feat(lsp+tool-edit-file): multi-server diagnostics + per-edit auto-append

LSP extension:
- Multi-server aggregation: query ALL connected servers matching the
  file's extension (not just the first), merge diagnostics tagged by source
- Incremental sync: capture each server's textDocumentSync.change during
  initialize; compute prefix/suffix diff ranges for change:2 servers;
  full content for change:1 (generic, works for any LSP)
- New diff.ts: pure computeChangeRange + offsetToPosition (O(n), tested)
- Buffer sync: change(filePath, newText) sends didChange with post-edit
  in-memory content; openWithText for first open; tracks open doc text
- languageId mapping: extended with .rb/.rbs/.c/.cpp/etc. (was 'unknown')
- waitForDiagnostics: accepts text override + timeoutMs; returns
  { formatted, slow, timedOut }; polls for publishDiagnostics push
- DiagnosticsStore: hasReceivedPush/clearReceived tracking; formatFiltered
  with minSeverity (1=Error, 2=Warning) for edit_file integration
- LspService.getDiagnostics: service method for cross-extension use

tool-edit-file:
- After successful edit, calls LSP getDiagnostics with post-edit buffer
- Only appends diagnostics with severity ≤ 2 (errors+warnings, no noise)
- Appends slow warning (>10s): 'LSP is taking unusually long...'
- 60s timeout; graceful degradation when no LSP available
- Optional dep on @dispatch/lsp (getService pattern, not manifest depOn)

1468 vitest pass (was 1453, +15 new diff tests).

12 files changed, 588 insertions, 53 deletions

diff --git a/packages/lsp/src/client.ts b/packages/lsp/src/client.ts
index 114b8eb..743fcb4 100644
--- a/packages/lsp/src/client.ts
+++ b/packages/lsp/src/client.ts
@@ -5,7 +5,9 @@
  */
 
 import { DiagnosticsStore, type PublishDiagnosticsParams } from "./diagnostics.js";
+import { computeChangeRange } from "./diff.js";
 import { FrameDecoder } from "./framing.js";
+import { languageId as resolveLanguageId } from "./language.js";
 import { JsonRpcConnection, type WriteFn } from "./rpc.js";
 import { FileChangeType, WatchedFilesRegistry } from "./watched-files.js";
 
@@ -97,7 +99,9 @@ export class LanguageServerClient {
 	private state: ClientState = "not-started";
 	private stateError: string | undefined;
 	private deps: ClientDeps;
-	private openDocuments = new Map<string, number>();
+	private openDocuments = new Map<string, { version: number; text: string }>();
+	/** Sync mode captured from the server's initialize capabilities: 1=Full, 2=Incremental. */
+	private textDocumentChange: 1 | 2 = 1;
 
 	constructor(deps: ClientDeps) {
 		this.deps = deps;
@@ -262,7 +266,22 @@ export class LanguageServerClient {
 			setTimeout(() => reject(new Error("Initialize timeout")), timeout);
 		});
 
-		await Promise.race([initPromise, timeoutPromise]);
+		const result = (await Promise.race([initPromise, timeoutPromise])) as {
+			readonly capabilities?: {
+				readonly textDocumentSync?:
+					| number
+					| { readonly openClose?: boolean; readonly change?: number }
+					| undefined;
+			};
+		};
+
+		// Capture the server's text document sync mode for didChange.
+		const sync = result.capabilities?.textDocumentSync;
+		if (typeof sync === "number") {
+			this.textDocumentChange = sync as 1 | 2;
+		} else if (sync && typeof sync === "object" && sync.change !== undefined) {
+			this.textDocumentChange = sync.change as 1 | 2;
+		}
 
 		rpc.sendNotification("initialized", {});
 
@@ -301,36 +320,99 @@ export class LanguageServerClient {
 		const rpc = this.rpc;
 		if (!rpc || this.state !== "connected") return;
 
-		const version = (this.openDocuments.get(filePath) ?? 0) + 1;
-		this.openDocuments.set(filePath, version);
-
 		try {
 			const text = await this.deps.fs.readText(filePath);
-			rpc.sendNotification("textDocument/didOpen", {
-				textDocument: {
-					uri: `file://${filePath}`,
-					languageId: "unknown",
-					version,
-					text,
-				},
-			});
+			await this.openWithText(filePath, text);
 		} catch {
 			// file may not exist
 		}
 	}
 
