feat: add typed multi-block simulation with capability errors

[koko1123]

What

Adds Provider.simulateV1 so callers can preview multiple calls across simulated blocks, inspect successful and reverted calls together, and distinguish an unsupported RPC method from other RPC errors. This is the simulation portion of #121; the tracing surface follows in a separate stacked PR.

Why

eth_call does not expose the complete outcome of a bundle or multi-block preview. Wallets and searchers need per-call gas, logs, return data, and failure details without changing live state.

How

• Typed payloads encode calls, block overrides, and the existing state-override serializer. An owned arena keeps all result data, including full per-call error payloads and optional raw block fields.
• Structured RPC results preserve diagnostics and map JSON-RPC -32601 to error.MethodNotFound; fallback remains limited to transport failures.
• Offline fixtures cover multiple blocks/calls, reverts, malformed input, and allocation failure. The Anvil suite tests overrides and unchanged live balance, and now accepts an isolated node port.
• Corrects transaction signature r/s parsing to accept RPC quantities with leading zeroes omitted, as exposed by the live pending-transaction test.

Validation: Zig 0.16.0 make ci, documentation production build, and the Anvil integration suite passed locally (25 passed, six mainnet-dependent checks skipped). Tracing is intentionally outside this PR; endpoint gas limits and supported methods still apply.

Checklist

• zig build test passes
• zig fmt --check src/ tests/ passes
• New functionality includes tests
• No external dependencies added
• CHANGELOG.md updated (if user-facing change)

Summary by CodeRabbit

• New Features

  • Added typed Ethereum transaction simulation through eth_simulateV1, including multi-block requests, state and block overrides, transfer tracing, revert details, and call results.
  • Added configurable Anvil ports for integration testing.
  • Improved transaction signature handling for shorter RPC integer values.

• Bug Fixes

  • Unsupported RPC methods now return explicit errors without triggering provider failover.
  • RPC errors and simulation failures retain detailed response data.

• Documentation

  • Added a simulation guide covering usage, validation, limits, overrides, errors, and integration testing.

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Project	Deployment	Actions	Updated
eth-zig	Ready	Preview	Sep 9, 2026 3:12am UTC

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📝 Walkthrough

Walkthrough

The PR adds typed eth_simulateV1 requests and results, provider methods, structured RPC error handling, reusable state overrides, response validation, configurable Anvil integration tests, and public documentation.

Changes

Simulation support

Layer / File(s)	Summary
Simulation contracts and request encoding src/simulation.zig, src/root.zig	Adds public request and result types, state and block overrides, simulation options, ownership helpers, and JSON-RPC request serialization.
Result parsing and validation src/simulation.zig	Parses multi-block results, call status, gas usage, logs, revert data, block data, and structured failures. It validates JSON shapes, quantities, fixed-size values, and allocation behavior.
Provider RPC integration and error handling src/json_rpc.zig, src/provider.zig, src/fallback_provider.zig	Adds eth_simulateV1, Provider.simulateV1, and Provider.requestJson. RPC errors distinguish unsupported methods from other failures. Transaction signature quantities accept short RPC integer forms.
Integration configuration and documentation build.zig, tests/integration_tests.zig, docs/content/docs/simulation.mdx, docs/content/docs/meta.json, CHANGELOG.md	Adds configurable Anvil ports, simulation integration coverage, navigation, usage documentation, and an Unreleased changelog entry.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🔵 Low · up to 7c371

Integration instructions may lead users to run tests against the wrong Anvil port when using custom configurations. The change is otherwise ready, but the documented execution path should be confirmed before merge.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant Provider
  participant Node
  participant SimulationParser
  Caller->>Provider: call simulateV1
  Provider->>Node: send eth_simulateV1 request
  Node-->>Provider: return simulation JSON or RPC error
  Provider->>SimulationParser: parse simulation result
  SimulationParser-->>Caller: return typed result or error

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main changes: typed multi-block simulation support and capability errors.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Create PR with unit tests
• Commit unit tests in branch koko/ethzig-simulation

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⚠️ Outside diff range comments (1)

tests/integration_tests.zig (1)

2-2: 📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Update the fixed-port instruction.

Line 2 states that Anvil must run on port 8545. The build now supports -Danvil-port, so this instruction is incorrect for the documented custom-port workflow. State that 8545 is the default, or document the matching build option.

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In `@tests/integration_tests.zig` at line 2, Update the setup comment near the
integration tests to state that port 8545 is Anvil’s default, and mention the
-Danvil-port build option for workflows using a custom port.

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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Outside diff comments:
In `@tests/integration_tests.zig`:
- Line 2: Update the setup comment near the integration tests to state that port
8545 is Anvil’s default, and mention the -Danvil-port build option for workflows
using a custom port.

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ℹ️ Review info

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between c01da28 and 5ae1054.

📒 Files selected for processing (10)

• CHANGELOG.md
• build.zig
• docs/content/docs/meta.json
• docs/content/docs/simulation.mdx
• src/fallback_provider.zig
• src/json_rpc.zig
• src/provider.zig
• src/root.zig
• src/simulation.zig
• tests/integration_tests.zig

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[koko1123]

Updated the integration setup comment in 7c37115 to identify 8545 as the default and show the matching custom-port options. Zig formatting and diff checks pass.