fix(edit): reduced hashline failures for weak models

- Added actionable reversed-range and block-anchor diagnostics.
- Accepted unambiguous single-dot ranges and routed affected models to replace mode.
- Preserved edit safety checks and added regression coverage.

Fixes #6671

packages/coding-agent/CHANGELOG.md

@@ -16,6 +16,7 @@
 
 ### Fixed
 
+- Fixed DeepSeek V4 Flash and Step 3.7 Flash models using hashline edit mode by default despite repeatedly misreading its range grammar; both now use the simpler replace-mode fallback unless explicitly overridden ([#6671](https://github.com/can1357/oh-my-pi/issues/6671)).
 - Fixed an Ask form appearing while the main prompt contains a draft hiding that text and consuming the next in-flight keystroke. The draft now remains visible and keeps receiving input until it is submitted or cleared; only then do form controls activate ([#6737](https://github.com/can1357/oh-my-pi/issues/6737)).
 - Fixed `glob` rejecting safe `memory://root/<directory>/**` patterns. Memory globs now resolve their directory prefix inside the project memory root while rejecting traversal and percent-encoded path separators across the complete glob path.
 - Fixed `omp --resume <id>` prompting to fork sessions from another existing directory instead of switching the process and cwd-scoped settings into the resumed session's recorded directory ([#6752](https://github.com/can1357/oh-my-pi/issues/6752)).

packages/coding-agent/src/utils/edit-mode.ts

@@ -16,6 +16,8 @@ export const EDIT_MODES = Object.keys(EDIT_MODE_IDS) as EditMode[];
 const HASHLINE_EXCLUDED_MODEL_MODES: Array<{ pattern: string; mode: EditMode }> = [
 	{ pattern: "kimi", mode: "replace" },
 	{ pattern: "mimo", mode: "replace" },
+	{ pattern: "deepseek-v4-flash", mode: "replace" },
+	{ pattern: "step-3.7-flash", mode: "replace" },
 ];
 
 function resolveHashlineExcludedModelMode(model: string | undefined): EditMode | null {

packages/coding-agent/test/core/block-replace.test.ts

@@ -196,6 +196,34 @@ describe("SWAP.BLK — native tree-sitter resolution end-to-end", () => {
 		});
 	});
 
+	it("suggests the next block opener for a blank anchor without modifying the file", async () => {
+		await withTempDir(async tempDir => {
+			const session = makeSession(tempDir);
+			const source = "\nfunction x() {\n  return 1;\n}\n";
+			const { filePath, header } = await seedFile(tempDir, session, "blank.ts", source);
+			const input = `${header}\nSWAP.BLK 1:\n+function y() {}`;
+
+			await expect(executeHashlineSingle(executeOptions(tempDir, input, session))).rejects.toThrow(
+				"Line 1 is blank; no syntactic block can begin there. The next multi-line block begins at line 2 and ends at line 4. Retry `SWAP.BLK 2:`.",
+			);
+			expect(await Bun.file(filePath).text()).toBe(source);
+		});
+	});
+
+	it("suggests the enclosing block and exact statement range without modifying the file", async () => {
+		await withTempDir(async tempDir => {
+			const session = makeSession(tempDir);
+			const source = "function x() {\n  run();\n}\n";
+			const { filePath, header } = await seedFile(tempDir, session, "statement.ts", source);
+			const input = `${header}\nSWAP.BLK 2:\n+  stop();`;
+
+			await expect(executeHashlineSingle(executeOptions(tempDir, input, session))).rejects.toThrow(
+				"For only this statement use `SWAP 2.=2:`. The nearest enclosing multi-line block begins at line 1 and ends at line 3; use `SWAP.BLK 1:` to target it.",
+			);
+			expect(await Bun.file(filePath).text()).toBe(source);
+		});
+	});
+
 	it("rejects a block edit on an unrecognized language", async () => {
 		await withTempDir(async tempDir => {
 			const session = makeSession(tempDir);

packages/coding-agent/test/edit-mode.test.ts

@@ -53,10 +53,18 @@ describe("resolveEditMode", () => {
 		expect(resolveEditMode(createSession({ activeModel: "xiaomi/MiMo-V2.5-Pro" }))).toBe("replace");
 	});
 