-	async waitForDiagnostics(filePath: string, timeoutMs = 10_000): Promise<string> {
-		await this.open(filePath);
-		return new Promise<string>((resolve) => {
-			const start = Date.now();
+	async openWithText(filePath: string, text: string, langId?: string): Promise<void> {
+		const rpc = this.rpc;
+		if (!rpc || this.state !== "connected") return;
+
+		// If already open, use didChange instead of re-opening.
+		if (this.openDocuments.has(filePath)) {
+			await this.change(filePath, text);
+			return;
+		}
+
+		const version = 1;
+		this.openDocuments.set(filePath, { version, text });
+
+		rpc.sendNotification("textDocument/didOpen", {
+			textDocument: {
+				uri: `file://${filePath}`,
+				languageId: langId ?? resolveLanguageId(filePath),
+				version,
+				text,
+			},
+		});
+	}
+
+	async change(filePath: string, newText: string): Promise<void> {
+		const rpc = this.rpc;
+		if (!rpc || this.state !== "connected") return;
+
+		const existing = this.openDocuments.get(filePath);
+		if (!existing) {
+			// Not open yet — didOpen instead.
+			await this.openWithText(filePath, newText);
+			return;
+		}
+
+		const version = existing.version + 1;
+		this.openDocuments.set(filePath, { version, text: newText });
+
+		if (this.textDocumentChange === 2) {
+			// Incremental sync — compute the minimal change range.
+			const changeEvent = computeChangeRange(existing.text, newText);
+			rpc.sendNotification("textDocument/didChange", {
+				textDocument: { uri: `file://${filePath}`, version },
+				contentChanges: [changeEvent],
+			});
+		} else {
+			// Full sync — send the entire content.
+			rpc.sendNotification("textDocument/didChange", {
+				textDocument: { uri: `file://${filePath}`, version },
+				contentChanges: [{ text: newText }],
+			});
+		}
+	}
+
+	async waitForDiagnostics(
+		filePath: string,
+		opts?: { readonly text?: string; readonly timeoutMs?: number; readonly minSeverity?: number },
+	): Promise<{ readonly formatted: string; readonly slow: boolean; readonly timedOut: boolean }> {
+		const timeoutMs = opts?.timeoutMs ?? 10_000;
+		const uri = `file://${filePath}`;
+
+		// Clear the "received" flag so we detect fresh publishDiagnostics after our sync.
+		this.diagnostics.clearReceived(uri);
+
+		// Sync the document: use didChange with the provided text (post-edit buffer)
+		// or fall back to didOpen reading from disk.
+		if (opts?.text !== undefined) {
+			await this.change(filePath, opts.text);
+		} else {
+			await this.open(filePath);
+		}
+
+		const slowThreshold = 10_000;
+		const start = Date.now();
+
+		// Poll until the server pushes diagnostics (even empty = done) or timeout.
+		return new Promise((resolve) => {
 			const check = () => {
-				const formatted = this.diagnostics.format(`file://${filePath}`);
-				if (formatted) {
-					resolve(formatted);
-					return;
-				}
-				if (Date.now() - start >= timeoutMs) {
-					resolve(this.diagnostics.format(`file://${filePath}`) || "");
+				const elapsed = Date.now() - start;
+				const received = this.diagnostics.hasReceivedPush(uri);
+				if (received || elapsed >= timeoutMs) {
+					resolve({
+						formatted: this.diagnostics.formatFiltered(uri, opts?.minSeverity),
+						slow: elapsed > slowThreshold,
+						timedOut: !received,
+					});
 					return;
 				}
 				setTimeout(check, 100);
diff --git a/packages/lsp/src/diagnostics.ts b/packages/lsp/src/diagnostics.ts
index ea18811..bc7ac0a 100644
--- a/packages/lsp/src/diagnostics.ts
+++ b/packages/lsp/src/diagnostics.ts
@@ -34,9 +34,11 @@ const severityNames: Record<number, string> = {
 export class DiagnosticsStore {
 	private pushDiagnostics = new Map<string, readonly Diagnostic[]>();
 	private pullDiagnostics = new Map<string, readonly Diagnostic[]>();
+	private pushReceived = new Set<string>();
 
 	setPushDiagnostics(params: PublishDiagnosticsParams): void {
 		this.pushDiagnostics.set(params.uri, params.diagnostics);
+		this.pushReceived.add(params.uri);
 	}
 
 	setPullDiagnostics(uri: string, report: DocumentDiagnosticReport): void {
@@ -45,14 +47,32 @@ export class DiagnosticsStore {
 		}
 	}
 
