st-eval

name	st-eval
description	Smalltalk code evaluator for Pharo via MCP. Use when executing Smalltalk expressions, verifying object state or intermediate values, debugging code incrementally, checking Pharo connection, or running quick experiments.
allowed-tools	mcp__smalltalk-interop__eval

Execute Smalltalk Code

Evaluate arbitrary Smalltalk expressions in the running Pharo image via mcp__smalltalk-interop__eval.

Essential Rules

Always use printString — MCP returns serialized output. Raw objects cause errors or unreadable results:

✅ MyClass new doSomething printString
✅ collection printString
❌ MyClass new doSomething

Wrap risky code in on:do: — Uncaught errors in eval crash the MCP call with no useful message:

| result |
result := Array new: 2.
[ result at: 1 put: (riskyOperation) printString ]
  on: Error do: [:ex | result at: 2 put: ex description].
^ result

Use fork for blocking operations — Code that opens a dialog or blocks the image will cause the MCP call to hang indefinitely. Detach it:

[ <blocking expression> ] fork.
^ 'started'

Common Patterns

Connection check

Smalltalk version

Inspect intermediate values

| step1 step2 |
step1 := objA computeStep1.
step2 := step1 processStep2.
^ { 'step1' -> step1 printString. 'step2' -> step2 printString } asDictionary printString

Inspect object state

{
  'class' -> obj class name.
  'value' -> obj printString.
  'size'  -> obj size printString
} asDictionary printString

Safe collection access

collection ifEmpty: ['empty'] ifNotEmpty: [:col | col first printString]

Error-capturing eval

| result |
result := Array new: 2.
[
  | obj |
  obj := MyClass new name: 'Test'.
  result at: 1 put: obj process printString.
] on: Error do: [:ex |
  result at: 2 put: ex description
].
^ result

Troubleshooting

Symptom	Cause	Fix
MCP call hangs / no response	Blocking operation (dialog, modal, infinite loop)	Wrap in fork
Error: cannot serialize	Returning raw object	Add printString
Debugger window opened in Pharo	Uncaught error triggered Pharo debugger	Close debugger in Pharo, then re-eval with on:do:
MessageNotUnderstood	Method doesn't exist or typo	Check with mcp__smalltalk-interop__search_implementors

Related Skills

• smalltalk-dev:smalltalk-debugger — Systematic debugging workflow using eval
• smalltalk-dev:st-init — Verify Pharo connection if eval fails unexpectedly