+export type OpenAiTextCompletionResponse = {
+  id: string;
+  object: string;
+  created: number;
+  model: string;
+  choices: {
+    text: string;
+    finish_reason: string | null;
+    index: number;
+    logprobs: null;
+  }[];
+};
+
+/**
+ * Given a list of OpenAI chat completion events, compiles them into a single
+ * finalized OpenAI text completion response so that non-streaming middleware
+ * can operate on it as if it were a blocking response.
+ */
+export function mergeEventsForOpenAIText(
+  events: OpenAIChatCompletionStreamEvent[]
+): OpenAiTextCompletionResponse {
+  let merged: OpenAiTextCompletionResponse = {
+    id: "",
+    object: "",
+    created: 0,
+    model: "",
+    choices: [],
+  };
+  merged = events.reduce((acc, event, i) => {
+    // The first event will only contain role assignment and response metadata
+    if (i === 0) {
+      acc.id = event.id;
+      acc.object = event.object;
+      acc.created = event.created;
+      acc.model = event.model;
+      acc.choices = [
+        {
+          text: "",
+          index: 0,
+          finish_reason: null,
+          logprobs: null,
+        },
+      ];
+      return acc;
+    }
+
+    acc.choices[0].finish_reason = event.choices[0].finish_reason;
+    if (event.choices[0].delta.content) {
+      acc.choices[0].text += event.choices[0].delta.content;
+    }
+
+    return acc;
+  }, merged);
+  return merged;
+}
diff --git a/src/proxy/middleware/response/streaming/event-aggregator.ts b/src/proxy/middleware/response/streaming/event-aggregator.ts
new file mode 100644
index 0000000..55f0fb3
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/event-aggregator.ts
@@ -0,0 +1,41 @@
+import { APIFormat } from "../../../../shared/key-management";
+import { assertNever } from "../../../../shared/utils";
+import {
+  mergeEventsForAnthropic,
+  mergeEventsForOpenAIChat,
+  mergeEventsForOpenAIText,
+  OpenAIChatCompletionStreamEvent
+} from "./index";
+
+/**
+ * Collects SSE events containing incremental chat completion responses and
+ * compiles them into a single finalized response for downstream middleware.
+ */
+export class EventAggregator {
+  private readonly format: APIFormat;
+  private readonly events: OpenAIChatCompletionStreamEvent[];
+
+  constructor({ format }: { format: APIFormat }) {
+    this.events = [];
+    this.format = format;
+  }
+
+  addEvent(event: OpenAIChatCompletionStreamEvent) {
+    this.events.push(event);
+  }
+
+  getFinalResponse() {
+    switch (this.format) {
+      case "openai":
+        return mergeEventsForOpenAIChat(this.events);
+      case "openai-text":
+        return mergeEventsForOpenAIText(this.events);
+      case "anthropic":
+        return mergeEventsForAnthropic(this.events);
+      case "google-palm":
+        throw new Error("Google PaLM API does not support streaming responses");
+      default:
+        assertNever(this.format);
+    }
+  }
+}
\ No newline at end of file
diff --git a/src/proxy/middleware/response/streaming/index.ts b/src/proxy/middleware/response/streaming/index.ts
new file mode 100644
index 0000000..6e232da
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/index.ts
@@ -0,0 +1,31 @@
+export type SSEResponseTransformArgs = {
+  data: string;
+  lastPosition: number;
+  index: number;
+  fallbackId: string;
+  fallbackModel: string;
+};
+
+export type OpenAIChatCompletionStreamEvent = {
+  id: string;
+  object: "chat.completion.chunk";
+  created: number;
+  model: string;
+  choices: {
+    index: number;
+    delta: { role?: string; content?: string };
+    finish_reason: string | null;
+  }[];
+}
+
+export type StreamingCompletionTransformer = (
+  params: SSEResponseTransformArgs
+) => { position: number; event?: OpenAIChatCompletionStreamEvent };
+
+export { openAITextToOpenAIChat } from "./transformers/openai-text-to-openai";
+export { anthropicV1ToOpenAI } from "./transformers/anthropic-v1-to-openai";
+export { anthropicV2ToOpenAI } from "./transformers/anthropic-v2-to-openai";
+export { mergeEventsForOpenAIChat } from "./aggregators/openai-chat";
+export { mergeEventsForOpenAIText } from "./aggregators/openai-text";
+export { mergeEventsForAnthropic } from "./aggregators/anthropic";
+
diff --git a/src/proxy/middleware/response/streaming/parse-sse.ts b/src/proxy/middleware/response/streaming/parse-sse.ts
new file mode 100644
index 0000000..d161fb5
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/parse-sse.ts
@@ -0,0 +1,29 @@
+export type ServerSentEvent = { id?: string; type?: string; data: string };
+
+/** Given a string of SSE data, parse it into a `ServerSentEvent` object. */
+export function parseEvent(event: string) {
+  const buffer: ServerSentEvent = { data: "" };
+  return event.split(/\r?\n/).reduce(parseLine, buffer)
+}
+
+function parseLine(event: ServerSentEvent, line: string) {
+  const separator = line.indexOf(":");
+  const field = separator === -1 ? line : line.slice(0,separator);
+  const value = separator === -1 ? "" : line.slice(separator + 1);
+
+  switch (field) {
+    case 'id':
+      event.id = value.trim()