+	/** True if the server has pushed at least one publishDiagnostics for this URI. */
+	hasReceivedPush(uri: string): boolean {
+		return this.pushReceived.has(uri);
+	}
+
+	/** Clear the "received" flag so the next waitForDiagnostics poll detects fresh pushes. */
+	clearReceived(uri: string): void {
+		this.pushReceived.delete(uri);
+	}
+
 	getMerged(uri: string): readonly Diagnostic[] {
 		const push = this.pushDiagnostics.get(uri) ?? [];
 		const pull = this.pullDiagnostics.get(uri) ?? [];
 		return dedupeDiagnostics([...push, ...pull]);
 	}
 
-	format(uri: string): string {
-		const diags = this.getMerged(uri);
+	/**
+	 * Format diagnostics for a URI, optionally filtering by minimum severity.
+	 * `minSeverity` includes only diagnostics with severity ≤ the given value
+	 * (1=Error, 2=Warning, 3=Info, 4=Hint). Omit to include all.
+	 */
+	formatFiltered(uri: string, minSeverity?: number): string {
+		let diags = this.getMerged(uri);
+		if (minSeverity !== undefined) {
+			diags = diags.filter((d) => (d.severity ?? 0) <= minSeverity);
+		}
 		if (diags.length === 0) return "";
 		const lines: string[] = [];
 		for (const d of diags) {
@@ -65,6 +85,10 @@ export class DiagnosticsStore {
 		}
 		return lines.join("\n");
 	}
+
+	format(uri: string): string {
+		return this.formatFiltered(uri);
+	}
 }
 
