diff --git a/packages/opencode/src/altimate/mcp-server/adapter.ts b/packages/opencode/src/altimate/mcp-server/adapter.ts
index dda116d5b9..f43c0a576f 100644
--- a/packages/opencode/src/altimate/mcp-server/adapter.ts
+++ b/packages/opencode/src/altimate/mcp-server/adapter.ts
@@ -9,7 +9,7 @@ import { Agent } from "@/agent/agent"
 import { Tool } from "@/tool/tool"
 import { Truncate } from "@/tool/truncate"
 import { outputLimits } from "./output-settings"
-import { MAX_BASIS_CHARACTERS, renderAnswer } from "../tools/answer-commit"
+import { MAX_BASIS_CHARACTERS, normalizeAnswerArgs, renderAnswer } from "../tools/answer-commit"
 import { AnswerCommitConjunction } from "../tools/answer-commit-conjunction"
 import { MAX_SCANNED_CHARACTERS } from "../tools/answer-commit-checks"
 import * as Precedence from "../workspace/precedence"
@@ -230,7 +230,10 @@ type TurnMessage = Tool.Context["messages"][number]
  * explanation never becomes part of the answer text a client copies. The basis is kept,
  * above that line, as context for the user.
  */
-export function finalAnswerText(args: Record<string, unknown>, committed: string, request?: unknown): string {
+export function finalAnswerText(raw: Record<string, unknown>, committed: string, request?: unknown): string {
+  // altimate_change start — #1700: read the same decoded shape the tool validated (JSON-string fields/rows)
+  const args = normalizeAnswerArgs(raw) as Record<string, unknown>
+  // altimate_change end
   // Answer-only only for a complete commit: rows, or fields covering every item the request names.
   // Otherwise keep the committed text (basis on the line) so an item kept only in the basis is not lost.
   const fields = Array.isArray(args.fields) ? args.fields.length : 0
diff --git a/packages/opencode/src/altimate/mcp-server/server.ts b/packages/opencode/src/altimate/mcp-server/server.ts
index 8ed203ac74..fda35a2603 100644
--- a/packages/opencode/src/altimate/mcp-server/server.ts
+++ b/packages/opencode/src/altimate/mcp-server/server.ts
@@ -9,6 +9,7 @@ import {
 } from "@modelcontextprotocol/sdk/types.js"
 import * as Registry from "../native/connections/registry"
 import { createAdapter, descriptions, resultFailed } from "./adapter"
+import { normalizeAnswerArgs } from "../tools/answer-commit"
 import { catalogSummary } from "../tools/warehouse-discovery"
 import { readOnlyCaveat } from "../native/connections/read-only"
 import path from "node:path"
@@ -183,7 +184,10 @@ export async function createServer(options: ServeOptions = {}) {
     result: { status?: string },
     signal: AbortSignal,
   ) => {
-    const readings = Array.isArray(args?.readings) ? (args.readings as { reading?: unknown }[]) : []
+    // altimate_change start — #1700: readings may arrive as a JSON string; read the decoded shape
+    const decoded = normalizeAnswerArgs(args) as Record<string, unknown> | undefined
+    const readings = Array.isArray(decoded?.readings) ? (decoded.readings as { reading?: unknown }[]) : []
+    // altimate_change end
     const choices = readings.map((item) => item.reading).filter((item): item is string => typeof item === "string" && !!item)
     if (elicited || result.status === "error" || choices.length < 2 || !server.getClientCapabilities()?.elicitation) return undefined
     elicited = true
diff --git a/packages/opencode/src/altimate/tools/answer-commit.ts b/packages/opencode/src/altimate/tools/answer-commit.ts
index 256568df00..982a34ec1c 100644
--- a/packages/opencode/src/altimate/tools/answer-commit.ts
+++ b/packages/opencode/src/altimate/tools/answer-commit.ts
@@ -183,13 +183,45 @@ export type AnswerCommitInput = z.input<typeof parameters>
 
 // Tolerate boolean aliases at runtime while advertising the canonical boolean schema.
 // Only answer_only is boolean; answer values and table cells must remain untouched.
-const parser = z.preprocess((input) => {
-  if (!input || typeof input !== "object" || Array.isArray(input) || !("answer_only" in input)) return input
-  const value = typeof input.answer_only === "string" ? input.answer_only.toLowerCase() : input.answer_only
-  const answer_only =
-    value === "true" || value === 1 ? true : value === "false" || value === 0 ? false : input.answer_only
-  return { ...input, answer_only }
-}, parameters)
+// altimate_change start — #1700: some models (e.g. Altimate Base) send list/object inputs as
+// JSON-encoded strings. A string is never valid for these keys, so decode it when it parses as
+// JSON and let the schema validate the result as usual; anything else is left for the normal error.
