@anthropic-ai/sdk/lib/middleware.mjs

The official TypeScript library for the Anthropic API

import { AnthropicError } from "../core/error.mjs";
import { Stream } from "../core/streaming.mjs";
import { isAbortError } from "../internal/errors.mjs";
import { appendHeaderValue } from "../internal/headers.mjs";
import { STAINLESS_HELPER_HEADER } from "../internal/stainless-helper-header.mjs";
import { safeJSON } from "../internal/utils/values.mjs";
export { BetaFallbackState } from "../internal/request-options.mjs";
const encoder = new TextEncoder();
/** Betas sent by default; override with {@link BetaRefusalFallbackOptions.betas}. */
const DEFAULT_BETAS = ['fallback-credit-2026-07-01'];
/**
 * Remove `fallback` blocks replayed in history. They only parse under the
 * server-side fallback beta, which belongs to the caller-owned server-side
 * `fallbacks` feature — this middleware never sends it, so a request
 * replaying them would 400. An assistant turn left empty is dropped whole.
 */
function stripFallbackBlocks(body) {
    const messages = body.messages
        .map((message) => Array.isArray(message.content) ?
        { ...message, content: message.content.filter((block) => block.type !== 'fallback') }
        : message)
        .filter((message) => !Array.isArray(message.content) || message.content.length > 0);
    return { ...body, messages };
}
/**
 * Apply one chain entry to the original request params as a patch: a field
 * set to a value overrides the original, a field explicitly `null` removes
 * the field from the retried request, and an absent (or `undefined`) field
 * keeps the original value. `output_config` patches one level deep — its
 * subfields follow the same set/`null`/absent rules against the original
 * `output_config` (created if the entry sets any subfield). Every hop patches
 * the original params — never a previous hop's patched body — and `body` is
 * never mutated.
 */
function applyFallbackPatch(body, entry) {
    const patched = { ...body };
    for (const [key, value] of Object.entries(entry)) {
        if (key === 'output_config' && value != null) {
            const merged = { ...(patched[key] ?? {}) };
            for (const [subKey, subValue] of Object.entries(value)) {
                patchField(merged, subKey, subValue);
            }
            patchField(patched, key, Object.keys(merged).length ? merged : null);
        }
        else {
            patchField(patched, key, value);
        }
    }
    return patched;
}
/** Set/`null`-unset/absent-keep one field on `target` (mutated). */
function patchField(target, key, value) {
    if (value === undefined)
        return;
    if (value === null) {
        delete target[key];
    }
    else {
        target[key] = value;
    }
}
/**
 * Middleware that retries refused `/v1/messages` requests down a fallback chain.
 *
 * Non-streaming: when a response comes back with `stop_reason: 'refusal'`, the
 * request is retried with each entry of `fallbacks` applied as a patch to the
 * original params (a set field overrides, an explicit `null` unsets, an absent
 * field keeps the original value; entries never patch each other's requests)
 * — passing along the refusal's `fallback_credit_token` — until a model
 * accepts or the chain is exhausted. A message served by a fallback carries a
 * `fallback` content block prepended at each model boundary — the same seam
 * block shape the server-side `fallbacks` param places in `content`, though
 * the rest of the envelope is the serving hop's as returned (see the
 * known-divergences note below); an exhausted chain surfaces the final
 * refusal verbatim.
 *
 * Streaming: when the stream ends in `stop_reason: 'refusal'`, a second
 * request is issued to the fallback model — carrying the refused model's
 * partial output as a trailing assistant prefill when the refusal grants one
 * (`fallback_has_prefill_claim`), plus the refusal's `fallback_credit_token`
 * — and the fallback's events are spliced onto the
 * still-open stream, so the client sees one continuous message in the
 * server-side `fallbacks` wire shape: a `fallback` content block at each model
 * boundary, monotonic block indices, and per-hop `usage.iterations` on the
 * final `message_delta`. Only `model` is honored from each entry on this path:
 * the credit token is redeemable only against the refused request's body, so
 * the other per-entry overrides (`max_tokens`, `thinking`, ...) would be
 * rejected.
