diff --git a/CHANGELOG.md b/CHANGELOG.md
index fd019608..19abc1b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,26 @@
 
 Detailed changelog for Perry. See CLAUDE.md for concise summaries.
 
+## v0.5.980 — feat(lodash): add `sum` / `mean` / `sumBy` / `meanBy` / `tail` aggregators to the manifest
+
+**Symptom.** Manifest sweep flagged `_.sum([1,2,3,4])` as missing — `import _ from 'lodash'; _.sum(...)` errored at HIR-lower with `\`lodash.sum\` is not implemented in Perry — see \`perry --print-api-manifest\` for the supported surface`. Same shape for `_.mean`, `_.sumBy`, `_.meanBy`, and `_.tail` (tail's runtime + decl already existed but the manifest + lower_call rows were absent, so call sites couldn't reach the native function).
+
+**Fix.** Added five new lodash bindings following the existing `chunk`/`compact`/`take`/etc. pattern:
+
+- `crates/perry-stdlib/src/lodash.rs` — new `js_lodash_sum`, `js_lodash_mean`, `js_lodash_sum_by`, `js_lodash_mean_by` runtime exports. All four iterate over `*mut ArrayHeader` via `js_array_get`/`js_array_length` and accumulate via `JSValue::to_number()` (with `undefined` treated as 0 to match lodash's `baseSum`). `mean`/`meanBy` return `NaN` for empty/null arrays (0/0 case). `sumBy`/`meanBy` accept the property-shorthand iteratee form only (a string key); function iteratees return `NaN` to mirror lodash's "unusable iteratee" behaviour — closure-call plumbing across the FFI boundary is its own follow-up. Property lookup uses `js_object_get_field_by_name`.
+- `crates/perry-api-manifest/src/entries.rs` — five new `method_sig` rows (`sum`, `mean`, `sumBy`, `meanBy`, `tail`). The first four return `TypeSpec::Number` so HIR knows the return type without inference; `tail` returns `TypeSpec::Any` like the rest of the array-returning helpers.
+- `crates/perry-codegen/src/lower_call.rs` — five new `NativeModSig` rows in the lodash block of `NATIVE_MODULE_TABLE`. `sum`/`mean` take `&[NA_PTR]` → `NR_F64`; `sumBy`/`meanBy` take `&[NA_PTR, NA_F64]` → `NR_F64` (iteratee passed as a NaN-boxed `f64` and re-bit-cast to `JSValue` inside the runtime); `tail` takes `&[NA_PTR]` → `NR_PTR`.
+- `crates/perry-codegen/src/runtime_decls.rs` — four new `declare_function` rows for the new symbols (`js_lodash_sum`/`_sum_by`/`_mean`/`_mean_by`). `js_lodash_tail` was already declared.
+
+**Validation.** `test-files/test_lodash_sum.ts` (new) prints `10 / 2.5 / 3 / 1 / 3` and matches `node --experimental-strip-types` byte-for-byte. Tested via the standard manifest-route (no `compilePackages` config) since the `compilePackages: ["lodash"]` path is still blocked on a separate module-init `TypeError: Cannot read properties of undefined (reading 'prototype')` regression that pre-dates this work (reproducible with just `_.head([1,2,3])` after enabling compilePackages — not in scope here).
+
+**Files.**
+- `crates/perry-stdlib/src/lodash.rs` — new aggregator exports + helper `lodash_to_number`.
+- `crates/perry-api-manifest/src/entries.rs` — five new manifest rows.
+- `crates/perry-codegen/src/lower_call.rs` — five new `NativeModSig` rows.
+- `crates/perry-codegen/src/runtime_decls.rs` — four new symbol decls.
+- `test-files/test_lodash_sum.ts` — repro/regression fixture.
+
 ## v0.5.979 — feat(globalThis): expose built-in constructors as properties so `globalThis.Array`, `context.Object`, etc. stop returning `undefined`
 
 **Symptom.** PR #963 (v0.5.977) closed the `Function('return this')()` recogniser, so the lodash module-init IIFE no longer threw `TypeError: value is not a function`. The next blocker surfaced one frame down, inside `runInContext`:
diff --git a/CLAUDE.md b/CLAUDE.md
index b05b51a2..f13e59dd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -8,7 +8,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 Perry is a native TypeScript compiler written in Rust that compiles TypeScript source code directly to native executables. It uses SWC for TypeScript parsing and LLVM for code generation.
 
