diff --git a/packages/producer/src/services/distributed/plan.test.ts b/packages/producer/src/services/distributed/plan.test.ts
index 91e87d59f..42efb383b 100644
--- a/packages/producer/src/services/distributed/plan.test.ts
+++ b/packages/producer/src/services/distributed/plan.test.ts
@@ -209,3 +209,82 @@ describe("plan() — golden planDir + planHash determinism", () => {
     TIMEOUT_MS,
   );
 });
+
+describe("plan() — codec knob", () => {
+  const TIMEOUT_MS = 30_000;
+
+  it(
+    "defaults `codec` to h264 (libx264-software) for mp4",
+    async () => {
+      const planDir = join(runRoot, "plan-codec-default");
+      mkdirSync(planDir, { recursive: true });
+      await plan(projectDir, { fps: 30, width: 320, height: 240, format: "mp4" }, planDir);
+      const encoder = JSON.parse(
+        readFileSync(join(planDir, "meta", "encoder.json"), "utf-8"),
+      ) as Record<string, unknown>;
+      expect(encoder.encoder).toBe("libx264-software");
+      expect(encoder.pixelFormat).toBe("yuv420p");
+    },
+    TIMEOUT_MS,
+  );
+
+  it(
+    'maps `codec: "h265"` to libx265-software for mp4',
+    async () => {
+      const planDir = join(runRoot, "plan-codec-h265");
+      mkdirSync(planDir, { recursive: true });
+      await plan(
+        projectDir,
+        { fps: 30, width: 320, height: 240, format: "mp4", codec: "h265" },
+        planDir,
+      );
+      const encoder = JSON.parse(
+        readFileSync(join(planDir, "meta", "encoder.json"), "utf-8"),
+      ) as Record<string, unknown>;
+      expect(encoder.encoder).toBe("libx265-software");
+      // SDR 8-bit yuv420p, same as h264. Distributed mode is SDR-only —
+      // anyone reading this and tempted to bump to 10-bit, that's HDR
+      // territory and lives in v1.5.
+      expect(encoder.pixelFormat).toBe("yuv420p");
+    },
+    TIMEOUT_MS,
+  );
+
+  it("rejects `codec` with format other than mp4", async () => {
+    const planDir = join(runRoot, "plan-codec-bad-format");
+    mkdirSync(planDir, { recursive: true });
+    let caught: unknown;
+    try {
+      await plan(
+        projectDir,
+        // @ts-expect-error — runtime check is the test's purpose.
+        { fps: 30, width: 320, height: 240, format: "mov", codec: "h265" },
+        planDir,
+      );
+    } catch (err) {
+      caught = err;
+    }
+    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 d9a3e6f8c..40d7d979b 100644
--- a/packages/producer/src/services/distributed/plan.ts
+++ b/packages/producer/src/services/distributed/plan.ts
@@ -80,6 +80,17 @@ export interface DistributedRenderConfig {
    * supports both.
    */
   format: "mp4" | "mov" | "png-sequence";
+  /**
+   * Codec selection for `format: "mp4"`. `"h264"` (the default) → libx264 +
+   * yuv420p; `"h265"` → libx265 + yuv420p with closed-GOP keyint params
+   * (`min-keyint=N:scenecut=0:open-gop=0:repeat-headers=1`) so chunked
+   * concat-copy round-trips losslessly the same way h264 does. Ignored for
+   * `format: "mov"` (always ProRes 4444) and `format: "png-sequence"`
+   * (no encoder). Passing `codec` with a non-mp4 format throws at plan
+   * time so caller errors surface immediately rather than producing a
+   * silently-wrong planDir.
+   */
+  codec?: "h264" | "h265";
   quality?: "draft" | "standard" | "high";
   /** Constant-rate-factor override; mutually exclusive with `bitrate`. */
   crf?: number;
@@ -425,7 +436,7 @@ function buildLockedRenderConfig(input: {
   runtimeEnv: Record<string, string>;
 }): LockedRenderConfig {
   const { config, forceScreenshot, deviceScaleFactor, ffmpegVersion } = input;
-  const { encoder, pixelFormat, preset } = FORMAT_ENCODER_TABLE[config.format];
+  const { encoder, pixelFormat, preset } = resolveEncoderTriple(config);
   return {
     captureMode: forceScreenshot ? "screenshot" : "beginframe",
     forceScreenshot,
@@ -455,18 +466,50 @@ function buildLockedRenderConfig(input: {
 }
 
 /**
- * Per-format encoder + pixel-format + preset triple. Distributed mode is
- * SDR-only: H.264 8-bit for mp4, ProRes 4444 for mov, raw RGBA for
- * png-sequence.
+ * Resolve the encoder + pixel-format + preset triple for a distributed
+ * render. Distributed mode is SDR-only: H.264 or H.265 8-bit for mp4,
+ * ProRes 4444 for mov, raw RGBA for png-sequence.
+ *
+ * `config.codec` is consulted only when `config.format === "mp4"`. Passing
+ * `codec` with a non-mp4 format throws at plan time — surfaces the
+ * caller error immediately rather than producing a silently-wrong planDir
+ * whose chunk worker would override the codec choice.
  */
-const FORMAT_ENCODER_TABLE: Record<
-  DistributedRenderConfig["format"],
-  { encoder: LockedRenderConfig["encoder"]; pixelFormat: string; preset: string }
-> = {
-  mp4: { encoder: "libx264-software", pixelFormat: "yuv420p", preset: "medium" },
-  mov: { encoder: "prores-software", pixelFormat: "yuva444p10le", preset: "4444" },
-  "png-sequence": { encoder: "png-sequence", pixelFormat: "rgba", preset: "lossless" },
-};
+function resolveEncoderTriple(config: DistributedRenderConfig): {
+  encoder: LockedRenderConfig["encoder"];
+  pixelFormat: string;
+  preset: string;
+} {
+  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 ${JSON.stringify(codec)}. Omit codec to default to h264.`,
+      );
+    }
+    if (codec === "h265") {
+      return { encoder: "libx265-software", pixelFormat: "yuv420p", preset: "medium" };
+    }
+    return { encoder: "libx264-software", pixelFormat: "yuv420p", preset: "medium" };
+  }
+  if (config.codec !== undefined) {
+    throw new Error(
+      `[plan] DistributedRenderConfig.codec is only valid for format="mp4"; received ` +
+        `codec=${JSON.stringify(config.codec)} with format=${JSON.stringify(config.format)}. ` +
+        `Omit codec for non-mp4 formats — mov is always ProRes 4444 and png-sequence has no encoder.`,
+    );
+  }
+  if (config.format === "mov") {
+    return { encoder: "prores-software", pixelFormat: "yuva444p10le", preset: "4444" };
+  }
+  return { encoder: "png-sequence", pixelFormat: "rgba", preset: "lossless" };
+}
 
 /**
  * Activity A of the distributed render pipeline. Produces a self-contained
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 17bee2f33..0cc494d8b 100644
--- a/packages/producer/src/services/distributed/renderChunk.ts
+++ b/packages/producer/src/services/distributed/renderChunk.ts
@@ -290,6 +290,26 @@ 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
@@ -552,7 +572,8 @@ export async function renderChunk(
       const presetFormat: "mp4" | "mov" | "webm" = isPngSequence
         ? "mp4"
         : (plan.dimensions.format as "mp4" | "mov");
-      const preset = getEncoderPreset(job.config.quality, presetFormat, undefined);
+      const basePreset = getEncoderPreset(job.config.quality, presetFormat, undefined);
+      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`