+const JSON_SHAPED_KEYS = ["fields", "asked", "rows", "unresolved", "readings"] as const
+
+/** Parse a string that looks like a JSON array or object; return any other value, or invalid JSON, unchanged. */
+function decodeJsonShaped(value: unknown): unknown {
+  if (typeof value !== "string" || !/^\s*[[{]/.test(value)) return value
+  try {
+    return JSON.parse(value)
+  } catch {
+    return value
+  }
+}
+// altimate_change end
+
+// altimate_change start — #1700: exported so raw-argument readers (MCP adapter/server) see the same decoded shape
+/**
+ * Normalize raw answer_commit arguments before validation: decode JSON-string list/object inputs
+ * and coerce string or numeric `answer_only` flags. Non-object input is returned unchanged.
+ */
+export function normalizeAnswerArgs(input: unknown): unknown {
+  if (!input || typeof input !== "object" || Array.isArray(input)) return input
+  const next: Record<string, unknown> = { ...input }
+  for (const key of JSON_SHAPED_KEYS) {
+    if (key in next) next[key] = decodeJsonShaped(next[key])
+  }
+  if (!("answer_only" in next)) return next
+  const value = typeof next.answer_only === "string" ? next.answer_only.toLowerCase() : next.answer_only
+  if (value === "true" || value === 1) {
+    next.answer_only = true
+  } else if (value === "false" || value === 0) {
+    next.answer_only = false
+  }
+  return next
+}
+
+const parser = z.preprocess(normalizeAnswerArgs, parameters)
+// altimate_change end
 
 function validationError(error: z.ZodError): string {
   const describe = (issues: z.core.$ZodIssue[], parent: PropertyKey[] = []): string[] =>
diff --git a/packages/opencode/test/altimate/mcp-server/final-answer.test.ts b/packages/opencode/test/altimate/mcp-server/final-answer.test.ts
index eaaa79f7b5..38a8837534 100644
--- a/packages/opencode/test/altimate/mcp-server/final-answer.test.ts
+++ b/packages/opencode/test/altimate/mcp-server/final-answer.test.ts
@@ -69,5 +69,23 @@ describe("finalAnswerText", () => {
   test("falls back to the committed text when the args cannot be re-rendered", () => {
     expect(finalAnswerText({ basis: "" }, "committed")).toBe("FINAL ANSWER: committed")
   })
+
+  // #1700: some models send list inputs as JSON strings; the FINAL ANSWER line must match a real array.
+  test("fields and asked sent as JSON strings give the same answer-only line", () => {
+    const fields = [{ name: "orders", value: 42 }]
+    const committed = `${basis}\n\n**Answer:** 42`
+    const expected = finalAnswerText({ fields, asked: ["orders"], basis }, committed)
+    expect(
+      finalAnswerText({ fields: JSON.stringify(fields), asked: JSON.stringify(["orders"]), basis }, committed),
+    ).toBe(expected)
+    expect(expected).toBe(`Basis: ${basis}\n\nFINAL ANSWER: 42`)
+  })
+
+  test("rows sent as a JSON string give the same answer-only line", () => {
+    const rows = { columns: ["city", "n"], values: [["Oslo", 3]] }
+    expect(finalAnswerText({ rows: JSON.stringify(rows), asked: ["city", "n"], basis }, "unused")).toBe(
+      finalAnswerText({ rows, asked: ["city", "n"], basis }, "unused"),
+    )
+  })
 })
 // altimate_change end
diff --git a/packages/opencode/test/altimate/mcp-server/server.test.ts b/packages/opencode/test/altimate/mcp-server/server.test.ts
index ed4dde9b83..29d7c8722f 100644
--- a/packages/opencode/test/altimate/mcp-server/server.test.ts
+++ b/packages/opencode/test/altimate/mcp-server/server.test.ts
@@ -855,6 +855,20 @@ mcpTest("answer_commit elicits a reading and asks for a recompute when another i
   expect(asked).toBe(1)
 })
 
+// #1700: readings sent as a JSON string still trigger the reading question.