 function diagnosticKey(d: Diagnostic): string {
diff --git a/packages/lsp/src/diff.test.ts b/packages/lsp/src/diff.test.ts
new file mode 100644
index 0000000..b5b6a7b
--- /dev/null
+++ b/packages/lsp/src/diff.test.ts
@@ -0,0 +1,117 @@
+import { describe, expect, it } from "vitest";
+import { computeChangeRange, offsetToPosition } from "./diff.js";
+
+describe("offsetToPosition", () => {
+	it("returns 0:0 for offset 0", () => {
+		expect(offsetToPosition("hello", 0)).toEqual({ line: 0, character: 0 });
+	});
+
+	it("counts characters on the first line", () => {
+		expect(offsetToPosition("hello", 3)).toEqual({ line: 0, character: 3 });
+	});
+
+	it("resets character count after newline", () => {
+		expect(offsetToPosition("ab\ncd", 4)).toEqual({ line: 1, character: 1 });
+	});
+
+	it("handles multiple lines", () => {
+		expect(offsetToPosition("a\nb\nc", 4)).toEqual({ line: 2, character: 0 });
+	});
+
+	it("clamps offset beyond text length", () => {
+		expect(offsetToPosition("ab", 100)).toEqual({ line: 0, character: 2 });
+	});
+
+	it("handles empty string", () => {
+		expect(offsetToPosition("", 0)).toEqual({ line: 0, character: 0 });
+	});
+});
+
+describe("computeChangeRange", () => {
+	it("detects a single-line insertion", () => {
+		const oldText = "hello world";
+		const newText = "hello cruel world";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 6 });
+		expect(change.text).toBe("cruel ");
+	});
+
+	it("detects a single-line deletion", () => {
+		const oldText = "hello cruel world";
+		const newText = "hello world";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 12 });
+		expect(change.text).toBe("");
+	});
+
+	it("detects a single-line replacement", () => {
+		const oldText = "hello world";
+		const newText = "hello earth";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 6 });
+		expect(change.range.end).toEqual({ line: 0, character: 11 });
+		expect(change.text).toBe("earth");
+	});
+
+	it("handles multi-line changes with correct line positions", () => {
+		const oldText = "line1\nline2\nline3";
+		const newText = "line1\nCHANGED\nline3";
+		const change = computeChangeRange(oldText, newText);
+		// Common prefix: "line1\n" → start at beginning of line 1
+		expect(change.range.start).toEqual({ line: 1, character: 0 });
+		// Common suffix: "\nline3" → end after "line2" on line 1
+		expect(change.range.end).toEqual({ line: 1, character: 5 });
+		expect(change.text).toBe("CHANGED");
+	});
+
+	it("handles insertion at end of file", () => {
+		const oldText = "abc";
+		const newText = "abcdef";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 3 });
+		expect(change.range.end).toEqual({ line: 0, character: 3 });
+		expect(change.text).toBe("def");
+	});
+
+	it("handles complete file replacement (no common prefix/suffix)", () => {
+		const oldText = "abc";
+		const newText = "xyz";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 0 });
+		expect(change.range.end).toEqual({ line: 0, character: 3 });
+		expect(change.text).toBe("xyz");
+	});
+
+	it("handles empty old text (new file)", () => {
+		const oldText = "";
+		const newText = "hello\nworld";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 0 });
+		expect(change.range.end).toEqual({ line: 0, character: 0 });
+		expect(change.text).toBe("hello\nworld");
+	});
+
+	it("handles identical text (no change)", () => {
+		const oldText = "same text";
+		const newText = "same text";
+		const change = computeChangeRange(oldText, newText);
+		expect(change.range.start).toEqual({ line: 0, character: 9 });
+		expect(change.range.end).toEqual({ line: 0, character: 9 });
+		expect(change.text).toBe("");
+	});
+
+	it("handles change spanning multiple lines", () => {
+		const oldText = "function foo() {\n  return 1;\n}\n";
+		const newText = "function foo() {\n  return 2;\n  console.log('hi');\n}\n";
+		const change = computeChangeRange(oldText, newText);
+		// Common prefix: "function foo() {\n  return " (26 chars)
+		// The change starts at "1" on line 1, character 9
+		expect(change.range.start).toEqual({ line: 1, character: 9 });
+		// Common suffix: ";\n}\n" (4 chars) → end at offset 27 (the ";")
+		// offset 27 = line 1, char 10 (after "  return 1")
+		expect(change.range.end).toEqual({ line: 1, character: 10 });
+		expect(change.text).toBe("2;\n  console.log('hi')");
+	});
+});
diff --git a/packages/lsp/src/diff.ts b/packages/lsp/src/diff.ts
new file mode 100644
index 0000000..ab40b36
--- /dev/null
+++ b/packages/lsp/src/diff.ts
@@ -0,0 +1,85 @@
+/**
+ * Pure diff utilities for computing LSP text document change ranges.
+ * Language-agnostic — works for any text content from any language server.
+ *
+ * Uses longest-common-prefix/suffix matching to find the minimal changed
+ * region between two versions of a file. O(n) in text length.
+ */
+
+export interface Position {
+	readonly line: number; // 0-based
+	readonly character: number; // 0-based
+}
+
+export interface TextDocumentContentChangeEvent {
+	readonly range: {
+		readonly start: Position;
+		readonly end: Position;
+	};
+	readonly text: string;
+}
+
+/**
+ * Compute the minimal change range that transforms `oldText` into `newText`.
+ * Returns a single `TextDocumentContentChangeEvent` suitable for
+ * `textDocument/didChange` with incremental sync (change: 2).
+ *
+ * Algorithm: find the longest common prefix and suffix of the two strings.
+ * The region between them is the changed range. The replacement text is
+ * the portion of `newText` between the prefix and suffix.
+ */
+export function computeChangeRange(
+	oldText: string,
+	newText: string,
+): TextDocumentContentChangeEvent {
+	const minLen = Math.min(oldText.length, newText.length);
+
+	// Longest common prefix
+	let prefixLen = 0;
+	while (prefixLen < minLen && oldText[prefixLen] === newText[prefixLen]) {
+		prefixLen++;
+	}
+
+	// Longest common suffix (must not overlap with prefix)
+	const oldRemaining = oldText.length - prefixLen;
+	const newRemaining = newText.length - prefixLen;
+	const maxSuffix = Math.min(oldRemaining, newRemaining);
+	let suffixLen = 0;
+	while (
+		suffixLen < maxSuffix &&
+		oldText[oldText.length - 1 - suffixLen] === newText[newText.length - 1 - suffixLen]
+	) {
+		suffixLen++;
+	}
+
+	const startOffset = prefixLen;
+	const endOffset = oldText.length - suffixLen;
+	const replacementText = newText.slice(prefixLen, newText.length - suffixLen);
+
+	return {
+		range: {
+			start: offsetToPosition(oldText, startOffset),
+			end: offsetToPosition(oldText, endOffset),
+		},
+		text: replacementText,
+	};
+}
+
+/**
+ * Convert a 0-based character offset into a text string to an LSP Position
+ * (0-based line and character). Scans for newlines up to the offset.
+ */
+export function offsetToPosition(text: string, offset: number): Position {
+	let line = 0;
+	let character = 0;
+	const limit = Math.min(offset, text.length);
+	for (let i = 0; i < limit; i++) {
+		if (text[i] === "\n") {
+			line++;
+			character = 0;
+		} else {
+			character++;
+		}
+	}
+	return { line, character };
+}
diff --git a/packages/lsp/src/extension.ts b/packages/lsp/src/extension.ts
index 486f66d..8e3178a 100644
--- a/packages/lsp/src/extension.ts
+++ b/packages/lsp/src/extension.ts
@@ -5,12 +5,18 @@
  * lspServiceHandle, and wires deactivate to manager.shutdownAll().
  */
 
