testing-simulink-models

Simulink Gherkin Tests

Requires Simulink Test. If unavailable, use simulating-simulink-models with manual assertions.

Use this skill when you need persistent, reusable pass/fail verification of model behavior.

When to Use

• Writing persistent pass/fail tests for a Simulink model or subsystem
• Verifying expected behavior against requirements or acceptance criteria
• Creating regression tests to catch future breakage
• Reproducing and validating bug fixes with structured assertions
• Collecting decision coverage metrics for a component

When NOT to Use

• Building or editing model structure → use building-simulink-models
• Running simulations for data exploration, sweeps, or custom analysis → use simulating-simulink-models
• Querying or resolving parameter values → use model_query_params / model_resolve_params
• Simulink Test is not installed → use simulating-simulink-models with manual assertions
• Component has no inputs or no outputs → model_test requires at least one Inport and one Outport; use simulating-simulink-models instead
• Component has physical modeling (Simscape) ports → test a parent subsystem with signal-based I/O instead

Workflow

1. Understand the component: Use model_overview and model_read on the target subsystem to identify inputs, outputs, and expected behavior.
2. Write the .feature file: Author a Gherkin test following the Syntax Reference below. Start with one scenario covering the primary nominal case.
3. Run in draft mode: Call model_test with draft_mode='true' for rapid iteration (~3s). Fix syntax or signal errors.
4. Run full compilation: Once draft passes, re-run with draft_mode='false' to validate against the actual compiled model (catches type/dimension mismatches).
5. Expand coverage: Add scenarios for edge cases, fault conditions, and boundary behavior. Use coverage='decision' to identify untested branches.

Syntax Reference

Write a .feature file in this format, then pass it to model_test:

# --- front-matter:toml ---                # REQUIRED: exactly one, must be first in file
model = "Model.slx"                        # model filename with .slx extension
component = "Model/Subsystem"              # optional; default = model name without .slx
[inputs]                                   # alias = "portReference" for each input port
Speed = "Speed"                            # scalar port: just the port name
Torque = "'Torque (Nm)'"                   # single quotes if name contains ( ) or .
Pos = "Position(2)"                        # vector element: "PortName(N)"
Cmd = "Control.Throttle"                   # bus element: "PortName.Element"
[outputs]                                  # alias = "portReference" for each output port
Output = "Output"                          # scalar port
Force = "'Force (N)'"                      # single-quoted scalar port
Yaw = "'Rate (deg/s)'.Filtered(2)"         # single-quoted port with vectorized bus element
# --- end front-matter ---                 # markers must be exact as shown

Feature: Descriptive title                 # exactly one Feature, colon required directly after keyword
  Description text here.                   # descriptions cannot start with keywords; prefix * to escape

Scenario: Unique scenario title            # at least one Scenario, unique titles, colon required
  Description of test case.
  Given inputs                             # exactly one Given; MUST have * line for EVERY declared input
    * Speed = const(50)                    # const(<value>)
    * Torque = step(0 -> 100 @ 1s)         # step(<from> -> <to> @ <time>)  time: Ns or Nms
  When simulate for 5s in Normal mode      # EXACT syntax; duration: Ns or Nms (>0); mode: Normal|SIL
  Then baseline "ref.mat" with tolerances: absTol=0.01, relTol=0.01, timeTol=50ms
    * Output: absTol=0.001                 # per-signal tolerance override; defaults are 0
  Then outputs                             # 1-2 Then blocks allowed (baseline and/or outputs)
    * Positive: Output > 0                 # operators: == != < > <= >=  (never vs another signal)
    * Bounded: Output == [10 .. 90]        # ranges with == only: [a..b] (a..b) [a..b) (a..b]
    * Settled: Output > 80 when t > 3s     # conditional: when t <op> <time>
    * InRange: Output == (0 .. 100]        # assessment names must be unique

Not supported: And But Rule Example @tags |tables| """

Description Line Escape

# ❌ WRONG - starts with keyword:
  When input changes, output responds
# ✅ Rephrase:
  If input changes, output responds
# ✅ Or escape with *:
  * When input changes, output responds

Best Practice

Prefer subsystem component-level tests—top level models can be large and slow to update/test, while subsystem components offer faster iteration, isolation, and clearer failure diagnosis. Use judgment based on what you're verifying.

The component under test must not contain physical modeling ports (PMIOPort / Simscape Connection Port blocks). Only components with standard Simulink Inport/Outport interfaces are supported. If the subsystem you want to test has physical ports, test a parent subsystem that wraps it with signal-based I/O instead.

If the model contains Simscape elements, set the model parameter SimscapeLogType to "none" before running tests. Simscape logging can interfere with test harness signal routing. This is a one-time model configuration, not a simulation command. Use: set_param('ModelName', 'SimscapeLogType', 'none').

Coverage

Pass coverage='none' (default) or coverage='decision' when calling model_test. The coverage parameter is required. Use 'decision' to collect both execution and decision coverage metrics (requires Simulink Coverage toolbox). Use 'none' when coverage is not needed.

Draft Mode

Pass draft_mode='true' for rapid test iteration (~3s vs ~60s). Draft mode skips the main model compile and uses a lightweight harness. Use 'true' by default during test development.

Limitation: Draft-mode harness uses double scalar for all inputs and outputs. If the model under test has non-double ports (boolean, integer, single, bus, vector), draft mode will fail with data type mismatch errors. In that case, re-run with draft_mode='false'.

Directory Warning

Don't change MATLAB's working directory while a model is open. Test harness cache is directory-tied—changing it causes stale harness errors.

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Copyright 2026 The MathWorks, Inc.

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