Custom prompt with long arg input does not expand the custom prompt

What version of Codex is running?

0.43.0-alpha.6

Which model were you using?

gpt-5-codex high

What platform is your computer?

Darwin 25.0.0 arm64 arm

What steps can reproduce the bug?

• Create a prompt. Could be as simple as a test.md file containing: Echo back: $1
• Call the prompt with a large argument string: /prompts:test "[Pasted Content 1105 chars]"
• See the expanded prompt as /prompts:test <LONG STRING EXPANDED HERE>

The prompt itself does not get expanded.

If I test with a small input like /prompts:test "Hello there", I see the prompt expand to: Echo back: Hello there.

What is the expected behavior?

The prompt should expand regardless of the length of the the arg string.

What do you see instead?

The prompt does not expand if the arg input is large.

Additional information

_No response_