feat: add /v3/transactions/simulate endpoint

[brice-stacks]

Added a new authenticated RPC endpoint POST /v3/transactions/simulate which simulates a single transaction on top of the canonical chain tip (discarding all state changes) and reports its result, events, and execution cost.

Applicable issues

• fixes Transaction simulation endpoint #7423

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[Copilot]

Pull request overview

Adds a new authenticated RPC endpoint to simulate a single signed Stacks transaction against the canonical chain tip, returning the execution result, events, and execution cost without persisting any state.

Changes:

• Implement POST /v3/transactions/simulate request/response handling and simulation logic in an ephemeral Nakamoto tenure/block context.
• Register the endpoint in the RPC router and add API tests covering success + auth/nonce failures.
• Document the endpoint in OpenAPI and add a changelog fragment.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
stackslib/src/net/api/txsimulate.rs	Implements the new /v3/transactions/simulate RPC handler, simulation logic, and client request/response helpers.
stackslib/src/net/api/tests/txsimulate.rs	Adds tests for request parsing, error cases, and successful simulation behavior.
stackslib/src/net/api/tests/mod.rs	Wires the new txsimulate test module into the API test suite.
stackslib/src/net/api/mod.rs	Registers the new RPC endpoint in StacksHttp.
docs/rpc/openapi.yaml	Documents the new endpoint contract and response shape.
changelog.d/tx-simulate-endpoint.added	Changelog entry announcing the new endpoint.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

[francesco-stacks]

Overall LGTM, just a couple of points:

• the OpenAPI CI step is failing
• could we also add the same memory limit mechanism we use on the miners/signers to guarantee that the transaction we are executing actually stays within the limits and would be accepted by the miners/singers?

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

[coveralls]

Coverage Report for CI Build 29847442772

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage increased (+0.8%) to 86.522%

Details

• Coverage increased (+0.8%) from the base build.
• Patch coverage: 59 uncovered changes across 1 file (218 of 277 lines covered, 78.7%).
• 12006 coverage regressions across 166 files.

Uncovered Changes

File	Changed	Covered	%
stackslib/src/net/api/txsimulate.rs	275	216	78.55%
Total (2 files)	277	218	78.7%

Coverage Regressions

12006 previously-covered lines in 166 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/chainstate/burn/db/sortdb.rs	641	90.47%
stackslib/src/chainstate/stacks/db/blocks.rs	525	89.96%
stackslib/src/chainstate/nakamoto/mod.rs	488	84.91%
stackslib/src/config/mod.rs	324	78.53%
stackslib/src/net/relay.rs	316	74.91%
stackslib/src/net/mod.rs	313	78.15%
stackslib/src/chainstate/stacks/index/storage.rs	277	82.41%
stackslib/src/chainstate/stacks/db/transactions.rs	269	97.13%
clarity/src/vm/database/clarity_db.rs	268	82.27%
stackslib/src/chainstate/stacks/miner.rs	267	83.12%

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Coverage Stats

Relevant Lines:	231623
Covered Lines:	200405
Line Coverage:	86.52%
Coverage Strength:	19619380.13 hits per line

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💛 - Coveralls

[rob-stacks]

I like it, given that the block simulation rpc endpoint supports running transactions in any arbitrary block in the chain, can we have the same feature here?