Tanstack AI Core Middleware

Middleware

Dependency note: This skill builds on ai-core. Read it first for critical rules.

Setup — Analytics Tracking Middleware

import { chat, toServerSentEventsResponse } from '@tanstack/ai'
import { openaiText } from '@tanstack/ai-openai'
import { trackAnalytics, reportError } from './analytics'

export async function POST(request: Request) {
  const { messages } = await request.json()

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    middleware: [
      {
        onStart: (ctx) => {
          console.log('Chat started:', ctx.model)
        },
        onFinish: (ctx, info) => {
          trackAnalytics({ model: ctx.model, tokens: info.usage?.totalTokens })
        },
        onError: (ctx, info) => {
          reportError(info.error)
        },
      },
    ],
  })

  return toServerSentEventsResponse(stream)
}

Hooks Reference

Every hook receives a ChatMiddlewareContext as its first argument, which provides requestId, streamId, phase, iteration, chunkIndex, model, provider, signal, abort(), defer(), and more.

Terminal hooks (onFinish, onAbort, onError) are mutually exclusive -- exactly one fires per chat() invocation.

Sampling in onConfig: temperature, topP, and maxTokens are not first-class fields on ChatMiddlewareConfig. To adjust sampling from middleware, return a partial that mutates config.modelOptions using the provider's native key (e.g. OpenAI temperature / max_output_tokens, Anthropic max_tokens, Ollama nested options.num_predict). Returning a top-level temperature/maxTokens has no effect.

Phase values

ctx.phase is one of:

Separate-finalization path (adapters without native-combined support):

• onStructuredOutputConfig fires before onConfig at the structured-output boundary.
• onConfig re-fires at the same boundary with ctx.phase === 'structuredOutput', receiving the post-onStructuredOutputConfig view of the config (minus outputSchema).
• onChunk and onUsage fire for every chunk and usage event emitted by the structured-output call, with ctx.phase === 'structuredOutput'.
• onIteration does not fire for finalization — it is agent-loop-only.
• Terminal info and structured-output: info.usage / info.finishReason / info.content reflect the agent loop's terminal state, NOT the finalization step. Finalization state is intentionally segregated to keep agent-loop semantics clean. For a tools-less chat({ outputSchema }) run, info.usage is undefined and info.finishReason is null (no agent-loop iteration produced RUN_FINISHED). To capture finalization tokens, use onUsage — it fires for both agent-loop iterations and the final call. For the structured-output result itself, observe the structured-output.complete CUSTOM event in onChunk.

Native-combined output:

• The schema-constrained JSON is produced by a normal agent-loop iteration. onStructuredOutputConfig does not fire, ctx.phase remains 'modelStream', and onIteration fires for that iteration.
• info.content includes the structured JSON because it is agent-loop text. Middleware observes the structured-output.complete event in onChunk during the same phase.

Both paths:

• On successful completion, onFinish fires once after the structured result completes. Terminal-hook exclusivity still holds.
• By onFinish, ctx.messages includes the completed terminal assistant messages. Native-combined output keeps the structured result on its terminal assistant message. The separate-finalization path can preserve the agent loop's plain-text message followed by a distinct structured-output message.

onStructuredOutputConfig

A dedicated config hook that fires only at the separate-finalization boundary. Use it to transform the JSON Schema sent to the provider (inject $defs, strip vendor-incompatible keywords) or to apply structured-output- specific config changes that should not affect the agent-loop adapter calls.

Signature:

import type {
  ChatMiddlewareContext,
  StructuredOutputMiddlewareConfig,
} from '@tanstack/ai'

// Excerpt of the `ChatMiddleware` interface exported by '@tanstack/ai'
interface ChatMiddleware {
  onStructuredOutputConfig?: (
    ctx: ChatMiddlewareContext,
    config: StructuredOutputMiddlewareConfig,
  ) =>
    | void
    | null
    | Partial<StructuredOutputMiddlewareConfig>
    | Promise<void | null | Partial<StructuredOutputMiddlewareConfig>>
}

StructuredOutputMiddlewareConfig shape:

import type { ChatMiddlewareConfig, JSONSchema } from '@tanstack/ai'

// As exported by '@tanstack/ai'
interface StructuredOutputMiddlewareConfig extends Omit<
  ChatMiddlewareConfig,
  'tools'
> {
  outputSchema: JSONSchema // The JSON Schema being sent to the provider
}

Note the Omit<…, 'tools'>: there is no config.tools on this hook. The structured-output call is the final, tool-free call, so reading or returning tools here is a compile error, not a no-op. Transform tools in onConfig instead.

