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VICE C64 Emulator MCP Server

by simen

toggleBreakpoint

Enable or disable debugging breakpoints in the VICE C64 emulator to pause program execution for inspection without removing breakpoint configurations.

Instructions

Enable or disable a breakpoint without deleting it.

Use this to temporarily disable breakpoints while keeping their configuration.

Related tools: setBreakpoint, deleteBreakpoint, listBreakpoints

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
breakpointIdYesBreakpoint ID from setBreakpoint
enabledYesTrue to enable, false to disable

Implementation Reference

  • src/index.ts:674-699 (registration)
    MCP tool registration for toggleBreakpoint, including input schema and inline handler function that delegates to ViceClient.toggleCheckpoint and formats response
      "toggleBreakpoint",
      {
        description: `Enable or disable a breakpoint without deleting it.
    
    Use this to temporarily disable breakpoints while keeping their configuration.
    
    Related tools: setBreakpoint, deleteBreakpoint, listBreakpoints`,
        inputSchema: z.object({
          breakpointId: z.number().describe("Breakpoint ID from setBreakpoint"),
          enabled: z.boolean().describe("True to enable, false to disable"),
        }),
      },
      async (args) => {
        try {
          await client.toggleCheckpoint(args.breakpointId, args.enabled);
          return formatResponse({
            success: true,
            breakpointId: args.breakpointId,
            enabled: args.enabled,
            message: `Breakpoint ${args.breakpointId} ${args.enabled ? "enabled" : "disabled"}`,
          });
        } catch (error) {
          return formatError(error as ViceError);
        }
      }
    );
  • Core implementation of toggle checkpoint (breakpoint/watchpoint) in ViceClient: sends CheckpointToggle command to VICE binary monitor protocol and updates local tracking
    async toggleCheckpoint(checkpointId: number, enabled: boolean): Promise<void> {
      const body = Buffer.alloc(5);
      body.writeUInt32LE(checkpointId, 0);
      body[4] = enabled ? 1 : 0;
      await this.sendCommand(Command.CheckpointToggle, body);
    
      // Update local tracking
      const cp = this.checkpoints.get(checkpointId);
      if (cp) {
        cp.enabled = enabled;
      }
    }
  • Zod input schema validation for toggleBreakpoint tool parameters: breakpointId (number) and enabled (boolean)
    inputSchema: z.object({
      breakpointId: z.number().describe("Breakpoint ID from setBreakpoint"),
      enabled: z.boolean().describe("True to enable, false to disable"),
    }),
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations are provided, so the description carries the full burden. It discloses that the tool toggles breakpoints without deletion, implying mutation but not specifying permissions, side effects, or response behavior. While it adds useful context about preserving configuration, it lacks details on error conditions, rate limits, or what happens if the breakpoint doesn't exist.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is highly concise and well-structured: the first sentence states the core purpose, the second provides usage context, and the third lists related tools. Every sentence adds value without redundancy, making it easy for an agent to parse quickly.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's moderate complexity (mutation operation with 2 parameters), no annotations, and no output schema, the description is reasonably complete. It covers purpose, usage context, and sibling relationships but lacks details on behavioral aspects like error handling or return values. It compensates well for the missing structured data but could be more comprehensive.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema fully documents both parameters (breakpointId and enabled). The description adds no additional parameter semantics beyond what's in the schema, such as format examples or constraints. Baseline 3 is appropriate when the schema handles all parameter documentation.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's purpose with specific verbs ('enable or disable') and resource ('breakpoint'), distinguishing it from siblings like deleteBreakpoint (which removes) and setBreakpoint (which creates). It explicitly mentions keeping configuration intact, which differentiates it from deletion.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides explicit guidance on when to use this tool ('temporarily disable breakpoints while keeping their configuration') and lists related tools (setBreakpoint, deleteBreakpoint, listBreakpoints), clearly positioning it among alternatives. This helps the agent understand the specific use case versus other breakpoint operations.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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