diff --git a/.changeset/eleven-terms-clap.md b/.changeset/eleven-terms-clap.md
new file mode 100644
index 0000000..a6d15b4
--- /dev/null
+++ b/.changeset/eleven-terms-clap.md
@@ -0,0 +1,5 @@
+---
+'token.js': minor
+---
+
+Upgrade the openai client library to 4.91.1
diff --git a/package.json b/package.json
index 2904552..3c8235c 100644
--- a/package.json
+++ b/package.json
@@ -40,7 +40,7 @@
     "cohere-ai": "7.10.6",
     "mime-types": "^2.1.35",
     "nanoid": "^5.0.7",
-    "openai": "4.52.2"
+    "openai": "4.91.1"
   },
   "devDependencies": {
     "@babel/eslint-parser": "^7.18.2",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index a2415fb..0eebdff 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -30,8 +30,8 @@ dependencies:
     specifier: ^5.0.7
     version: 5.0.8
   openai:
-    specifier: 4.52.2
-    version: 4.52.2
+    specifier: 4.91.1
+    version: 4.91.1
 
 devDependencies:
   '@babel/eslint-parser':
@@ -6609,9 +6609,17 @@ packages:
       mimic-fn: 2.1.0
     dev: true
 
-  /openai@4.52.2:
-    resolution: {integrity: sha512-mMc0XgFuVSkcm0lRIi8zaw++otC82ZlfkCur1qguXYWPETr/+ZwL9A/vvp3YahX+shpaT6j03dwsmUyLAfmEfg==}
+  /openai@4.91.1:
+    resolution: {integrity: sha512-DbjrR0hIMQFbxz8+3qBsfPJnh3+I/skPgoSlT7f9eiZuhGBUissPQULNgx6gHNkLoZ3uS0uYS6eXPUdtg4nHzw==}
     hasBin: true
+    peerDependencies:
+      ws: ^8.18.0
+      zod: ^3.23.8
+    peerDependenciesMeta:
+      ws:
+        optional: true
+      zod:
+        optional: true
     dependencies:
       '@types/node': 18.19.64
       '@types/node-fetch': 2.6.12
@@ -6620,7 +6628,6 @@ packages:
       form-data-encoder: 1.7.2
       formdata-node: 4.4.1
       node-fetch: 2.7.0
-      web-streams-polyfill: 3.3.3
     transitivePeerDependencies:
       - encoding
     dev: false
diff --git a/src/handlers/ai21.ts b/src/handlers/ai21.ts
index f4a1033..3b4910c 100644
--- a/src/handlers/ai21.ts
+++ b/src/handlers/ai21.ts
@@ -10,7 +10,7 @@ import {
 } from '../userTypes/index.js'
 import { BaseHandler } from './base.js'
 import { InputError } from './types.js'
-import { getTimestamp } from './utils.js'
+import { convertMessageContentToString, getTimestamp } from './utils.js'
 
 type AI21ChatCompletionParams = {
   model: string
@@ -73,7 +73,7 @@ const convertMessages = (
     if (i === 0 && message.role === 'system') {
       output.push({
         role: 'system',
-        content: message.content,
+        content: convertMessageContentToString(message.content),
       })
     } else if (
       message.role === 'user' ||
@@ -268,6 +268,10 @@ export class AI21Handler extends BaseHandler<AI21Model> {
       const convertedChoices = data.choices.map((choice) => {
         return {
           ...choice,
+          message: {
+            ...choice.message,
+            refusal: null,
+          },
           logprobs: null,
         }
       })
diff --git a/src/handlers/anthropic.ts b/src/handlers/anthropic.ts
index d356239..f7a9455 100644
--- a/src/handlers/anthropic.ts
+++ b/src/handlers/anthropic.ts
@@ -28,6 +28,7 @@ import { BaseHandler } from './base.js'
 import { InputError, InvariantError } from './types.js'
 import {
   consoleWarn,
+  convertMessageContentToString,
   fetchThenParseImage,
   getTimestamp,
   isEmptyObject,
@@ -247,12 +248,14 @@ const toChatCompletionChoiceMessage = (
     const messageContent = content.every(isToolUseBlock) ? null : ''
     return {
       role,
+      refusal: null,
       content: messageContent,
       tool_calls: toolCalls,
     }
   } else {
     return {
       role,
+      refusal: null,
       content: textBlocks.map((textBlock) => textBlock.text).join('\n'),
       tool_calls: toolCalls,
     }
@@ -361,7 +364,7 @@ export const convertMessages = async (
   // unchanged.
   let systemMessage: string | undefined
   if (clonedMessages.length > 0 && clonedMessages[0].role === 'system') {
-    systemMessage = clonedMessages[0].content
+    systemMessage = convertMessageContentToString(clonedMessages[0].content)
     clonedMessages.shift()
   }
 
