fix(rivetkit): bind methods through createWriteThroughProxy

[abcxff]

Fixes an error that occurs when inspector state attempts to serialize non trivial values (ie Date).

[railway-app]

🚅 Deployed to the rivet-pr-4987 environment in rivet-frontend

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[abcxff]

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• fix(pegboard): validate drain_grace_period < request_lifespan #4998 : 2 dependent PRs (#5010 , #5012 )
• fix(rivetkit): bind methods through createWriteThroughProxy #4987 👈 (View in Graphite)
• chore(rivetkit): log inspector errors + standardize pino errorKey error #4986
• [slop]fix(rivetkit): remove async drizzle db proxy #4984 : 2 other dependent PRs (#4985 , #4991 )
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[claude]

Code Review: fix(rivetkit): bind methods through createWriteThroughProxy

Overview

This 3-line change adds method binding inside the get trap of createWriteThroughProxy so that functions retrieved from a proxied state object receive the correct this (the raw inner target). The stated motivation is to fix serialization errors when inspector state contains non-trivial values like Date.

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What the fix solves

Without this change, calling prototype methods on built-in types stored in actor state throws a TypeError at runtime because those types use internal slots that a Proxy cannot satisfy. For example, date.toISOString() requires this to have a [[DateValue]] internal slot, which the proxy wrapper cannot expose. Binding to innerTarget makes those calls work correctly.

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Critical issue: mutation tracking silently broken for array/object methods

The binding is applied unconditionally to every function, including methods on plain arrays and objects. This breaks write-through mutation tracking.

Before this PR: arr.push(x) calls push with this = proxy, which fires the set trap (push sets this[length]), which calls commit(). Mutation is tracked.

After this PR: result.bind(innerTarget) means push runs with this = raw array, bypassing the set trap entirely. commit() is never called. Mutation is silently dropped.

The same regression applies to pop, splice, sort, reverse, and any other method that mutates via index assignment. Actor state modified through these methods will silently not persist.

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Suggested fix

Only bind to innerTarget for built-in types whose methods require an internal slot. For plain objects and arrays, keep the proxy as the receiver so mutations still flow through the set trap:

get(innerTarget, property, receiver) {
    const result = Reflect.get(innerTarget, property, receiver);
    if (typeof result === "function") {
        const internalSlotTypes = [Date, Map, Set, WeakMap, WeakSet, RegExp, ArrayBuffer, DataView];
        if (internalSlotTypes.some(T => innerTarget instanceof T)) {
            return result.bind(innerTarget);
        }
        return result.bind(proxy);
    }
    return result && typeof result === "object"
        ? wrap(result as object)
        : result;
},

(proxy can be forward-referenced with a let proxy: object declaration before the handler literal, then assigned after new Proxy(...). The same pattern is already used for proxies.set(target, proxy).)

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Additional concerns

• No tests. Two behaviors need coverage: (a) Date/Map/Set method calls through the proxy work without throwing, and (b) Array.push / splice through the proxy still triggers commit. The driver test suite (rivetkit-typescript/packages/rivetkit/tests/driver/) is the right home.
• Draft state / unfilled template. The PR description still has the template checklist and no root-cause context. Please fill it in before requesting review.
• Comment the invariant. A short inline comment explaining that internal-slot types require binding to the raw target would help future readers avoid regressing this again.

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Summary

The intent is correct. Bound receivers are required for built-in types with internal slots. But binding unconditionally to all functions silently disables write-through mutation tracking for arrays and objects, which is the core purpose of this proxy. Please scope the binding to known internal-slot types, add test coverage for both the fixed case and the mutation-tracking case, and fill in the PR description before merging.