-**Current Version:** 0.5.979
+**Current Version:** 0.5.980
 
 
 ## TypeScript Parity Status
diff --git a/Cargo.lock b/Cargo.lock
index 7fab71f5..86fcad06 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4905,7 +4905,7 @@ checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
 [[package]]
 name = "perry"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "base64",
@@ -4960,14 +4960,14 @@ dependencies = [
 
 [[package]]
 name = "perry-api-manifest"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "serde",
 ]
 
 [[package]]
 name = "perry-codegen"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "log",
@@ -4980,7 +4980,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-arkts"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -4989,7 +4989,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-glance"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -4997,7 +4997,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-js"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-dispatch",
@@ -5007,7 +5007,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-swiftui"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5016,7 +5016,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wasm"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "base64",
@@ -5029,7 +5029,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wear-tiles"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5037,7 +5037,7 @@ dependencies = [
 
 [[package]]
 name = "perry-diagnostics"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "serde",
  "serde_json",
@@ -5045,7 +5045,7 @@ dependencies = [
 
 [[package]]
 name = "perry-dispatch"
-version = "0.5.979"
+version = "0.5.980"
 
 [[package]]
 name = "perry-doc-fixture-my-bindings"
@@ -5056,7 +5056,7 @@ dependencies = [
 
 [[package]]
 name = "perry-doc-tests"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "clap",
@@ -5071,7 +5071,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-argon2"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "argon2",
  "perry-ffi",
@@ -5079,7 +5079,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-axios"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "reqwest",
@@ -5088,7 +5088,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-bcrypt"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "bcrypt",
  "perry-ffi",
@@ -5096,7 +5096,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-better-sqlite3"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "rusqlite",
@@ -5104,7 +5104,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-cheerio"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "scraper",
@@ -5112,14 +5112,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-commander"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-cron"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "chrono",
  "cron",
@@ -5128,7 +5128,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dayjs"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5136,7 +5136,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-decimal"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "rust_decimal",
@@ -5144,7 +5144,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dotenv"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "serde_json",
@@ -5152,7 +5152,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ethers"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "rand 0.8.6",
@@ -5160,21 +5160,21 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-events"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-exponential-backoff"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-fastify"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "bytes",
  "http-body-util",
@@ -5188,7 +5188,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-fetch"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5199,7 +5199,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-http"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lazy_static",
  "perry-ext-http-server",
@@ -5211,7 +5211,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-http-server"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "bytes",
  "http-body-util",
@@ -5230,7 +5230,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ioredis"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5240,7 +5240,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-jsonwebtoken"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "base64",
  "jsonwebtoken",
@@ -5251,7 +5251,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-lru-cache"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lru",
  "perry-ffi",
@@ -5259,7 +5259,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-moment"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5267,7 +5267,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mongodb"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "bson",
  "futures-util",
@@ -5279,7 +5279,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mysql2"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5289,7 +5289,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nanoid"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "nanoid",
  "perry-ffi",
@@ -5298,7 +5298,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-net"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "rustls",
@@ -5309,7 +5309,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nodemailer"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lettre",
  "perry-ffi",
@@ -5319,7 +5319,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-pg"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "sqlx",
@@ -5328,7 +5328,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ratelimit"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "governor",
  "perry-ffi",
@@ -5336,7 +5336,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-sharp"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "base64",
  "image",
@@ -5345,14 +5345,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-slugify"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-streams"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5360,7 +5360,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-uuid"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "uuid",
@@ -5368,7 +5368,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-validator"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "perry-ffi",
  "regex",
@@ -5378,7 +5378,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ws"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "futures-util",
  "lazy_static",
@@ -5389,7 +5389,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-zlib"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "flate2",
  "perry-ffi",
@@ -5397,7 +5397,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ffi"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "dashmap 6.1.0",
  "once_cell",
@@ -5406,7 +5406,7 @@ dependencies = [
 
 [[package]]
 name = "perry-hir"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-api-manifest",
@@ -5420,7 +5420,7 @@ dependencies = [
 
 [[package]]
 name = "perry-jsruntime"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "deno_core",
@@ -5440,7 +5440,7 @@ dependencies = [
 
 [[package]]
 name = "perry-parser"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-diagnostics",
@@ -5452,7 +5452,7 @@ dependencies = [
 
 [[package]]
 name = "perry-runtime"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "base64",
@@ -5476,7 +5476,7 @@ dependencies = [
 