@@ -448,7 +451,7 @@ export const convertMessages = async (
               return {
                 type: 'text',
                 text,
-              }
+              } as TextBlockParam
             } else {
               const parsedImage = await fetchThenParseImage(e.image_url.url)
               return {
@@ -458,7 +461,7 @@ export const convertMessages = async (
                   media_type: parsedImage.mimeType,
                   type: 'base64',
                 },
-              }
+              } as ImageBlockParam
             }
           })
         )
diff --git a/src/handlers/bedrock.ts b/src/handlers/bedrock.ts
index e267d78..ccc8b69 100644
--- a/src/handlers/bedrock.ts
+++ b/src/handlers/bedrock.ts
@@ -9,7 +9,10 @@ import {
   ConverseStreamCommandOutput,
   ImageFormat,
   SystemContentBlock,
+  Tool,
   ToolChoice,
+  ToolConfiguration,
+  ToolSpecification,
 } from '@aws-sdk/client-bedrock-runtime'
 import { ChatCompletionMessageToolCall } from 'openai/resources/index'
 
@@ -29,6 +32,7 @@ import { BaseHandler } from './base.js'
 import { InputError, InvariantError, MIMEType } from './types.js'
 import {
   consoleWarn,
+  convertMessageContentToString,
   fetchThenParseImage,
   getTimestamp,
   normalizeTemperature,
@@ -84,6 +88,7 @@ const toChatCompletionChoiceMessage = (
 ): CompletionResponse['choices'][0]['message'] => {
   if (output?.message?.content === undefined) {
     return {
+      refusal: null,
       content: '',
       role: 'assistant',
     }
@@ -146,6 +151,7 @@ const toChatCompletionChoiceMessage = (
       : ''
     return {
       role,
+      refusal: null,
       content: messageContent,
       tool_calls: toolCalls,
     }
@@ -153,6 +159,7 @@ const toChatCompletionChoiceMessage = (
     const content = textBlocks.map((textBlock) => textBlock.text).join('\n')
     return {
       role,
+      refusal: null,
       content,
       tool_calls: toolCalls,
     }
@@ -187,7 +194,10 @@ export const convertMessages = async (
   const systemMessages: Array<SystemContentBlock> = []
   if (supportsSystemMessages(model)) {
     while (clonedMessages.length > 0 && clonedMessages[0].role === 'system') {
-      systemMessages.push({ text: clonedMessages[0].content })
+      const messageContent = convertMessageContentToString(
+        clonedMessages[0].content
+      )
+      systemMessages.push({ text: messageContent })
       clonedMessages.shift()
     }
   }
@@ -247,7 +257,7 @@ export const convertMessages = async (
           toolUseId: message.tool_call_id,
           content: [
             {
-              text: message.content,
+              text: convertMessageContentToString(message.content),
             },
           ],
         },
@@ -288,7 +298,7 @@ export const convertMessages = async (
             const text = makeTextContent(message.role, e.text)
             return {
               text,
-            }
+            } as ContentBlock.TextMember
           } else {
             const parsedImage = await fetchThenParseImage(e.image_url.url)
             return {
@@ -298,7 +308,7 @@ export const convertMessages = async (
                   bytes: Buffer.from(parsedImage.content, 'base64'),
                 },
               },
-            }
+            } as ContentBlock.ImageMember
           }
         })
       )
@@ -354,27 +364,24 @@ export const convertToolParams = (
     return undefined
   }
 
