diff --git a/packages/coding-agent/src/tools/read.ts b/packages/coding-agent/src/tools/read.ts
index 454aa828d..1a9140064 100644
--- a/packages/coding-agent/src/tools/read.ts
+++ b/packages/coding-agent/src/tools/read.ts
@@ -3244,6 +3244,7 @@ export class ReadTool implements AgentTool<typeof readSchema, ReadToolDetails> {
 		if (!rawSelector && artifact.size > MAX_ARTIFACT_RAW_INLINE_BYTES) {
 			outputText += `\n\n[${this.#formatArtifactWorkflowNotice(artifact, artifactUrl)}]`;
 		}
+		if (reachedEof) details.totalLines = totalFileLines;
 		if (displayContent) details.displayContent = displayContent;
 		if (truncationInfo) details.truncation = truncationInfo.result;
 		const resultBuilder = toolResult<ReadToolDetails>(details)
diff --git a/packages/coding-agent/test/tools/read-artifact-large.test.ts b/packages/coding-agent/test/tools/read-artifact-large.test.ts
index 46831a30f..389bebc2b 100644
--- a/packages/coding-agent/test/tools/read-artifact-large.test.ts
+++ b/packages/coding-agent/test/tools/read-artifact-large.test.ts
@@ -104,6 +104,12 @@ describe("read tool large artifact handling", () => {
 		expect(output).not.toContain("line-032");
 	});
 
+	it("records the source line count for an open-ended artifact range that reaches EOF", async () => {
+		const result = await tool.execute("call-raw-tail", { path: "artifact://0:raw:301-" });
+
+		expect(result.details?.totalLines).toBe(400);
+	});
+
 	it("shortens artifact paths under the user's home dir instead of leaking the absolute path", async () => {
 		const homeSpy = spyOn(os, "homedir").mockReturnValue(testDir);
 		try {