+      break
+    case 'event':
+      event.type = value.trim()
+      break
+    case 'data':
+      event.data += value.trimStart()
+      break
+    default:
+      break
+  }
+
+  return event
+}
\ No newline at end of file
diff --git a/src/proxy/middleware/response/streaming/sse-message-transformer.ts b/src/proxy/middleware/response/streaming/sse-message-transformer.ts
new file mode 100644
index 0000000..33d2882
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/sse-message-transformer.ts
@@ -0,0 +1,123 @@
+import { Transform, TransformOptions } from "stream";
+import { logger } from "../../../../logger";
+import { APIFormat } from "../../../../shared/key-management";
+import { assertNever } from "../../../../shared/utils";
+import {
+  anthropicV1ToOpenAI,
+  anthropicV2ToOpenAI,
+  OpenAIChatCompletionStreamEvent,
+  openAITextToOpenAIChat,
+  StreamingCompletionTransformer,
+} from "./index";
+import { passthroughToOpenAI } from "./transformers/passthrough-to-openai";
+
+const genlog = logger.child({ module: "sse-transformer" });
+
+type SSEMessageTransformerOptions = TransformOptions & {
+  requestedModel: string;
+  requestId: string;
+  inputFormat: APIFormat;
+  inputApiVersion?: string;
+  logger?: typeof logger;
+};
+
+/**
+ * Transforms SSE messages from one API format to OpenAI chat.completion.chunks.
+ * Emits the original string SSE message as an "originalMessage" event.
+ */
+export class SSEMessageTransformer extends Transform {
+  private lastPosition: number;
+  private msgCount: number;
+  private readonly transformFn: StreamingCompletionTransformer;
+  private readonly log;
+  private readonly fallbackId: string;
+  private readonly fallbackModel: string;
+
+  constructor(options: SSEMessageTransformerOptions) {
+    super({ ...options, readableObjectMode: true });
+    this.log = options.logger?.child({ module: "sse-transformer" }) ?? genlog;
+    this.lastPosition = 0;
+    this.msgCount = 0;
+    this.transformFn = getTransformer(
+      options.inputFormat,
+      options.inputApiVersion
+    );
+    this.fallbackId = options.requestId;
+    this.fallbackModel = options.requestedModel;
+    this.log.debug(
+      {
+        fn: this.transformFn.name,
+        format: options.inputFormat,
+        version: options.inputApiVersion,
+      },
+      "Selected SSE transformer"
+    );
+  }
+
+  _transform(chunk: Buffer, _encoding: BufferEncoding, callback: Function) {
+    try {
+      const originalMessage = chunk.toString();
+      const { event: transformedMessage, position: newPosition } =
+        this.transformFn({
+          data: originalMessage,
+          lastPosition: this.lastPosition,
+          index: this.msgCount++,
+          fallbackId: this.fallbackId,
+          fallbackModel: this.fallbackModel,
+        });
+      this.lastPosition = newPosition;
+
+      this.emit("originalMessage", originalMessage);
+
+      // Some events may not be transformed, e.g. ping events
+      if (!transformedMessage) return callback();
+
+      if (this.msgCount === 1) {
+        this.push(createInitialMessage(transformedMessage));
+      }
+      this.push(transformedMessage);
+      callback();
+    } catch (err) {
+      this.log.error(err, "Error transforming SSE message");
+      callback(err);
+    }
+  }
+}
+
+function getTransformer(
+  responseApi: APIFormat,
+  version?: string
+): StreamingCompletionTransformer {
+  switch (responseApi) {
+    case "openai":
+      return passthroughToOpenAI;
+    case "openai-text":
+      return openAITextToOpenAIChat;
+    case "anthropic":
+      return version === "2023-01-01"
+        ? anthropicV1ToOpenAI
+        : anthropicV2ToOpenAI;
+    case "google-palm":
+      throw new Error("Google PaLM does not support streaming responses");
+    default:
+      assertNever(responseApi);
+  }
+}
+
+/**
+ * OpenAI streaming chat completions start with an event that contains only the
+ * metadata and role (always 'assistant') for the response.  To simulate this
+ * for APIs where the first event contains actual content, we create a fake
+ * initial event with no content but correct metadata.
+ */
+function createInitialMessage(
+  event: OpenAIChatCompletionStreamEvent
+): OpenAIChatCompletionStreamEvent {
+  return {
+    ...event,
+    choices: event.choices.map((choice) => ({
+      ...choice,
+      delta: { role: "assistant", content: "" },
+    })),
+  };
+}
diff --git a/src/proxy/middleware/response/sse-stream-adapter.ts b/src/proxy/middleware/response/streaming/sse-stream-adapter.ts
similarity index 66%
rename from src/proxy/middleware/response/sse-stream-adapter.ts
rename to src/proxy/middleware/response/streaming/sse-stream-adapter.ts
index bbe473c..7ae1996 100644
--- a/src/proxy/middleware/response/sse-stream-adapter.ts
+++ b/src/proxy/middleware/response/streaming/sse-stream-adapter.ts
@@ -1,11 +1,11 @@
 import { Transform, TransformOptions } from "stream";
 // @ts-ignore
 import { Parser } from "lifion-aws-event-stream";
-import { logger } from "../../../logger";
+import { logger } from "../../../../logger";
 