-  const convertedTools =
+  const convertedTools: (Tool | undefined)[] =
     tools.length > 0
       ? tools.map((tool) => {
-          const inputSchema = tool.function.parameters
-            ? {
+          const inputSchema: ToolSpecification['inputSchema'] | undefined = tool
+            .function.parameters
+            ? ({
                 // Bedrock and OpenAI's function parameter types are incompatible even though they both
                 // adhere to the JSON schema, so we set the type to `any` to prevent a TypeScript error.
                 json: tool.function.parameters as any,
-                // TypeScript throws a type error if we don't define this field:
-                $unknown: undefined,
-              }
+              } satisfies ToolSpecification['inputSchema'])
             : undefined
           return {
-            // TypeScript throws a type error if we don't define this field:
-            $unknown: undefined,
             toolSpec: {
               name: tool.function.name,
               description: tool.function.description,
               inputSchema,
             },
-          }
+          } satisfies Tool
         })
       : undefined
 
@@ -387,7 +394,10 @@ export const convertToolParams = (
     convertedToolChoice = { tool: { name: toolChoice.function.name } }
   }
 
-  return { toolChoice: convertedToolChoice, tools: convertedTools }
+  return {
+    toolChoice: convertedToolChoice,
+    tools: convertedTools,
+  } satisfies ToolConfiguration
 }
 
 const convertStopReason = (
diff --git a/src/handlers/cohere.ts b/src/handlers/cohere.ts
index 5db1c17..dae0b66 100644
--- a/src/handlers/cohere.ts
+++ b/src/handlers/cohere.ts
@@ -28,7 +28,11 @@ import {
 } from '../userTypes/index.js'
 import { BaseHandler } from './base.js'
 import { InputError, InvariantError, MessageRole } from './types.js'
-import { consoleWarn, getTimestamp } from './utils.js'
+import {
+  consoleWarn,
+  convertMessageContentToString,
+  getTimestamp,
+} from './utils.js'
 
 type CohereMessageRole = 'CHATBOT' | 'SYSTEM' | 'USER' | 'TOOL'
 
@@ -41,6 +45,8 @@ const convertRole = (role: MessageRole): CohereMessageRole => {
     return 'TOOL'
   } else if (role === 'user') {
     return 'USER'
+  } else if (role === 'developer') {
+    return 'SYSTEM'
   } else {
     throw new InputError(`Unknown role: ${role}`)
   }
@@ -259,12 +265,13 @@ const toToolResult = (
     )
   }
 
+  const tollCallContentStr = convertMessageContentToString(toolMessage.content)
   const toolResult: ToolResult = {
     call: {
       name: toolCall.function.name,
       parameters: JSON.parse(toolCall.function.arguments),
     },
-    outputs: [JSON.parse(toolMessage.content)],
+    outputs: [JSON.parse(tollCallContentStr)],
   }
   return toolResult
 }
@@ -318,9 +325,10 @@ export const convertMessages = (
         })
       }
     } else if (message.role === 'assistant') {
+      const messageContentStr = convertMessageContentToString(message.content)
       chatHistory.push({
         role: convertRole(message.role),
-        message: message.content ?? '',
+        message: messageContentStr,
         toolCalls: message.tool_calls?.map((toolCall) => {
           return {
             name: toolCall.function.name,
@@ -567,6 +575,7 @@ export class CohereHandler extends BaseHandler<CohereModel> {
             logprobs: null,
             message: {
               role: 'assistant',
+              refusal: null, // openai requires this field, fill in if Cohere ever supports
               content: response.text,
               tool_calls: toolCalls,
             },
diff --git a/src/handlers/gemini.ts b/src/handlers/gemini.ts
index 2c3b570..bceb967 100644
--- a/src/handlers/gemini.ts
+++ b/src/handlers/gemini.ts
@@ -10,6 +10,7 @@ import {
   GenerateContentResult,
   GenerateContentStreamResult,
   GoogleGenerativeAI,
+  InlineDataPart,
   Part,
   TextPart,
   Tool,
@@ -31,7 +32,12 @@ import {
 } from '../userTypes/index.js'
 import { BaseHandler } from './base.js'
 import { InputError } from './types.js'
-import { consoleWarn, fetchThenParseImage, getTimestamp } from './utils.js'
+import {
+  consoleWarn,
+  convertMessageContentToString,
+  fetchThenParseImage,
+  getTimestamp,
+} from './utils.js'
 
