diff --git a/packages/lint/src/rules/media.test.ts b/packages/lint/src/rules/media.test.ts
index 1d621e8a51..ec14b9d421 100644
--- a/packages/lint/src/rules/media.test.ts
+++ b/packages/lint/src/rules/media.test.ts
@@ -379,6 +379,62 @@ describe("media rules", () => {
     expect(finding).toBeUndefined();
   });
 
+  it("warns on raw src mutations targeting existing video and audio", async () => {
+    const html = `
+<html><body>
+  <div data-composition-id="c1" data-width="1920" data-height="1080">
+    <video id="clip" src="video-a.mp4"></video>
+    <audio id="voice" src="audio-a.wav"></audio>
+  </div>
+  <script>
+    const clip = document.getElementById("clip");
+    clip.src = "video-b.mp4";
+    document.querySelector("#voice").setAttribute("src", "audio-b.wav");
+  </script>
+</body></html>`;
+
+    const result = await lintHyperframeHtml(html);
+    const findings = result.findings.filter(
+      (finding) => finding.code === "media_runtime_src_mutation",
+    );
+    expect(findings).toHaveLength(2);
+    expect(findings.map((finding) => finding.elementId).sort()).toEqual(["clip", "voice"]);
+    expect(findings.every((finding) => finding.fixHint?.includes("data-var-src"))).toBe(true);
+  });
+
+  it("does not flag img or script src mutations", async () => {
+    const html = `
+<html><body>
+  <div data-composition-id="c1" data-width="1920" data-height="1080">
+    <img id="poster" src="a.png"><script id="loader"></script>
+  </div>
+  <script>
+    document.getElementById("poster").src = "b.png";
+    document.getElementById("loader").setAttribute("src", "loader-b.js");
+  </script>
+</body></html>`;
+
+    const result = await lintHyperframeHtml(html);
+    expect(
+      result.findings.find((finding) => finding.code === "media_runtime_src_mutation"),
+    ).toBeUndefined();
+  });
+
+  it("warns when a source child of existing media is mutated", async () => {
+    const html = `
+<html><body>
+  <div data-composition-id="c1" data-width="1920" data-height="1080">
+    <video id="clip"><source id="clip-source" src="video-a.mp4"></video>
+  </div>
+  <script>document.getElementById("clip-source").src = "video-b.mp4";</script>
+</body></html>`;
+
+    const result = await lintHyperframeHtml(html);
+    expect(
+      result.findings.find((finding) => finding.code === "media_runtime_src_mutation"),
+    ).toMatchObject({ severity: "warning", elementId: "clip" });
+  });
+
   it("does not flag <video> inside a sub-composition (runtime drives nested media)", async () => {
     // The runtime's global media sweep (querySelectorAll("video, audio")) drives
     // media at any nesting depth, and startResolver re-bases each nested clip's
diff --git a/packages/lint/src/rules/media.ts b/packages/lint/src/rules/media.ts
index a2509ad09c..60d792b135 100644
--- a/packages/lint/src/rules/media.ts
+++ b/packages/lint/src/rules/media.ts
@@ -1,6 +1,8 @@
 import type { LintContext, HyperframeLintFinding, OpenTag } from "../context";
 import { readAttr, readDecodedAttr, stripJsComments, truncateSnippet, isMediaTag } from "../utils";
 import { validateColorGradingContract } from "@hyperframes/parsers/color-grading-contract";
+import { extractMediaSrcMutations } from "@hyperframes/parsers";
+import { parseHTML } from "linkedom";
 
 /**
  * Does the GSAP call that names `#id` also set `volume` in the same call?