 *
 * The fallback-credit beta the credit tokens require is sent by default on
 * every request the middleware handles; the `betas` option controls this.
 *
 * In both modes a fallback that itself refuses with a fresh credit token
 * continues down the chain. A streaming fallback whose prefill the server
 * rejects (HTTP 400) is retried once without it; a fallback whose request
 * fails outright is skipped — its token was never redeemed, so it carries to
 * the next entry.
 *
 * To keep later requests on the model that accepted, pass a
 * {@link BetaFallbackState} via the `fallbackState` request option; requests
 * sharing that state start directly at the pinned fallback. Reuse one state
 * across whatever scope the pin should apply to — typically a conversation.
 *
 * @example
 * ```ts
 * const client = new Anthropic({
 *   middleware: [betaRefusalFallbackMiddleware([{ model: 'claude-opus-4-8' }])],
 * });
 *
 * const fallbackState = new BetaFallbackState();
 * const message = await client.beta.messages.create(params, { fallbackState });
 * ```
 */
export function betaRefusalFallbackMiddleware(fallbacks, options = {}) {
    let warnedMissingState = false;
    return async (request, next, ctx) => {
        // This middleware only applies to the beta messages API
        // (`client.beta.messages`, which posts to `/v1/messages?beta=true`).
        // An empty chain also disables this middleware.
        const [path, query] = (ctx.options?.path ?? '').split('?');
        if (fallbacks.length === 0 ||
            ctx.options?.method !== 'post' ||
            path !== '/v1/messages' ||
            new URLSearchParams(query).get('beta') !== 'true' ||
            typeof ctx.options.body !== 'object' ||
            ctx.options.body == null) {
            return next(request);
        }
        if (ctx.options.body.fallbacks != null) {
            throw new AnthropicError('Sending the `fallbacks:` request param is not supported when using the `betaRefusalFallbackMiddleware`. ' +
                'You should either remove the middleware and send `fallbacks:` with the `server-side-fallback-2026-07-01` beta header to let the API handle refusal fallbacks, ' +
                "or omit the `fallbacks:` param if you'd like `betaRefusalFallbackMiddleware` to handle fallbacks on the client side.");
        }
        const onError = options.onError ??
            ((error) => ctx.logger.error(`anthropic-sdk: betaRefusalFallbackMiddleware: ${error.message}`));
        // Send the configured betas on this and every hop request derived from it,
        // and tag this and every hop with the middleware's helper telemetry.
        request = withMiddlewareHeaders(request, options.betas ?? DEFAULT_BETAS);
        const body = stripFallbackBlocks(ctx.options.body);
        const state = ctx.options.fallbackState;
        // start from the pinned fallback (-1 = the original params)
        const startIndex = state?.index ?? -1;
        if (!Number.isInteger(startIndex) || startIndex < -1 || startIndex >= fallbacks.length) {
            throw new AnthropicError(`fallbackState.index ${startIndex} is out of bounds for a chain of ${fallbacks.length} fallback(s); was the state shared with a different middleware?`);
        }
        // pin requests sharing the state to the entry being tried
        const pin = (index) => {
            if (state) {
                state.index = index;
            }
            else if (!warnedMissingState) {
                warnedMissingState = true;
                ctx.logger.warn('anthropic-sdk: betaRefusalFallbackMiddleware fell back without a `fallbackState` request option; follow-up requests will retry models that already refused. Pass a shared `{ fallbackState: new BetaFallbackState() }` to pin them to the accepted model.');
            }
        };
        const initialBody = startIndex === -1 ? body : applyFallbackPatch(body, fallbacks[startIndex]);
        // a non-string body can't be respliced or redeemed against — leave the
        // request untouched (the streaming path stands down on it below too)
        const initialRequest = typeof request.body !== 'string' ? request : { ...request, body: JSON.stringify(initialBody) };
        const response = await next(initialRequest);
        if (!response.ok) {
            return response;
        }
        if (ctx.options.stream === true) {
            const firstHop = startIndex + 1;
            // Splicing needs at least one entry left to hop to and the JSON request
            // body the credit token is redeemable against (an earlier middleware
            // may have rewritten it to another BodyInit); otherwise the stream
            // passes through untouched.