Ordering rule:

• onStructuredOutputConfig fires before onConfig at the structured-output boundary.
• onConfig re-fires at the same boundary with ctx.phase === 'structuredOutput', receiving the post-onStructuredOutputConfig view of the config (minus outputSchema).
• Use onConfig for general-purpose transforms that apply to every adapter call (agent-loop iterations and the final structured-output call).
• Use onStructuredOutputConfig when you need to transform the JSON Schema or apply structured-output-specific behavior.

Core Patterns

Pattern 1: Analytics and Logging Middleware

Use onStart, onFinish, onUsage, and onError for comprehensive observability. Use ctx.defer() for non-blocking async side effects that should not block the stream.

import {
  chat,
  toServerSentEventsResponse,
  type ChatMiddleware,
} from '@tanstack/ai'
import { openaiText } from '@tanstack/ai-openai'

const analytics: ChatMiddleware = {
  name: 'analytics',
  onStart: (ctx) => {
    console.log(`[${ctx.requestId}] Chat started — model: ${ctx.model}`)
  },
  onUsage: (ctx, usage) => {
    console.log(`[${ctx.requestId}] Tokens: ${usage.totalTokens}`)
  },
  onFinish: (ctx, info) => {
    ctx.defer(
      fetch('/api/analytics', {
        method: 'POST',
        body: JSON.stringify({
          requestId: ctx.requestId,
          model: ctx.model,
          duration: info.duration,
          tokens: info.usage?.totalTokens,
          finishReason: info.finishReason,
        }),
      }),
    )
  },
  onError: (ctx, info) => {
    ctx.defer(
      fetch('/api/errors', {
        method: 'POST',
        body: JSON.stringify({
          requestId: ctx.requestId,
          error: String(info.error),
          duration: info.duration,
        }),
      }),
    )
  },
}

export async function POST(request: Request) {
  const { messages } = await request.json()

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    middleware: [analytics],
  })

  return toServerSentEventsResponse(stream)
}

Pattern 2: Tool Interception Middleware

Use onBeforeToolCall to validate, gate, or transform tool arguments before execution. Use onAfterToolCall to log results and timing. The first middleware that returns a non-void decision from onBeforeToolCall short-circuits remaining middleware for that call.

import type { ChatMiddleware } from '@tanstack/ai'

const toolGuard: ChatMiddleware = {
  name: 'tool-guard',
  onBeforeToolCall: (ctx, hookCtx) => {
    // Block dangerous tools
    if (hookCtx.toolName === 'deleteDatabase') {
      return { type: 'abort', reason: 'Dangerous operation blocked' }
    }

    // Enforce default arguments. `hookCtx.args` is `unknown` — the provider
    // sent it — so narrow before reading it. No `as` casts.
    if (hookCtx.toolName === 'search') {
      const args =
        typeof hookCtx.args === 'object' && hookCtx.args !== null
          ? hookCtx.args
          : {}
      if (!('limit' in args)) {
        return {
          type: 'transformArgs',
          args: { ...args, limit: 10 },
        }
      }
    }

    // Return void to continue normally
  },
  onAfterToolCall: (ctx, info) => {
    if (info.ok) {
      console.log(`${info.toolName} completed in ${info.duration}ms`)
    } else {
      console.error(`${info.toolName} failed:`, info.error)
    }
  },
}

onBeforeToolCall decision types:

Pattern 3: Structured-Output Middleware

On the separate-finalization path, the final structured-output adapter call flows through the same middleware chain as the agent loop with ctx.phase === 'structuredOutput'. Native-combined output has no separate provider call: middleware observes its chunks during modelStream, and onStructuredOutputConfig does not fire. Middleware cannot transform the native-combined schema.

