Runtime MCP mutations can return before tool registry reconciliation #39902

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1. kitlangton

   added

   bugSomething isn't workingSomething isn't working

   on Jul 31, 2026
2. omidsaffari commented on Aug 1, 2026

   

   omidsaffari

   on Aug 1, 2026

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   I’d like to take this if the lifecycle-handler reconciliation fence explored in #39898 is still the preferred direction. I’ll keep it limited to awaiting the existing serialized reconciliation after runtime MCP add/remove/connect/disconnect, add the deterministic no-sleep regression described here, and avoid making the global event bus synchronous. If that scope is acceptable, could you assign it to me?
3. cestercian commented on Sep 17, 2026

   

   cestercian

   on Sep 17, 2026

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   Looking into the register-time / reconciliation ordering gap — will land a fix that waits for ToolRegistry reconciliation before MCP mutation endpoints return.
4. DeepxD-code

   added a commit that references this issue on Oct 18, 2026

   fix(mcp): await tool registry reconciliation after mutations

   ...

   This issue will close once commit 579e73f is merged into the 'dev' branch.

   579e73f
5. DeepxD-code commented on Oct 18, 2026

   

   DeepxD-code

   on Oct 18, 2026

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   Fix pushed in PR #55231 (commit 579e73f on truncated-diff-patches-v2).

   Root cause: MCP mutation methods (add/remove/connect/disconnect) published McpEvent.ToolsChanged/StatusChanged bus events, but McpTool consumed them on a debounced async stream (100ms). The mutation returned before ToolRegistry reconciled.

   Fix: McpTool registers its reconcile function in a shared mutable cell (mcp-reconcile.ts) during layer construction. Mcp mutation methods now await it after mutating state. Breaks the circular dependency (McpTool depends on Mcp) without layer wiring changes.

   Verified: mcp test suite 104/104 green, normalization 37/37, git 16/16, tsc clean.
6. opencodejr commented on Oct 28, 2026

   

   opencodejrBot

   on Oct 28, 2026

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   Thanks for the detailed write-up. I could reproduce this on dev (1c49e51). The code involved hasn't changed since v2.2.0.

   The test below uses a mocked MCP catalog that starts empty. It adds voice.list_open_tabs, publishes mcp.tools.changed the way MCP.add/remove/connect/disconnect do, and then takes a ToolRegistry snapshot right away. That snapshot contains only ["execute"]. voice_list_open_tabs only appears after waiting for reconciliation.

   The cause matches your description. In packages/core/src/tool/mcp.ts, McpTool handles the event on a stream it forks off and debounces by 100 ms before it runs reconcile. The mcp.add, mcp.remove, mcp.connect and mcp.disconnect handlers in packages/server/src/handlers/mcp.ts return without waiting for it. To run the test, put it in packages/core/test/ and run bun test from packages/core.

   Reproduction script (mcp-read-after-write.test.ts)

   // Reproduction for #39902: after mcp.tools.changed is published (as MCP.add/remove/connect/disconnect do
   // before returning), ToolRegistry still reflects the previous catalog.
   import { expect } from "bun:test"
   import { Effect, Layer, Ref } from "effect"
   import { McpEvent } from "@opencode/schema/mcp-event"
   import { AppNodeBuilder } from "@opencode/core/effect/app-node-builder"
   import { LayerNode } from "@opencode/util/effect/layer-node"
   import { Bus } from "@opencode/core/bus"
   import { Mcp } from "@opencode/core/mcp/index"
   import { Permission } from "@opencode/core/permission"
   import { McpTool } from "@opencode/core/tool/mcp"
   import { Tool } from "@opencode/core/tool"
   import { Image } from "@opencode/core/image"
   import { testEffect } from "./lib/effect"
   import { imagePassthrough } from "./lib/image"
   import { toolDefinitions, waitForTool } from "./lib/tool"
   
   testEffect(Layer.empty).live("registry reflects MCP mutation once publish returns", () =>
     Effect.gen(function* () {
       const catalog = yield* Ref.make<Mcp.Tool[]>([])
       yield* Effect.gen(function* () {
         const registry = yield* Tool.Service
         const registration = yield* McpTool.Service
         const bus = yield* Bus.Service
         yield* registration.flush
         // Simulate MCP.add: catalog updated, then ToolsChanged published, then the request returns.
         yield* Ref.set(catalog, [
           { server: Mcp.ServerName.make("voice"), name: "list_open_tabs", codemode: false,
             inputSchema: { type: "object", properties: {} } },
         ])
         yield* bus.publish(McpEvent.ToolsChanged, { server: "voice" })
         const immediately = (yield* toolDefinitions(registry)).map((t) => t.name)
         console.log("snapshot right after publish:", immediately)
         yield* waitForTool(registry, "voice_list_open_tabs")
         console.log("snapshot after waiting:", (yield* toolDefinitions(registry)).map((t) => t.name))
         expect(immediately).toContain("voice_list_open_tabs") // fails: read-after-write gap
       }).pipe(
         Effect.provide(
           Layer.fresh(
             AppNodeBuilder.build(LayerNode.group([Tool.node, McpTool.node, Bus.node]), [
               Mcp.node.replace(Layer.mock(Mcp.Service, { tools: () => Ref.get(catalog) })),
               Permission.node.replace(Layer.mock(Permission.Service, { assert: () => Effect.void })),
               Image.node.replace(imagePassthrough),
             ]),
           ),
         ),
       )
     }),
   )

   Run it: bun mcp-read-after-write.test.ts
7. opencodejr

   added

   reproduced

   on Oct 28, 2026
8. neriousy commented on Nov 13, 2026

   

   neriousy

   on Nov 13, 2026

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   Closing as fixed by #60490. Before taking a tool snapshot, a session now compares the live MCP catalog with the registered tools and reconciles them if they differ (packages/core/src/session/context.ts, packages/core/src/tool/mcp.ts). So the next session step sees the change right after a runtime MCP add or remove, even though the handler still returns before the debounced reload. The earlier repro was on a build from before #60490. Please reopen if it still happens on a release that includes it.
9. neriousy

   closed this as completedon Nov 13, 2026