diff --git a/packages/shared/src/tx/escrow/postChatEscrow.ts b/packages/shared/src/tx/escrow/postChatEscrow.ts
index fba3f53..a66dddd 100644
--- a/packages/shared/src/tx/escrow/postChatEscrow.ts
+++ b/packages/shared/src/tx/escrow/postChatEscrow.ts
@@ -42,6 +42,7 @@ import { loadBlueprint } from "../blueprint.js";
 import { encodeTxBody, sha256Hex } from "../internal/testTxBody.js";
 import { mockSlotToWallclockMs, NETWORK_BUFFER_MS } from "../internal/constants.js";
 import { detectCborBackend } from "../internal/cborBackend.js";
+import { escrowLockFloor } from "../internal/minAdaFloor.js";
 import type { LiveOgmiosProvider } from "../../chain/LiveOgmiosProvider.js";
 
 export interface PostChatEscrowParams {
@@ -132,7 +133,7 @@ export async function buildPostChatEscrowTx(
   const requestSpecHash = sha256Utf8Hex(requestSpecCanonical);
   const promptHash = chatSessionPromptHash({ session_nonce });
 
-  const totalLocked =
+  const economicTotal =
     advertDatum.price_lovelace +
     advertDatum.buyer_bond_lovelace +
     advertDatum.supplier_bond_lovelace;
@@ -154,6 +155,11 @@ export async function buildPostChatEscrowTx(
     state: "Open",
   };
 
+  // Lock enough that the escrow output satisfies min-ada in its LARGEST
+  // future state (Submitted) - value_equal on Claim/Submit forbids any
+  // later bump (min-ada floor).
+  const totalLocked = escrowLockFloor(escrowDatum, economicTotal);
+
   // Live path: reuse the capability-agnostic chat builder (messages = []).
   // The on-chain commitment is the escrowDatum (carrying our session-init
   // prompt_hash); messages are only used for the mock cbor body, which we omit.
diff --git a/packages/shared/src/tx/escrow/postEscrow.ts b/packages/shared/src/tx/escrow/postEscrow.ts
index 2cd53b3..9fe8094 100644
--- a/packages/shared/src/tx/escrow/postEscrow.ts
+++ b/packages/shared/src/tx/escrow/postEscrow.ts
@@ -38,6 +38,7 @@ import { loadBlueprint } from "../blueprint.js";
 import { encodeTxBody, sha256Hex } from "../internal/testTxBody.js";
 import { mockSlotToWallclockMs, NETWORK_BUFFER_MS } from "../internal/constants.js";
 import { detectCborBackend } from "../internal/cborBackend.js";
+import { escrowLockFloor } from "../internal/minAdaFloor.js";
 import type { LiveOgmiosProvider } from "../../chain/LiveOgmiosProvider.js";
 
 export interface PostEscrowParams {
@@ -129,7 +130,7 @@ export async function buildPostEscrowTx(
   // supplier-side validation in M1-C can verify role/content/system integrity.
   const promptHash = sha256Utf8Hex(canonicalize(messages));
 
-  const totalLocked =
+  const economicTotal =
     advertDatum.price_lovelace +
     advertDatum.buyer_bond_lovelace +
     advertDatum.supplier_bond_lovelace;
@@ -151,6 +152,11 @@ export async function buildPostEscrowTx(
     state: "Open",
   };
 
+  // Lock enough that the escrow output satisfies min-ada in its LARGEST
+  // future state (Submitted) - value_equal on Claim/Submit forbids any
+  // later bump (min-ada floor).
+  const totalLocked = escrowLockFloor(escrowDatum, economicTotal);
+
   // Live path: produce real Cardano CBOR via lucid-evolution.
   if (detectCborBackend(chain) === "live") {
     const liveCborPath = "../internal/liveCbor.js";
diff --git a/packages/shared/src/tx/escrow/postTtsEscrow.ts b/packages/shared/src/tx/escrow/postTtsEscrow.ts
index 932b532..0b5bcb7 100644
--- a/packages/shared/src/tx/escrow/postTtsEscrow.ts
+++ b/packages/shared/src/tx/escrow/postTtsEscrow.ts
@@ -31,6 +31,7 @@ import { loadBlueprint } from "../blueprint.js";
 import { encodeTxBody, sha256Hex } from "../internal/testTxBody.js";
 import { mockSlotToWallclockMs, NETWORK_BUFFER_MS } from "../internal/constants.js";
 import { detectCborBackend } from "../internal/cborBackend.js";
+import { escrowLockFloor } from "../internal/minAdaFloor.js";
 import type { LiveOgmiosProvider } from "../../chain/LiveOgmiosProvider.js";
 
 /** TTS request envelope. The buyer-side hash and the supplier-side hash
@@ -149,7 +150,7 @@ export async function buildPostTtsEscrowTx(
   const requestSpecHash = sha256Utf8Hex(requestSpecCanonical);
   const promptHash = ttsPromptHash(request);
 