Example A — Observability (tracing every chunk, including separate finalization):

import type { ChatMiddleware } from '@tanstack/ai'
import { trace } from '@opentelemetry/api'

const tracing: ChatMiddleware = {
  name: 'tracing',
  onChunk(ctx, chunk) {
    trace
      .getActiveSpan()
      ?.addEvent('chunk', { phase: ctx.phase, type: chunk.type })
  },
}

On the separate-finalization path, this middleware observes every chunk from the final structured-output call with ctx.phase === 'structuredOutput'. On the native-combined path, it observes the structured stream with ctx.phase === 'modelStream'.

Example B — Schema rewriting (inject shared $defs):

import type { ChatMiddleware } from '@tanstack/ai'
import { sharedDefs } from './defs'

const injectDefs: ChatMiddleware = {
  name: 'inject-defs',
  onStructuredOutputConfig(_ctx, config) {
    return {
      outputSchema: { ...config.outputSchema, $defs: { ...sharedDefs } },
    }
  },
}

onStructuredOutputConfig is the right hook here on the separate-finalization path because it has direct access to config.outputSchema. Native-combined schema transformation is not exposed through middleware.

Pattern 4: Multiple Middleware Composition

Middleware executes in array order (left-to-right). Ordering matters for hooks that pipe or short-circuit:

import {
  chat,
  toolDefinition,
  toServerSentEventsResponse,
  type ChatMiddleware,
} from '@tanstack/ai'
import { toolCacheMiddleware } from '@tanstack/ai/middlewares'
import { openaiText } from '@tanstack/ai-openai'
import { z } from 'zod'

const weatherTool = toolDefinition({
  name: 'getWeather',
  description: 'Get the current weather for a city',
  inputSchema: z.object({ city: z.string() }),
}).server(async ({ city }) => ({ city, tempC: 21 }))

const stockTool = toolDefinition({
  name: 'getStock',
  description: 'Get the latest price for a ticker symbol',
  inputSchema: z.object({ symbol: z.string() }),
}).server(async ({ symbol }) => ({ symbol, price: 123.45 }))

const logging: ChatMiddleware = {
  name: 'logging',
  onStart: (ctx) => console.log(`[${ctx.requestId}] started`),
  onChunk: (ctx, chunk) => {
    console.log(`[${ctx.requestId}] chunk: ${chunk.type}`)
  },
  onFinish: (ctx, info) => {
    console.log(`[${ctx.requestId}] done in ${info.duration}ms`)
  },
}

const configTransform: ChatMiddleware = {
  name: 'config-transform',
  onConfig: (ctx, config) => {
    if (ctx.phase === 'init') {
      return {
        systemPrompts: [...config.systemPrompts, 'Always respond in JSON.'],
        // Sampling options are NOT first-class config fields — mutate them
        // through `config.modelOptions` using the provider's native key.
        // (e.g. OpenAI `temperature` / `max_output_tokens`.)
        modelOptions: { ...config.modelOptions, temperature: 0.2 },
      }
    }
  },
}

export async function POST(request: Request) {
  const { messages } = await request.json()

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    tools: [weatherTool, stockTool],
    middleware: [
      logging, // Runs first
      configTransform, // Transforms config second
      toolCacheMiddleware({ ttl: 60_000 }), // Caches tool results third
    ],
  })

  return toServerSentEventsResponse(stream)
}

Composition rules by hook:

Pattern: tool-call budget (app-owned)

Not a built-in. Cap fan-out with onBeforeToolCall skip + onShouldContinue. See docs/chat/agentic-cycle.md ("Tool-call budgets").

