diff --git a/packages/opencode/src/session/llm.ts b/packages/opencode/src/session/llm.ts
index b06112f2c..0e7ec2589 100644
--- a/packages/opencode/src/session/llm.ts
+++ b/packages/opencode/src/session/llm.ts
@@ -13,6 +13,9 @@ import {
 } from "ai"
 import { mergeDeep, pipe } from "remeda"
 import { ProviderTransform } from "@/provider/transform"
+// altimate_change start — tool retrieval
+import { Retrieval } from "@/tool/retrieval"
+// altimate_change end
 import { Config } from "@/config/config"
 import { Instance } from "@/project/instance"
 import type { Agent } from "@/agent/agent"
@@ -179,6 +182,26 @@ export namespace LLM {
     }
     // altimate_change end
 
+    // altimate_change start — tool retrieval
+    // Expose only the relevant top-k tools this turn (flag-gated). Keeps the
+    // always-on core + any in-flight (referenced) tools; no-op for small sets.
+    if (Retrieval.enabled()) {
+      const lastUser = [...input.messages].reverse().find((m) => m.role === "user")
+      const c = lastUser?.content as any
+      const query =
+        typeof c === "string"
+          ? c
+          : Array.isArray(c)
+            ? c.map((p: any) => (typeof p === "string" ? p : (p?.text ?? ""))).join(" ")
+            : ""
+      const list = Object.entries(tools).map(([name, t]) => ({ name, description: (t as any)?.description }))
+      const keep = Retrieval.select(query, list, { keep: referencedTools })
+      for (const name of Object.keys(tools)) {
+        if (name !== "invalid" && !keep.has(name)) delete tools[name]
+      }
+    }
+    // altimate_change end
+
     return streamText({
       onError(error) {
         l.error("stream error", {
diff --git a/packages/opencode/src/tool/retrieval.ts b/packages/opencode/src/tool/retrieval.ts
new file mode 100644
index 000000000..d02a3d372
--- /dev/null
+++ b/packages/opencode/src/tool/retrieval.ts
@@ -0,0 +1,77 @@
+/**
+ * Tool retrieval — pick a relevant subset of tools per turn.
+ *
+ * With ~78 tools, sending the full set every turn floods context and adds
+ * distractors that hurt tool SELECTION. This picks a relevant subset per turn:
+ * a fixed always-on CORE + lexically-ranked top-k of the rest, and NEVER drops a
+ * tool that's mid-trajectory (referenced by an in-flight tool call) — dropping
+ * those would corrupt the conversation.
+ *
+ * v1 is lexical (dependency-free, deterministic, testable). An embedding +
+ * cross-encoder rerank pass is a later enhancement; the `select` signature is
+ * stable so wiring doesn't change.
+ */
+
+export namespace Retrieval {
+  /** Always-available agent essentials — never retrieved out. */
+  export const CORE = [
+    "bash", "read", "write", "edit", "glob", "grep", "ls",
+    "task", "todowrite", "skill",
+  ]
+
+  export interface Tool {
+    name: string
+    description?: string
+  }
+
+  export interface Options {
+    /** target number of tools to expose (incl. core). */
+    topk?: number
+    /** names that MUST stay (e.g. tools referenced by in-flight tool calls). */
+    keep?: Iterable<string>
+    /** only retrieve when the tool count exceeds this (no-op for small sets). */
+    minToolsToRetrieve?: number
+  }
+
+  export function enabled(): boolean {
+    return process.env["ALTIMATE_TOOL_RETRIEVAL"] === "1"
+  }
+
+  function score(query: string, t: Tool): number {
+    const words = new Set(query.toLowerCase().match(/[a-z_]+/g) ?? [])
+    const hay = (t.name + " " + (t.description ?? "")).toLowerCase()
+    let s = 0
+    for (const w of words) if (w.length > 3 && hay.includes(w)) s += 1
+    // small boost for a direct name mention
+    if (words.has(t.name.toLowerCase())) s += 3
+    return s
+  }
+
+  /**
+   * Return the SUBSET of tool names to expose this turn. Caller deletes the rest.
+   * Deterministic: core + forced-keep first, then highest-scoring others up to topk
+   * (ties broken by original order for stability).
+   */
+  export function select(query: string, tools: Tool[], opts: Options = {}): Set<string> {
+    const topk = opts.topk ?? 12
+    const minToRetrieve = opts.minToolsToRetrieve ?? topk
+    const all = new Set(tools.map((t) => t.name))
+    // No-op for small tool sets — nothing to gain.
+    if (tools.length <= minToRetrieve) return all
+
+    const keep = new Set<string>()
+    for (const n of opts.keep ?? []) if (all.has(n)) keep.add(n)
+    for (const n of CORE) if (all.has(n)) keep.add(n)
+
+    const rest = tools.filter((t) => !keep.has(t.name))
+    const ranked = rest
+      .map((t, i) => ({ name: t.name, s: score(query, t), i }))
+      .sort((a, b) => b.s - a.s || a.i - b.i)
+
+    for (const r of ranked) {
+      if (keep.size >= topk) break
+      keep.add(r.name)
+    }
+    return keep
+  }
+}
diff --git a/packages/opencode/test/tool/retrieval.test.ts b/packages/opencode/test/tool/retrieval.test.ts
new file mode 100644
index 000000000..cb1adb885
--- /dev/null
+++ b/packages/opencode/test/tool/retrieval.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, test } from "bun:test"
+import { Retrieval } from "../../src/tool/retrieval"
+
+const TOOLS = [
+  ...Retrieval.CORE.map((name) => ({ name })),
+  ...Array.from({ length: 20 }, (_, i) => ({ name: `warehouse_op${i}`, description: `warehouse operation ${i}` })),
+  { name: "dbt_run", description: "run dbt models build" },
+  { name: "sql_execute", description: "execute SQL query against warehouse" },
+]
+
+describe("Retrieval.select", () => {
+  test("always keeps core tools", () => {
+    const sel = Retrieval.select("run the dbt models", TOOLS, { topk: 12 })
+    expect(sel.has("bash")).toBe(true)
+    expect(sel.has("read")).toBe(true)
+  })
+
+  test("picks lexically relevant tools", () => {
+    expect(Retrieval.select("run the dbt models and build", TOOLS, { topk: 12 }).has("dbt_run")).toBe(true)
+    expect(Retrieval.select("execute a SQL query on the warehouse", TOOLS, { topk: 12 }).has("sql_execute")).toBe(true)
+  })
+
+  test("never drops in-flight (keep) tools, even if irrelevant", () => {
+    const sel = Retrieval.select("hello", TOOLS, { topk: 12, keep: ["warehouse_op19"] })
+    expect(sel.has("warehouse_op19")).toBe(true)
+  })
+
+  test("no-op for small tool sets (returns all)", () => {
+    const small = [{ name: "a" }, { name: "b" }]
+    expect(Retrieval.select("x", small, { topk: 12 }).size).toBe(2)
+  })
+
+  test("enabled() reads the env flag", () => {
+    const prev = process.env["ALTIMATE_TOOL_RETRIEVAL"]
+    process.env["ALTIMATE_TOOL_RETRIEVAL"] = "1"
+    expect(Retrieval.enabled()).toBe(true)
+    delete process.env["ALTIMATE_TOOL_RETRIEVAL"]
+    expect(Retrieval.enabled()).toBe(false)
+    if (prev !== undefined) process.env["ALTIMATE_TOOL_RETRIEVAL"] = prev
+  })
+})