-	test("does not exclude DeepSeek Flash models", () => {
+	test("falls back from hashline to replace for DeepSeek V4 Flash models", () => {
 		delete Bun.env.PI_EDIT_VARIANT;
 
-		expect(resolveEditMode(createSession({ activeModel: "deepseek/deepseek-v4-flash" }))).toBe("hashline");
+		expect(resolveEditMode(createSession({ activeModel: "tensormesh/deepseek-ai/DeepSeek-V4-Flash" }))).toBe(
+			"replace",
+		);
+	});
+
+	test("falls back from hashline to replace for Step 3.7 Flash models", () => {
+		delete Bun.env.PI_EDIT_VARIANT;
+
+		expect(resolveEditMode(createSession({ activeModel: "kilo/stepfun/step-3.7-flash:free" }))).toBe("replace");
 	});
 
 	test("does not exclude non-Kimi Moonshot models", () => {

packages/hashline/CHANGELOG.md

@@ -2,6 +2,13 @@
 
 ## [Unreleased]
 
+## [17.1.5] - 2026-07-27
+
+### Changed
+
+- Improved reversed-range and invalid block-anchor diagnostics with absolute endpoint corrections plus nearby syntactic opener suggestions, without auto-applying the suggested edit ([#6671](https://github.com/can1357/oh-my-pi/issues/6671)).
+- Accepted a single dot between integer range endpoints, such as `DEL 235.258`, as an unambiguous range separator ([#6671](https://github.com/can1357/oh-my-pi/issues/6671)).
+
 ## [17.1.2] - 2026-07-24
 
 ### Changed

packages/hashline/src/block.ts

@@ -15,12 +15,57 @@
 import { STRUCTURAL_CLOSER_RE } from "./apply";
 import {
 	BLOCK_RESOLVER_UNAVAILABLE,
+	type BlockDiagnosticSuggestions,
 	blockSingleLineMessage,
 	blockUnresolvedMessage,
 	insertAfterBlockCloserLoweredWarning,
 	insertAfterBlockUnresolvedLoweredWarning,
 } from "./messages";
-import type { BlockResolution, BlockResolver, Cursor, Edit } from "./types";
+import type { BlockResolution, BlockResolver, BlockSpan, Cursor, Edit } from "./types";
+
+/** Maximum nearby lines inspected only after a block anchor has already failed. */
+const BLOCK_SUGGESTION_SCAN_LIMIT = 64;
+
+function resolveDiagnosticBlock(resolver: BlockResolver, path: string, text: string, line: number): BlockSpan | null {
+	try {
+		return resolver({ path, text, line });
+	} catch {
+		// Suggestions are best-effort and must never hide the authoritative anchor error.
+		return null;
+	}
+}
+
+function findNextBlock(
+	anchorLine: number,
+	lines: readonly string[],
+	path: string,
+	text: string,
+	resolver: BlockResolver,
+): BlockSpan | null {
+	const lastLine = Math.min(lines.length, anchorLine + BLOCK_SUGGESTION_SCAN_LIMIT);
+	for (let line = anchorLine + 1; line <= lastLine; line++) {
+		if (lines[line - 1]?.trim().length === 0) continue;
+		const span = resolveDiagnosticBlock(resolver, path, text, line);
+		if (span?.start === line && span.end > line) return span;
+	}
+	return null;
+}
+
+function findEnclosingBlock(
+	anchorLine: number,
+	lines: readonly string[],
+	path: string,
+	text: string,
+	resolver: BlockResolver,
+): BlockSpan | null {
+	const firstLine = Math.max(1, anchorLine - BLOCK_SUGGESTION_SCAN_LIMIT);
+	for (let line = anchorLine - 1; line >= firstLine; line--) {
+		if (lines[line - 1]?.trim().length === 0) continue;
+		const span = resolveDiagnosticBlock(resolver, path, text, line);
+		if (span?.start === line && span.end >= anchorLine && span.end > line) return span;
+	}
+	return null;
+}
 