            if (firstHop >= fallbacks.length || typeof initialRequest.body !== 'string') {
                return response;
            }
            return spliceFallbackStream({
                request: initialRequest,
                response,
                next,
                ctx,
                fallbacks,
                firstHop,
                onError,
                pin,
            });
        }
        let index = startIndex;
        let res = response;
        // The model the current hop was requested as — the caller's spelling, not
        // the server's `message.model` echo; the seam block's `from` carries it.
        let requestedModel = initialBody.model;
        const fallbackBlocks = [];
        while (index < fallbacks.length - 1) {
            const message = await ctx.parse(res);
            if (message?.type !== 'message' || message.stop_reason !== 'refusal') {
                break;
            }
            index += 1;
            pin(index);
            const entry = fallbacks[index];
            // One `fallback` seam block per model boundary, prepended to the serving
            // hop's content below — the same block shape the server places in
            // `content`, not a claim of full envelope parity.
            fallbackBlocks.push({
                type: 'fallback',
                // `requestedModel` is always set for a typed body; the `??` defends
                // against an untyped body that carried no `model` field.
                from: { model: requestedModel ?? message.model },
                to: { model: entry.model },
                trigger: { type: 'refusal', category: message.stop_details?.category ?? null },
            });
            requestedModel = entry.model;
            res = await next({
                ...request,
                body: JSON.stringify({
                    ...applyFallbackPatch(body, entry),
                    ...(message.stop_details?.fallback_credit_token ?
                        { fallback_credit_token: creditTokenParam(message.stop_details.fallback_credit_token) }
                        : undefined),
                }),
            });
        }
        if (fallbackBlocks.length === 0) {
            return res;
        }
        const served = await ctx.parse(res);
        // Chain exhausted on a refusal (or an error/malformed body): surface it
        // verbatim. The array guard keeps a message-shaped body with non-array
        // `content` from throwing at the spread below.
        if (served?.type !== 'message' || served.stop_reason === 'refusal' || !Array.isArray(served.content)) {
            return res;
        }
        // A fallback hop served (or exhausted the chain with output): prepend the
        // seam blocks so the app-visible `content` opens with one `fallback` block
        // per model boundary. Response init is preserved (same `_request_id`);
        // `content-length` is dropped since the body grew.
        const headers = new Headers(res.headers);
        headers.delete('content-length');
        return new Response(JSON.stringify({ ...served, content: [...fallbackBlocks, ...served.content] }), {
            status: res.status,
            statusText: res.statusText,
            headers,
        });
    };
}
/**
 * Wrap stream A in a response whose body passes events through until a
 * retryable refusal, then splices the fallback chain's events on (see
 * {@link splicedEvents}). Cancelling the returned body tears down whichever
 * stream is being read and aborts any in-flight fallback request or retry
 * backoff: hop requests run under `controller`'s signal, which fires on
 * cancel and mirrors the original request's signal — a user abort has no
 * other way to reach a hop, since this synthetic body isn't fetch-backed.
 */
function spliceFallbackStream(args) {
    const controller = new AbortController();
    const signal = args.request.signal;
    if (signal?.aborted) {
        controller.abort(signal.reason);
    }
    else {
        signal?.addEventListener('abort', makeAbort(controller, signal), { once: true });
    }
    const iter = splicedEvents(args, controller);
    const body = new ReadableStream({
        async pull(ctrl) {
            try {
                const { value, done } = await iter.next();
                if (done)
                    return ctrl.close();
                ctrl.enqueue(value);
            }
            catch (err) {
                ctrl.error(err);
            }
        },
        async cancel() {
            controller.abort();
            await iter.return?.(undefined);
        },
    });
    return new Response(body, args.response);
}
async function* splicedEvents({ request, response, next, ctx, fallbacks, firstHop, onError, pin }, controller) {
    // --- stream A: pass through until a chainable refusal ---
    const a = yield* consumeHop({
        response,
        controller,
        indexBase: 0,
        hasNext: true, // the caller guarantees firstHop < fallbacks.length
        onError,
        splice: null,
    });
    if (!a.refused)
        return; // non-refusal or not-retryable: pure pass-through.
