stan-dev/stan

Include (unconstrained) parameter names along with the inverse metric?

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#3,398 opened on Jul 21, 2026

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good first issuei/ointerface

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Description

CmdStan writes the inverse metric to a file, but it doesn't include the names of the unconstrained parameters corresponding to the rows/columns. See stan-dev/cmdstanr#716.

To do that I think it would just involve including the names returned by

  model.unconstrained_param_names(names, false, false);

in the JSON file, right? For example,

  {
    "stepsize": 0.1,
    "metric_type": "diag_e",
    "unconstrained_param_names": ["alpha", "beta.1", "beta.2"],
    "inv_metric": [0.1, 0.2, 0.3]
  }

This would make it much easier for users to figure out which unconstrained parameter elements correspond to which elements of the matrix.

Contributor guide

Research direction
Investigate how the inverse metric is currently written to file and modify the code to also output the unconstrained parameter names using model.unconstrained param names(...).
Tech stack
cpp
Domain
backend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40