diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 00b5106e4..ea8b1a407 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -6,6 +6,10 @@
 
 - Restored the legacy project-scoped session directory naming scheme and removed its automatic migration ([#7646](https://github.com/can1357/oh-my-pi/issues/7646)).
 
+### Fixed
+
+- Fixed LSP diagnostics being dropped when servers normalize file URI percent-encoding or Windows path casing.
+
 ## [17.2.8] - 2026-08-04
 
 ### Changed
diff --git a/packages/coding-agent/src/lsp/client.ts b/packages/coding-agent/src/lsp/client.ts
index f4df6cb7b..a90b4fd02 100644
--- a/packages/coding-agent/src/lsp/client.ts
+++ b/packages/coding-agent/src/lsp/client.ts
@@ -17,7 +17,7 @@ import type {
 	ServerConfig,
 	WorkspaceEdit,
 } from "./types";
-import { detectLanguageId, fileToUri } from "./utils";
+import { detectLanguageId, EquivalentUriMap, fileToUri } from "./utils";
 
 // =============================================================================
 // Client State
@@ -787,7 +787,7 @@ export async function getOrCreateClient(
 			proc,
 			config,
 			requestId: 0,
-			diagnostics: new Map(),
+			diagnostics: new EquivalentUriMap(),
 			diagnosticsVersion: 0,
 			dynamicCapabilityRegistrations: new Map(),
 			openFiles: new Map(),
diff --git a/packages/coding-agent/src/lsp/utils.ts b/packages/coding-agent/src/lsp/utils.ts
index d9eeac7ac..d96f7f97b 100644
--- a/packages/coding-agent/src/lsp/utils.ts
+++ b/packages/coding-agent/src/lsp/utils.ts
@@ -78,6 +78,35 @@ function laxUriToFile(uri: string): string {
 	return filePath;
 }
 
+/** Map that treats equivalent file URI spellings as the same key. */
+export class EquivalentUriMap<Value> extends Map<string, Value> {
+	#key(uri: string): string {
+		if (!uri.startsWith("file://")) return uri;
+		const filePath = path.normalize(uriToFile(uri));
+		return process.platform === "win32" ? filePath.toLowerCase() : filePath;
+	}
+
+	override delete(uri: string): boolean {
+		const key = this.#key(uri);
+		return super.delete(key);
+	}
+
+	override get(uri: string): Value | undefined {
+		const key = this.#key(uri);
+		return super.get(key);
+	}
+
+	override has(uri: string): boolean {
+		const key = this.#key(uri);
+		return super.has(key);
+	}
+
+	override set(uri: string, value: Value): this {
+		const key = this.#key(uri);
+		return super.set(key, value);
+	}
+}
+
 // =============================================================================
 // Diagnostic Formatting
 // =============================================================================
diff --git a/packages/coding-agent/test/tools/lsp-diagnostics-freshness.test.ts b/packages/coding-agent/test/tools/lsp-diagnostics-freshness.test.ts
index 13ed0a57c..4f967c1ad 100644
--- a/packages/coding-agent/test/tools/lsp-diagnostics-freshness.test.ts
+++ b/packages/coding-agent/test/tools/lsp-diagnostics-freshness.test.ts
@@ -5,7 +5,7 @@ import { createLspWritethrough, type FileDiagnosticsResult, FileFormatResult } f
 import * as lspClient from "@oh-my-pi/pi-coding-agent/lsp/client";
 import * as lspConfig from "@oh-my-pi/pi-coding-agent/lsp/config";
 import type { Diagnostic, LinterClient, LspClient, ServerConfig } from "@oh-my-pi/pi-coding-agent/lsp/types";
-import { fileToUri } from "@oh-my-pi/pi-coding-agent/lsp/utils";
+import { EquivalentUriMap, fileToUri } from "@oh-my-pi/pi-coding-agent/lsp/utils";
 import type { DeferredDiagnosticsEntry, ToolSession } from "@oh-my-pi/pi-coding-agent/tools";
 import { WriteTool } from "@oh-my-pi/pi-coding-agent/tools/write";
 import { type ptree, TempDir } from "@oh-my-pi/pi-utils";
@@ -47,7 +47,7 @@ function createClient(cwd: string, config: ServerConfig): LspClient {
 		config,
 		proc: {} as ptree.ChildProcess<"pipe">,
 		requestId: 0,
-		diagnostics: new Map(),
+		diagnostics: new EquivalentUriMap(),
 		diagnosticsVersion: 0,
 		openFiles: new Map(),
 		pendingRequests: new Map(),
@@ -438,6 +438,51 @@ describe("LSP diagnostics freshness", () => {
 		expect(result?.messages.some(m => m.includes("stale error"))).toBe(false);
 	});
 
+	it("matches published diagnostics when the server renormalizes the document URI", async () => {
+		const filePath = path.join(tempDir.path(), "renormalized.ts");
+		const uri = fileToUri(filePath);
+		const serverUri = uri.replace("/renormalized.ts", "/%72enormalized.ts");
+		const client = createClient(tempDir.path(), TEST_SERVER);
+		const clock = new VirtualClock(Date.now());
+		installVirtualTime(clock);
+
+		vi.spyOn(lspConfig, "loadConfig").mockReturnValue({ servers: {}, idleTimeoutMs: undefined });
+		vi.spyOn(lspConfig, "getServersForFile").mockReturnValue([["test-lsp", TEST_SERVER]]);
+		vi.spyOn(lspClient, "getOrCreateClient").mockResolvedValue(client);
+		vi.spyOn(lspClient, "syncContent").mockImplementation(async (mockClient, syncedFilePath) => {
+			const syncedUri = fileToUri(syncedFilePath);
+			mockClient.openFiles.set(syncedUri, { version: 1, languageId: "typescript" });
+		});
+		vi.spyOn(lspClient, "notifySaved").mockImplementation(async mockClient => {
+			clock.in(10, () => {
+				publishDiagnostics(mockClient, serverUri, [createDiagnostic("renormalized URI error")], 1);
+			});
+		});
+
+		const writethrough = createLspWritethrough(tempDir.path(), {
+			enableFormat: false,
+			enableDiagnostics: true,
+		});
+		const result = await writethrough(filePath, "export const value = missing;\n");
+
+		expect(result?.errored).toBe(true);
+		expect(result?.messages.some(message => message.includes("renormalized URI error"))).toBe(true);
+	});
+
+	it("matches Windows drive-letter case and percent-encoding differences", () => {
+		const platformDescriptor = Object.getOwnPropertyDescriptor(process, "platform");
+		if (!platformDescriptor) throw new Error("process.platform descriptor is unavailable");
+		Object.defineProperty(process, "platform", { ...platformDescriptor, value: "win32" });
+		try {
+			const diagnostics = new EquivalentUriMap<string>();
+			diagnostics.set("file:///c%3A/Users/serge/doc.md", "published");
+
+			expect(diagnostics.get("file:///C:/Users/serge/doc.md")).toBe("published");
+		} finally {
+			Object.defineProperty(process, "platform", platformDescriptor);
+		}
+	});
+
 	it("returns completed pull diagnostics inside the inline write window", async () => {
 		const filePath = path.join(tempDir.path(), "pull-only.ts");
 		const uri = fileToUri(filePath);