    // --- fallback chain: try each entry in order ---
    // `base` is the assistant-turn content the current token's request already
    // carried — the token is redeemable only with it resent verbatim. `partial`
    // is the newest refused hop's output, included only when its refusal
    // granted a prefill claim (any other change to the body is a 400).
    let nextIndex = a.nextIndex; // monotonic block index across all spliced streams
    let token = a.refused.token;
    let base = [];
    let partial = a.refused.hasPrefillClaim ? toPrefillBlocks(a.blocks) : [];
    let fromModel = a.model ?? '';
    let lastUsage = a.refused.usage;
    // The refusal whose token is currently in flight — surfaced verbatim (with a
    // recommended_model added) if every fallback request fails and we degrade.
    let refusalDetails = a.refused.stopDetails;
    // One `message` entry per refused hop, in order — A first. Failed hops are
    // skipped (no usage came back); the serving hop is appended as
    // `fallback_message` when its message_delta arrives.
    const iterations = [
        toIterationUsage('message', a.model ?? '', a.refused.usage),
    ];
    for (let hop = firstHop; hop < fallbacks.length; hop++) {
        const model = fallbacks[hop].model;
        const hasNext = hop + 1 < fallbacks.length;
        pin(hop);
        // --- boundary: a `fallback` content block at the next monotonic index ---
        // Emitted before the request, so a hop that fails leaves its boundary in
        // place and the next attempt emits its own (still `from: fromModel` — the
        // last model that contributed output).
        const fbIndex = nextIndex++;
        yield emit('content_block_start', {
            type: 'content_block_start',
            index: fbIndex,
            content_block: {
                type: 'fallback',
                from: { model: fromModel },
                to: { model },
                trigger: { type: 'refusal', category: refusalDetails?.category ?? null },
            },
        });
        yield emit('content_block_stop', {
            type: 'content_block_stop',
            index: fbIndex,
        });
        // --- build the request: appended-assistant continuation ---
        // First attempt carries the newest partial appended (when its refusal
        // granted a prefill claim); a 400 on that form means the server rejected
        // the prefill, so the hop is retried once without it — the same-body
        // form the token always supports.
        let continuation = [...base, ...partial];
        let resB = null;
        let failure = null;
        for (let attempt = 0; attempt < 2; attempt++) {
            const reqB = buildFallbackRequest(request, { model, creditToken: token, continuation });
            // controller mirrors the original signal and additionally fires when the
            // spliced body is cancelled — either must abort an in-flight hop request.
            reqB.signal = controller.signal;
            try {
                resB = await next(reqB);
            }
            catch (err) {
                // the consumer cancelled (or the original request was aborted): unwind
                if (isAbortError(err))
                    throw err;
                failure = {
                    kind: 'request_failed',
                    message: `fallback request failed: ${err}`,
                    model,
                    status: null,
                    detail: err,
                };
                break;
            }
            if (resB.ok)
                break;
            // ctx.parse reads through an internal clone, so it works even though
            // the client will also read this body; resB.text() would conflict.
            const errBody = await ctx.parse(resB).catch(() => null);
            if (attempt === 0 && resB.status === 400 && partial.length) {
                ctx.logger.warn(`anthropic-sdk: betaRefusalFallbackMiddleware: fallback request with the partial output appended was rejected (HTTP 400: ${JSON.stringify(errBody)}); retrying without it`);
                continuation = base;
                resB = null;
                continue;
            }
            failure = {
                kind: 'request_failed',
                message: `fallback request failed: HTTP ${resB.status}: ${JSON.stringify(errBody)}`,
                model,
                status: resB.status,
                detail: errBody,
            };
            break;
        }
        if (failure) {
            onError(failure);
            // The token was never redeemed — retry it against the next entry.
            if (hasNext)
                continue;
            // Surface the held refusal verbatim — its category/explanation and the
            // still-unredeemed credit token — and point recommended_model at the hop
            // we last tried.