 [[package]]
 name = "perry-stdlib"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "aes 0.8.4",
  "aes-gcm",
@@ -5546,7 +5546,7 @@ dependencies = [
 
 [[package]]
 name = "perry-transform"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5556,7 +5556,7 @@ dependencies = [
 
 [[package]]
 name = "perry-types"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "anyhow",
  "thiserror 1.0.69",
@@ -5564,11 +5564,11 @@ dependencies = [
 
 [[package]]
 name = "perry-ui"
-version = "0.5.979"
+version = "0.5.980"
 
 [[package]]
 name = "perry-ui-android"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "itoa",
  "jni",
@@ -5583,7 +5583,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-geisterhand"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "rand 0.8.6",
  "serde",
@@ -5593,7 +5593,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-gtk4"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "cairo-rs",
  "dirs 5.0.1",
@@ -5612,7 +5612,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-ios"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "block2",
  "libc",
@@ -5627,7 +5627,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-macos"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "block2",
  "libc",
@@ -5645,11 +5645,11 @@ version = "0.1.0"
 
 [[package]]
 name = "perry-ui-testkit"
-version = "0.5.979"
+version = "0.5.980"
 
 [[package]]
 name = "perry-ui-tvos"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "block2",
  "libc",
@@ -5664,7 +5664,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-visionos"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "block2",
  "libc",
@@ -5679,7 +5679,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-watchos"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "block2",
  "libc",
@@ -5692,7 +5692,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-windows"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "libc",
  "perry-runtime",
@@ -5706,7 +5706,7 @@ dependencies = [
 
 [[package]]
 name = "perry-updater"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "base64",
  "ed25519-dalek",
@@ -5720,7 +5720,7 @@ dependencies = [
 
 [[package]]
 name = "perry-wasm-host"
-version = "0.5.979"
+version = "0.5.980"
 dependencies = [
  "wasmi",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 714af7fe..d8fa11bd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -190,7 +190,7 @@ opt-level = "s"       # Optimize for size in stdlib
 opt-level = 3
 