import {
  chat,
  maxIterations,
  toolDefinition,
  toServerSentEventsResponse,
  type ChatMiddleware,
} from '@tanstack/ai'
import { openaiText } from '@tanstack/ai-openai'
import { z } from 'zod'

const weatherTool = toolDefinition({
  name: 'getWeather',
  description: 'Get the current weather for a city',
  inputSchema: z.object({ city: z.string() }),
}).server(async ({ city }) => ({ city, tempC: 21 }))

function toolCallBudget(opts: {
  max?: number
  maxPerTurn?: number
}): ChatMiddleware {
  let perTurn = 0
  return {
    onIteration: () => {
      perTurn = 0
    },
    onToolPhaseComplete: () => {
      perTurn = 0
    },
    onBeforeToolCall: () => {
      if (opts.maxPerTurn == null) return undefined
      if (++perTurn > opts.maxPerTurn) {
        return {
          type: 'skip',
          result: {
            error: `Skipped: exceeded maxToolCallsPerTurn (${opts.maxPerTurn})`,
          },
        }
      }
      return undefined
    },
    onShouldContinue: (_ctx, state) =>
      opts.max != null && state.toolCallCount >= opts.max ? false : undefined,
  }
}

export async function POST(request: Request) {
  const { messages } = await request.json()

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    tools: [weatherTool],
    agentLoopStrategy: maxIterations(20),
    middleware: [toolCallBudget({ maxPerTurn: 10, max: 20 })],
  })

  return toServerSentEventsResponse(stream)
}

Built-in: toolCacheMiddleware

Caches tool call results by name + arguments. Import from @tanstack/ai/middlewares:

import { chat, toolDefinition, toServerSentEventsResponse } from '@tanstack/ai'
import { toolCacheMiddleware } from '@tanstack/ai/middlewares'
import { openaiText } from '@tanstack/ai-openai'
import { z } from 'zod'

const weatherTool = toolDefinition({
  name: 'getWeather',
  description: 'Get the current weather for a city',
  inputSchema: z.object({ city: z.string() }),
}).server(async ({ city }) => ({ city, tempC: 21 }))

export async function POST(request: Request) {
  const { messages } = await request.json()

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    tools: [weatherTool],
    middleware: [
      toolCacheMiddleware({
        ttl: 60_000, // Cache entries expire after 60 seconds
        maxSize: 50, // Max 50 entries (LRU eviction)
        toolNames: ['getWeather'], // Only cache specific tools
      }),
    ],
  })

  return toServerSentEventsResponse(stream)
}

Options: maxSize (default 100), ttl (default Infinity), toolNames (default all), keyFn (custom cache key), storage (custom backend like Redis). See docs/advanced/middleware.md for custom storage examples.

Server State Persistence: withPersistence

withPersistence(persistence) (from @tanstack/ai-persistence) is a ChatMiddleware that persists state for chat() — thread messages, run records (status/timing/usage/errors), and interrupt state — to a backend store. Add it to the middleware array like any other middleware. It never mutates the chunk stream; replaying a dropped/reloaded stream is a separate transport-layer concern (see ../tanstack-ai-core-chat-experience/SKILL.md resumability, not this middleware).

import {
  chat,
  chatParamsFromRequest,
  toServerSentEventsResponse,
} from '@tanstack/ai'
import { openaiText } from '@tanstack/ai-openai'
import { withPersistence, memoryPersistence } from '@tanstack/ai-persistence'

// memoryPersistence() is the in-process reference backend (dev/tests). For a
// durable one, implement the store contracts against your database — see the
// @tanstack/ai-persistence skills.
const persistence = memoryPersistence()

export async function POST(request: Request) {
  const params = await chatParamsFromRequest(request)

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages: params.messages,
    threadId: params.threadId,
    runId: params.runId,
    ...(params.resume ? { resume: params.resume } : {}),
    middleware: [withPersistence(persistence)],
  })

  return toServerSentEventsResponse(stream)
}

Authoritative-history contract

The middleware treats each request's messages as the source of truth for the thread:

• Non-empty messages → on a successful finish (and at an interrupt boundary) the middleware overwrites the entire stored thread with that array. Post the complete transcript, never just the newest message(s) — a delta would replace and destroy the stored history.
• Empty messages → the middleware loads the stored thread and runs the turn from the server's copy. This is how you continue a conversation without resending history from the client.

Backends

@tanstack/ai-persistence ships contracts, not a database backend. It provides the four store interfaces (messages, runs, interrupts, metadata), the middleware that drives them, memoryPersistence() for dev/tests, and a conformance testkit. For anything durable you implement the stores against your own database and pass the result to withPersistence.