            const stopDetails = {
                ...refusalDetails,
                recommended_model: model,
            };
            yield emit('message_delta', {
                type: 'message_delta',
                context_management: null,
                delta: {
                    stop_reason: 'refusal',
                    stop_sequence: null,
                    container: null,
                    stop_details: stopDetails,
                },
                usage: (lastUsage ?? {}),
            });
            yield emit('message_stop', { type: 'message_stop' });
            return;
        }
        // --- splice: monotonic indices, suppressed message_start, usage.iterations ---
        const b = yield* consumeHop({
            response: resB,
            controller,
            indexBase: nextIndex,
            hasNext,
            onError,
            splice: { iterations, model },
        });
        if (!b.refused)
            return;
        // This hop refused too, with a fresh token: its emitted partial stays in
        // the client's message, becomes the next partial segment, and the chain
        // continues.
        token = b.refused.token;
        refusalDetails = b.refused.stopDetails;
        base = continuation;
        partial = b.refused.hasPrefillClaim ? toPrefillBlocks(b.blocks) : [];
        iterations.push(toIterationUsage('message', model, b.refused.usage));
        lastUsage = b.refused.usage;
        fromModel = model;
        nextIndex = b.nextIndex;
    }
}
/**
 * Consume one hop's SSE events, forwarding them to the client while
 * accumulating its content blocks (returned in the outcome).
 *
 * Stream A (`splice: null`) is forwarded in its original wire bytes; a
 * spliced hop (`splice` set) has its message_start suppressed (the client
 * already saw A's), its block indices shifted by `indexBase`, and its
 * terminal message_delta's usage rewritten to the `usage.iterations`
 * chain shape.
 *
 * A refusal that can be chained — it carries a `fallback_credit_token` and a
 * fallback entry remains — ends the hop early: open blocks are closed, the
 * terminal message_delta + message_stop are suppressed, and the token+usage
 * are returned so the caller can issue the next hop. Any other refusal is
 * reported through `onError` and passes through to the client.
 */
async function* consumeHop(args) {
    const { response, controller, indexBase, hasNext, onError, splice } = args;
    const tracker = new BlockTracker(indexBase);
    let model;
    let startUsage = null;
    for await (const sse of Stream.rawEvents(response, controller)) {
        const p = safeJSON(sse.data);
        switch (p?.type) {
            case 'message_start': {
                model = p.message.model;
                startUsage = p.message.usage;
                if (splice)
                    continue;
                break;
            }
            case 'content_block_start': {
                tracker.start(p);
                if (splice) {
                    yield emit(p.type, p);
                    continue;
                }
                break;
            }
            case 'content_block_delta': {
                tracker.delta(p);
                if (splice) {
                    yield emit(p.type, p);
                    continue;
                }
                break;
            }
            case 'content_block_stop': {
                tracker.stop(p);
                if (splice) {
                    yield emit(p.type, p);
                    continue;
                }
                break;
            }
            case 'message_delta': {
                if (p.delta.stop_reason === 'refusal') {
                    // `fallback_credit_token` is null when the refusal isn't eligible
                    // for a fallback credit; without one we don't retry.
                    const details = p.delta.stop_details?.type === 'refusal' ? p.delta.stop_details : null;
                    if (details?.fallback_credit_token && hasNext) {
                        const usage = backfill(p.usage, startUsage);
                        yield* tracker.closeOpenBlocks();
                        // suppress this hop's message_delta + message_stop
                        return {
                            refused: {
                                token: details.fallback_credit_token,
                                hasPrefillClaim: details.fallback_has_prefill_claim === true,
                                usage,
                                stopDetails: details,
                            },
                            model,
                            blocks: tracker.contentBlocks(),
                            nextIndex: tracker.nextIndex,
                        };
                    }
                    if (!details?.fallback_credit_token) {
                        onError({
                            kind: 'no_credit_token',
                            message: 'refusal stop_details has no fallback_credit_token',
                            event: p,
                        });
                    }
                    else {
                        onError({
                            kind: 'chain_exhausted',
                            message: 'refusal but no fallback entries remain',
                            event: p,
                        });
                    }
                }
                if (splice) {
                    // Terminal hop. Replace iterations, don't append: this hop's own
                    // message_delta self-reports a single `{type:"message",
                    // model:undefined}` iteration (a fresh non-fallback request counts
                    // itself as one message hop). Server-side `fallbacks` relabels the
                    // whole chain instead — refused hops as `message`, the serving hop
                    // as `fallback_message` — so spreading the self-report would
                    // prepend a spurious `message:undefined` entry.
