fix: strip mcp_ prefix from tool names before dispatch

The model emits tool names like mcp_background_output but the runtime
registry has them as background_output. While transformToolName already
handles the prefix for display purposes, the tool dispatch path in
tool-execute-before was not stripping it, causing 'unavailable tool' errors.

This adds mcp_ prefix stripping at the earliest point in the tool
execution pipeline, fixing background_output, background_cancel, and
all nocturne-memory_* tools.

Closes #2697

src/plugin/tool-execute-before-mcp-prefix.test.ts

@@ -0,0 +1,102 @@
+import { describe, expect, test, mock, beforeEach } from "bun:test"
+import { createToolExecuteBeforeHandler } from "./tool-execute-before"
+import type { PluginContext } from "./types"
+import type { CreatedHooks } from "../create-hooks"
+
+function createMockContext(): PluginContext {
+  return {
+    directory: "/tmp/test-dir",
+    sessionId: "test-session",
+  } as unknown as PluginContext
+}
+
+function createMockHooks(): CreatedHooks {
+  return {
+    ralphLoop: null,
+    startWork: null,
+    autoSlashCommand: null,
+  } as unknown as CreatedHooks
+}
+
+describe("tool-execute-before mcp_ prefix stripping", () => {
+  test("should strip mcp_ prefix from mcp_background_output", async () => {
+    // given
+    const handler = createToolExecuteBeforeHandler({
+      ctx: createMockContext(),
+      hooks: createMockHooks(),
+    })
+    const input = { tool: "mcp_background_output", sessionID: "ses_123", callID: "call_123" }
+    const output = { args: {} }
+
+    // when
+    await handler(input, output)
+
+    // then
+    expect(input.tool).toBe("background_output")
+  })
+
+  test("should strip mcp_ prefix from mcp_background_cancel", async () => {
+    // given
+    const handler = createToolExecuteBeforeHandler({
+      ctx: createMockContext(),
+      hooks: createMockHooks(),
+    })
+    const input = { tool: "mcp_background_cancel", sessionID: "ses_123", callID: "call_123" }
+    const output = { args: {} }
+
+    // when
+    await handler(input, output)
+
+    // then
+    expect(input.tool).toBe("background_cancel")
+  })
+
+  test("should strip mcp_ prefix from mcp_nocturne-memory_read_memory", async () => {
+    // given
+    const handler = createToolExecuteBeforeHandler({
+      ctx: createMockContext(),
+      hooks: createMockHooks(),
+    })
+    const input = { tool: "mcp_nocturne-memory_read_memory", sessionID: "ses_123", callID: "call_123" }
+    const output = { args: {} }
+
+    // when
+    await handler(input, output)
+
+    // then
+    expect(input.tool).toBe("nocturne-memory_read_memory")
+  })
+
+  test("should NOT strip mcp_ prefix from tools that already work (e.g., mcp_bash)", async () => {
+    // given — mcp_bash works fine because OpenCode handles it natively,
+    // but our prefix stripping should still normalize it
+    const handler = createToolExecuteBeforeHandler({
+      ctx: createMockContext(),
+      hooks: createMockHooks(),
+    })
+    const input = { tool: "mcp_bash", sessionID: "ses_123", callID: "call_123" }
+    const output = { args: { command: "echo hello" } }
+
+    // when
+    await handler(input, output)
+
+    // then — prefix stripped, bash handler still processes it
+    expect(input.tool).toBe("bash")
+  })
+
+  test("should not modify tool names without mcp_ prefix", async () => {
+    // given
+    const handler = createToolExecuteBeforeHandler({
+      ctx: createMockContext(),
+      hooks: createMockHooks(),
+    })
+    const input = { tool: "background_output", sessionID: "ses_123", callID: "call_123" }
+    const output = { args: {} }
+
+    // when
+    await handler(input, output)
+
+    // then
+    expect(input.tool).toBe("background_output")
+  })
+})

src/plugin/tool-execute-before.ts

@@ -52,6 +52,19 @@ export function createToolExecuteBeforeHandler(args: {
   }
 
   return async (input, output): Promise<void> => {
+    // Strip mcp_ prefix from tool names — the model may emit mcp_background_output
+    // but the runtime registry has it as background_output (fixes #2697)
+    if (/^mcp_/i.test(input.tool)) {
+      const stripped = input.tool.replace(/^mcp_/i, "")
+      log("[tool-execute-before] Stripped mcp_ prefix from tool name", {
+        original: input.tool,
+        resolved: stripped,
+        sessionID: input.sessionID,
+        callID: input.callID,
+      })
+      input.tool = stripped
+    }
+
     if (input.tool.toLowerCase() === "bash" && typeof output.args.command === "string") {
       if (output.args.command.includes("\x00")) {
         replaceToolArgs(output, { command: output.args.command.replace(/\x00/g, "") })