$ARGUMENTS substitution does not work in skills with context: fork

Bug

When a skill with context: fork in frontmatter is invoked by another skill via the Skill() tool, $ARGUMENTS, $ARGUMENTS[0], and $ARGUMENTS[1] are not substituted. The forked context receives the raw template with literal placeholder text. The model then misinterprets $ARGUMENTS[1] (the literal text in the second argument definition) as the value of $ARGUMENTS[0].

Without context: fork, argument substitution works correctly.

Reproduction

1. Create skill-a (the caller):

---
name: skill-a
---
Invoke Skill("skill-b") with args "hello world".
Report what skill-b returned.

2. Create skill-b (the callee, with context: fork):

---
name: skill-b
context: fork
argument-hint: [arg1] [arg2]
---
## Inputs
- FIRST = $ARGUMENTS[0]
- SECOND = $ARGUMENTS[1]

Report the values of FIRST and SECOND.

3. User invokes skill-a: /skill-a

4. Expected — skill-b should see after substitution:

- FIRST = hello
- SECOND = world

5. Actual — no substitution occurs. The model sees the raw template and misinterprets the literal placeholder text:

- FIRST = $ARGUMENTS[1]   (model reads the next line as the value)
- SECOND = (empty)

Because no substitution happened, the model confuses literal $ARGUMENTS[1] text on the next line as the value of FIRST, and SECOND ends up empty.

Notes

• Tested with both claude-opus-4-6 and claude-sonnet-4-6 — same behavior on both.
• Removing context: fork from skill-b fixes the issue immediately — same invocation correctly substitutes both arguments.
• context: subdialog has the same problem.
• Direct user invocation (/skill-b hello world) substitutes correctly even with context: fork — the bug only occurs in the skill-a → skill-b chain.

Environment

• Claude Code CLI v2.1.74
• macOS Darwin 25.2.0