 export interface ResolveBlockEditsOptions {
 	/**
@@ -105,11 +150,18 @@ export function resolveBlockEdits(
 				continue;
 			}
 			if (onUnresolved === "drop") continue;
-			throw new Error(
-				`line ${edit.lineNum}: ${
-					resolver ? blockUnresolvedMessage(edit.anchor.line, op, text.split("\n")) : BLOCK_RESOLVER_UNAVAILABLE
-				}`,
-			);
+			if (!resolver) throw new Error(`line ${edit.lineNum}: ${BLOCK_RESOLVER_UNAVAILABLE}`);
+			const lines = text.split("\n");
+			const nextBlock =
+				lines[edit.anchor.line - 1]?.trim().length === 0
+					? findNextBlock(edit.anchor.line, lines, path, text, resolver)
+					: null;
+			const enclosingBlock =
+				nextBlock === null ? findEnclosingBlock(edit.anchor.line, lines, path, text, resolver) : null;
+			const suggestions: BlockDiagnosticSuggestions = {};
+			if (nextBlock) suggestions.nextBlock = nextBlock;
+			if (enclosingBlock) suggestions.enclosingBlock = enclosingBlock;
+			throw new Error(`line ${edit.lineNum}: ${blockUnresolvedMessage(edit.anchor.line, op, lines, suggestions)}`);
 		}
 		if (span.start === span.end) {
 			// A single-line block resolution means line N is a bare statement, not
@@ -118,7 +170,12 @@ export function resolveBlockEdits(
 			// and its `break;`). The plain op is exact for one line, so reject and
 			// point at it; drop instead on the lenient preview path.
 			if (onUnresolved === "drop") continue;
-			throw new Error(`line ${edit.lineNum}: ${blockSingleLineMessage(edit.anchor.line, op)}`);
+			const enclosingBlock = resolver
+				? findEnclosingBlock(edit.anchor.line, text.split("\n"), path, text, resolver)
+				: null;
+			throw new Error(
+				`line ${edit.lineNum}: ${blockSingleLineMessage(edit.anchor.line, op, enclosingBlock ?? undefined)}`,
+			);
 		}
 		options.onResolved?.({
 			anchorLine: edit.anchor.line,

packages/hashline/src/messages.ts

@@ -1,6 +1,7 @@
 /** Centralized error/warning text for the hashline parser, applier, and patcher. */
 
 import { formatNumberedLine, HL_FILE_HASH_SEP, HL_FILE_PREFIX, HL_FILE_SUFFIX, HL_RANGE_SEP } from "./format";
+import type { BlockSpan } from "./types";
 
 /** Lines of context shown either side of a hash mismatch. */
 export const MISMATCH_CONTEXT = 2;
@@ -29,6 +30,39 @@ export function formatAnchoredContext(anchorLines: readonly number[], fileLines:
 	}
 	return rows;
 }
+/** Concrete range operation rejected because its absolute end precedes its start. */
+export type AbsoluteRangeOp = "replace" | "delete";
+
+/** Explain absolute range endpoints and provide safe, non-applying retry forms. */
+export function invalidAbsoluteRangeMessage(
+	patchLine: number,
+	start: number,
+	end: number,
+	op: AbsoluteRangeOp,
+	block?: BlockSpan,
+): string {
+	const single = op === "replace" ? `SWAP ${start}${HL_RANGE_SEP}${start}:` : `DEL ${start}`;
+	const countedEnd = start + end - 1;
+	const counted =
+		Number.isSafeInteger(countedEnd) && countedEnd >= start
+			? op === "replace"
+				? `SWAP ${start}${HL_RANGE_SEP}${countedEnd}:`
+				: `DEL ${start}${HL_RANGE_SEP}${countedEnd}`
+			: null;
+	const blockForm = op === "replace" ? `SWAP.BLK ${start}:` : `DEL.BLK ${start}`;
+	let message =
+		`line ${patchLine}: Invalid absolute range: start ${start}, end ${end}. ` +
+		`The value after \`${HL_RANGE_SEP}\` is an absolute source line, not a line count or replacement length. ` +
+		`For one line use \`${single}\`.`;
+	if (counted !== null) {
+		message += ` For ${end} lines starting at ${start}, use \`${counted}\`.`;
+	}
+	if (block?.start === start && block.end > start) {
+		message +=
+			` The syntactic block beginning at ${start} ends at ${block.end}, ` + `so \`${blockForm}\` is also valid.`;
+	}
+	return message;
+}
 