+mcpTest("answer_commit elicits a reading when readings arrive as a JSON string", async () => {
+  const { client, call } = await fixture(false, { elicitation: {} })
+  let asked = 0
+  client.setRequestHandler(ElicitRequestSchema, async () => {
+    asked++
+    return { action: "accept", content: { reading: "by id" } }
+  })
+  const result = await call("answer_commit", { ...twoReadings, readings: JSON.stringify(twoReadings.readings) })
+  expect(result.isError).toBe(true)
+  expect(text(result)).toContain("by id")
+  expect(asked).toBe(1)
+})
+
 mcpTest("answer_commit degrades silently when the client cancels or lacks elicitation", async () => {
   const cancelled = await fixture(false, { elicitation: {} })
   cancelled.client.setRequestHandler(ElicitRequestSchema, async () => ({ action: "cancel" }))
diff --git a/packages/opencode/test/altimate/tools/answer-commit.test.ts b/packages/opencode/test/altimate/tools/answer-commit.test.ts
index 6bcffbeaa8..dc0a02a675 100644
--- a/packages/opencode/test/altimate/tools/answer-commit.test.ts
+++ b/packages/opencode/test/altimate/tools/answer-commit.test.ts
@@ -50,6 +50,79 @@ describe("answer_commit boolean tolerance", () => {
   })
 })
 
+// altimate_change start — #1700: some models (e.g. Altimate Base) send list/object inputs as
+// JSON-encoded strings. Every commit was rejected with "fields: Invalid input: expected array,
+// received string", the model retried with the same shape, and the structured answer was lost.
+describe("answer_commit JSON-string tolerance (#1700)", () => {
+  const basis = "Counted orders at order grain using stg_orders.order_id."
+  const fields = [{ name: "Is anything wrong", value: "Yes, a join fan-out inflates number_of_orders" }]
+
+  test("fields sent as a JSON string renders the same answer as a real array", async () => {
+    const expected = AnswerCommit.renderAnswer({ basis, fields })
+    const payload = { basis, fields: JSON.stringify(fields) }
+    expect(AnswerCommit.renderAnswer(payload)).toBe(expected)
+    const tool = await initTool(AnswerCommit.AnswerCommitTool)
+    expect((await tool.execute(payload, context)).output).toBe(expected)
+  })
+
+  test("the exact shape from the #1700 trace is accepted", async () => {
+    // Pretty-printed with a space after the colon, as the model sent it.
+    const payload = {
+      basis,
+      asked: '["is anything wrong"]',
+      fields: '[{"name": "diagnosis", "value": 1}]',
+    }
+    const tool = await initTool(AnswerCommit.AnswerCommitTool)
+    expect((await tool.execute(payload, context)).output).toBe(`${basis}\n\n**Answer:** diagnosis: 1`)
+  })
+
+  test("rows and unresolved sent as JSON strings are accepted", async () => {
+    const rows = { columns: ["customer_id", "orders"], values: [[51, 3]] }
+    expect(AnswerCommit.renderAnswer({ basis, rows: JSON.stringify(rows) })).toBe(
+      AnswerCommit.renderAnswer({ basis, rows }),
+    )
+    const unresolved = { reason: "The warehouse was unreachable." }
+    expect(AnswerCommit.renderAnswer({ basis, unresolved: JSON.stringify(unresolved) })).toBe(
+      AnswerCommit.renderAnswer({ basis, unresolved }),
+    )
+  })
+
+  test("readings and asked sent as JSON strings are decoded into metadata", async () => {
+    const readings = [{ clause: "order counts", reading: "distinct orders", result_summary: "99", decided_by: "request" }]
+    const tool = await initTool(AnswerCommit.AnswerCommitTool)
+    const result = await tool.execute(
+      { basis, fields, asked: '["Is anything wrong"]', readings: JSON.stringify(readings) },
+      context,
+    )
+    expect(result.metadata.readings).toEqual(readings)
+    expect(result.metadata.asked).toEqual(["Is anything wrong"])
+    expect(result.output).toBe(AnswerCommit.renderAnswer({ basis, fields }))
+  })
+
+  test("a JSON string that is not valid JSON is still rejected with the normal error", async () => {
+    const payload = { basis, fields: '[{"name": "diagnosis", "value": 1}' }
+    expect(() => AnswerCommit.renderAnswer(payload)).toThrow(/fields.*expected array/i)
+    const tool = await initTool(AnswerCommit.AnswerCommitTool)
+    await rejects(tool.execute(payload, context), /fields.*expected array/i)
+  })
+
+  test("a plain string that does not look like JSON is still rejected", () => {
+    expect(() => AnswerCommit.renderAnswer({ basis, fields: "Yes, a join fan-out" })).toThrow(/fields.*expected array/i)
+  })
+
+  test("decoded values still go through full validation", () => {
+    expect(() => AnswerCommit.renderAnswer({ basis, fields: '[{"name": "x", "value": ""}]' })).toThrow(
+      /non-empty/i,
+    )
+  })
+
+  test("string answer values that look like JSON are left untouched", () => {
+    const value = '[{"looks": "like json"}]'
+    expect(AnswerCommit.renderAnswer({ basis, fields: [{ name: "Raw", value }] })).toContain(value)
+  })
+})
+// altimate_change end
+
 describe("answer_commit required basis", () => {
   test.each([undefined, false, "false", 0, null, "", [], {}, true, "true", "TRUE", "TrUe", 1].map((value) => [value]))(
     "answer_only %j requires basis for every answer shape",