+import { extname, join } from "node:path";
 import type { Extension, HostAPI, ServiceHandle } from "@dispatch/kernel";
 import { defineService } from "@dispatch/kernel";
 import type { SpawnedProcess } from "./client.js";
 import { LspManager } from "./manager.js";
 import { createLspTool } from "./tool.js";
-import type { LspServerStatus, LspService } from "./types.js";
+import type {
+	DiagnosticsResult,
+	GetDiagnosticsOpts,
+	LspServerStatus,
+	LspService,
+} from "./types.js";
 
 export const lspServiceHandle: ServiceHandle<LspService> = defineService<LspService>("lsp");
 
@@ -101,6 +107,53 @@ export const extension: Extension = {
 			async status(cwd: string): Promise<readonly LspServerStatus[]> {
 				return manager.status(cwd);
 			},
+			async getDiagnostics(opts: GetDiagnosticsOpts): Promise<DiagnosticsResult> {
+				const timeoutMs = opts.timeoutMs ?? 60_000;
+				const slowThreshold = 10_000;
+				const fileExt = extname(opts.filePath).toLowerCase();
+				const absolutePath = opts.filePath.startsWith("/")
+					? opts.filePath
+					: join(opts.cwd, opts.filePath);
+
+				// Get all connected servers matching this file's extension.
+				const statuses = await manager.status(opts.cwd);
+				const matching = statuses.filter(
+					(s) => s.state === "connected" && s.extensions.some((ext) => ext === fileExt),
+				);
+
+				if (matching.length === 0) {
+					return { formatted: "", slow: false, timedOut: false };
+				}
+
+				const parts: string[] = [];
+				let anySlow = false;
+				let anyTimedOut = false;
+				const start = Date.now();
+
+				for (const s of matching) {
+					const client = manager.getClient(s.id, s.root);
+					if (!client) continue;
+					const waitOpts: { text?: string; timeoutMs?: number; minSeverity?: number } = {
+						timeoutMs,
+					};
+					if (opts.text !== undefined) waitOpts.text = opts.text;
+					if (opts.minSeverity !== undefined) waitOpts.minSeverity = opts.minSeverity;
+					const result = await client.waitForDiagnostics(absolutePath, waitOpts);
+					if (result.slow) anySlow = true;
+					if (result.timedOut) anyTimedOut = true;
+					if (result.formatted) {
+						parts.push(`[${s.name}]\n${result.formatted}`);
+					}
+				}
+
+				const elapsed = Date.now() - start;
+
+				return {
+					formatted: parts.join("\n\n"),
+					slow: anySlow || elapsed > slowThreshold,
+					timedOut: anyTimedOut,
+				};
+			},
 		};
 		host.provideService(lspServiceHandle, service);
 
diff --git a/packages/lsp/src/language.ts b/packages/lsp/src/language.ts
index 214294e..a10dbed 100644
--- a/packages/lsp/src/language.ts
+++ b/packages/lsp/src/language.ts
@@ -26,6 +26,22 @@ const extensionMap: Record<string, string> = {
 	".sh": "shellscript",
 	".bash": "shellscript",
 	".zsh": "shellscript",
+	".rb": "ruby",
+	".rbs": "ruby",
+	".c": "c",
+	".h": "c",
+	".cpp": "cpp",
+	".cc": "cpp",
+	".cxx": "cpp",
+	".hpp": "cpp",
+	".hxx": "cpp",
+	".java": "java",
+	".kt": "kotlin",
+	".swift": "swift",
+	".php": "php",
+	".cs": "csharp",
+	".sql": "sql",
+	".dockerfile": "dockerfile",
 };
 
 export function languageId(filePath: string): string {
diff --git a/packages/lsp/src/tool.ts b/packages/lsp/src/tool.ts
index bc4b41f..8d282ec 100644
--- a/packages/lsp/src/tool.ts
+++ b/packages/lsp/src/tool.ts
@@ -4,7 +4,7 @@
  * Operations: diagnostics, hover, definition, references, documentSymbol.
  */
 