 /** Optional patch envelope start marker; silently consumed. */
 export const BEGIN_PATCH_MARKER = "*** Begin Patch";
@@ -70,6 +104,14 @@ export const EMPTY_REPLACE = `\`SWAP N${HL_RANGE_SEP}M:\` needs at least one \`+
 /** `replace_block N:` hunk with no body. */
 export const EMPTY_BLOCK = "`SWAP.BLK N:` needs at least one `+TEXT` body row. To delete a block, use `DEL.BLK N`.";
 
+/** Optional source-aware suggestions appended to block-anchor diagnostics. */
+export interface BlockDiagnosticSuggestions {
+	/** Closest following multi-line block that begins after the authored anchor. */
+	nextBlock?: BlockSpan;
+	/** Closest preceding multi-line block whose span contains the authored anchor. */
+	enclosingBlock?: BlockSpan;
+}
+
 /**
  * Block-anchored replace/delete could not resolve to a syntactic block
  * (unsupported language, blank/out-of-range line, no node beginning on N, or
@@ -82,12 +124,31 @@ export function blockUnresolvedMessage(
 	line: number,
 	op: "replace" | "delete" = "replace",
 	fileLines?: readonly string[],
+	suggestions: BlockDiagnosticSuggestions = {},
 ): string {
 	const phrase = op === "delete" ? `DEL.BLK ${line}` : `SWAP.BLK ${line}:`;
 	const fallback = op === "delete" ? `DEL ${line}${HL_RANGE_SEP}M` : `SWAP ${line}${HL_RANGE_SEP}M:`;
-	let message =
-		`\`${phrase}\` could not resolve a syntactic block beginning on line ${line} ` +
-		`(unsupported language, blank/closer line, or parse error). Use \`${fallback}\` with explicit lines.`;
+	const anchorText = fileLines?.[line - 1];
+	const nextBlock = suggestions.nextBlock;
+	let message: string;
+	if (anchorText !== undefined && anchorText.trim().length === 0 && nextBlock) {
+		const retry = op === "delete" ? `DEL.BLK ${nextBlock.start}` : `SWAP.BLK ${nextBlock.start}:`;
+		message =
+			`Line ${line} is blank; no syntactic block can begin there. ` +
+			`The next multi-line block begins at line ${nextBlock.start} and ends at line ${nextBlock.end}. ` +
+			`Retry \`${retry}\`.`;
+	} else {
+		message =
+			`\`${phrase}\` could not resolve a syntactic block beginning on line ${line} ` +
+			`(unsupported language, blank/closer line, or parse error). Use \`${fallback}\` with explicit lines.`;
+	}
+	const enclosingBlock = suggestions.enclosingBlock;
+	if (enclosingBlock) {
+		const retry = op === "delete" ? `DEL.BLK ${enclosingBlock.start}` : `SWAP.BLK ${enclosingBlock.start}:`;
+		message +=
+			` The nearest enclosing multi-line block begins at line ${enclosingBlock.start} ` +
+			`and ends at line ${enclosingBlock.end}; use \`${retry}\` to target it.`;
+	}
 	if (fileLines) {
 		const context = formatAnchoredContext([line], fileLines);
 		if (context.length > 0) message += `\n\n${context.join("\n")}`;
@@ -344,7 +405,7 @@ export type BlockOp = "replace" | "delete" | "insert_after";
  * form only earns its keep when it spares counting a closing line you cannot
  * see. Reject and point at both fixes.
  */
-export function blockSingleLineMessage(line: number, op: BlockOp): string {
+export function blockSingleLineMessage(line: number, op: BlockOp, enclosingBlock?: BlockSpan): string {
 	const blockForm = op === "insert_after" ? "INS.BLK.POST" : op === "delete" ? "DEL.BLK" : "SWAP.BLK";
 	const plainForm =
 		op === "insert_after"
@@ -352,9 +413,19 @@ export function blockSingleLineMessage(line: number, op: BlockOp): string {
 			: op === "delete"
 				? `DEL ${line}`
 				: `SWAP ${line}${HL_RANGE_SEP}${line}:`;
-	return (
+	let message =
 		`\`${blockForm} ${line}\` resolved a single-line block — line ${line} is a bare statement, not the opening line ` +
-		`of a multi-line construct. For that one line use \`${plainForm}\`; to act on an enclosing construct, anchor ${blockForm} ` +
-		`on the line that OPENS it (e.g. its \`function\`/\`if\`/\`case\` header), never a statement inside it.`
-	);
+		`of a multi-line construct. For only this statement use \`${plainForm}\`.`;
+	if (enclosingBlock) {
+		const enclosingForm =
+			op === "insert_after"
+				? `INS.BLK.POST ${enclosingBlock.start}:`
+				: op === "delete"
+					? `DEL.BLK ${enclosingBlock.start}`
+					: `SWAP.BLK ${enclosingBlock.start}:`;
+		message +=
+			` The nearest enclosing multi-line block begins at line ${enclosingBlock.start} ` +
+			`and ends at line ${enclosingBlock.end}; use \`${enclosingForm}\` to target it.`;
+	}
+	return message;
 }