Annotate your factory with a named shape (ChatPersistence / ChatTranscriptPersistence) — bare AIPersistence is the all-optional bag and withPersistence rejects it.

Locks are separate from state and are not a stores key: wire a LockStore with withLocks(lockStore).

The runs store in that list is typed against RunStore, which ships in @tanstack/ai alongside RunRecord, RunStatus, TerminalRunStatus, RunError, isTerminalRunStatus, defineRunStore, and InMemoryRunStore. A RunRecord tracks one run: runId, threadId, status, startedAt, plus optional finishedAt, error, usage, sandboxKey, detachedSince, cancelRequested, and driverEpoch. A backend must round-trip all of them: cancelRequested is the durable out-of-band cancel channel (requestRunCancel writes it, wasCancelRequested reads it), and driverEpoch is the monotonic fencing token each host bumps when it claims a run, so a superseded host can discover it lost. Omit either and a durable sandboxed run loses a mechanism silently — Stop stops reaching a remote driver, or nothing fences a dead host's writes. error is a structured RunError ({ message: string, code?: string }), not a bare string: message is the provider's prose, code is the stable, machine-branchable classification a consumer switches on. Only createOrResume, update, get, and findActiveRun are required on a RunStore; listByThread and listReclaimable are optional, so a backend can leave either out and callers feature-detect (store.listReclaimable?.(opts)). Shape your own store with defineRunStore for autocomplete without a separate : RunStore annotation, matching defineLock; defineRunStore<const T extends RunStore>(store: T): T returns the argument's own type, so an optional method your store implements stays known-present on the result instead of collapsing to | undefined. isTerminalRunStatus(status) is a type predicate narrowing RunStatus to TerminalRunStatus, so code inside the guard can pass status where a TerminalRunStatus is required without a cast. When a backend omits an optional RunStore method, declare the omission when running the conformance testkit (ai-persistence/stores's skipMethods option) rather than leaving it undeclared.

StreamDurability.snapshot()

A StreamDurability (the event-log backend memoryStream / durableStream implement, and what @tanstack/ai-sandbox's run driver resolves per run — its RunDeps.durability / sandboxRunDriver({ durability }) is a factory (runId) => StreamDurability, because one log is bound to one run) requires a snapshot() method alongside append, read, and close:

import type { StreamChunk } from '@tanstack/ai'

// Excerpt of the `StreamDurability` interface exported by '@tanstack/ai'
interface StreamDurability<TOffset extends string = string> {
  snapshot: () => Promise<Array<{ offset: TOffset; chunk: StreamChunk }>>
}

It returns everything stored for a run right now, in append order, then resolves. Use it, not read(), when a caller needs to inspect a run's stored prefix and get an answer back: read() tails and only resolves once the log is terminalized with close() or the caller aborts, so it never resolves against a producer that crashed without calling close(), and its log stays open indefinitely. snapshot() resolves immediately with what is stored, including while the log is still open, and resolves to an empty array for a run with nothing stored yet.

Full guidance lives in the package's own skills — start at node_modules/@tanstack/ai-persistence/skills/ai-persistence/SKILL.md, which routes to the server, client, stores, locks, and adapter-recipe (Drizzle / Prisma / Cloudflare) sub-skills.

Resume reconstruction is the middleware's job (server-authoritative path)

When a thread has pending interrupts, the middleware records them and gates new input: a request that carries pending interrupts must include a resume batch that references them, or onConfig throws. On a valid resume batch the middleware also builds ChatResumeToolState (approvals / client-tool results) and clears config.resume so the chat engine skips its ephemeral reconstruction — that path needs client message history the persistence flow deliberately omits when the server owns the transcript. Resumes accepted in onConfig are committed (marked resolved/cancelled) only once the run reaches a successful boundary, so a provider failure between accepting a resume and finishing leaves the interrupt pending and a retry with the same resume succeeds.

A companion withGenerationPersistence(persistence) tracks run records for non-chat generation activities (image, audio, TTS, video, transcription).