 const log = logger.child({ module: "sse-stream-adapter" });
 
-type SSEStreamAdapterOptions = TransformOptions & { isAwsStream?: boolean };
+type SSEStreamAdapterOptions = TransformOptions & { contentType?: string };
 type AwsEventStreamMessage = {
   headers: { ":message-type": "event" | "exception" };
   payload: { message?: string /** base64 encoded */; bytes?: string };
@@ -15,24 +15,25 @@ type AwsEventStreamMessage = {
  * Receives either text chunks or AWS binary event stream chunks and emits
  * full SSE events.
  */
-export class ServerSentEventStreamAdapter extends Transform {
+export class SSEStreamAdapter extends Transform {
   private readonly isAwsStream;
   private parser = new Parser();
   private partialMessage = "";
 
   constructor(options?: SSEStreamAdapterOptions) {
     super(options);
-    this.isAwsStream = options?.isAwsStream || false;
+    this.isAwsStream =
+      options?.contentType === "application/vnd.amazon.eventstream";
 
     this.parser.on("data", (data: AwsEventStreamMessage) => {
       const message = this.processAwsEvent(data);
       if (message) {
-        this.push(Buffer.from(message, "utf8"));
+        this.push(Buffer.from(message + "\n\n"), "utf8");
       }
     });
   }
 
