[vishal-bala]

Motivation

A code tool is an async function whose first parameter is a ToolContext and whose other parameters are the arguments the model supplies. This PR turns such a function into a registered MCP tool, with the same rails a built-in gets, applied around the function rather than left to its author. It builds on the ToolContext from the previous PR. Users still cannot reach it, because the server registers nothing yet.

Changes

Checked where the tool is declared

build_code_tool_spec(fn, ...) validates a function and returns a frozen CodeToolSpec. It refuses:

• a sync function, since ctx.index is async and a sync tool could not use it;
• a first parameter that is not a positional ToolContext. The wrapper passes the context positionally, so a keyword-only one would pass a shape check and then fail on every call;
• any other parameter that is unannotated, *args, **kwargs or positional-only. The annotations become the tool's input schema;
• a parameter type that no input schema can describe, such as a plain class. Otherwise it would surface only at startup, after every binding had connected;
• a missing description. The docstring is used unless description= is given, because the description is what the model selects tools by.

The name is the function's name with hyphens for underscores, or name=, prefixed by namespace= when given. It is checked by validate_tool_name, now factored out of profile validation, so profiles and code tools share one naming rule. Annotations are resolved with typing.get_type_hints, so a module using postponed annotations is checked against the real types.

Applied on every call

register_code_tool(server, spec, binding_id) registers a wrapper whose advertised signature is the function's minus the context. This follows how register_profile_tool narrows its signature, so FastMCP never offers the context to the model, and it rejects a call that tries to pass one. On each call the wrapper:

1. checks the server's read scope, plus the write scope for a tool declared with writes=True, whose handle can both read and write, then the tool's own scope= on top. A tool can therefore be stricter than the built-ins, never looser;
2. refuses a writing tool whose binding is now read-only;
3. builds the ToolContext, giving a writing tool the writable handle;
4. bounds the whole call by the binding's request_timeout_seconds, so the tool's own code cannot hold a request open indefinitely. A timeout returns a retryable backend_unavailable that names the tool;
5. maps any other exception through map_exception, so a ValueError reaches the client as invalid_request and no traceback leaks. A raised RedisVLMCPError passes through unchanged.

Startup will also refuse a writing tool on a read-only binding (next PR). The per-call check in step 2 is defence in depth, as in upsert_records, so a write never depends on that one check.

Notes

The whole-call timeout and the per-call handle timeout use the same value, so the outer bound usually fires first. That is deliberate: the per-call bound protects the semaphore, and the outer bound protects the request.

A test runs a timed-out call against the real run_guarded. It checks that the call releases its concurrency slot and the active-request count, since a leak there would remove one slot per timeout until the server stopped answering.

The whole-call timeout can also cancel a handle call while it is still queued for a concurrency slot. run_guarded now acquires the slot explicitly and closes the Redis call it never started, which previously logged "coroutine was never awaited" once per such timeout.

Release Notes

An async function becomes a code tool only if it takes a positional ToolContext, annotates its other parameters and has a description; every call is then checked against the read scope, the write scope for a writing tool, and any per-tool scope=, bounded by the binding's request timeout, and mapped to the MCP error contract.

Next Steps

The next PR in the stack adds server.code_tool(...) and the startup checks that pair each tool with its binding.

Reviewer Brief

Start here: redisvl/mcp/tools/code.py:179, register_code_tool. Its wrapper applies the scopes, the read-only check, the whole-call timeout and the error mapping around every code tool.

Where it could fail

• redisvl/mcp/tools/code.py:187: every call needs the read scope, a writing tool the write scope too, then the tool's own scope=; all three must run before the binding is resolved.
• redisvl/mcp/server.py:251: run_guarded now acquires its slot explicitly, so every path after the acquire must release it exactly once, and a call cancelled while queued must close the coroutine it never started.
• redisvl/mcp/server.py:251 (pre-existing): the admission and bookkeeping steps hold _request_state_lock without awaiting inside it; an await added there later would let a timeout skip the active-request decrement and hang shutdown.

Not verified: FastMCP 2.x; cluster mode.

Beyond CI: mutants removing the server-wide scope check, the read-scope check for writing tools, the unstarted-call close, fullmatch and the whole-call timeout each fail a unit test; 3,000 run_guarded calls under random cancellation left no leaked slot and no never-awaited warning.

Since the last brief: 971de0f refuses a scope= with surrounding whitespace, which could never match a token.

_{Reviewed 8b8dee6, fixes in f6f3c49 · Claude Code (claude-opus-5-5) · lenses: correctness, security, design, tests, docs, Redis behaviour}

---

Note

Medium Risk
Changes request admission (run_guarded) affect all MCP tools, and new code-tool wrappers enforce auth scopes and timeouts on every call—important for security but with broad runtime impact.

Overview
Adds code tools: async Python functions with a positional ToolContext first argument can be validated via build_code_tool_spec and registered with register_code_tool, using the same safety rails as built-ins (scopes, read-only writes, binding timeout, MCP error mapping). The model-facing schema hides ToolContext and is built from annotated parameters.

Shared naming: validate_tool_name is extracted from profile config validation and uses fullmatch so names with trailing newlines (e.g. foo\n) are rejected; custom profile tools delegate to this helper.

Concurrency fix: run_guarded explicitly acquires/releases the semaphore so timeouts or cancellation while queued still release the slot and close coroutines that never started—preventing leaked concurrency capacity and “coroutine was never awaited” warnings.

Server startup does not register code tools yet (follow-up PR); this PR is the library + tests only.

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[limjoobin]

Found an issue with permission-scoping that should be resolved before merging!

[limjoobin]

LGTM, thanks! The permission-scoping issue has been addressed with appropriate regression coverage.

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