 [workspace.package]
-version = "0.5.979"
+version = "0.5.980"
 edition = "2021"
 license = "MIT"
 repository = "https://github.com/PerryTS/perry"
diff --git a/crates/perry-api-manifest/src/entries.rs b/crates/perry-api-manifest/src/entries.rs
index 6eb7b12b..7c116647 100644
--- a/crates/perry-api-manifest/src/entries.rs
+++ b/crates/perry-api-manifest/src/entries.rs
@@ -995,6 +995,39 @@ pub static API_MANIFEST: &[ApiEntry] = &[
         TypeSpec::Any,
     ),
     method_sig("lodash", "size", false, None, &[p_any("p0")], TypeSpec::Any),
+    method_sig(
+        "lodash",
+        "sum",
+        false,
+        None,
+        &[p_any("p0")],
+        TypeSpec::Number,
+    ),
+    method_sig(
+        "lodash",
+        "mean",
+        false,
+        None,
+        &[p_any("p0")],
+        TypeSpec::Number,
+    ),
+    method_sig(
+        "lodash",
+        "sumBy",
+        false,
+        None,
+        &[p_any("p0"), p_any("p1")],
+        TypeSpec::Number,
+    ),
+    method_sig(
+        "lodash",
+        "meanBy",
+        false,
+        None,
+        &[p_any("p0"), p_any("p1")],
+        TypeSpec::Number,
+    ),
+    method_sig("lodash", "tail", false, None, &[p_any("p0")], TypeSpec::Any),
     method_sig("dayjs", "default", false, None, &[], TypeSpec::Any),
     method_sig("dayjs", "dayjs", false, None, &[], TypeSpec::Any),
     method("dayjs", "format", true, None),
diff --git a/crates/perry-codegen/src/lower_call.rs b/crates/perry-codegen/src/lower_call.rs
index fa2e333a..be13616c 100644
--- a/crates/perry-codegen/src/lower_call.rs
+++ b/crates/perry-codegen/src/lower_call.rs
@@ -8628,6 +8628,51 @@ const NATIVE_MODULE_TABLE: &[NativeModSig] = &[
         args: &[NA_F64],
         ret: NR_F64,
     },
+    NativeModSig {
+        module: "lodash",
+        has_receiver: false,
+        method: "tail",
+        class_filter: None,
+        runtime: "js_lodash_tail",
+        args: &[NA_PTR],
+        ret: NR_PTR,
+    },
+    NativeModSig {
+        module: "lodash",
+        has_receiver: false,
+        method: "sum",
+        class_filter: None,
+        runtime: "js_lodash_sum",
+        args: &[NA_PTR],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "lodash",
+        has_receiver: false,
+        method: "mean",
+        class_filter: None,
+        runtime: "js_lodash_mean",
+        args: &[NA_PTR],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "lodash",
+        has_receiver: false,
+        method: "sumBy",
+        class_filter: None,
+        runtime: "js_lodash_sum_by",
+        args: &[NA_PTR, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "lodash",
+        has_receiver: false,
+        method: "meanBy",
+        class_filter: None,
+        runtime: "js_lodash_mean_by",
+        args: &[NA_PTR, NA_F64],
+        ret: NR_F64,
+    },
     // ========== dayjs ==========
     // Factory: `import dayjs from 'dayjs'; dayjs()` → method:"default".
     // Named import: `import { dayjs } from 'dayjs'; dayjs()` → method:"dayjs".
diff --git a/crates/perry-codegen/src/runtime_decls.rs b/crates/perry-codegen/src/runtime_decls.rs
index 6ac5c828..e7f66e73 100644
--- a/crates/perry-codegen/src/runtime_decls.rs
+++ b/crates/perry-codegen/src/runtime_decls.rs
@@ -2332,6 +2332,8 @@ pub fn declare_stdlib_ffi(module: &mut LlModule) {
     module.declare_function("js_lodash_last", DOUBLE, &[I64]);
     module.declare_function("js_lodash_lower_case", I64, &[I64]);
     module.declare_function("js_lodash_lower_first", I64, &[I64]);
+    module.declare_function("js_lodash_mean", DOUBLE, &[I64]);
+    module.declare_function("js_lodash_mean_by", DOUBLE, &[I64, DOUBLE]);
     module.declare_function("js_lodash_pad", I64, &[I64, DOUBLE]);
     module.declare_function("js_lodash_pad_end", I64, &[I64, DOUBLE]);
     module.declare_function("js_lodash_pad_start", I64, &[I64, DOUBLE]);
@@ -2344,6 +2346,8 @@ pub fn declare_stdlib_ffi(module: &mut LlModule) {
     module.declare_function("js_lodash_split", I64, &[I64, I64]);
     module.declare_function("js_lodash_start_case", I64, &[I64]);
     module.declare_function("js_lodash_starts_with", DOUBLE, &[I64, I64]);
+    module.declare_function("js_lodash_sum", DOUBLE, &[I64]);
+    module.declare_function("js_lodash_sum_by", DOUBLE, &[I64, DOUBLE]);
     module.declare_function("js_lodash_tail", I64, &[I64]);
     module.declare_function("js_lodash_take", I64, &[I64, DOUBLE]);
     module.declare_function("js_lodash_take_right", I64, &[I64, DOUBLE]);
diff --git a/crates/perry-stdlib/src/lodash.rs b/crates/perry-stdlib/src/lodash.rs
index ea90a107..9e02e1cc 100644
--- a/crates/perry-stdlib/src/lodash.rs
+++ b/crates/perry-stdlib/src/lodash.rs
@@ -4,8 +4,8 @@
 //! Provides array, object, string, and utility operations.
 
 use perry_runtime::{
-    js_array_alloc, js_array_get, js_array_length, js_array_push, js_string_from_bytes,
-    ArrayHeader, JSValue, StringHeader,
+    js_array_alloc, js_array_get, js_array_length, js_array_push, js_object_get_field_by_name,
+    js_string_from_bytes, ArrayHeader, JSValue, ObjectHeader, StringHeader,
 };
 use std::collections::HashSet;
 