-  processAwsEvent(event: AwsEventStreamMessage): string | null {
+  protected processAwsEvent(event: AwsEventStreamMessage): string | null {
     const { payload, headers } = event;
     if (headers[":message-type"] === "exception" || !payload.bytes) {
       log.error(
@@ -42,7 +43,14 @@ export class ServerSentEventStreamAdapter extends Transform {
       const message = JSON.stringify(event);
       return getFakeErrorCompletion("proxy AWS error", message);
     } else {
-      return `data: ${Buffer.from(payload.bytes, "base64").toString("utf8")}`;
+      const { bytes } = payload;
+      // technically this is a transformation but we don't really distinguish
+      // between aws claude and anthropic claude at the APIFormat level, so
+      // these will short circuit the message transformer
+      return [
+        "event: completion",
+        `data: ${Buffer.from(bytes, "base64").toString("utf8")}`,
+      ].join("\n");
     }
   }
 
@@ -55,11 +63,15 @@ export class ServerSentEventStreamAdapter extends Transform {
         // so we need to buffer and emit separate stream events for full
         // messages so we can parse/transform them properly.
         const str = chunk.toString("utf8");
+
         const fullMessages = (this.partialMessage + str).split(/\r?\n\r?\n/);
         this.partialMessage = fullMessages.pop() || "";
 
         for (const message of fullMessages) {
-          this.push(message);
+          // Mixing line endings will break some clients and our request queue
+          // will have already sent \n for heartbeats, so we need to normalize
+          // to \n.
+          this.push(message.replace(/\r\n/g, "\n") + "\n\n");
         }
       }
       callback();
@@ -72,7 +84,7 @@ export class ServerSentEventStreamAdapter extends Transform {
 