packages/hashline/src/parser.ts

@@ -5,11 +5,13 @@
  */
 import { HL_PAYLOAD_REPLACE, HL_RANGE_SEP } from "./format";
 import {
+	type AbsoluteRangeOp,
 	BARE_BODY_AUTO_PIPED_WARNING,
 	DELETE_BLOCK_TAKES_NO_BODY,
 	DELETE_TAKES_NO_BODY,
 	EMPTY_BLOCK,
 	EMPTY_INSERT,
+	invalidAbsoluteRangeMessage,
 	MINUS_BULLET_AUTO_PIPED_WARNING,
 	MINUS_ROW_REJECTED,
 	MOVE_TAKES_NO_BODY,
@@ -17,13 +19,36 @@ import {
 } from "./messages";
 import { stripOneLeadingHashlinePrefix } from "./prefixes";
 import { type BlockTarget, cloneCursor, type ParsedRange, type Token, Tokenizer } from "./tokenizer";
-import type { Anchor, Cursor, Edit, FileOp } from "./types";
+import type { Anchor, BlockSpan, Cursor, Edit, FileOp } from "./types";
+/** Parser error carrying enough range metadata for source-aware diagnostic enrichment. */
+export class InvalidAbsoluteRangeError extends Error {
+	/** Patch-language line containing the invalid range header. */
+	readonly patchLine: number;
+	/** Absolute first source line authored in the range. */
+	readonly startLine: number;
+	/** Invalid absolute last source line authored in the range. */
+	readonly endLine: number;
+	/** Operation whose range was invalid. */
+	readonly op: AbsoluteRangeOp;
 
-function validateRangeOrder(range: ParsedRange, lineNum: number): void {
+	constructor(patchLine: number, startLine: number, endLine: number, op: AbsoluteRangeOp, block?: BlockSpan) {
+		super(invalidAbsoluteRangeMessage(patchLine, startLine, endLine, op, block));
+		this.name = "InvalidAbsoluteRangeError";
+		this.patchLine = patchLine;
+		this.startLine = startLine;
+		this.endLine = endLine;
+		this.op = op;
+	}
+
+	/** Rebuild this error with a proven syntactic-block endpoint suggestion. */
+	withBlock(block: BlockSpan): InvalidAbsoluteRangeError {
+		return new InvalidAbsoluteRangeError(this.patchLine, this.startLine, this.endLine, this.op, block);
+	}
+}
+
+function validateRangeOrder(range: ParsedRange, lineNum: number, op: AbsoluteRangeOp): void {
 	if (range.end.line < range.start.line) {
-		throw new Error(
-			`line ${lineNum}: range ${range.start.line}${HL_RANGE_SEP}${range.end.line} ends before it starts.`,
-		);
+		throw new InvalidAbsoluteRangeError(lineNum, range.start.line, range.end.line, op);
 	}
 }
 