-import { resolve } from "node:path";
+import { extname, resolve } from "node:path";
 import type { ToolContract, ToolExecuteContext, ToolResult } from "@dispatch/kernel";
 import type { LspManager } from "./manager.js";
 
@@ -153,30 +153,61 @@ export function createLspTool(manager: LspManager): ToolContract {
 					return { content: "No language server configured for this workspace.", isError: true };
 				}
 
-				const connected = statuses.find((s) => s.state === "connected");
-				if (!connected) {
-					const first = statuses[0];
-					const detail = first
-						? `"${first.name}" is not connected (state: ${first.state})`
-						: "is not connected";
-					return {
-						content: `Language server ${detail}.`,
-						isError: true,
-					};
-				}
-
-				// Find the client for this server
-				const client = manager.getClient(connected.id, connected.root);
-				if (!client) {
-					return { content: "Language server client not available.", isError: true };
-				}
+				const fileExt = extname(absolutePath).toLowerCase();
 
 				switch (operation) {
 					case "diagnostics": {
-						const diags = await client.waitForDiagnostics(absolutePath);
-						return { content: diags || "No diagnostics found." };
+						// Query ALL connected servers whose extensions match this file.
+						const matching = statuses.filter(
+							(s) => s.state === "connected" && s.extensions.some((ext) => ext === fileExt),
+						);
+
+						if (matching.length === 0) {
+							// No matching server — fall back to any connected server.
+							const connected = statuses.find((s) => s.state === "connected");
+							if (!connected) {
+								const first = statuses[0];
+								const detail = first
+									? `"${first.name}" is not connected (state: ${first.state})`
+									: "is not connected";
+								return {
+									content: `Language server ${detail}.`,
+									isError: true,
+								};
+							}
+							const client = manager.getClient(connected.id, connected.root);
+							if (!client) {
+								return { content: "Language server client not available.", isError: true };
+							}
+							const result = await client.waitForDiagnostics(absolutePath);
+							return { content: result.formatted || "No diagnostics found." };
+						}
+
+						// Query each matching server and merge results, tagged by source.
+						const parts: string[] = [];
+						let anyTimedOut = false;
+						for (const s of matching) {
+							const client = manager.getClient(s.id, s.root);
+							if (!client) continue;
+							const result = await client.waitForDiagnostics(absolutePath, { timeoutMs: 60_000 });
+							if (result.timedOut) anyTimedOut = true;
+							if (result.slow) {
+								parts.push(
+									`⚠️ LSP is taking unusually long. If this happens more than once, raise it to the user.`,
+								);
+							}
+							if (result.formatted) {
+								parts.push(`[${s.name}]\n${result.formatted}`);
+							}
+						}
+						if (anyTimedOut && parts.length === 0) {
+							parts.push("Diagnostics timed out (server may still be indexing).");
+						}
+						return { content: parts.length > 0 ? parts.join("\n\n") : "No diagnostics found." };
 					}
 					case "hover": {
+						const client = await getFirstMatchingClient(manager, statuses, fileExt);
+						if (!client) return { content: "No language server available.", isError: true };
 						const result = await client.request("textDocument/hover", {
 							textDocument: { uri: `file://${absolutePath}` },
 							position: toPosition(line, character),
@@ -190,6 +221,8 @@ export function createLspTool(manager: LspManager): ToolContract {
 						return { content };
 					}
 					case "definition": {
+						const client = await getFirstMatchingClient(manager, statuses, fileExt);
+						if (!client) return { content: "No language server available.", isError: true };
 						const result = await client.request("textDocument/definition", {
 							textDocument: { uri: `file://${absolutePath}` },
 							position: toPosition(line, character),
@@ -198,6 +231,8 @@ export function createLspTool(manager: LspManager): ToolContract {
 						return { content: JSON.stringify(result) };
 					}
 					case "references": {
+						const client = await getFirstMatchingClient(manager, statuses, fileExt);
+						if (!client) return { content: "No language server available.", isError: true };
 						const result = await client.request("textDocument/references", {
 							textDocument: { uri: `file://${absolutePath}` },
 							position: toPosition(line, character),
@@ -207,6 +242,8 @@ export function createLspTool(manager: LspManager): ToolContract {
 						return { content: JSON.stringify(result) };
 					}
 					case "documentSymbol": {
+						const client = await getFirstMatchingClient(manager, statuses, fileExt);
+						if (!client) return { content: "No language server available.", isError: true };
 						const result = await client.request("textDocument/documentSymbol", {
 							textDocument: { uri: `file://${absolutePath}` },
 						});
@@ -223,3 +260,29 @@ export function createLspTool(manager: LspManager): ToolContract {
 		},
 	};
 }
+
+/**
+ * Find the first connected client whose server claims the file's extension.
+ * Falls back to any connected server if no extension match is found.
+ * Used by hover/definition/references/documentSymbol (single-server ops).
+ */
+async function getFirstMatchingClient(
+	manager: LspManager,
+	statuses: readonly {
+		readonly id: string;
+		readonly name: string;
+		readonly root: string;
+		readonly extensions: readonly string[];
+		readonly state: string;
+	}[],
+	fileExt: string,
+): Promise<
+	{ readonly request: (method: string, params?: unknown) => Promise<unknown> } | undefined
+> {
+	const matching = statuses.filter(
+		(s) => s.state === "connected" && s.extensions.some((ext) => ext === fileExt),
+	);
+	const target = matching[0] ?? statuses.find((s) => s.state === "connected");
+	if (!target) return undefined;
+	return manager.getClient(target.id, target.root);
+}
diff --git a/packages/lsp/src/types.ts b/packages/lsp/src/types.ts
index cc33e03..1f72bdf 100644
--- a/packages/lsp/src/types.ts
+++ b/packages/lsp/src/types.ts
@@ -20,6 +20,25 @@ export interface LspServerStatus {
 	readonly configSource?: string | undefined;
 }
 