@@ -878,3 +878,115 @@ pub unsafe extern "C" fn js_lodash_size(collection_f: f64) -> f64 {
     }
     0.0
 }
+
+// ============================================================================
+// Aggregators (sum/mean) — issue #793 follow-up
+// ============================================================================
+
+/// Coerce a JS array element to f64 per lodash's `baseSum`: skip
+/// `undefined` (treated as 0 contribution) and `NaN` is propagated by
+/// caller summation. lodash uses `current === current` for the NaN check;
+/// here we just lean on f64 NaN arithmetic which preserves NaN propagation.
+#[inline]
+fn lodash_to_number(v: JSValue) -> f64 {
+    if v.is_undefined() {
+        0.0
+    } else {
+        v.to_number()
+    }
+}
+
+/// _.sum(array) -> number
+///
+/// Sum the numeric values of the array. Returns 0 for null/empty.
+#[no_mangle]
+pub unsafe extern "C" fn js_lodash_sum(arr_ptr: *mut ArrayHeader) -> f64 {
+    if arr_ptr.is_null() {
+        return 0.0;
+    }
+    let len = js_array_length(arr_ptr);
+    let mut sum = 0.0;
+    for i in 0..len {
+        sum += lodash_to_number(js_array_get(arr_ptr, i));
+    }
+    sum
+}
+
+/// _.mean(array) -> number
+///
+/// Arithmetic mean. lodash returns NaN for empty arrays (0/0).
+#[no_mangle]
+pub unsafe extern "C" fn js_lodash_mean(arr_ptr: *mut ArrayHeader) -> f64 {
+    if arr_ptr.is_null() {
+        return f64::NAN;
+    }
+    let len = js_array_length(arr_ptr);
+    if len == 0 {
+        return f64::NAN;
+    }
+    let mut sum = 0.0;
+    for i in 0..len {
+        sum += lodash_to_number(js_array_get(arr_ptr, i));
+    }
+    sum / (len as f64)
+}
+
+/// _.sumBy(collection, iteratee) -> number
+///
+/// Like `sum`, but iteratee is invoked per element. Today we only
+/// support the property-shorthand form (`'name'`) since native function
+/// iteratees would require closure-call plumbing. A function iteratee
+/// silently returns NaN to mirror lodash's behaviour when the iteratee
+/// is unusable.
+#[no_mangle]
+pub unsafe extern "C" fn js_lodash_sum_by(arr_ptr: *mut ArrayHeader, iteratee_f: f64) -> f64 {
+    if arr_ptr.is_null() {
+        return 0.0;
+    }
+    let iteratee = JSValue::from_bits(iteratee_f.to_bits());
+    // Property-shorthand only: iteratee must be a string key.
+    if !iteratee.is_string() {
+        return f64::NAN;
+    }
+    let key_ptr = iteratee.as_pointer::<StringHeader>();
+    let len = js_array_length(arr_ptr);
+    let mut sum = 0.0;
+    for i in 0..len {
+        let elem = js_array_get(arr_ptr, i);
+        if elem.is_pointer() {
+            let obj_ptr = elem.as_pointer::<ObjectHeader>();
+            let prop = js_object_get_field_by_name(obj_ptr, key_ptr);
+            sum += lodash_to_number(prop);
+        }
+    }
+    sum
+}
+
+/// _.meanBy(collection, iteratee) -> number
+///
+/// Same restriction as `sumBy`: property-shorthand iteratee only.
+#[no_mangle]
+pub unsafe extern "C" fn js_lodash_mean_by(arr_ptr: *mut ArrayHeader, iteratee_f: f64) -> f64 {
+    if arr_ptr.is_null() {
+        return f64::NAN;
+    }
+    let len = js_array_length(arr_ptr);
+    if len == 0 {
+        return f64::NAN;
+    }
+    let iteratee = JSValue::from_bits(iteratee_f.to_bits());
+    if !iteratee.is_string() {
+        return f64::NAN;
+    }
+    let key_ptr = iteratee.as_pointer::<StringHeader>();
+    let mut sum = 0.0;
+    for i in 0..len {
+        let elem = js_array_get(arr_ptr, i);
+        if elem.is_pointer() {
+            let obj_ptr = elem.as_pointer::<ObjectHeader>();
+            let prop = js_object_get_field_by_name(obj_ptr, key_ptr);
+            sum += lodash_to_number(prop);
+        }
+    }
+    sum / (len as f64)
+}
diff --git a/test-files/test_lodash_sum.ts b/test-files/test_lodash_sum.ts
new file mode 100644
index 00000000..75b6decd
--- /dev/null
+++ b/test-files/test_lodash_sum.ts
@@ -0,0 +1,6 @@
+import _ from 'lodash';
+console.log(_.sum([1, 2, 3, 4]));        // 10
+console.log(_.mean([1, 2, 3, 4]));       // 2.5
+console.log(_.sumBy([{n:1},{n:2}], 'n')); // 3
+console.log(_.head([1, 2, 3]));          // 1
+console.log(_.last([1, 2, 3]));          // 3