 // Google's `GenerateContentCandidate.content` field should be optional, but it's a required field
 // in Google's types. This field can be undefined if a content filter is triggered when the user
@@ -81,29 +87,28 @@ export const convertContentsToParts = async (
       },
     ]
   } else {
-    return Promise.all(
-      contents.map(async (part) => {
-        if (part.type === 'text') {
-          return {
-            text: `${systemPrefix}${part.text}`,
-          }
-        } else if (part.type === 'image_url') {
-          const imageData = await fetchThenParseImage(part.image_url.url)
-          return {
-            inlineData: {
-              mimeType: imageData.mimeType,
-              data: imageData.content,
-            },
-          }
-        } else {
-          throw new InputError(
-            `Invalid content part type: ${
-              (part as any).type
-            }. Must be "text" or "image_url".`
-          )
-        }
-      })
-    )
+    const allParts: Promise<Part>[] = contents.map(async (part) => {
+      if (part.type === 'text') {
+        return {
+          text: `${systemPrefix}${part.text}`,
+        } as TextPart
+      } else if (part.type === 'image_url') {
+        const imageData = await fetchThenParseImage(part.image_url.url)
+        return {
+          inlineData: {
+            mimeType: imageData.mimeType,
+            data: imageData.content,
+          },
+        } satisfies InlineDataPart
+      } else {
+        throw new InputError(
+          `Invalid content part type: ${
+            (part as any).type
+          }. Must be "text" or "image_url".`
+        )
+      }
+    })
+    return Promise.all(allParts)
   }
 }
 
@@ -164,7 +169,9 @@ export const convertMessageToContent = async (
           {
             functionResponse: {
               name: message.tool_call_id,
-              response: JSON.parse(message.content),
+              response: JSON.parse(
+                convertMessageContentToString(message.content)
+              ),
             },
           },
         ],
@@ -304,6 +311,7 @@ export const convertResponseMessage = (
     content: candidate.content?.parts.map((part) => part.text).join('') ?? null,
     role: 'assistant',
     tool_calls: convertToolCalls(candidate),
+    refusal: null,
   }
 }
 
diff --git a/src/handlers/mistral.ts b/src/handlers/mistral.ts
index 715f930..6f2969a 100644
--- a/src/handlers/mistral.ts
+++ b/src/handlers/mistral.ts
@@ -26,6 +26,7 @@ import {
 } from '../userTypes/index.js'
 import { BaseHandler } from './base.js'
 import { InputError } from './types.js'
+import { convertMessageContentToString } from './utils.js'
 
 export const findLinkedToolCallName = (
   messages: ChatCompletionMessage[],
@@ -46,6 +47,7 @@ export const convertMessages = (
   messages: (ChatCompletionMessageParam | ChatCompletionMessage)[]
 ): Array<Message | ChatCompletionResponseChoice['message']> => {
   return messages.map((message) => {
+    const messageContent = convertMessageContentToString(message.content)
     if (message.role === 'tool') {
       const name = findLinkedToolCallName(
         messages as ChatCompletionMessage[],
@@ -55,7 +57,7 @@ export const convertMessages = (
       return {
         name,
         role: 'tool',
-        content: message.content,
+        content: messageContent,
         tool_call_id: message.tool_call_id,
       }
     }
@@ -63,12 +65,12 @@ export const convertMessages = (
     if (message.role === 'system') {
       return {
         role: message.role,
-        content: message.content ?? '',
+        content: messageContent,
       }
     } else if (message.role === 'assistant') {
       return {
         role: message.role,
-        content: message.content ?? '',
+        content: messageContent,
         tool_calls: message.tool_calls ?? null,
       }
     } else if (message.role === 'user') {
@@ -239,6 +241,7 @@ const toCompletionResponse = (
         index: choice.index,
         message: {
           role: 'assistant',
+          refusal: null,
           content: choice.message.content,
           tool_calls: convertToolCalls(choice.message.tool_calls),
         },
diff --git a/src/handlers/types.ts b/src/handlers/types.ts
index ebcb286..598e505 100644
--- a/src/handlers/types.ts
+++ b/src/handlers/types.ts
@@ -1,4 +1,10 @@
-export type MessageRole = 'system' | 'user' | 'assistant' | 'tool' | 'function'
+export type MessageRole =
+  | 'system'
+  | 'user'
+  | 'assistant'
+  | 'tool'
+  | 'function'
+  | 'developer'
 export type MIMEType = 'image/jpeg' | 'image/png' | 'image/gif' | 'image/webp'
 