Source: docs/persistence/overview.md

Sandbox File-Event Hooks (sandbox group)

Declare a sandbox: ChatSandboxHooks group on defineChatMiddleware to react to every file created/changed/deleted inside a sandbox provided by withSandbox (from @tanstack/ai-sandbox). These fire per-run, server-side, and each handler receives the run's ChatMiddlewareContext as the first argument:

import { defineChatMiddleware } from '@tanstack/ai'
import { db } from './db'

const auditMiddleware = defineChatMiddleware({
  name: 'audit',
  sandbox: {
    onFile: (ctx, e) => console.log(ctx.runId, e.type, e.path),
    onFileCreate: (ctx, e) => db.log({ run: ctx.runId, event: e }),
  },
})

These are independent of the stream: the engine also emits a sandbox.file CUSTOM chunk per change regardless of whether any sandbox hooks are registered, so a client can react to the same edits without middleware. See ../tanstack-ai-core-ag-ui-protocol/SKILL.md for reading that chunk (and the opt-in sandbox.file.diff chunk) off ChatStream.

before() / after() / diff() — lazy, git-backed content accessors

Each hook receives a SandboxFileHookEvent: the serializable { type, path, timestamp } plus three lazy accessors for the file's content:

interface SandboxFileHookEvent {
  type: 'create' | 'change' | 'delete'
  path: string
  timestamp: number
  before(): Promise<string> // content at the session baseline ('' if new / non-git)
  after(): Promise<string> // current content ('' if deleted)
  diff(): Promise<string> // unified patch vs the baseline
}

import { defineChatMiddleware } from '@tanstack/ai'
import { db } from './db'

const auditMiddleware = defineChatMiddleware({
  name: 'audit',
  sandbox: {
    onFileChange: async (ctx, e) => {
      const [before, after] = await Promise.all([e.before(), e.after()])
      db.log({ run: ctx.runId, path: e.path, before, after })
    },
  },
})

Lazy — path-only hooks pay nothing. before(), after(), and diff() are methods, not fields: each only reads the file or shells out to git when called. A hook that only reads e.path/e.type (like the onFile logger above) never touches the filesystem or spawns a process.

Git session baseline. The sandbox snapshots git rev-parse HEAD once at setup as the session baseline (empty string if the workspace isn't a git repo or has no commits). before() and diff() always diff against that same fixed baseline for the rest of the run, so onFileChange reports the file's cumulative change since the run started, not just the delta since the last poll. after() always reads current on-disk content. None of the three accessors throw: a deleted file resolves after() to '' (it still has before()); a new file resolves before() to '' (it still has after()); a non-git workspace resolves both before() and after() to '' and makes diff() fall back to a synthesized add-patch built from after() — except for a delete event in a non-git workspace, where there's nothing to synthesize and diff() resolves to ''. In a git workspace a file git isn't tracking yet (a file the agent created, and every later edit to it) diffs empty because git diff ignores untracked files, so diff() falls back to the same synthesized add-patch whenever the file is absent at the baseline — a create-or-edit of an untracked file never streams an empty diff. An empty diff for a tracked file (identical to the baseline) stays empty, as it should. A git-ignored file is withheld: the file event still fires (you're notified it changed) but diff() returns '', so a secret like a .env never has its contents surfaced in the diff feed.

Failures are logged, not silent. Every git/exec/fs failure behind these accessors (and behind the find-poll watcher) still falls back to ''/an empty snapshot, but logs first: real anomalies (a failed git diff, an unreadable file, a lost find poll) under the errors category (on by default); expected-empty conditions (a new file's before(), a non-git baseline) under the sandbox debug category.

Hook errors are swallowed per hook. A throwing sandbox hook is caught and logged under the errors category (on by default) — it cannot break the run or stop other hooks (or the sandbox.file chunk) from continuing.