@@ -170,7 +195,7 @@ export class Executor {
 			case "op-block":
 				this.#discardPendingSkippableComments();
 				if (token.target.kind === "replace" || token.target.kind === "delete") {
-					validateRangeOrder(token.target.range, token.lineNum);
+					validateRangeOrder(token.target.range, token.lineNum, token.target.kind);
 				}
 				if (token.target.kind === "rem") {
 					this.#flushPending();

packages/hashline/src/patcher.ts

@@ -38,9 +38,10 @@ import {
 } from "./messages";
 import { MismatchError } from "./mismatch";
 import { detectLineEnding, type LineEnding, normalizeToLF, restoreLineEndings, stripBom } from "./normalize";
+import { InvalidAbsoluteRangeError } from "./parser";
 import { Recovery, type RecoveryResult } from "./recovery";
 import type { Snapshot, SnapshotStore } from "./snapshots";
-import type { ApplyResult, BlockResolution, BlockResolver, Edit, FileOp } from "./types";
+import type { ApplyResult, BlockResolution, BlockResolver, BlockSpan, Edit, FileOp } from "./types";
 
 /**
  * Upper bound on the number of unseen anchor lines whose actual file content
@@ -277,6 +278,25 @@ export class Patcher {
 		}
 	}
 
+	async #parseWithRangeDiagnostics(section: PatchSection) {
+		try {
+			return section.parse();
+		} catch (error) {
+			if (!(error instanceof InvalidAbsoluteRangeError) || !this.blockResolver) throw error;
+			let span: BlockSpan | null = null;
+			try {
+				const read = await this.#tryRead(section.path);
+				if (read.exists) {
+					const normalized = normalizeToLF(stripBom(read.rawContent).text);
+					span = this.blockResolver({ path: section.path, text: normalized, line: error.startLine });
+				}
+			} catch {
+				// Source-aware enrichment is best-effort; preserve the actionable parser error.
+			}
+			throw span?.start === error.startLine && span.end > span.start ? error.withBlock(span) : error;
+		}
+	}
+
 	/**
 	 * Read a section's target file, parse the section, validate the snapshot
 	 * tag (with recovery), and apply the edits in memory. Returns a
@@ -287,7 +307,7 @@ export class Patcher {
 	 * tag mismatch ({@link MismatchError}).
 	 */
 	async prepare(section: PatchSection): Promise<PreparedSection> {
-		const parsed = section.parse();
+		const parsed = await this.#parseWithRangeDiagnostics(section);
 		const parseWarnings = [...parsed.warnings];
 		const fileOp = parsed.fileOp;
 		assertSectionHashPresent(section.path, section.fileHash);

packages/hashline/src/tokenizer.ts

@@ -171,14 +171,18 @@ function scanRangeSeparator(line: string, index: number, end: number): number |
 			consumedSeparator = true;
 			continue;
 		}
-		if (
-			code === CHAR_DOT &&
-			cursor + 1 < end &&
-			(line.charCodeAt(cursor + 1) === CHAR_DOT || line.charCodeAt(cursor + 1) === CHAR_EQUALS)
-		) {
-			cursor += 2;
-			consumedSeparator = true;
-			continue;
+		if (code === CHAR_DOT && cursor + 1 < end) {
+			const next = line.charCodeAt(cursor + 1);
+			if (next === CHAR_DOT || next === CHAR_EQUALS) {
+				cursor += 2;
+				consumedSeparator = true;
+				continue;
+			}
+			if (isNonZeroDigitCode(next)) {
+				cursor++;
+				consumedSeparator = true;
+				continue;
+			}
 		}
 		break;
 	}

packages/hashline/test/block.test.ts

@@ -104,6 +104,30 @@ describe("resolveBlockEdits", () => {
 		expect(error?.message).not.toContain("foxtrot");
 	});
 
