Use exact assertions in fingerprint tests instead of fuzzy matching

Replace toContain() with toBe() for canonical form assertions. When output
is deterministic, test the exact value. Control inputs by mocking error.stack
to make outputs predictable.

Add testing guidance to CLAUDE.md about preferring exact assertions.

Author: petyosi

CLAUDE.md

@@ -175,6 +175,7 @@ The SDK uses ULID (Universally Unique Lexicographically Sortable Identifier) for
 - Tests use Vitest
 - Some packages have minimal tests (`--passWithNoTests` flag in package.json)
 - Test files are located alongside source files with `.test.ts` extension
+- **Prefer exact assertions over fuzzy matching**: When testing functions that produce stable, canonical output, use exact equality (`toBe`, `toEqual`) rather than fuzzy matchers (`toContain`, `toMatch`). If a value should be stable, test the exact value. For example, instead of `expect(result).toContain('foo')`, use `expect(result).toBe('expected\nfull\nvalue')`. Control inputs (like mocking `error.stack`) to make outputs deterministic.
 
 ## Node Version
 

packages/logfire-api/src/fingerprint.test.ts

@@ -3,12 +3,18 @@ import { describe, expect, test } from 'vitest'
 import { canonicalizeError, computeFingerprint } from './fingerprint'
 
 describe('canonicalizeError', () => {
-  test('includes error type', () => {
+  test('produces stable canonical form with error type and stack frames', () => {
     const error = new TypeError('test message')
+    error.stack = `TypeError: test message
+    at testFunction (test.js:10:5)
+    at main (test.js:20:3)`
+
     const canonical = canonicalizeError(error)
 
-    expect(canonical).toContain('TypeError')
-    expect(canonical).toContain('----')
+    expect(canonical).toBe(`TypeError
+----
+test:testFunction
+test:main`)
   })
 
   test('includes function names from stack', () => {
@@ -32,34 +38,73 @@ describe('canonicalizeError', () => {
 
   test('handles error.cause chain', () => {
     const cause = new Error('root cause')
+    cause.stack = `Error: root cause
+    at causeFunction (cause.js:5:1)`
+
     const error = new Error('wrapper', { cause })
+    error.stack = `Error: wrapper
+    at wrapperFunction (wrapper.js:10:5)`
 
     const canonical = canonicalizeError(error)
 
-    expect(canonical).toContain('----CAUSE----')
-    expect(canonical.split('----CAUSE----').length).toBe(2)
+    expect(canonical).toBe(`Error
+----
+wrapper:wrapperFunction
+----CAUSE----
+Error
+----
+cause:causeFunction`)
   })
 
   test('handles AggregateError', () => {
-    const errors = [new Error('first'), new TypeError('second')]
-    const aggregate = new AggregateError(errors, 'multiple errors')
+    const error1 = new Error('first')
+    error1.stack = `Error: first
+    at firstFn (first.js:1:1)`
+
+    const error2 = new TypeError('second')
+    error2.stack = `TypeError: second
+    at secondFn (second.js:2:2)`
+
+    const aggregate = new AggregateError([error1, error2], 'multiple errors')
+    aggregate.stack = `AggregateError: multiple errors
+    at aggregateFn (aggregate.js:10:5)`
 
     const canonical = canonicalizeError(aggregate)
 
-    expect(canonical).toContain('AggregateError')
-    expect(canonical).toContain('----AGGREGATE----')
-    expect(canonical).toContain('Error')
-    expect(canonical).toContain('TypeError')
+    expect(canonical).toBe(`AggregateError
+----
+aggregate:aggregateFn
+----AGGREGATE----
+Error
+----
+first:firstFn
+----
+TypeError
+----
+second:secondFn`)
   })
 
   test('handles circular cause references', () => {
     const error1 = new Error('error 1')
+    error1.stack = `Error: error 1
+    at fn1 (file1.js:1:1)`
+
     const error2 = new Error('error 2', { cause: error1 })
+    error2.stack = `Error: error 2
+    at fn2 (file2.js:2:2)`
     ;(error1 as Error & { cause: Error }).cause = error2
 
     const canonical = canonicalizeError(error1)
 
-    expect(canonical).toContain('[circular]')
+    expect(canonical).toBe(`Error
+----
+file1:fn1
+----CAUSE----
+Error
+----
+file2:fn2
+----CAUSE----
+[circular]`)
   })
 
   test('deduplicates repeated frames (recursion)', () => {
@@ -83,9 +128,14 @@ describe('canonicalizeError', () => {
 
   test('does not include error message in canonical form', () => {
     const error = new Error('this message should not appear')
+    error.stack = `Error: this message should not appear
+    at testFn (test.js:1:1)`
+
     const canonical = canonicalizeError(error)
 
-    expect(canonical).not.toContain('this message should not appear')
+    expect(canonical).toBe(`Error
+----
+test:testFn`)
   })
 })
 
@@ -217,7 +267,9 @@ otherFunction@http://different.com/src/script.js:888:2`
     expect(fp1).toBe(fp2)
 
     const canonical = canonicalizeError(error1)
-    expect(canonical).toContain('myFunction')
-    expect(canonical).toContain('otherFunction')
+    expect(canonical).toBe(`Error
+----
+src/script:myFunction
+src/script:otherFunction`)
   })
 })