 export class InputError extends Error {
diff --git a/src/handlers/utils.ts b/src/handlers/utils.ts
index 07c00a5..162afd1 100644
--- a/src/handlers/utils.ts
+++ b/src/handlers/utils.ts
@@ -1,5 +1,6 @@
 import chalk from 'chalk'
 import { lookup } from 'mime-types'
+import OpenAI from 'openai'
 
 import { LLMChatModel, LLMProvider } from '../chat/index.js'
 import { models } from '../models.js'
@@ -270,3 +271,21 @@ export const isObject = (variable: any): boolean => {
     variable && typeof variable === 'object' && variable.constructor === Object
   )
 }
+
+export const convertMessageContentToString = (
+  messageContent: OpenAI.Chat.Completions.ChatCompletionMessageParam['content']
+): string => {
+  if (!messageContent) {
+    return ''
+  }
+
+  return (
+    (typeof messageContent === 'string'
+      ? messageContent
+      : messageContent
+          .map(
+            (m: OpenAI.Chat.Completions.ChatCompletionContentPartText) => m.text
+          )
+          .join('\n')) ?? ''
+  )
+}
diff --git a/test/automated/handlers/cohere.test.ts b/test/automated/handlers/cohere.test.ts
index 6300f33..167a9e2 100644
--- a/test/automated/handlers/cohere.test.ts
+++ b/test/automated/handlers/cohere.test.ts
@@ -299,6 +299,13 @@ describe('convertMessages', () => {
         role: 'tool',
         content: '{"temperature":"22","unit":"fahrenheit"}',
       },
+      {
+        tool_call_id: 'call_lhMKBlOSnwwq5BZDCGo5SVTJ',
+        role: 'tool',
+        content: [
+          { text: '{"temperature":"100","unit":"fahrenheit"}', type: 'text' },
+        ],
+      },
     ]
 
