diff --git a/packages/producer/src/services/distributed/plan.test.ts b/packages/producer/src/services/distributed/plan.test.ts
index 6e630ee59..42efb383b 100644
--- a/packages/producer/src/services/distributed/plan.test.ts
+++ b/packages/producer/src/services/distributed/plan.test.ts
@@ -267,4 +267,24 @@ describe("plan() — codec knob", () => {
     expect(caught).toBeInstanceOf(Error);
     expect((caught as Error).message).toMatch(/codec.*only valid for format="mp4"/);
   });
+
+  it("rejects unknown codec strings for format=mp4 (no silent fall-through to h264)", async () => {
+    const planDir = join(runRoot, "plan-codec-unknown");
+    mkdirSync(planDir, { recursive: true });
+    let caught: unknown;
+    try {
+      await plan(
+        projectDir,
+        // @ts-expect-error — runtime check is the test's purpose. Catches
+        // typos ("H265") and future codec additions ("av1") that a JS
+        // caller building config from JSON might pass.
+        { fps: 30, width: 320, height: 240, format: "mp4", codec: "h266" },
+        planDir,
+      );
+    } catch (err) {
+      caught = err;
+    }
+    expect(caught).toBeInstanceOf(Error);
+    expect((caught as Error).message).toMatch(/codec must be "h264" or "h265"/);
+  });
 });
diff --git a/packages/producer/src/services/distributed/plan.ts b/packages/producer/src/services/distributed/plan.ts
index d813e9eb3..40d7d979b 100644
--- a/packages/producer/src/services/distributed/plan.ts
+++ b/packages/producer/src/services/distributed/plan.ts
@@ -482,6 +482,17 @@ function resolveEncoderTriple(config: DistributedRenderConfig): {
 } {
   if (config.format === "mp4") {
     const codec = config.codec ?? "h264";
+    // Explicit unknown-codec throw rather than silent fall-through to h264.
+    // A JS caller building config from JSON who passes `codec: "h266"` or
+    // `codec: "H265"` (typo / wrong case) would otherwise produce h264
+    // output with no signal. The non-mp4-format branch below already throws
+    // for the symmetric "wrong combination" case — match that shape.
+    if (codec !== "h264" && codec !== "h265") {
+      throw new Error(
+        `[plan] DistributedRenderConfig.codec must be "h264" or "h265" for format="mp4"; ` +
+          `received ${JSON.stringify(codec)}. Omit codec to default to h264.`,
+      );
+    }
     if (codec === "h265") {
       return { encoder: "libx265-software", pixelFormat: "yuv420p", preset: "medium" };
     }
diff --git a/packages/producer/src/services/distributed/renderChunk.test.ts b/packages/producer/src/services/distributed/renderChunk.test.ts
index 375ff6ec1..61e79c5ea 100644
--- a/packages/producer/src/services/distributed/renderChunk.test.ts
+++ b/packages/producer/src/services/distributed/renderChunk.test.ts
@@ -29,6 +29,7 @@ import {
   PLAN_HASH_MISMATCH,
   renderChunk,
   RenderChunkValidationError,
+  resolvePresetForLockedEncoder,
 } from "./renderChunk.js";
 
 // Tiny fixture: 5 frames at 30fps. Captures finish in a few seconds on the
@@ -309,3 +310,41 @@ describe("renderChunk()", () => {
     TIMEOUT_MS,
   );
 });
+
+describe("resolvePresetForLockedEncoder", () => {
+  // Tiny fast tests for the codec-override helper. No Chrome, no ffmpeg —
+  // exists so a refactor that moves the override (e.g. into
+  // `getEncoderPreset` itself) gets caught here before the heavyweight
+  // Docker fixture is even run.
