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.

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.

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.

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.