fix(session): clone tool input before passing to EventV2 to prevent Immer freeze

[stephanschielke]

Issue for this PR

Closes #25873

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes TypeError: Attempted to assign to readonly property that occurs on every tool call when OPENCODE_EXPERIMENTAL=true and a plugin mutates tool args in tool.execute.before hooks (e.g. oh-my-openagent).

Root cause: In processor.ts:329, the tool-call event handler passes value.input by reference to EventV2.run(). This reference is the same object as the tool's args in execute(args, options). The call flows through SyncEvent.run -> SessionMessageUpdater.update() -> Immer produce(), which calls Object.freeze() on the entire produced state tree, including the original args object. When a plugin hook subsequently tries to mutate args.command, it throws.

Fix: Clone the input before passing to EventV2.run so Immer freezes the clone instead of the original:

-              input: value.input,
+              input: structuredClone(value.input),

Instrumented proof:

BEFORE EventV2.run: value.input frozen=false
AFTER EventV2.run:  value.input frozen=true

How did you verify your code works?

1. Built v1.14.34 binary with OPENCODE_EXPERIMENTAL=true + oh-my-openagent plugin
2. Unpatched: 8/8 tool calls crash with Attempted to assign to readonly property
3. Patched: 8/8 tool calls succeed
4. OPENCODE_EXPERIMENTAL=false: 8/8 succeed (confirms the flag gates the code path)
5. git bisect across 12,213 commits identified commit a3bc5d35b (PR Refactor v2 session events as schemas #24512) as the first bad commit
6. File-level revert of only processor.ts fixes the crash
7. Stubbing EventV2.run as a no-op fixes the crash

Screenshots / recordings

N/A (not a UI change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[stephanschielke]

@thdxr You are working on the new event system, right?

As code-yeongyu/oh-my-openagent#3816 rightfully points out plugins should not be allowed to change/modify arguments like tool calls to prevent them from braking out. However, as the current v1 event system (probably unconsciously) established, plugins rely on / make use of receiving mutable arguments as a plugin system "feature".

What are your plans for the new v2 event system? Should plugins receive a mutable reference of the arguments, or a copy? How should plugins return a modification of arguments?