web_search: fall back to DuckDuckGo when Bing returns zero results

[Hmbown]

Problem

web_search defaults to Bing HTML scraping. Bing works for simple one-word queries ("test") but silently returns zero results for technical/compound queries ("Claude Code tool architecture agent loop", "rust async traits stabilization"). This is a known Bing HTML search behavior — their search page is ad-driven and doesn't surface organic results for specific technical queries.

Meanwhile, web.run (which uses DuckDuckGo) returns 5 perfectly relevant results for the same queries.

Evidence (macOS, from US IP):

Tool	Query	Result
web_search (Bing)	"test"	2 results
web_search (Bing)	"Claude Code tool architecture..."	empty
web.run (DuckDuckGo)	"Claude Code tool architecture agent loop"	5 results, perfectly relevant

Proposed Fix

In crates/tui/src/tools/web_search.rs, the DuckDuckGo path already falls back to Bing when DDG returns a bot challenge or empty results (lines 254-281). Add the symmetric fallback: when Bing returns zero results, try DuckDuckGo before reporting empty.

This is a ~15-line change in the SearchProvider::Bing branch (around line 213-217):

// Current (line 213-217):
if matches!(context.search_provider, SearchProvider::Bing) {
    check_policy(decider, BING_HOST)?;
    let results = run_bing_search(&client, &query, max_results).await?;
    return search_tool_result(query, "bing", results, None);
}

// Proposed: add zero-result fallback to DDG
if matches!(context.search_provider, SearchProvider::Bing) {
    check_policy(decider, BING_HOST)?;
    let results = run_bing_search(&client, &query, max_results).await?;
    if !results.is_empty() {
        return search_tool_result(query, "bing", results, None);
    }
    // Bing returned zero — try DuckDuckGo as fallback
    check_policy(decider, DUCKDUCKGO_HOST)?;
    // ... run DDG search, return with "bing returned no results; used DuckDuckGo fallback" message
}

Acceptance Criteria

• web_search "rust async traits stabilization" returns results (via DDG fallback)
• web_search "test" still returns Bing results directly (no fallback needed)
• Message includes provenance: "Bing returned no results; used DuckDuckGo fallback"
• Network policy gates applied for both hosts independently
• Spam detection (web_search / web.run returns unusable results — DDG blocked, Bing garbage #964) still runs on fallback results
• Existing tests pass (cargo test -p codewhale-tui web_search)

Related

• Closes or partially addresses web_search / web.run returns unusable results — DDG blocked, Bing garbage #964
• Config workaround (already works): [search] provider = "duckduckgo" in ~/.deepseek/config.toml
• Longer-term: Web search is not usable in China; support region-aware search providers #1681 (region-aware providers), Defer-load low-value tools to shrink prefix-cache footprint #2076 (defer-load tools)