A DeepSeek-backed WebSearchProvider for the harness web capability seam (ctx.web). It calls DeepSeek's Anthropic-compatible Messages API (POST {baseURL}/messages) with the native web_search_20250305 server tool enabled, and maps the structured web_search_tool_result blocks DeepSeek returns into the seam's normalized WebSearchResult.

This is an implementation package: it registers a provider into ctx.web, it does not own the key and it does not register a model-facing tool. Like @deepseek-ai/dsh-llm-deepseek, it is a function/namespace plugin (inject: ['web']). The Anthropic wire shape is a provider-private detail — it does not make this provider depend on ctx.llm.

How it differs from a dedicated search endpoint

Exa and Perplexity expose dedicated search endpoints; DeepSeek does not. Instead this provider issues a full Messages model call carrying the web_search server tool, so one search costs a complete model turn in latency and tokens — heavier than a pure retrieval endpoint. DeepSeek runs the search server-side and returns structured web_search_tool_result blocks; the provider parses those blocks and never scrapes URLs out of model prose.

Strict mode: if the response carries no web_search_tool_result block (native search did not trigger), the provider throws WebError WEB_PROVIDER_ERROR rather than degrading to prose-scraping — honest and debuggable.

It reuses $DEEPSEEK_API_KEY (no new secret) but not $DEEPSEEK_BASE_URL: the search endpoint is the Anthropic-compatible base (https://api.deepseek.com/anthropic/v1), distinct from the chat-completions base (https://api.deepseek.com) the LLM adapter uses.

Config

Key	Default	Meaning
`apiKey`	`$DEEPSEEK_API_KEY`	DeepSeek API key. Empty/absent makes the provider unavailable. Sent as both `x-api-key` and `Authorization: Bearer` (official vs Anthropic-compatible proxy).
`baseURL`	`https://api.deepseek.com/anthropic/v1`	Anthropic-compatible endpoint base; `/messages` is appended. Use a separate env var such as `$DEEPSEEK_SEARCH_BASE_URL` when overriding it; do not reuse `$DEEPSEEK_BASE_URL`, which belongs to the chat-completions LLM adapter. An unparseable value makes the provider unavailable.
`model`	`deepseek-v4-flash`	Anthropic-format model name.
`apiVersion`	`2023-06-01`	`anthropic-version` header value.
`maxTokens`	`4096`	Positive-integer upper bound on generated tokens for the Messages request.
`maxUses`	`5`	Positive-integer maximum `web_search` server-tool uses per request.

- id: web-search-deepseek
  name: '@deepseek-ai/dsh-web-search-deepseek'
  config:
    apiKey: !!js process.env.DEEPSEEK_API_KEY
    baseURL: !!js process.env.DEEPSEEK_SEARCH_BASE_URL

Mapping

DeepSeek returns no provider-generated answer surface this provider trusts as content, so content is omitted. sources[] comes from web_search_result items inside web_search_tool_result blocks: url ← url, title ← title, and publishedAt ← page_age. Snippets live separately as URL-keyed cited_text entries in a text block's citations[]; the provider joins them, leaving snippet absent when no excerpt exists.

Results are deduplicated by URL because one request may surface the same page across searches. DeepSeek exposes maxUses, not a result-count knob, so the seam enforces maxResults by truncating sources[] and setting truncated.

Provider failures become WEB_PROVIDER_ERROR; caller cancellation becomes WEB_ABORTED. HTTP redirects are rejected before the Location target is contacted and surface as WEB_PROVIDER_ERROR.

Model Experience

Auxiliary DeepSeek search request

What the model sees

A separate DeepSeek model receives exactly Perform a web search for the query: <query> as its user text and one native web_search server-tool definition. This request is not part of the conversation model's context.

Token effect

Separate provider input and output tokens are incurred for each search; maxTokens caps generated output and maxUses caps native search uses.

KV Cache effect

Independent of the conversation request cache. The auxiliary instruction and native tool definition can form a stable prefix, but each changed query or model route prevents reuse from its first difference.

Conversation tool result, indirectly

What the model sees

Through dsh-tool-web, the conversation model sees deduplicated URLs, titles, dates, and citation snippets from structured search blocks; provider prose is not trusted as an answer. This provider's exact failures are DeepSeek search aborted, DeepSeek search request failed: <error>, DeepSeek returned no web_search_tool_result blocks; the request may not have triggered native web search, and DeepSeek returned an unprocessable response body: <error>; HTTP failures preserve the provider message. The consumer owns the error wrapper.

Token effect

Zero direct conversation tokens from registration. Result tokens scale with returned sources and snippets, then the seam enforces the requested source bound.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Known Limitations and Deferred Work

One search costs a full Messages model turn — latency plus generated tokens, with up to maxUses server-side searches; DeepSeek exposes no dedicated retrieval endpoint.
Over-returned sources still cost tokens — with no result-count knob on the wire, maxResults is enforced only post-hoc by seam truncation.
Uncited results carry no snippet — a source gains one only when a text block citation (cited_text) matches its URL.
Abort classification is error-shape-based — only a DOMException named AbortError maps to WEB_ABORTED; an abort carrying a custom reason (e.g. dsh-timeout's TimeoutReason) surfaces as WEB_PROVIDER_ERROR.