[praveenperera]

What changed

Each upstream server now takes a max_concurrent setting (default 8, from 1 to 4096). The gateway sends a server at most that many tool calls at once. When a script's asyncio.gather makes more calls than that, the extra calls wait for a free slot instead of all being sent together. A call that gets no slot within the tool timeout fails with a TimeoutError and is never sent. Nothing is retried, because a tool may not be safe to run twice.

Builtin servers such as secrets run inside the daemon and are not capped. That way, all of a script's 1Password finds still reach the owner as one approval prompt.

When a tool's error looks like a rate limit (rate limit, ratelimit or too many requests), the error the script sees names the cap and the file where it can be lowered. For a server kept in an included file, that is the included file. The whole hint goes through the secret redactor.

When a spoke forwards a call to the hub, it now waits twice the tool timeout plus 30 seconds. That covers the time the call may spend queued on the hub as well as the call itself.

Why

An agent ran about 45 Linear searches in one asyncio.gather, and Linear rate limited 14 of them. The gateway had no per-server limit and never retries a call, so the agent had to notice the failures and rerun the searches one at a time.

Verification

• New integration test tests/gateway/concurrency.rs:
  • A server capped at 2 never has more than 2 of 6 parallel calls running, and a server with the default cap peaks at 8 of 12.
  • A rate-limit error gets the hint and an unrelated error doesn't.
  • For a server defined in an included file, the hint names that file.
• cargo test, cargo clippy --all-targets and cargo fmt --check pass.
• Two GPT-6.1 Sol reviews at xhigh effort. Their five findings are all fixed:
  • Queue wait could outlast the spoke's deadline.
  • The config path in the hint bypassed redaction.
  • A huge cap panicked the daemon at startup.
  • The busy error was raised as the wrong exception type.
  • The hint named the root config for servers kept in included files.

Summary by CodeRabbit

• New Features
  • Non-builtin servers can limit simultaneous calls, with a default limit of 8 and configurable values from 1 to 4096. Built-in servers are not capped.
  • Calls that cannot get a slot before the timeout expire without being sent or retried.
  • Recognized rate-limit errors now identify the concurrency limit and the configuration file where it is set.
• Documentation
  • Updated operations guidance to describe concurrency limits, queue timeouts, and rate-limit errors.

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Reviewing files that changed from the base of the PR and between e0dca77 and 6b6630e.

📒 Files selected for processing: 5

• docs/operations.md
• src/gateway.rs
• src/hub.rs
• src/hub/client.rs
• src/upstream.rs

📝 Walkthrough

Non-builtin servers now limit simultaneous tool calls using a configurable max_concurrent setting. Calls that cannot acquire a slot within the timeout return Busy without being sent. Gateway and hub paths classify these calls as timeouts, and gateway errors can include configuration guidance for recognized rate-limit messages.

Changes: Server tool concurrency

Layer / File(s)	Summary
Concurrency configuration and source tracking src/config.rs	ServerConfig adds max_concurrent, with a default of 8 and a valid range of 1–4096. Config records the setting’s source file and provides a lookup that falls back to the main configuration path.
Call-slot enforcement and concurrency checks src/upstream.rs, tests/fixtures/fixture_server.rs, tests/gateway/concurrency.rs, tests/gateway/main.rs, docs/operations.md	Non-builtin servers use a semaphore sized by max_concurrent. Calls wait up to call_timeout for a slot and return Busy without being sent if the wait expires. Fixture tools and integration tests check concurrency limits and error guidance. The operations documentation describes the setting and timeout behavior.
Timeout mapping and rate-limit guidance src/gateway.rs, src/hub.rs, src/hub/client.rs	Gateway and hub paths classify Busy as a timeout. Gateway tool failures add redacted guidance for recognized rate-limit messages when the server limit is available. The hub request timeout now allows twice tool_timeout plus QUICK.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Gateway
  participant Upstream
  participant Semaphore
  participant MCPServer
  Gateway->>Upstream: call_tool
  Upstream->>Semaphore: Request slot
  Semaphore-->>Upstream: Slot acquired
  Upstream->>MCPServer: Send tool call
  MCPServer-->>Upstream: Return tool result
  Upstream-->>Gateway: Return result
  Semaphore-->>Upstream: Slot unavailable before timeout
  Upstream-->>Gateway: Return Busy without sending call

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Merge Risk: 🟡 Moderate · up to e0dca

Forwarded tool calls can report a timeout even when the hub completes them. Align the deadline with the hub’s queue and execution budgets before merging.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the primary change: limiting concurrent calls for each upstream server.
Docstring Coverage		Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 8 files. (1 skipped: 1 …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 8 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

---

• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/hub/client.rs:
- Line 290: Update the forwarded /call deadline passed to self.post to use the
hub’s configured timeout, covering its queue and execution budgets, instead of
doubling the spoke’s tool_timeout; preserve any required QUICK allowance.

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📒 Files selected for processing: 9

• docs/operations.md
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• src/hub.rs
• src/hub/client.rs
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• tests/fixtures/fixture_server.rs
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