@@ -322,6 +324,46 @@ function findImperativeMediaControlFindings(ctx: LintContext): HyperframeLintFin
   return findings;
 }
 
+function findRuntimeMediaSrcMutationFindings(ctx: LintContext): HyperframeLintFinding[] {
+  const { document } = parseHTML(ctx.source);
+  const findings: HyperframeLintFinding[] = [];
+  for (const script of ctx.scripts) {
+    for (const mutation of extractMediaSrcMutations(script.content)) {
+      let targets: Element[];
+      try {
+        const id = /^#[A-Za-z_][\w-]*$/.test(mutation.selector) ? mutation.selector.slice(1) : null;
+        const idTarget = id ? document.getElementById(id) : null;
+        targets = id
+          ? idTarget
+            ? [idTarget]
+            : []
+          : [...document.querySelectorAll(mutation.selector)];
+      } catch {
+        continue;
+      }
+      const mediaTargets = targets
+        .map((element) => {
+          const name = element.tagName.toLowerCase();
+          if (name === "video" || name === "audio") return element;
+          return name === "source" ? element.closest("video, audio") : null;
+        })
+        .filter((element): element is Element => element !== null);
+      if (mediaTargets.length === 0) continue;
+      findings.push({
+        code: "media_runtime_src_mutation",
+        severity: "warning",
+        message: `Inline script mutates the source of existing managed media via ${mutation.operation === "src_assignment" ? ".src assignment" : "setAttribute('src', ...)"}. Browser probing can reconcile synchronous writes, but external or delayed writes can still diverge between preview and extraction.`,
+        elementId: mediaTargets[0]?.getAttribute("id") || undefined,
+        selector: mutation.selector,
+        fixHint:
+          "Author the final static src, or bind data-var-src to a declared image/string variable so the selected source is applied before media discovery and extraction.",
+        snippet: truncateSnippet(mutation.raw),
+      });
+    }
+  }
+  return findings;
+}
+
 export const mediaRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
   // duplicate_media_id + duplicate_media_discovery_risk
   ({ tags }) => {
@@ -727,6 +769,7 @@ export const mediaRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> =
 
   // imperative_media_control
   findImperativeMediaControlFindings,
+  findRuntimeMediaSrcMutationFindings,
 
   // audio_volume_double_automation
   findVolumeDoubleAutomationFindings,
diff --git a/packages/parsers/src/index.ts b/packages/parsers/src/index.ts
index 2baf100c85..7570ac5354 100644
--- a/packages/parsers/src/index.ts
+++ b/packages/parsers/src/index.ts
@@ -14,6 +14,7 @@ export * from "./compositionContract.js";
 // browser-safe.
 export { decodeUrlPathVariants } from "./utils/urlPath.js";
 export { scanVariableUsage, type VariableUsageScan } from "./variableUsage.js";
+export { extractMediaSrcMutations, type MediaSrcMutation } from "./mediaSrcMutation.js";
 export {
   FONT_ALIAS_MAP,
   FONT_ALIAS_KEYS,
diff --git a/packages/parsers/src/mediaSrcMutation.ts b/packages/parsers/src/mediaSrcMutation.ts
new file mode 100644
index 0000000000..23812f8374
--- /dev/null
+++ b/packages/parsers/src/mediaSrcMutation.ts
@@ -0,0 +1,85 @@
+import * as acorn from "acorn";
+import * as acornWalk from "acorn-walk";
+
+export interface MediaSrcMutation {
+  selector: string;
+  operation: "src_assignment" | "set_attribute";
+  raw: string;
+}
+
+function parseProgram(script: string): any {
+  try {
+    return acorn.parse(script, { ecmaVersion: "latest", sourceType: "script" });
+  } catch {
+    return acorn.parse(script, { ecmaVersion: "latest", sourceType: "module" });
+  }
+}
+
+function literalString(node: any): string | undefined {
+  if (node?.type === "Literal" && typeof node.value === "string") return node.value;
+  if (node?.type === "TemplateLiteral" && node.expressions?.length === 0) {
+    return node.quasis?.[0]?.value?.cooked;
+  }
+  return undefined;
+}
+
+function memberName(node: any): string | undefined {
+  if (node?.type !== "MemberExpression") return undefined;
+  if (!node.computed && node.property?.type === "Identifier") return node.property.name;
+  return literalString(node.property);
+}
+
+function lookupSelector(node: any, bindings: ReadonlyMap<string, string>): string | undefined {
+  if (node?.type === "Identifier") return bindings.get(node.name);