                    const usage = backfill(p.usage, startUsage);
                    usage.iterations = [
                        ...splice.iterations,
                        toIterationUsage('fallback_message', splice.model, usage),
                    ];
                    p.usage = usage;
                    yield emit('message_delta', p);
                    continue;
                }
                break;
            }
        }
        // message_stop, ping, error, unrecognised — and for stream A every
        // event — pass through in their original wire bytes.
        yield passthroughSSE(sse);
    }
    return { refused: null, model, blocks: tracker.contentBlocks(), nextIndex: tracker.nextIndex };
}
/**
 * Block bookkeeping for one stream of the splice: accumulates each content
 * block from its deltas (for the continuation prefill), shifts wire indices
 * by `indexBase` so they stay monotonic across hops, and tracks which blocks
 * are still open so a refusal that cuts mid-block can close them.
 */
class BlockTracker {
    constructor(indexBase = 0) {
        this.indexBase = indexBase;
        /** The stream's accumulated blocks keyed by their original wire index. */
        this.blocks = [];
        /** Shifted indices of blocks started but not yet stopped. */
        this.open = [];
        this.nextIndex = indexBase;
    }
    /** The accumulated content blocks, in start order. */
    contentBlocks() {
        return this.blocks.map((b) => b.block);
    }
    /** Track a content_block_start, shifting `event.index`. */
    start(event) {
        this.blocks.push({ index: event.index, block: { ...event.content_block } });
        event.index += this.indexBase;
        this.open.push(event.index);
        this.nextIndex = Math.max(this.nextIndex, event.index + 1);
    }
    /** Apply a content_block_delta to its accumulating block, shifting `event.index`. */
    delta(event) {
        applyDelta(this.blocks, event.index, event.delta);
        event.index += this.indexBase;
    }
    /** Track a content_block_stop, shifting `event.index`. */
    stop(event) {
        event.index += this.indexBase;
        const i = this.open.indexOf(event.index);
        if (i !== -1)
            this.open.splice(i, 1);
        this.nextIndex = Math.max(this.nextIndex, event.index + 1);
    }
    /** content_block_stop events for any blocks still open. */
    *closeOpenBlocks() {
        for (const index of this.open) {
            yield emit('content_block_stop', {
                type: 'content_block_stop',
                index,
            });
        }
        this.open.length = 0;
    }
}
// --- fallback request construction (appended-assistant continuation) -------
/**
 * Object form of the redeemed credit token. `best_effort` keeps the retry
 * served if the token layer rejects it — the retry then proceeds at normal
 * price and the outcome lands on `usage.fallback_credit` — where the
 * bare-string (`strict`) form would 400 the whole request.
 */
function creditTokenParam(token) {
    return { token, mode: 'best_effort' };
}
function buildFallbackRequest(orig, { model, creditToken, continuation, }) {
    // the caller guarantees a JSON string body (checked before stream A is read)
    const body = JSON.parse(orig.body);
    body.model = model;
    body.fallback_credit_token = creditTokenParam(creditToken);
    // Append the continuation (decided by the chain loop) as a trailing
    // assistant turn; everything else must stay identical to the refused
    // request. When the refusal granted no prefill claim, omit the turn
    // entirely and send the same-body form.
    if (continuation.length) {
        body.messages = [...body.messages, { role: 'assistant', content: continuation }];
    }
    // Do NOT touch max_tokens (or any other render-shaping field): the token is
    // only redeemable against the same request body as the refused request —
    // model, fallback_credit_token, and the one appended assistant turn are the
    // only permitted deltas; anything else is a 400 ("request body ... does not
    // match the original refused request"). This is also why the per-entry
    // BetaFallbackParam overrides are ignored on the streaming path.
