diff --git a/src/core/diff/strategies/__tests__/multi-search-replace.spec.ts b/src/core/diff/strategies/__tests__/multi-search-replace.spec.ts
index 297e64fa83..07ffc419a4 100644
--- a/src/core/diff/strategies/__tests__/multi-search-replace.spec.ts
+++ b/src/core/diff/strategies/__tests__/multi-search-replace.spec.ts
@@ -905,6 +905,29 @@ replacement content here
 			expect(error).toContain("Best Match Length: 0 characters")
 		})
 
+		it("should include a whole-lines tip when no sufficiently similar match is found", async () => {
+			const originalContent = "line one\nline two"
+			const diffContent = `test.ts
+<<<<<<< SEARCH
+:start_line:999
+-------
+non-existent content that cannot be found in the file
+=======
+replacement content here
+>>>>>>> REPLACE`
+
+			const result = await strategy.applyDiff(originalContent, diffContent)
+			expect(result.success).toBe(false)
+			const error =
+				!result.success && result.failParts?.[0]
+					? "error" in result.failParts[0]
+						? result.failParts[0].error
+						: ""
+					: ""
+			expect(error).toContain("No sufficiently similar match found")
+			expect(error).toContain("Tip: The SEARCH block must contain complete, whole lines")
+		})
+
 		it("should include scoped original content when search fails with start_line that has a low-score match", async () => {
 			const originalContent = "function existing() {\n    return 42;\n}\n"
 			const diffContent = `test.ts
diff --git a/src/core/diff/strategies/multi-search-replace.ts b/src/core/diff/strategies/multi-search-replace.ts
index a8ba0215d3..c8af32c70a 100644
--- a/src/core/diff/strategies/multi-search-replace.ts
+++ b/src/core/diff/strategies/multi-search-replace.ts
@@ -577,7 +577,7 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 
 					diffResults.push({
 						success: false,
-						error: `No sufficiently similar match found${lineRange} (${Math.floor(bestMatchScore * 100)}% similar, needs ${Math.floor(this.fuzzyThreshold * 100)}%)\n\nDebug Info:\n- Similarity Score: ${Math.floor(bestMatchScore * 100)}%\n- Required Threshold: ${Math.floor(this.fuzzyThreshold * 100)}%\n- Search Range: ${startLine ? `starting at line ${startLine}` : "start to end"}\n- Levenshtein Distance: ${levenDist >= 0 ? `${levenDist} characters` : "N/A"}\n- Search Length: ${searchChunk.length} characters\n- Best Match Length: ${bestMatchContent ? bestMatchContent.length : 0} characters\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n\nSearch Content:\n${searchChunk}${bestMatchSection}${originalContentSection}`,
+						error: `No sufficiently similar match found${lineRange} (${Math.floor(bestMatchScore * 100)}% similar, needs ${Math.floor(this.fuzzyThreshold * 100)}%)\n\nDebug Info:\n- Similarity Score: ${Math.floor(bestMatchScore * 100)}%\n- Required Threshold: ${Math.floor(this.fuzzyThreshold * 100)}%\n- Search Range: ${startLine ? `starting at line ${startLine}` : "start to end"}\n- Levenshtein Distance: ${levenDist >= 0 ? `${levenDist} characters` : "N/A"}\n- Search Length: ${searchChunk.length} characters\n- Best Match Length: ${bestMatchContent ? bestMatchContent.length : 0} characters\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n- Tip: The SEARCH block must contain complete, whole lines — partial-line (substring) matching is not supported. If the target line is very long, copy the entire line into the SEARCH block\n\nSearch Content:\n${searchChunk}${bestMatchSection}${originalContentSection}`,
 					})
 					continue
 				}
diff --git a/src/core/prompts/tools/native-tools/__tests__/apply_diff.spec.ts b/src/core/prompts/tools/native-tools/__tests__/apply_diff.spec.ts
new file mode 100644
index 0000000000..5df5488242
--- /dev/null
+++ b/src/core/prompts/tools/native-tools/__tests__/apply_diff.spec.ts
@@ -0,0 +1,40 @@
+import type OpenAI from "openai"
+import { apply_diff } from "../apply_diff"
+
+// Helper type to access function tools
+type FunctionTool = OpenAI.Chat.ChatCompletionTool & { type: "function" }
+
+// Helper to get function definition from tool
+const getFunctionDef = (tool: OpenAI.Chat.ChatCompletionTool) => (tool as FunctionTool).function
+
+// OpenAI types the parameter schema as FunctionParameters (Record<string, unknown>).
+// Narrow it with runtime checks instead of assertions so the spec tracks schema changes.
+function asRecord(value: unknown): Record<string, unknown> {
+	if (typeof value !== "object" || value === null) {
+		throw new Error("expected an object value in the apply_diff parameter schema")
+	}
+	return value as Record<string, unknown>
+}
+
+const getDiffParameterDescription = (): string => {
+	const parameters = asRecord(getFunctionDef(apply_diff).parameters)
+	const properties = asRecord(parameters["properties"])
+	const diff = asRecord(properties["diff"])
+	const description = diff["description"]
+	if (typeof description !== "string") {
+		throw new Error("diff parameter must declare a string description")
+	}
+	return description
+}
+
+describe("apply_diff diff parameter description", () => {
+	const diffDescription = getDiffParameterDescription()
+
+	it("should require complete, whole lines in the SEARCH block", () => {
+		expect(diffDescription).toContain("The SEARCH block must contain complete, whole lines")
+	})
+
+	it("should warn that partial-line (substring) matching is not supported", () => {
+		expect(diffDescription).toContain("partial-line (substring) matching is not supported")
+	})
+})
diff --git a/src/core/prompts/tools/native-tools/apply_diff.ts b/src/core/prompts/tools/native-tools/apply_diff.ts
index cdbd99df33..720dddef3a 100644
--- a/src/core/prompts/tools/native-tools/apply_diff.ts
+++ b/src/core/prompts/tools/native-tools/apply_diff.ts
@@ -14,6 +14,7 @@ const DIFF_PARAMETER_DESCRIPTION = `A string containing one or more search/repla
 CRITICAL:
 - The ':start_line:[line_number]' header is strongly recommended for accurate matching. When provided, it must follow the exact syntax ':start_line:[integer]' (for example: ':start_line:220'). Do not write headers with shorthand forms like ':220' or variations like ':start_line=220'.
 - Copy the exact lines from the source file for a 100% string match including all whitespace, indentation, and newlines.
+- The SEARCH block must contain complete, whole lines — partial-line (substring) matching is not supported. When the target line is very long (e.g. a Markdown paragraph), copy the entire line into the SEARCH block.
 - Ensure the separator '-------' is on its own line immediately following ':start_line:[line_number]' with a newline.`
 
 export const apply_diff = {