refactor: Extract the substitution behaviour used by custom slash commands to its own file so that it can be more easily reused by new features.

packages/opencode/src/config/substitute.ts

@@ -0,0 +1,27 @@
+const placeholderRegex = /\$(\d+)/g
+
+export function substituteArguments(
+  template: string,
+  args: string[],
+): { result: string; hasPlaceholders: boolean } {
+  const placeholders = template.match(placeholderRegex) ?? []
+  let last = 0
+  for (const item of placeholders) {
+    const value = Number(item.slice(1))
+    if (value > last) last = value
+  }
+
+  const hasPlaceholders = placeholders.length > 0
+
+  let result = template.replaceAll(placeholderRegex, (_, index) => {
+    const position = Number(index)
+    const argIndex = position - 1
+    if (argIndex >= args.length) return ""
+    if (position === last) return args.slice(argIndex).join(" ")
+    return args[argIndex]
+  })
+
+  result = result.replaceAll("$ARGUMENTS", args.join(" "))
+
+  return { result, hasPlaceholders }
+}

packages/opencode/src/session/prompt.ts

@@ -33,6 +33,7 @@ import { spawn } from "child_process"
 import { Command } from "../command"
 import { $, fileURLToPath, pathToFileURL } from "bun"
 import { ConfigMarkdown } from "../config/markdown"
+import { substituteArguments } from "../config/substitute"
 import { SessionSummary } from "./summary"
 import { NamedError } from "@opencode-ai/util/error"
 import { fn } from "@/util/fn"
@@ -1733,7 +1734,6 @@ NOTE: At any point in time through this workflow you should feel free to ask the
   const bashRegex = /!`([^`]+)`/g
   // Match [Image N] as single token, quoted strings, or non-space sequences
   const argsRegex = /(?:\[Image\s+\d+\]|"[^"]*"|'[^']*'|[^\s"']+)/gi
-  const placeholderRegex = /\$(\d+)/g
   const quoteTrimRegex = /^["']|["']$/g
   /**
    * Regular expression to match @ file references in text
@@ -1751,27 +1751,13 @@ NOTE: At any point in time through this workflow you should feel free to ask the
 
     const templateCommand = await command.template
 
-    const placeholders = templateCommand.match(placeholderRegex) ?? []
-    let last = 0
-    for (const item of placeholders) {
-      const value = Number(item.slice(1))
-      if (value > last) last = value
-    }
-
-    // Let the final placeholder swallow any extra arguments so prompts read naturally
-    const withArgs = templateCommand.replaceAll(placeholderRegex, (_, index) => {
-      const position = Number(index)
-      const argIndex = position - 1
-      if (argIndex >= args.length) return ""
-      if (position === last) return args.slice(argIndex).join(" ")
-      return args[argIndex]
-    })
+    const { result: withArgs, hasPlaceholders } = substituteArguments(templateCommand, args)
     const usesArgumentsPlaceholder = templateCommand.includes("$ARGUMENTS")
     let template = withArgs.replaceAll("$ARGUMENTS", input.arguments)
 
     // If command doesn't explicitly handle arguments (no $N or $ARGUMENTS placeholders)
     // but user provided arguments, append them to the template
-    if (placeholders.length === 0 && !usesArgumentsPlaceholder && input.arguments.trim()) {
+    if (!hasPlaceholders && !usesArgumentsPlaceholder && input.arguments.trim()) {
       template = template + "\n\n" + input.arguments
     }
 

packages/opencode/test/config/substitute.test.ts

@@ -0,0 +1,35 @@
+import { test, expect } from "bun:test"
+import { substituteArguments } from "../../src/config/substitute"
+
+test("substituteArguments - no placeholders", () => {
+  const { result, hasPlaceholders } = substituteArguments("hello world", ["a", "b"])
+  expect(result).toBe("hello world")
+  expect(hasPlaceholders).toBe(false)
+})
+
+test("substituteArguments - single placeholder", () => {
+  const { result, hasPlaceholders } = substituteArguments("hello $1", ["world"])
+  expect(result).toBe("hello world")
+  expect(hasPlaceholders).toBe(true)
+})
+
+test("substituteArguments - multiple placeholders", () => {
+  const { result, hasPlaceholders } = substituteArguments("$1 and $2", ["first", "second"])
+  expect(result).toBe("first and second")
+  expect(hasPlaceholders).toBe(true)
+})
+
+test("substituteArguments - last placeholder swallows remaining", () => {
+  const { result } = substituteArguments("$1 $2", ["a", "b", "c", "d"])
+  expect(result).toBe("a b c d")
+})
+
+test("substituteArguments - missing argument returns empty", () => {
+  const { result } = substituteArguments("$1 and $3", ["only", "two"])
+  expect(result).toBe("only and ")
+})
+
+test("substituteArguments - $ARGUMENTS replaced", () => {
+  const { result } = substituteArguments("args: $ARGUMENTS", ["a", "b", "c"])
+  expect(result).toBe("args: a b c")
+})