    return { ...orig, headers: new Headers(orig.headers), body: JSON.stringify(body) };
}
// --- block accumulation & prefill conversion -------------------------------
/** Apply a content_block_delta to the accumulating block at `index`. */
function applyDelta(blocks, index, delta) {
    const block = blocks.find((x) => x.index === index)?.block;
    if (!block)
        return;
    switch (delta.type) {
        case 'text_delta': {
            block.text = (block.text ?? '') + delta.text;
            break;
        }
        case 'input_json_delta': {
            block._partial_json = (block._partial_json ?? '') + delta.partial_json;
            break;
        }
        case 'citations_delta':
            (block.citations ?? (block.citations = [])).push(delta.citation);
            break;
        case 'thinking_delta': {
            block.thinking = (block.thinking ?? '') + delta.thinking;
            break;
        }
        case 'signature_delta': {
            block.signature = delta.signature;
            break;
        }
        case 'compaction_delta': {
            break;
        }
        default:
            ((_) => { })(delta);
    }
}
/**
 * Convert a hop's accumulated response blocks to the appended assistant turn,
 * as-is: a `fallback_has_prefill_claim` refusal guarantees the partial output
 * is resendable verbatim, so no client-side filtering is applied. The only
 * rewrite is reassembling tool inputs from their accumulated
 * `input_json_delta` JSON (content_block_start carries `input: {}`).
 */
function toPrefillBlocks(responseBlocks) {
    return responseBlocks.map((b) => {
        if (typeof b?._partial_json !== 'string')
            return b;
        const { _partial_json, ...block } = b;
        return { ...block, input: safeJSON(_partial_json) ?? block.input };
    });
}
// --- helpers --------------------------------------------------------------
/**
 * A copy of `request` with `betas` appended to its `anthropic-beta` header,
 * skipping values already present (set by the caller or another middleware).
 */
function withMiddlewareHeaders(request, betas) {
    const headers = new Headers(request.headers);
    const existing = new Set(headers
        .get('anthropic-beta')
        ?.split(',')
        .map((s) => s.trim()));
    for (const beta of betas) {
        if (!existing.has(beta)) {
            headers.append('anthropic-beta', beta);
            existing.add(beta);
        }
    }
    headers.set(STAINLESS_HELPER_HEADER, appendHeaderValue(headers.get(STAINLESS_HELPER_HEADER), 'fallback-refusal-middleware'));
    return { ...request, headers };
}
function emit(event, payload) {
    const sse = { event, data: JSON.stringify(payload), raw: [] };
    return encoder.encode(serializeSSE(sse));
}
/**
 * Forward a decoded event in its original wire bytes, preserving SSE fields
 * the decoder doesn't model (`id:`, `retry:`, comment lines). Falls back to
 * re-serializing for events with no raw lines.
 */
function passthroughSSE(sse) {
    return encoder.encode(sse.raw.length ? sse.raw.join('\n') + '\n\n' : serializeSSE(sse));
}
function toIterationUsage(type, model, u) {
    return {
        type,
        model,
        input_tokens: u?.input_tokens ?? 0,
        output_tokens: u?.output_tokens ?? 0,
        cache_read_input_tokens: u?.cache_read_input_tokens ?? 0,
        cache_creation_input_tokens: u?.cache_creation_input_tokens ?? 0,
        cache_creation: u?.cache_creation ?? null,
    };
}
/** Fill null/undefined fields on `primary` from `fallback`. */
function backfill(primary, fallback) {
    const out = { ...(fallback ?? {}), ...(primary ?? {}) };
    for (const k of Object.keys(out)) {
        if (out[k] == null && fallback?.[k] != null)
            out[k] = fallback[k];
    }
    return out;
}
/**
 * Serialize a {@link ServerSentEvent} back to its SSE wire form
 * (`event: ...\ndata: ...\n\n`). Multi-line `data` is emitted as one
 * `data:` line per line, matching the spec. The inverse of the decoder
 * behind {@link Stream.rawEvents}.
 */
function serializeSSE(sse) {
    let out = '';
    if (sse.event !== null)
        out += `event: ${sse.event}\n`;
    for (const line of sse.data.split('\n'))
        out += `data: ${line}\n`;
    return out + '\n';
}
function makeAbort(controller, signal) {
    return () => controller.abort(signal.reason);
}
//# sourceMappingURL=middleware.mjs.map