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Hermes-agent
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---
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title: Tool Search
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sidebar_position: 95
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---
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# Tool Search
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When you have many MCP servers or non-core plugin tools attached to a
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session, their JSON schemas can consume a substantial fraction of the
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context window on every turn — even when only a few of them are relevant
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to what the user actually asked for.
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**Tool Search** is Hermes' opt-in progressive-disclosure layer for that
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problem. When activated, MCP and plugin tools are replaced in the
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model-visible tools array by three bridge tools, and the model loads each
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specific tool's schema on demand.
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:::info Built-in Hermes tools never defer
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The tools that make up Hermes' core capability set (`terminal`,
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`read_file`, `write_file`, `patch`, `search_files`, `todo`, `memory`,
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`browser_*`, `web_search`, `web_extract`, `clarify`, `execute_code`,
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`delegate_task`, `session_search`, `send_message`, and the rest of
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`_HERMES_CORE_TOOLS`) are *always* loaded directly. Only MCP tools and
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non-core plugin tools are eligible for deferral.
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:::
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## How it works
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When Tool Search activates for a turn, the model sees three new tools in
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place of the deferred ones:
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```
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tool_search(query, limit?) — search the deferred-tool catalog
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tool_describe(name) — load the full schema for one tool
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tool_call(name, arguments) — invoke a deferred tool
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```
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A typical interaction looks like:
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```
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Model: tool_search("create a github issue")
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→ { matches: [{ name: "mcp_github_create_issue", ... }, ...] }
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Model: tool_describe("mcp_github_create_issue")
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→ { parameters: { type: "object", properties: { ... } } }
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Model: tool_call("mcp_github_create_issue", { title: "...", body: "..." })
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→ { ok: true, issue_number: 42 }
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```
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When the model invokes `tool_call`, Hermes **unwraps the bridge** and
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dispatches the underlying tool exactly as if the model had called it
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directly. Pre-tool-call hooks, guardrails, approval prompts, and
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post-tool-call hooks all run against the real tool name — not against
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`tool_call`. The activity feed in the CLI and gateway also unwraps so you
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see the underlying tool, not the bridge.
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## When does it activate?
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By default Tool Search runs in `auto` mode: it activates only when the
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deferrable tool schemas would consume at least 10% of the active model's
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context window. Below that, the tools-array assembly is a pure
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pass-through and you pay no overhead.
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This decision is re-evaluated every time the tools array is built, so:
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- A session with just a few MCP tools and a long context model never
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activates Tool Search.
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- A session with many MCP servers attached (15+ tools typically) starts
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activating it.
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- Removing MCP servers mid-session correctly returns to direct exposure
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on the next assembly.
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## Configuration
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```yaml
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tools:
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tool_search:
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enabled: auto # auto (default), on, or off
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threshold_pct: 10 # percentage of context — only used in auto mode
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search_default_limit: 5
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max_search_limit: 20
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```
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| Key | Default | Meaning |
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| --- | --- | --- |
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| `enabled` | `auto` | `auto` activates above threshold; `on` always activates if there's at least one deferrable tool; `off` disables entirely. |
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| `threshold_pct` | `10` | Percentage of context length at which `auto` mode kicks in. Range 0–100. |
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| `search_default_limit` | `5` | Hits returned when the model calls `tool_search` without a `limit`. |
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| `max_search_limit` | `20` | Hard upper bound the model can request via `limit`. Range 1–50. |
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You can also flip the legacy boolean shape:
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```yaml
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tools:
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tool_search: true # equivalent to {enabled: auto}
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```
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## When NOT to use it
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Tool Search trades a fixed per-turn token cost (the three bridge tool
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schemas, ~300 tokens) and at least one extra round trip (search →
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describe → call) for the savings on the deferred schemas. It's a clear
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win when you have many tools and use few per turn; it's overhead when
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you have few tools total.
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The `auto` default handles this for you. If you set `enabled: on`
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unconditionally, expect a slight per-turn cost on small toolsets.
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## Trade-offs that don't go away
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These come from the prompt-cache integrity invariant — they are inherent
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to any progressive-disclosure design, not specific to this implementation:
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- **One extra round trip on cold tools.** The first time the model needs
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a deferred tool, it spends one or two extra model calls to find and
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load the schema. The token savings on the static side are real, but a
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portion is paid back at runtime.
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- **No cache benefit on deferred schemas.** A loaded `tool_describe`
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result enters the conversation history (so it does get cached on
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subsequent turns) but it never benefits from the system-prompt cache
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prefix.
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- **Model-quality dependence.** Tool Search assumes the model can write a
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reasonable search query for the tool it wants. Smaller models do this
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less well; the published Anthropic numbers (49% → 74% on Opus 4 with
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vs. without tool search) show the upside but also that ~26 points of
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accuracy is still retrieval failure.
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- **Toolset edits invalidate cache.** Adding or removing a tool mid-
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session changes the bridge tools' descriptions (which include the
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count of deferred tools) and the catalog, so the prompt cache is
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invalidated. This is the same trade-off as any toolset edit.
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## Implementation details
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- **Retrieval:** BM25 over tokenized tool name + description + parameter
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names. Falls back to a literal substring match on the tool name when
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BM25 returns no positive-score hits, which protects against
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zero-IDF degenerate cases (e.g. searching `"github"` against a
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catalog where every tool name contains "github").
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- **Catalog is stateless across turns.** It rebuilds from the current
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tool-defs list every assembly — no session-keyed `Map`. This avoids
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the class of bug where a stored catalog drifts out of sync with the
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live tool registry.
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- **The catalog is scoped to the session's toolsets.** `tool_search`,
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`tool_describe`, and `tool_call` only ever see and invoke tools the
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session was actually granted. A subagent, kanban worker, or gateway
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session restricted to a subset of toolsets cannot use the bridge to
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discover or call a tool outside that subset — the deferred catalog is
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the deferrable slice of the session's own enabled/disabled toolsets,
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not the whole process registry.
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- **No JS sandbox.** Hermes uses the simpler "structured tools" mode
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(search / describe / call as plain functions). The JS-sandbox "code
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mode" some other implementations offer is a large surface area; we
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skip it.
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## See also
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- `tools/tool_search.py` — the implementation
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- `tests/tools/test_tool_search.py` — the regression suite
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- The `openclaw-tool-search-report` PDF in the original implementation
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PR for the research that shaped the design
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