[Bug]: Code Mode search silently returns an empty list for a namespace prefix or a multi-word query #33542

What happened

search in Code Mode returns { items: [], remaining: 0 } — silently and with no error — for two ordinary inputs. Because an agent cannot tell that apart from "the tool does not exist", it concludes the capability is missing and falls back to shell/CLI workarounds.

Reproduction

Server 0.1.8-nightly.20261107.4314, harness opencode, thread t3-code-<threadId> MCP server.

1. search({ query: "delegate_task" }) → returns delegate_task, task_cancel, task_status, … (works).
2. search({ query: "delegate_task", namespace: "t3-code" }) → { items: [], remaining: 0 }. The real namespace is t3-code-<threadId>, so a prefix silently matches nothing.
3. search({ query: "delegate orchestrator task" }) → { items: [], remaining: 0 }. A multi-word query is matched as one literal string, not tokenized.
4. search({ query: "delegate", namespace: "t3-code-<full-thread-id>" }) → works.

Expected

• Prefix (or substring) namespace matching, or an explicit error for an unknown namespace.
• Tokenized query matching, or a distinct "no match" signal so an empty result is not confused with an invalid filter.

Either fix removes the silent failure. An error would be enough — silence is the part that misleads.

Impact

In one session this cost about 15 tool calls and a user correction before orchestrator_capabilities was found by dropping the namespace and the extra words. The agent had a working delegation path available the whole time and could not see it.

Workaround

Use one keyword and the exact namespace. Never pass a phrase.