+export interface DiagnosticsResult {
+	/** Formatted diagnostic string (filtered by minSeverity). Empty if none. */
+	readonly formatted: string;
+	/** True if diagnostics took >10s. */
+	readonly slow: boolean;
+	/** True if the 60s timeout was hit before all servers responded. */
+	readonly timedOut: boolean;
+}
+
+export interface GetDiagnosticsOpts {
+	readonly filePath: string;
+	/** Post-edit buffer content. If omitted, the server reads from disk. */
+	readonly text?: string;
+	readonly cwd: string;
+	readonly timeoutMs?: number;
+	/** Only include diagnostics with severity ≤ this (1=Error, 2=Warning). Omit for all. */
+	readonly minSeverity?: number;
+}
+
 export interface LspService {
 	/**
 	 * Resolve the language servers configured for `cwd`, ensure each is spawned +
@@ -27,4 +46,12 @@ export interface LspService {
 	 * server's failure — reflect it as state:"error" with a short `error`.
 	 */
 	status(cwd: string): Promise<readonly LspServerStatus[]>;
+
+	/**
+	 * Query ALL connected language servers matching the file's extension for
+	 * diagnostics. Merges results tagged by source server. Sends didOpen/didChange
+	 * with the provided text (post-edit buffer) so the server checks the in-memory
+	 * version, not stale disk content.
+	 */
+	getDiagnostics(opts: GetDiagnosticsOpts): Promise<DiagnosticsResult>;
 }
diff --git a/packages/tool-edit-file/package.json b/packages/tool-edit-file/package.json
index f71ad80..7194cce 100644
--- a/packages/tool-edit-file/package.json
+++ b/packages/tool-edit-file/package.json
@@ -6,6 +6,7 @@
 	"main": "dist/index.js",
 	"types": "dist/index.d.ts",
 	"dependencies": {
-		"@dispatch/kernel": "workspace:*"
+		"@dispatch/kernel": "workspace:*",
+		"@dispatch/lsp": "workspace:*"
 	}
 }
diff --git a/packages/tool-edit-file/src/edit-file.ts b/packages/tool-edit-file/src/edit-file.ts
index 36f99e0..1719ea3 100644
--- a/packages/tool-edit-file/src/edit-file.ts
+++ b/packages/tool-edit-file/src/edit-file.ts
@@ -103,13 +103,35 @@ export function computeReplacement(
 	};
 }
 