 function getFakeErrorCompletion(type: string, message: string) {
   const content = `\`\`\`\n[${type}: ${message}]\n\`\`\`\n`;
-  const fakeEvent = {
+  const fakeEvent = JSON.stringify({
     log_id: "aws-proxy-sse-message",
     stop_reason: type,
     completion:
@@ -80,6 +92,6 @@ function getFakeErrorCompletion(type: string, message: string) {
     truncated: false,
     stop: null,
     model: "",
-  };
-  return `data: ${JSON.stringify(fakeEvent)}\n\n`;
+  });
+  return ["event: completion", `data: ${fakeEvent}\n\n`].join("\n");
 }
diff --git a/src/proxy/middleware/response/streaming/transformers/anthropic-v1-to-openai.ts b/src/proxy/middleware/response/streaming/transformers/anthropic-v1-to-openai.ts
new file mode 100644
index 0000000..f145290
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/transformers/anthropic-v1-to-openai.ts
@@ -0,0 +1,67 @@
+import { StreamingCompletionTransformer } from "../index";
+import { parseEvent, ServerSentEvent } from "../parse-sse";
+import { logger } from "../../../../../logger";
+
+const log = logger.child({
+  module: "sse-transformer",
+  transformer: "anthropic-v1-to-openai",
+});
+
+type AnthropicV1StreamEvent = {
+  log_id?: string;
+  model?: string;
+  completion: string;
+  stop_reason: string;
+};
+
+/**
+ * Transforms an incoming Anthropic SSE (2023-01-01 API) to an equivalent
+ * OpenAI chat.completion.chunk SSE.
+ */
+export const anthropicV1ToOpenAI: StreamingCompletionTransformer = (params) => {
+  const { data, lastPosition } = params;
+
+  const rawEvent = parseEvent(data);
+  if (!rawEvent.data || rawEvent.data === "[DONE]") {
+    return { position: lastPosition };
+  }
+
+  const completionEvent = asCompletion(rawEvent);
+  if (!completionEvent) {
+    return { position: lastPosition };
+  }
+
+  // Anthropic sends the full completion so far with each event whereas OpenAI
+  // only sends the delta. To make the SSE events compatible, we remove
+  // everything before `lastPosition` from the completion.
+  const newEvent = {
+    id: "ant-" + (completionEvent.log_id ?? params.fallbackId),
+    object: "chat.completion.chunk" as const,
+    created: Date.now(),
+    model: completionEvent.model ?? params.fallbackModel,
+    choices: [
+      {
+        index: 0,
+        delta: { content: completionEvent.completion?.slice(lastPosition) },
+        finish_reason: completionEvent.stop_reason,
+      },
+    ],
+  };
+
+  return { position: completionEvent.completion.length, event: newEvent };
+};
+
+function asCompletion(event: ServerSentEvent): AnthropicV1StreamEvent | null {
+  try {
+    const parsed = JSON.parse(event.data);
+    if (parsed.completion !== undefined && parsed.stop_reason !== undefined) {
+      return parsed;
+    } else {
+      // noinspection ExceptionCaughtLocallyJS
+      throw new Error("Missing required fields");
+    }
+  } catch (error) {
+    log.warn({ error: error.stack, event }, "Received invalid event");
+  }
+  return null;
+}
diff --git a/src/proxy/middleware/response/streaming/transformers/anthropic-v2-to-openai.ts b/src/proxy/middleware/response/streaming/transformers/anthropic-v2-to-openai.ts
new file mode 100644
index 0000000..9db1c0f
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/transformers/anthropic-v2-to-openai.ts
@@ -0,0 +1,66 @@
+import { StreamingCompletionTransformer } from "../index";
+import { parseEvent, ServerSentEvent } from "../parse-sse";
+import { logger } from "../../../../../logger";
+
+const log = logger.child({
+  module: "sse-transformer",
+  transformer: "anthropic-v2-to-openai",
+});
+
+type AnthropicV2StreamEvent = {
+  log_id?: string;
+  model?: string;
+  completion: string;
+  stop_reason: string;
+};
+
+/**
+ * Transforms an incoming Anthropic SSE (2023-06-01 API) to an equivalent
+ * OpenAI chat.completion.chunk SSE.
+ */
+export const anthropicV2ToOpenAI: StreamingCompletionTransformer = (params) => {
+  const { data } = params;
+
+  const rawEvent = parseEvent(data);
+  if (!rawEvent.data || rawEvent.data === "[DONE]") {
+    return { position: -1 };
+  }
+
+  const completionEvent = asCompletion(rawEvent);
+  if (!completionEvent) {
+    return { position: -1 };
+  }
+
+  const newEvent = {
+    id: "ant-" + (completionEvent.log_id ?? params.fallbackId),
+    object: "chat.completion.chunk" as const,
+    created: Date.now(),
+    model: completionEvent.model ?? params.fallbackModel,
+    choices: [
+      {
+        index: 0,
+        delta: { content: completionEvent.completion },
+        finish_reason: completionEvent.stop_reason,
+      },
+    ],
+  };