+  if (node?.type !== "CallExpression" || node.callee?.type !== "MemberExpression") {
+    return undefined;
+  }
+  const method = memberName(node.callee);
+  if (method !== "getElementById" && method !== "querySelector") return undefined;
+  const value = literalString(node.arguments?.[0]);
+  if (!value) return undefined;
+  return method === "getElementById" ? `#${value}` : value;
+}
+
+/** Find literal source writes whose target is a statically resolvable DOM lookup. */
+export function extractMediaSrcMutations(script: string): MediaSrcMutation[] {
+  try {
+    const ast = parseProgram(script);
+    const bindings = new Map<string, string>();
+    acornWalk.simple(ast, {
+      VariableDeclarator(node: any) {
+        if (node.id?.type !== "Identifier") return;
+        const selector = lookupSelector(node.init, bindings);
+        if (selector) bindings.set(node.id.name, selector);
+      },
+    });
+
+    const mutations: MediaSrcMutation[] = [];
+    acornWalk.simple(ast, {
+      AssignmentExpression(node: any) {
+        if (node.operator !== "=" || memberName(node.left) !== "src") return;
+        const selector = lookupSelector(node.left.object, bindings);
+        if (!selector) return;
+        mutations.push({
+          selector,
+          operation: "src_assignment",
+          raw: script.slice(node.start, node.end),
+        });
+      },
+      CallExpression(node: any) {
+        if (memberName(node.callee) !== "setAttribute") return;
+        if (literalString(node.arguments?.[0])?.toLowerCase() !== "src") return;
+        const selector = lookupSelector(node.callee.object, bindings);
+        if (!selector) return;
+        mutations.push({
+          selector,
+          operation: "set_attribute",
+          raw: script.slice(node.start, node.end),
+        });
+      },
+    });
+    return mutations;
+  } catch {
+    return [];
+  }
+}
diff --git a/packages/producer/src/services/render/observability.ts b/packages/producer/src/services/render/observability.ts
index 6dbbc7143a..c757b986bd 100644
--- a/packages/producer/src/services/render/observability.ts
+++ b/packages/producer/src/services/render/observability.ts
@@ -174,7 +174,7 @@ export interface RenderExtractionObservability {
    * a coarse proxy for the ts=1784144554 field signal shape (147-clip
    * composition, 130 word-level caption divs authored-clip-count-scaled
    * failure). Static scan; dynamic script-inserted timed clips land in
-   * the probe-stage's `hasRuntimeInsertedMedia` path (PR #2474).
+   * the probe-stage's `hasRuntimeMediaChanges` path (PR #2474).
    */
   authoredTimedClipCount?: number;
 }
diff --git a/packages/producer/src/services/render/stages/probeStage.test.ts b/packages/producer/src/services/render/stages/probeStage.test.ts
index dd0052388d..08caa14f29 100644
--- a/packages/producer/src/services/render/stages/probeStage.test.ts
+++ b/packages/producer/src/services/render/stages/probeStage.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it, mock } from "bun:test";
+import { createHash } from "node:crypto";
 import {
   hasAutoStartVideos,
   hasScriptedAudioVolumeAutomation,
@@ -583,6 +584,113 @@ describe("runProbeStage — forceScreenshot threading", () => {
     expect(capturedCfgs.length).toBeGreaterThan(0);
   });
 
+  it("probes and reconciles synchronous src mutations on existing video and audio", async () => {
+    resetRetryMocks();
+    capturedCfgs.length = 0;
+    const hash = (value: string) => createHash("sha256").update(value).digest("hex");
+    const videoA = `assets/video-a-${hash("sanitized-video-a")}.mp4`;
+    const videoB = `assets/video-b-${hash("sanitized-video-b")}.mp4`;
+    const audioA = `assets/audio-a-${hash("sanitized-audio-a")}.wav`;
+    const audioB = `assets/audio-b-${hash("sanitized-audio-b")}.wav`;
+    expect(hash("sanitized-video-a")).not.toBe(hash("sanitized-video-b"));
+    expect(hash("sanitized-audio-a")).not.toBe(hash("sanitized-audio-b"));
+    browserMediaResults = [
+      {
+        id: "clip",
+        tagName: "video",
+        src: videoB,
+        start: 0,
+        end: 5,
+        duration: 5,
+        mediaStart: 0,
+        loop: false,
+        hasAudio: false,
+        volume: 1,
+        muted: true,
+      },
+      {
+        id: "voice",
+        tagName: "audio",
+        src: audioB,
+        start: 0,
+        end: 5,
+        duration: 5,
+        mediaStart: 0,
+        loop: false,
+        hasAudio: true,