-  const totalLocked =
+  const economicTotal =
     advertDatum.price_lovelace +
     advertDatum.buyer_bond_lovelace +
     advertDatum.supplier_bond_lovelace;
@@ -171,6 +172,11 @@ export async function buildPostTtsEscrowTx(
     state: "Open",
   };
 
+  // Lock enough that the escrow output satisfies min-ada in its LARGEST
+  // future state (Submitted) - value_equal on Claim/Submit forbids any
+  // later bump (min-ada floor).
+  const totalLocked = escrowLockFloor(escrowDatum, economicTotal);
+
   // Live path: reuse the chat builder. The live cbor path is already
   // capability-agnostic — `messages` is `void`-marked there. We pass an
   // empty array so the type checker is satisfied; the actual on-chain
diff --git a/packages/shared/src/tx/internal/minAdaFloor.ts b/packages/shared/src/tx/internal/minAdaFloor.ts
new file mode 100644
index 0000000..5c86ee2
--- /dev/null
+++ b/packages/shared/src/tx/internal/minAdaFloor.ts
@@ -0,0 +1,67 @@
+/**
+ * minAdaFloor.ts — min-ada floor for the escrow's LARGEST future datum.
+ *
+ * The escrow validator requires value_equal(continuing, in) on Claim and
+ * Submit, so the lovelace locked at POST time must already satisfy min-ada
+ * for every datum the output will ever carry. The largest is the Submitted
+ * state: submitted_at None -> Some(POSIX ms) and result_receipt_hash
+ * None -> Some(32 bytes) add ~43 bytes over Open/Claimed. If the posted
+ * lovelace sits below that state's min-ada, lucid bumps the Submit
+ * continuing output to min-ada, value_equal fails, and the validator
+ * errors with no traces (the 2026-08-07 mainnet incident: the 33-byte
+ * "ocr.page.extract.chandra-ocr-2.v1" capability_id pushed the Submitted
+ * output 11,030 lovelace over a 2,200,000 lock; the 20-byte chat
+ * capability stayed 49,310 under, which is why only OCR Submits failed).
+ *
+ * escrowLockFloor() returns the lovelace to lock: the economic total
+ * (payment + both bonds), raised to the Submitted-state min-ada plus a
+ * safety margin when the total alone is too small.
+ */
+
+import type { EscrowDatum } from "../../cbor/types.js";
+import { encodeEscrowDatum } from "../../cbor/EscrowDatum.js";
+
+/** Conway/Babbage utxoCostPerByte on Vector mainnet (protocol params). */
+const UTXO_COST_PER_BYTE = 4310n;
+
+/** Ledger constant: min-ada = (160 + serialized output bytes) * costPerByte. */
+const OUTPUT_OVERHEAD_BYTES = 160n;
+
+/** Covers CBOR framing drift between this estimate and the ledger's exact
+ * serialization (address, coin width, datum wrapper). Generous on purpose:
+ * 50k lovelace = 0.05 AP3X, dwarfed by per-leg tx fees. */
+const SAFETY_MARGIN_LOVELACE = 50_000n;
+
+/** Serialized-output byte estimate for an enterprise script address output
+ * carrying `lovelace` and an inline datum of `datumBytes`. Mirrors the
+ * arithmetic validated against mainnet on 2026-08-07 (minada-calc). */
+function estimateOutputBytes(datumBytes: number, lovelace: bigint): bigint {
+  const addr = 1n + 29n;
+  const coin = 1n + (lovelace > 0xffffffffn ? 8n : 4n);
+  const datumField = 1n + 2n + 3n + BigInt(datumBytes);
+  const mapOverhead = 3n;
+  return addr + coin + datumField + mapOverhead;
+}
+
+/** Min-ada for the escrow output when it carries `datum`. */
+export function minAdaForEscrowDatum(datum: EscrowDatum, lovelace: bigint): bigint {
+  const datumBytes = encodeEscrowDatum(datum).length / 2;
+  const size = estimateOutputBytes(datumBytes, lovelace);
+  return (OUTPUT_OVERHEAD_BYTES + size) * UTXO_COST_PER_BYTE;
+}
+
+/** The lovelace a POST must lock so the escrow output satisfies min-ada in
+ * every reachable state, including Submitted. `economicTotal` is
+ * payment + buyer_bond + supplier_bond. */
+export function escrowLockFloor(openDatum: EscrowDatum, economicTotal: bigint): bigint {
+  const submittedShape: EscrowDatum = {
+    ...openDatum,
+    // Representative worst-case Submit fields: a 13-digit POSIX ms stamp
+    // and a 32-byte receipt hash — sizes, not values, matter here.
+    submitted_at: openDatum.deliver_by,
+    result_receipt_hash: "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+    state: "Submitted",