+
+  return { position: completionEvent.completion.length, event: newEvent };
+};
+
+function asCompletion(event: ServerSentEvent): AnthropicV2StreamEvent | null {
+  if (event.type === "ping") return null;
+
+  try {
+    const parsed = JSON.parse(event.data);
+    if (parsed.completion !== undefined && parsed.stop_reason !== undefined) {
+      return parsed;
+    } else {
+      // noinspection ExceptionCaughtLocallyJS
+      throw new Error("Missing required fields");
+    }
+  } catch (error) {
+    log.warn({ error: error.stack, event }, "Received invalid event");
+  }
+  return null;
+}
diff --git a/src/proxy/middleware/response/streaming/transformers/openai-text-to-openai.ts b/src/proxy/middleware/response/streaming/transformers/openai-text-to-openai.ts
new file mode 100644
index 0000000..c85795a
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/transformers/openai-text-to-openai.ts
@@ -0,0 +1,68 @@
+import { SSEResponseTransformArgs } from "../index";
+import { parseEvent, ServerSentEvent } from "../parse-sse";
+import { logger } from "../../../../../logger";
+
+const log = logger.child({
+  module: "sse-transformer",
+  transformer: "openai-text-to-openai",
+});
+
+type OpenAITextCompletionStreamEvent = {
+  id: string;
+  object: "text_completion";
+  created: number;
+  choices: {
+    text: string;
+    index: number;
+    logprobs: null;
+    finish_reason: string | null;
+  }[];
+  model: string;
+};
+
+export const openAITextToOpenAIChat = (params: SSEResponseTransformArgs) => {
+  const { data } = params;
+
+  const rawEvent = parseEvent(data);
+  if (!rawEvent.data || rawEvent.data === "[DONE]") {
+    return { position: -1 };
+  }
+
+  const completionEvent = asCompletion(rawEvent);
+  if (!completionEvent) {
+    return { position: -1 };
+  }
+
+  const newEvent = {
+    id: completionEvent.id,
+    object: "chat.completion.chunk" as const,
+    created: completionEvent.created,
+    model: completionEvent.model,
+    choices: [
+      {
+        index: completionEvent.choices[0].index,
+        delta: { content: completionEvent.choices[0].text },
+        finish_reason: completionEvent.choices[0].finish_reason,
+      },
+    ],
+  };
+
+  return { position: -1, event: newEvent };
+};
+
+function asCompletion(
+  event: ServerSentEvent
+): OpenAITextCompletionStreamEvent | null {
+  try {
+    const parsed = JSON.parse(event.data);
+    if (Array.isArray(parsed.choices) && parsed.choices[0].text !== undefined) {
+      return parsed;
+    } else {
+      // noinspection ExceptionCaughtLocallyJS
+      throw new Error("Missing required fields");
+    }
+  } catch (error) {
+    log.warn({ error: error.stack, event }, "Received invalid data event");
+  }
+  return null;
+}
diff --git a/src/proxy/middleware/response/streaming/transformers/passthrough-to-openai.ts b/src/proxy/middleware/response/streaming/transformers/passthrough-to-openai.ts
new file mode 100644
index 0000000..2edcf60
--- /dev/null
+++ b/src/proxy/middleware/response/streaming/transformers/passthrough-to-openai.ts
@@ -0,0 +1,38 @@
+import {
+  OpenAIChatCompletionStreamEvent,
+  SSEResponseTransformArgs,
+} from "../index";
+import { parseEvent, ServerSentEvent } from "../parse-sse";
+import { logger } from "../../../../../logger";
+
+const log = logger.child({
+  module: "sse-transformer",
+  transformer: "openai-to-openai",
+});
+
+export const passthroughToOpenAI = (params: SSEResponseTransformArgs) => {
+  const { data } = params;
+
+  const rawEvent = parseEvent(data);
+  if (!rawEvent.data || rawEvent.data === "[DONE]") {
+    return { position: -1 };
+  }
+
+  const completionEvent = asCompletion(rawEvent);
+  if (!completionEvent) {
+    return { position: -1 };
+  }
+
+  return { position: -1, event: completionEvent };
+};
+
+function asCompletion(
+  event: ServerSentEvent
+): OpenAIChatCompletionStreamEvent | null {
+  try {
+    return JSON.parse(event.data);
+  } catch (error) {
+    log.warn({ error: error.stack, event }, "Received invalid event");
+  }
+  return null;
+}
diff --git a/src/proxy/queue.ts b/src/proxy/queue.ts
index fc98ad3..66aa37c 100644
--- a/src/proxy/queue.ts
+++ b/src/proxy/queue.ts
@@ -23,10 +23,11 @@ import {
   getOpenAIModelFamily,
   ModelFamily,
 } from "../shared/models";
+import { initializeSseStream } from "../shared/streaming";
+import { assertNever } from "../shared/utils";
 import { logger } from "../logger";
 import { AGNAI_DOT_CHAT_IP } from "./rate-limit";
 import { buildFakeSseMessage } from "./middleware/common";
-import { assertNever } from "../shared/utils";
 
