diff --git a/src/cli/environmentCommands.ts b/src/cli/environmentCommands.ts
index 229d4deb27..f84d4967a6 100644
--- a/src/cli/environmentCommands.ts
+++ b/src/cli/environmentCommands.ts
@@ -11,6 +11,7 @@ import { logCliCommand } from "../cliLogging.js";
 import { parseEnvironmentName } from "../config/environmentName.js";
 import {
   parseMcpHttpEndpoint,
+  mcpHttpListenerPortSchema,
   type McpHttpEndpoint,
 } from "../config/mcpHttpEndpoint.js";
 import type { AnalysisError } from "../domain/analysisErrorBase.js";
@@ -364,11 +365,7 @@ export const registerEnvironmentCommands = (
         .min(1)
         .optional()
         .describe("Listener address; default MCP_HOST, else 127.0.0.1"),
-      port: z
-        .number()
-        .int()
-        .min(0)
-        .max(65535)
+      port: mcpHttpListenerPortSchema
         .optional()
         .describe(
           "Listener port; default MCP_PORT, else 0 for an ephemeral port",
diff --git a/src/config/mcpHttpEndpoint.ts b/src/config/mcpHttpEndpoint.ts
index 4604497ffb..06980156ba 100644
--- a/src/config/mcpHttpEndpoint.ts
+++ b/src/config/mcpHttpEndpoint.ts
@@ -11,12 +11,15 @@ export const DEFAULT_MCP_HTTP_HOST = "127.0.0.1";
 /** Port 0 asks the host for an ephemeral port, reported by the listening log line. */
 export const DEFAULT_MCP_HTTP_PORT = 0;
 
+/** A listener may request an ephemeral port with 0; connected endpoints may not. */
+export const mcpHttpListenerPortSchema = z.number().int().min(0).max(65535);
+
 /** Decimal TCP port text, shared by the `--port` flag and `MCP_PORT`. */
 export const mcpHttpPortSchema = z
   .string()
   .regex(/^(?:0|[1-9]\d*)$/u)
   .transform(Number)
-  .pipe(z.number().int().min(0).max(65535));
+  .pipe(mcpHttpListenerPortSchema);
 
 const mcpHttpHostSchema = z.string().trim().min(1);
 
diff --git a/src/main/mcpHttpArguments.ts b/src/main/mcpHttpArguments.ts
index 634c58ff32..6072857c3d 100644
--- a/src/main/mcpHttpArguments.ts
+++ b/src/main/mcpHttpArguments.ts
@@ -2,6 +2,7 @@ import { Help, z } from "incur";
 
 import {
   MCP_HTTP_PORT_CONSTRAINT,
+  mcpHttpListenerPortSchema,
   mcpHttpPortSchema,
 } from "../config/mcpHttpEndpoint.js";
 
@@ -18,11 +19,7 @@ const MCP_HTTP_OPTIONS = z.object({
     .min(1)
     .optional()
     .describe("Listener address; default MCP_HOST, else 127.0.0.1"),
-  port: z
-    .number()
-    .int()
-    .min(0)
-    .max(65535)
+  port: mcpHttpListenerPortSchema
     .optional()
     .describe("Listener port; default MCP_PORT, else 0 for an ephemeral port"),
   help: z.boolean().optional().describe("Show help"),