+// --- Diagnostics hook ---
+
+/**
+ * Optional post-edit diagnostics hook. Returns formatted diagnostics string
+ * (empty if none) + timing metadata. Injected by the extension from the LSP
+ * service; absent when no LSP is available (graceful degradation).
+ */
+export type DiagnosticsHook = (opts: {
+	readonly filePath: string;
+	readonly text: string;
+	readonly cwd: string;
+}) => Promise<{
+	readonly formatted: string;
+	readonly slow: boolean;
+	readonly timedOut: boolean;
+}>;
+
 // --- Shell / edge ---
 
 /**
  * Factory: create an edit_file ToolContract bound to a working directory.
  * The working directory is injected so the tool is testable.
+ * `diagnostics` is optional — when provided, errors+warnings from LSP servers
+ * are appended to successful edit results (only when errors exist).
  */
-export function createEditFileTool(workingDirectory: string): ToolContract {
+export function createEditFileTool(
+	workingDirectory: string,
+	diagnostics?: DiagnosticsHook,
+): ToolContract {
 	const workdir = resolve(workingDirectory);
 
 	return {
@@ -202,7 +224,36 @@ export function createEditFileTool(workingDirectory: string): ToolContract {
 			}
 
 			const plural = result.count === 1 ? "" : "s";
-			return { content: `Replaced ${result.count} occurrence${plural} in "${relPath}".` };
+			let baseContent = `Replaced ${result.count} occurrence${plural} in "${relPath}".`;
+
+			// After a successful edit, query LSP diagnostics (if available).
+			// Only append if there are actual errors/warnings (no noise on clean edits).
+			if (diagnostics) {
+				try {
+					const cwd = ctx.cwd ?? process.cwd();
+					const diag = await diagnostics({
+						filePath: resolvedPath,
+						text: result.content,
+						cwd,
+					});
+					const suffix: string[] = [];
+					if (diag.slow) {
+						suffix.push(
+							"⚠️ LSP is taking unusually long. If this happens more than once, raise it to the user.",
+						);
+					}
+					if (diag.formatted) {
+						suffix.push(diag.formatted);
+					}
+					if (suffix.length > 0) {
+						baseContent += `\n\n${suffix.join("\n\n")}`;
+					}
+				} catch {
+					// LSP diagnostics failure is non-fatal — the edit already succeeded.
+				}
+			}
+
+			return { content: baseContent };
 		},
 	};
 }
diff --git a/packages/tool-edit-file/src/extension.ts b/packages/tool-edit-file/src/extension.ts
index a4bb19e..bbd8256 100644
--- a/packages/tool-edit-file/src/extension.ts
+++ b/packages/tool-edit-file/src/extension.ts
@@ -1,5 +1,6 @@
 import type { Extension } from "@dispatch/kernel";
-import { createEditFileTool } from "./edit-file.js";
+import { lspServiceHandle } from "@dispatch/lsp";
+import { createEditFileTool, type DiagnosticsHook } from "./edit-file.js";
 
 export const extension: Extension = {
 	manifest: {
@@ -13,6 +14,21 @@ export const extension: Extension = {
 		contributes: { tools: ["edit_file"] },
 	},
 	activate(host) {
-		host.defineTool(createEditFileTool(process.cwd()));
+		// Optional LSP integration: if the lsp extension is loaded, wire its
+		// getDiagnostics service as the post-edit diagnostics hook. If absent,
+		// edits proceed without diagnostics (graceful degradation).
+		const lspService = host.getService(lspServiceHandle);
+		const diagnostics: DiagnosticsHook | undefined = lspService
+			? async (opts) =>
+					lspService.getDiagnostics({
+						filePath: opts.filePath,
+						text: opts.text,
+						cwd: opts.cwd,
+						timeoutMs: 60_000,
+						minSeverity: 2, // errors + warnings only
+					})
+			: undefined;
+
+		host.defineTool(createEditFileTool(process.cwd(), diagnostics));
 	},
 };
diff --git a/packages/tool-edit-file/tsconfig.json b/packages/tool-edit-file/tsconfig.json
index ff99a43..38a7610 100644
--- a/packages/tool-edit-file/tsconfig.json
+++ b/packages/tool-edit-file/tsconfig.json
@@ -2,5 +2,5 @@
 	"extends": "../../tsconfig.base.json",
 	"compilerOptions": { "rootDir": "src", "outDir": "dist", "composite": true },
 	"include": ["src/**/*.ts"],
-	"references": [{ "path": "../kernel" }]
+	"references": [{ "path": "../kernel" }, { "path": "../lsp" }]
 }