+	it("suggests the next multi-line opener for a blank anchor without applying it", () => {
+		const text = "alpha\n\nfunction x() {\n  return 1;\n}";
+		const resolver: BlockResolver = ({ line }) => (line === 3 ? { start: 3, end: 5 } : null);
+		const edits = parsePatch("SWAP.BLK 2:\n+function y() {}").edits;
+
+		expect(() => resolveBlockEdits(edits, text, PATH, resolver)).toThrow(
+			"Line 2 is blank; no syntactic block can begin there. The next multi-line block begins at line 3 and ends at line 5. Retry `SWAP.BLK 3:`.",
+		);
+	});
+
+	it("suggests both the exact statement range and nearest enclosing block", () => {
+		const text = "function x() {\n  run();\n}";
+		const resolver: BlockResolver = ({ line }) => {
+			if (line === 2) return { start: 2, end: 2 };
+			if (line === 1) return { start: 1, end: 3 };
+			return null;
+		};
+		const edits = parsePatch("SWAP.BLK 2:\n+  stop();").edits;
+
+		expect(() => resolveBlockEdits(edits, text, PATH, resolver)).toThrow(
+			"For only this statement use `SWAP 2.=2:`. The nearest enclosing multi-line block begins at line 1 and ends at line 3; use `SWAP.BLK 1:` to target it.",
+		);
+	});
+
 	it("omits the context preview when the anchor line is out of range", () => {
 		const edits = parsePatch("SWAP.BLK 9:\n+X").edits;
 		let error: Error | undefined;
@@ -219,6 +243,20 @@ describe("Patcher with a block resolver", () => {
 		expect(result.sections[0]?.blockResolutions).toEqual([{ anchorLine: 2, start: 2, end: 3, op: "replace" }]);
 	});
 
+	it("enriches reversed absolute ranges with the resolved block endpoint without writing", async () => {
+		const source = Array.from({ length: 255 }, (_, index) => `line ${index + 1}`).join("\n");
+		const fs = new InMemoryFilesystem([[PATH, source]]);
+		const snapshots = new InMemorySnapshotStore();
+		const tag = snapshots.record(PATH, source);
+		const resolver: BlockResolver = ({ line }) => (line === 195 ? { start: 195, end: 255 } : null);
+		const patcher = new Patcher({ fs, snapshots, blockResolver: resolver });
+
+		await expect(patcher.apply(Patch.parse(`[${PATH}#${tag}]\nSWAP 195.=61:\n+replacement`))).rejects.toThrow(
+			"Invalid absolute range: start 195, end 61. The value after `.=` is an absolute source line, not a line count or replacement length. For one line use `SWAP 195.=195:`. For 61 lines starting at 195, use `SWAP 195.=255:`. The syntactic block beginning at 195 ends at 255, so `SWAP.BLK 195:` is also valid.",
+		);
+		expect(fs.get(PATH)).toBe(source);
+	});
+
 	it("resolves against the tagged snapshot and recovers onto drifted content", async () => {
 		const snapshotText = "line0\nline1\nline2\nline3\nline4\n";
 		// The live file gained a trailing line after the read minted the tag.

packages/hashline/test/format-v2.test.ts

@@ -23,6 +23,12 @@ describe("hashline format v4", () => {
 		expect(applyPatch(text, "DEL 2.=3")).toBe("a\nd");
 	});
 
+	it("accepts a single dot between integer range endpoints", () => {
+		const text = "a\nb\nc\nd";
+		expect(applyPatch(text, "DEL 2.3")).toBe("a\nd");
+		expect(applyPatch(text, "SWAP 2.3:\n+middle")).toBe("a\nmiddle\nd");
+	});
+
 	it("inserts before and after concrete anchors", () => {
 		const text = "a\nb\nc";
 		const diff = ["INS.PRE 2:", "+before", "INS.POST 2:", "+after"].join("\n");