 const queue: Request[] = [];
 const log = logger.child({ module: "request-queue" });
@@ -352,14 +353,8 @@ function killQueuedRequest(req: Request) {
 }
 
 function initStreaming(req: Request) {
-  req.log.info(`Initiating streaming for new queued request.`);
   const res = req.res!;
-  res.statusCode = 200;
-  res.setHeader("Content-Type", "text/event-stream");
-  res.setHeader("Cache-Control", "no-cache");
-  res.setHeader("Connection", "keep-alive");
-  res.setHeader("X-Accel-Buffering", "no"); // nginx-specific fix
-  res.flushHeaders();
+  initializeSseStream(res);
 
   if (req.query.badSseParser) {
     // Some clients have a broken SSE parser that doesn't handle comments
@@ -368,7 +363,6 @@ function initStreaming(req: Request) {
     return;
   }
 
-  res.write("\n");
   res.write(": joining queue\n\n");
 }
 
diff --git a/src/proxy/routes.ts b/src/proxy/routes.ts
index a2ffda6..424f104 100644
--- a/src/proxy/routes.ts
+++ b/src/proxy/routes.ts
@@ -7,6 +7,14 @@ import { googlePalm } from "./palm";
 import { aws } from "./aws";
 
 const proxyRouter = express.Router();
+proxyRouter.use((req, _res, next) => {
+  if (req.headers.expect) {
+    // node-http-proxy does not like it when clients send `expect: 100-continue`
+    // and will stall. none of the upstream APIs use this header anyway.
+    delete req.headers.expect;
+  }
+  next();
+});
 proxyRouter.use(
   express.json({ limit: "1536kb" }),
   express.urlencoded({ extended: true, limit: "1536kb" })
diff --git a/src/shared/streaming.ts b/src/shared/streaming.ts
new file mode 100644
index 0000000..60b8455
--- /dev/null
+++ b/src/shared/streaming.ts
@@ -0,0 +1,34 @@
+import { Response } from "express";
+import { IncomingMessage } from "http";
+
+export function initializeSseStream(res: Response) {
+  res.statusCode = 200;
+  res.setHeader("Content-Type", "text/event-stream; charset=utf-8");
+  res.setHeader("Cache-Control", "no-cache");
+  res.setHeader("Connection", "keep-alive");
+  res.setHeader("X-Accel-Buffering", "no"); // nginx-specific fix
+  res.flushHeaders();
+}
+
+/**
+ * Copies headers received from upstream API to the SSE response, excluding
+ * ones we need to set ourselves for SSE to work.
+ */
+export function copySseResponseHeaders(
+  proxyRes: IncomingMessage,
+  res: Response
+) {
+  const toOmit = [
+    "content-length",
+    "content-encoding",
+    "transfer-encoding",
+    "content-type",
+    "connection",
+    "cache-control",
+  ];
+  for (const [key, value] of Object.entries(proxyRes.headers)) {
+    if (!toOmit.includes(key) && value) {
+      res.setHeader(key, value);
+    }
+  }
+}
diff --git a/src/shared/users/user-store.ts b/src/shared/users/user-store.ts
index 3af078b..8924952 100644
--- a/src/shared/users/user-store.ts
+++ b/src/shared/users/user-store.ts
@@ -277,7 +277,7 @@ function cleanupExpiredTokens() {
       deleted++;
     }
   }
-  log.debug({ disabled, deleted }, "Expired tokens cleaned up.");
+  log.trace({ disabled, deleted }, "Expired tokens cleaned up.");
 }
 
 function refreshAllQuotas() {