     const { messages, lastUserMessage, toolResults } =
@@ -379,6 +386,20 @@ describe('convertMessages', () => {
           },
         ],
       },
+      {
+        call: {
+          name: 'get_current_weather',
+          parameters: {
+            location: 'Paris, France',
+          },
+        },
+        outputs: [
+          {
+            temperature: '100',
+            unit: 'fahrenheit',
+          },
+        ],
+      },
     ])
   })
 })
diff --git a/test/automated/handlers/gemini.test.ts b/test/automated/handlers/gemini.test.ts
index fd658c4..34e5853 100644
--- a/test/automated/handlers/gemini.test.ts
+++ b/test/automated/handlers/gemini.test.ts
@@ -143,6 +143,7 @@ describe('convertAssistantMessage', () => {
   it('should convert a message with text content and no tool calls', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'assistant',
+      refusal: null,
       content: 'Hello, world!',
       tool_calls: undefined,
     }
@@ -158,6 +159,7 @@ describe('convertAssistantMessage', () => {
   it('should convert a message with tool calls and no text content', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'assistant',
+      refusal: null,
       content: null,
       tool_calls: [
         {
@@ -181,6 +183,7 @@ describe('convertAssistantMessage', () => {
   it('should convert a message with both text content and tool calls', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'assistant',
+      refusal: null,
       content: 'Hello, world!',
       tool_calls: [
         {
@@ -205,6 +208,7 @@ describe('convertAssistantMessage', () => {
   it('should convert a message with null content and no tool calls', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'assistant',
+      refusal: null,
       content: null,
       tool_calls: undefined,
     }
@@ -220,6 +224,7 @@ describe('convertAssistantMessage', () => {
   it('should handle messages with multiple tool calls', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'assistant',
+      refusal: null,
       content: null,
       tool_calls: [
         {
@@ -249,6 +254,7 @@ describe('convertAssistantMessage', () => {
   it('should throw an InputError for an unexpected message role', () => {
     const message: OpenAI.Chat.Completions.ChatCompletionMessage = {
       role: 'unexpectedRole' as any,
+      refusal: null,
       content: 'Hello, world!',
       tool_calls: undefined,
     }
@@ -1012,6 +1018,7 @@ describe('convertResponseMessage', () => {
     expect(result).toEqual({
       content: 'Hello world',
       role: 'assistant',
+      refusal: null,
       tool_calls: [
         {
           id: 'mockId',
@@ -1040,6 +1047,7 @@ describe('convertResponseMessage', () => {
     expect(result).toEqual({
       content: 'Hello world',
       role: 'assistant',
+      refusal: null,
       tool_calls: undefined,
     })
   })
@@ -1058,6 +1066,7 @@ describe('convertResponseMessage', () => {
     expect(result).toEqual({
       content: '',
       role: 'assistant',
+      refusal: null,
       tool_calls: undefined,
     })
   })
@@ -1079,6 +1088,7 @@ describe('convertResponseMessage', () => {
     expect(result).toEqual({
       content: '',
       role: 'assistant',
+      refusal: null,
       tool_calls: [
         {
           id: 'mockId',
@@ -1421,6 +1431,7 @@ describe('GeminiHandler', () => {
           index: 0,
           message: {
             role: 'assistant',
+            refusal: null,
             content:
               mockBasicChatResponse.response.candidates![0].content.parts[0]
                 .text,
@@ -1464,6 +1475,7 @@ describe('GeminiHandler', () => {
           index: 0,
           message: {
             role: 'assistant',
+            refusal: null,
             content: '',
             tool_calls: [
               {
diff --git a/test/automated/handlers/mistral.test.ts b/test/automated/handlers/mistral.test.ts
index 44dff16..62e9953 100644
--- a/test/automated/handlers/mistral.test.ts
+++ b/test/automated/handlers/mistral.test.ts
@@ -776,6 +776,7 @@ describe('MistralHandler', () => {
           index: 0,
           message: {
             role: 'assistant',
+            refusal: null,
             content: mockBasicChatResponse.choices[0].message.content,
             tool_calls: undefined,
           },
@@ -809,6 +810,7 @@ describe('MistralHandler', () => {
           index: 0,
           message: {
             role: 'assistant',
+            refusal: null,
             content: mockToolChatResponse.choices[0].message.content,
             tool_calls: [
               {
diff --git a/tsconfig.json b/tsconfig.json
index 897c535..1b8fee4 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,26 +1,25 @@
 {
   "compilerOptions": {
+    "composite": true,
+    "declaration": true,
+    "declarationDir": "./dist",
+    "emitDeclarationOnly": true,
+    "emitDecoratorMetadata": true,
+    "esModuleInterop": true,
+    "experimentalDecorators": true,
     "module": "NodeNext",
-    "target": "ES2015",
     "moduleResolution": "NodeNext",
-    "sourceMap": true,
-    "esModuleInterop": true,
-    "composite": true,
     "noImplicitAny": false,
-    "removeComments": true,
     "noLib": false,
-    "emitDecoratorMetadata": true,
-    "experimentalDecorators": true,
-    "strictNullChecks": true,
-    "typeRoots": [
-      "node_modules/@types"
-    ],
-    "rootDir": "./src",
     "outDir": "./dist",
+    "removeComments": true,
+    "rootDir": "./src",
     "skipLibCheck": true,
-    "declaration": true,
-    "declarationDir": "./dist",
-    "emitDeclarationOnly": true
+    "sourceMap": true,
+    "target": "ES2015",
+    "typeRoots": [
+      "node_modules/@types"
+    ]
   },
   "exclude": [
     "node_modules",