+  it("flips codec from h264 to h265 when encoder is libx265-software", () => {
+    const base = { preset: "medium", quality: 18, codec: "h264" as const, pixelFormat: "yuv420p" };
+    const out = resolvePresetForLockedEncoder(base, "libx265-software");
+    expect(out.codec).toBe("h265");
+    expect(out.preset).toBe("medium");
+    expect(out.quality).toBe(18);
+    expect(out.pixelFormat).toBe("yuv420p");
+  });
+
+  it("leaves the preset unchanged for libx264-software", () => {
+    const base = { preset: "medium", quality: 18, codec: "h264" as const, pixelFormat: "yuv420p" };
+    const out = resolvePresetForLockedEncoder(base, "libx264-software");
+    expect(out).toBe(base);
+  });
+
+  it("leaves the preset unchanged for prores-software", () => {
+    const base = {
+      preset: "4444",
+      quality: 18,
+      codec: "prores" as const,
+      pixelFormat: "yuva444p10le",
+    };
+    const out = resolvePresetForLockedEncoder(base, "prores-software");
+    expect(out).toBe(base);
+  });
+
+  it("leaves the preset unchanged for png-sequence", () => {
+    const base = { preset: "medium", quality: 18, codec: "h264" as const, pixelFormat: "yuv420p" };
+    const out = resolvePresetForLockedEncoder(base, "png-sequence");
+    expect(out).toBe(base);
+  });
+});
diff --git a/packages/producer/src/services/distributed/renderChunk.ts b/packages/producer/src/services/distributed/renderChunk.ts
index 717a5663a..980b5d8db 100644
--- a/packages/producer/src/services/distributed/renderChunk.ts
+++ b/packages/producer/src/services/distributed/renderChunk.ts
@@ -290,6 +290,27 @@ function hashChunkOutput(outputPath: string, kind: "file" | "frame-dir"): string
   return sha256Hex(lines.join("\0"));
 }
 
+/**
+ * Apply the planDir's locked-encoder choice on top of an
+ * `EncoderPreset` from `getEncoderPreset`. `getEncoderPreset` returns
+ * h265 only on the HDR branch, but distributed mode is SDR-only — for
+ * an `libx265-software` planDir we still need to flip the preset's
+ * codec to h265 so `runEncodeStage` invokes libx265. Exported so a
+ * unit test can pin the override independently of the heavyweight
+ * Docker fixture: a refactor that moves the override (e.g. into
+ * `getEncoderPreset` itself) shouldn't be able to silently regress
+ * the contract without a fast-test signal.
+ */
+export function resolvePresetForLockedEncoder<P extends { codec: "h264" | "h265" | "vp9" | "prores" }>(
+  basePreset: P,
+  lockedEncoder: LockedRenderConfig["encoder"],
+): P {
+  if (lockedEncoder === "libx265-software") {
+    return { ...basePreset, codec: "h265" as const };
+  }
+  return basePreset;
+}
+
 /**
  * Activity B: render a single chunk of the planDir. The `outputChunkPath`
  * argument is a file for mp4/mov outputs and a directory for png-sequence
@@ -553,13 +574,7 @@ export async function renderChunk(
         ? "mp4"
         : (plan.dimensions.format as "mp4" | "mov");
       const basePreset = getEncoderPreset(job.config.quality, presetFormat, undefined);
-      // Override the preset's codec from the planDir's locked encoder so
-      // h265 mp4 chunks call libx265 instead of getEncoderPreset's default
-      // h264. `getEncoderPreset` only returns h265 in HDR paths today;
-      // distributed mode is SDR-only, so the override here is the
-      // canonical way for chunks to honor `DistributedRenderConfig.codec`.
-      const preset: typeof basePreset =
-        encoder.encoder === "libx265-software" ? { ...basePreset, codec: "h265" } : basePreset;
+      const preset = resolvePresetForLockedEncoder(basePreset, encoder.encoder);
       const effectiveQuality = encoder.crf ?? preset.quality;
       const effectiveBitrate = encoder.crf != null ? undefined : encoder.bitrate;
       // For non-pngseq, encodeStage writes to `outputPath` when `isPngSequence`