+  };
+  const floor = minAdaForEscrowDatum(submittedShape, economicTotal) + SAFETY_MARGIN_LOVELACE;
+  return floor > economicTotal ? floor : economicTotal;
+}
diff --git a/tests/unit/tx-minada-floor.test.ts b/tests/unit/tx-minada-floor.test.ts
new file mode 100644
index 0000000..51cf11f
--- /dev/null
+++ b/tests/unit/tx-minada-floor.test.ts
@@ -0,0 +1,84 @@
+/**
+ * tx-minada-floor.test.ts — the escrow min-ada floor.
+ *
+ * The escrow output locks payment + both bonds and the validator requires
+ * exact value preservation (value_equal) on Claim and Submit. Submit grows
+ * the inline datum (submitted_at None->Some, result_receipt_hash None->Some
+ * 32 bytes), so the lovelace locked at POST time must already cover min-ada
+ * for the Submitted state. A capability_id long enough to push the
+ * Submitted output's min-ada past the economic total made lucid bump the
+ * continuing output, value_equal failed, and the validator errored with no
+ * traces — so escrows with short capability ids settled and long ones never
+ * could. escrowLockFloor closes that by locking the max of the economic
+ * total and the Submitted-state min-ada plus a margin.
+ */
+
+import { describe, it, expect } from "vitest";
+import {
+  escrowLockFloor,
+  minAdaForEscrowDatum,
+} from "../../packages/shared/src/tx/internal/minAdaFloor.js";
+import type { EscrowDatum } from "../../packages/shared/src/cbor/types.js";
+
+function openDatum(capabilityId: string): EscrowDatum {
+  return {
+    buyer_pkh: "3b2fdaed0398485ff35887fb7d0ad2025348864407854f7831dc6a55",
+    supplier_pkh: "401af418a7e6fb106277292881fcdeaa1cc15b5b112e9c8bdb2d46c9",
+    advert_ref: { txHash: "a6".repeat(32), index: 0 },
+    capability_id: capabilityId,
+    request_spec_hash: "1b".repeat(32),
+    prompt_hash: "23".repeat(32),
+    payment_lovelace: 200_000n,
+    buyer_bond_lovelace: 1_000_000n,
+    supplier_bond_lovelace: 1_000_000n,
+    deliver_by: 1_786_105_955_257,
+    posted_at: 1_786_105_625_257,
+    submitted_at: null,
+    result_receipt_hash: null,
+    state: "Open",
+  };
+}
+
+const ECONOMIC_TOTAL = 2_200_000n; // 0.2 payment + 1.0 + 1.0 bonds
+
+describe("escrowLockFloor", () => {
+  it("barely lifts a short-capability escrow (margin only, no capability-driven bump)", () => {
+    // The 20-byte chat capability's Submitted-state min-ada is ~2.15M, under
+    // the 2.2M economic total; only the safety margin nudges the lock, so it
+    // stays within a fraction of a percent of payment + bonds.
+    const datum = openDatum("llm.text.generate.v1");
+    const floor = escrowLockFloor(datum, ECONOMIC_TOTAL);
+    expect(floor).toBeGreaterThanOrEqual(ECONOMIC_TOTAL);
+    expect(floor).toBeLessThan(ECONOMIC_TOTAL + 100_000n); // << the OCR lift
+  });
+
+  it("raises a long-capability escrow above the economic total", () => {
+    // The 33-byte OCR capability that could not settle: its Submitted-state
+    // output needs more than 2.2M, so the floor must lift the lock.
+    const datum = openDatum("ocr.page.extract.chandra-ocr-2.v1");
+    const floor = escrowLockFloor(datum, ECONOMIC_TOTAL);
+    expect(floor).toBeGreaterThan(ECONOMIC_TOTAL);
+    // The locked value must satisfy the Submitted-state min-ada it is sized for.
+    const submitted: EscrowDatum = {
+      ...datum,
+      submitted_at: datum.deliver_by,
+      result_receipt_hash: "ff".repeat(32),
+      state: "Submitted",
+    };
+    expect(floor).toBeGreaterThanOrEqual(minAdaForEscrowDatum(submitted, floor));
+  });
+
+  it("never returns less than the economic total", () => {
+    for (const cap of ["a", "x".repeat(64), "tts.speak.v1"]) {
+      expect(escrowLockFloor(openDatum(cap), ECONOMIC_TOTAL)).toBeGreaterThanOrEqual(
+        ECONOMIC_TOTAL,
+      );
+    }
+  });
+
+  it("the floor is monotonic in capability-id length", () => {
+    const short = escrowLockFloor(openDatum("a.b.v1"), ECONOMIC_TOTAL);
+    const long = escrowLockFloor(openDatum("a.very.long.capability.id.that.keeps.going.v1"), ECONOMIC_TOTAL);
+    expect(long).toBeGreaterThanOrEqual(short);
+  });
+});