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fix: enhance null edit detection for Gemini models #7362

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fix: enhance null edit detection for Gemini models #7362

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134 changes: 134 additions & 0 deletions src/core/diff/strategies/__tests__/null-edit-detection.spec.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,134 @@
import { MultiSearchReplaceDiffStrategy } from "../multi-search-replace"

describe("null edit detection", () => {
let strategy: MultiSearchReplaceDiffStrategy

beforeEach(() => {
strategy = new MultiSearchReplaceDiffStrategy()
})

it("should detect and reject null edits (identical search and replace)", async () => {
const originalContent = 'function hello() {\n console.log("hello")\n}\n'
const diffContent = [
"<<<<<<< SEARCH",
"function hello() {",
' console.log("hello")',
"}",
"=======",
"function hello() {",
' console.log("hello")',
"}",
">>>>>>> REPLACE",
].join("\n")

const result = await strategy.applyDiff(originalContent, diffContent)
expect(result.success).toBe(false)
if (!result.success && result.failParts && result.failParts.length > 0) {
const firstError = result.failParts[0]
if (!firstError.success && firstError.error) {
expect(firstError.error).toContain("NULL EDIT DETECTED")
expect(firstError.error).toContain("Search and replace content are identical")
expect(firstError.error).toContain("This is a common issue with AI models (especially Gemini)")
expect(firstError.error).toContain("Model hallucination")
}
}
})

it("should detect null edits in multi-block diffs", async () => {
const originalContent =
'function hello() {\n console.log("hello")\n}\nfunction world() {\n return "world"\n}'
const diffContent = [
"<<<<<<< SEARCH",
"function hello() {",
' console.log("hello")',
"}",
"=======",
"function hello() {",
' console.log("hello world")',
"}",
">>>>>>> REPLACE",
"",
"<<<<<<< SEARCH",
"function world() {",
' return "world"',
"}",
"=======",
"function world() {",
' return "world"',
"}",
">>>>>>> REPLACE",
].join("\n")

const result = await strategy.applyDiff(originalContent, diffContent)
// Should partially succeed but report the null edit
expect(result.success).toBe(true)
if (result.success) {
expect(result.failParts).toBeDefined()
if (result.failParts && result.failParts[0] && !result.failParts[0].success) {
expect(result.failParts[0].error).toContain("NULL EDIT DETECTED")
}
}
})

it("should detect null edits with line numbers", async () => {
const originalContent = "function test() {\n return true;\n}\n"
const diffContent = [
"<<<<<<< SEARCH",
":start_line:1",
"-------",
"function test() {",
" return true;",
"}",
"=======",
"function test() {",
" return true;",
"}",
">>>>>>> REPLACE",
].join("\n")

const result = await strategy.applyDiff(originalContent, diffContent)
expect(result.success).toBe(false)
if (!result.success && result.failParts && result.failParts.length > 0) {
const firstError = result.failParts[0]
if (!firstError.success && firstError.error) {
expect(firstError.error).toContain("NULL EDIT DETECTED")
expect(firstError.error).toContain("phantom")
expect(firstError.error).toContain("Ensure the REPLACE block contains the actual modified content")
}
}
})

it("should not trigger null edit detection for legitimate empty replacements (deletions)", async () => {
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@roomote roomote bot Aug 23, 2025

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Great test coverage! Consider adding a comment here explaining why legitimate deletions (empty replace blocks) should not trigger null edit detection. This distinction is important for future maintainers to understand the design decision.

const originalContent = "function test() {\n // Remove this comment\n return true;\n}\n"
const diffContent = ["<<<<<<< SEARCH", " // Remove this comment", "=======", ">>>>>>> REPLACE"].join("\n")

const result = await strategy.applyDiff(originalContent, diffContent)
expect(result.success).toBe(true)
// Should not contain null edit error
if (!result.success && result.error) {
expect(result.error).not.toContain("NULL EDIT DETECTED")
}
})

it("should not trigger for actual changes even if similar", async () => {
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@roomote roomote bot Aug 23, 2025

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Could we add an edge case test for when search and replace blocks have different whitespace but are otherwise identical? This would help document the expected behavior when whitespace types differ (tabs vs spaces).

const originalContent = "function test() {\n return true;\n}\n"
const diffContent = [
"<<<<<<< SEARCH",
"function test() {",
" return true;",
"}",
"=======",
"function test() {",
" return false;",
"}",
">>>>>>> REPLACE",
].join("\n")

const result = await strategy.applyDiff(originalContent, diffContent)
expect(result.success).toBe(true)
// Should not contain null edit error
if (!result.success && result.error) {
expect(result.error).not.toContain("NULL EDIT DETECTED")
}
})
})
13 changes: 10 additions & 3 deletions src/core/diff/strategies/multi-file-search-replace.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -515,10 +515,17 @@ Each file requires its own path, start_line, and diff elements.
diffResults.push({
success: false,
error:
`Search and replace content are identical - no changes would be made\n\n` +
`NULL EDIT DETECTED: Search and replace content are identical - no changes would be made\n\n` +
`This is a common issue with AI models (especially Gemini) producing "phantom" edits.\n\n` +
`Debug Info:\n` +
`- Search and replace must be different to make changes\n` +
`- Use read_file to verify the content you want to change`,
`- The SEARCH block and REPLACE block contain exactly the same content\n` +
`- This means the edit would have zero effect on the file\n` +
`- To fix: Ensure the REPLACE block contains the actual modified content\n` +
`- Use read_file to verify the current file content before making changes\n\n` +
`Common causes:\n` +
`1. Model hallucination - thinking it made changes when it didn't\n` +
`2. Incorrect copy/paste of content between blocks\n` +
`3. Missing the actual modification in the REPLACE block`,
})
continue
}
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13 changes: 10 additions & 3 deletions src/core/diff/strategies/multi-search-replace.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -432,10 +432,17 @@ Only use a single line of '=======' between search and replacement content, beca
diffResults.push({
success: false,
error:
`Search and replace content are identical - no changes would be made\n\n` +
`NULL EDIT DETECTED: Search and replace content are identical - no changes would be made\n\n` +
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@roomote roomote bot Aug 23, 2025

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Consider extracting this error message to a shared constant since it's duplicated in multi-file-search-replace.ts (lines 518-528). This would ensure consistency and make future updates easier.

Also, while the error message is comprehensive, we might want to add a test case for when search and replace blocks differ only in whitespace type (tabs vs spaces) to ensure we're not triggering false positives.

`This is a common issue with AI models (especially Gemini) producing "phantom" edits.\n\n` +
`Debug Info:\n` +
`- Search and replace must be different to make changes\n` +
`- Use read_file to verify the content you want to change`,
`- The SEARCH block and REPLACE block contain exactly the same content\n` +
`- This means the edit would have zero effect on the file\n` +
`- To fix: Ensure the REPLACE block contains the actual modified content\n` +
`- Use read_file to verify the current file content before making changes\n\n` +
`Common causes:\n` +
`1. Model hallucination - thinking it made changes when it didn't\n` +
`2. Incorrect copy/paste of content between blocks\n` +
`3. Missing the actual modification in the REPLACE block`,
})
continue
}
Expand Down
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