Source: docs/sandbox/observability.md

Common Mistakes

a. MEDIUM: Trying to modify StreamChunks in middleware

import type { ChatMiddleware } from '@tanstack/ai'

// WRONG -- mutating the chunk object directly
const broken: ChatMiddleware = {
  name: 'broken',
  onChunk: (ctx, chunk) => {
    if (chunk.type === 'TEXT_MESSAGE_CONTENT') {
      chunk.delta = 'modified' // Mutation does nothing; chunk is not modified in-place
    }
  },
}

// CORRECT -- return a new chunk to replace the original
const correct: ChatMiddleware = {
  name: 'correct',
  onChunk: (ctx, chunk) => {
    if (chunk.type === 'TEXT_MESSAGE_CONTENT') {
      return { ...chunk, delta: chunk.delta.replace(/secret/g, '[REDACTED]') }
    }
    // Return void to pass through unchanged
  },
}

Middleware onChunk hooks are functional transforms. Return a new chunk, an array of chunks, null (to drop), or void (to pass through). Mutating the input object has no effect on the stream output.

Source: docs/advanced/middleware.md

b. MEDIUM: Middleware exceptions breaking the stream — in onChunk / onConfig

Know which hooks the framework already guards. The terminal hooks (onFinish, onAbort, onError) are individually wrapped by core's runTerminalHook: a throw there is logged on the errors channel and the next middleware's terminal hook still runs, so a failed analytics POST in onFinish cannot break the stream or replace the abort reason. Guarding those is about keeping your own bookkeeping intact, not about protecting the run.

onChunk and onConfig are NOT guarded, deliberately — they are transforms on the data path, where swallowing a throw would forward a chunk or a config the middleware had decided to reject. A throw from either fails the whole stream. That is where an unhandled error actually costs you a response:

import type { ChatMiddleware } from '@tanstack/ai'
import { logChunk, requireEnv } from './logging'

// WRONG -- an unhandled error in onChunk kills the entire streaming response
const fragile: ChatMiddleware = {
  name: 'fragile-chunk-logger',
  onChunk: (ctx, chunk) => {
    // A logger that throws on an unexpected chunk shape takes the stream with it
    logChunk(chunk)
  },
  onConfig: (ctx, config) => {
    // Same for a config transform that reads an env var that is not set
    return {
      modelOptions: {
        ...config.modelOptions,
        temperature: Number(requireEnv('TEMPERATURE')),
      },
    }
  },
}

// CORRECT -- own the failure inside the unguarded hooks
const resilient: ChatMiddleware = {
  name: 'resilient-chunk-logger',
  onChunk: (ctx, chunk) => {
    try {
      logChunk(chunk)
    } catch (err) {
      console.error('Logging failed:', err)
    }
    // Return void to pass through
  },
  onConfig: (ctx, config) => {
    const temperature = process.env.TEMPERATURE
    // Decide, do not throw: no override means no transform.
    if (temperature === undefined) return undefined
    return {
      modelOptions: {
        ...config.modelOptions,
        temperature: Number(temperature),
      },
    }
  },
  onFinish: (ctx, info) => {
    // Already guarded by core — but prefer ctx.defer() anyway, so a slow
    // analytics call does not delay the terminal fan-out at all.
    ctx.defer(
      fetch('/api/analytics', {
        method: 'POST',
        body: JSON.stringify({ duration: info.duration }),
      }),
    )
  },
}

Rule: put the try-catch where the framework has none — onChunk and onConfig (and the other transform hooks: onStructuredOutputConfig, onBeforeToolCall, onAfterToolCall). For async side effects in the terminal hooks, prefer ctx.defer(), which runs after the terminal hook and isolates failures.

Source: docs/advanced/middleware.md, packages/ai/src/activities/chat/middleware/compose.ts

Cross-References

• See also: ../tanstack-ai-core-chat-experience/SKILL.md -- Middleware hooks into the chat lifecycle
• See also: ../tanstack-ai-core-structured-outputs/SKILL.md -- Separate finalization uses onStructuredOutputConfig for JSON-Schema transforms; native-combined schema transformation is not exposed through middleware
• See also: ../tanstack-ai-core-ag-ui-protocol/SKILL.md -- Reading the sandbox.file / sandbox.file.diff CUSTOM chunks the sandbox runtime emits alongside these sandbox hooks, via ChatStream's typed KnownCustomEvent narrowing
• See also: @tanstack/ai-persistence skills (../tanstack-ai-persistence/SKILL.md in that package) -- Full persistence suite (withPersistence, client storage, store contracts, adapter recipes, locks). This file only sketches server withPersistence.