+        volume: 1,
+        muted: false,
+      },
+    ];
+    const { runProbeStage } = await import("./probeStage.js");
+    const input = makeProbeInput({});
+    input.composition.duration = 5;
+    input.composition.videos.push({
+      id: "clip",
+      src: videoA,
+      start: 0,
+      end: 5,
+      mediaStart: 0,
+      loop: false,
+      hasAudio: false,
+    });
+    input.composition.audios.push({
+      id: "voice",
+      src: audioA,
+      start: 0,
+      end: 5,
+      mediaStart: 0,
+      layer: 0,
+      volume: 1,
+      type: "audio",
+    });
+    input.compiled.html = `<video id="clip" src="${videoA}"></video>
+      <audio id="voice" src="${audioA}"></audio>
+      <script>
+        const clip = document.getElementById("clip");
+        clip.src = ${JSON.stringify(videoB)};
+        document.querySelector("#voice").setAttribute("src", ${JSON.stringify(audioB)});
+      </script>`;
+
+    await runProbeStage(input);
+
+    expect(capturedCfgs.length).toBeGreaterThan(0);
+    expect(input.composition.videos[0]?.src).toBe(videoB);
+    expect(input.composition.audios[0]?.src).toBe(audioB);
+    expect(mediaPreflightComposition).toBe(input.composition);
+  });
+
+  it("does not probe for img or script src mutations", async () => {
+    resetRetryMocks();
+    capturedCfgs.length = 0;
+    const { runProbeStage } = await import("./probeStage.js");
+    const input = makeProbeInput({});
+    input.composition.duration = 5;
+    input.compiled.html = `<img id="poster" src="a.png"><script id="loader"></script>
+      <script>
+        document.getElementById("poster").src = "b.png";
+        document.getElementById("loader").setAttribute("src", "loader-b.js");
+      </script>`;
+
+    await runProbeStage(input);
+
+    expect(capturedCfgs).toHaveLength(0);
+  });
+
+  it("probes when a source child of existing media is mutated", async () => {
+    resetRetryMocks();
+    capturedCfgs.length = 0;
+    const { runProbeStage } = await import("./probeStage.js");
+    const input = makeProbeInput({});
+    input.composition.duration = 5;
+    input.compiled.html = `<video id="clip"><source id="clip-source" src="video-a.mp4"></video>
+      <script>document.getElementById("clip-source").src = "video-b.mp4";</script>`;
+
+    await runProbeStage(input);
+
+    expect(capturedCfgs.length).toBeGreaterThan(0);
+  });
+
   it("launches a probe when a static-duration composition uses the Audio constructor", async () => {
     capturedCfgs.length = 0;
     const { runProbeStage } = await import("./probeStage.js");
diff --git a/packages/producer/src/services/render/stages/probeStage.ts b/packages/producer/src/services/render/stages/probeStage.ts
index 3e234f24a2..429c915872 100644
--- a/packages/producer/src/services/render/stages/probeStage.ts
+++ b/packages/producer/src/services/render/stages/probeStage.ts
@@ -43,6 +43,7 @@ import {
   probeBeginFrameLiveness,
 } from "@hyperframes/engine";
 import { fpsToNumber } from "@hyperframes/core";
+import { extractMediaSrcMutations } from "@hyperframes/parsers";
 import type { CompiledComposition } from "../../htmlCompiler.js";
 import {
   discoverMediaFromBrowser,
@@ -193,16 +194,41 @@ function reconcileBrowserMediaEnd(
  * extraction, even when the root duration is already known. External script
  * sources have no inline text to inspect and remain a known heuristic gap.
  */
-function hasRuntimeInsertedMedia(html: string): boolean {
+function hasRuntimeMediaChanges(html: string): boolean {
   const { document } = parseHTML(html);
-  const scriptBodies = [...document.querySelectorAll("script")]
-    .map((script) => script.textContent ?? "")
-    .join("\n");
-  return (
-    /\bcreateElement\s*\(\s*["'`](?:video|audio)["'`]\s*\)/i.test(scriptBodies) ||
-    /\bnew\s+(?:Audio|Video)\s*\(/i.test(scriptBodies) ||
-    /<(?:video|audio)\b[^>]*>/i.test(scriptBodies)
+  const scriptBodies = [...document.querySelectorAll("script")].map(
+    (script) => script.textContent ?? "",
+  );
+  const insertedMedia = scriptBodies.some(
+    (script) =>
+      /\bcreateElement\s*\(\s*["'`](?:video|audio)["'`]\s*\)/i.test(script) ||
+      /\bnew\s+(?:Audio|Video)\s*\(/i.test(script) ||
+      /<(?:video|audio)\b[^>]*>/i.test(script),
   );
+  if (insertedMedia) return true;
+
+  const isManagedMedia = (element: Element): boolean => {
+    const name = element.tagName.toLowerCase();
+    if (name === "video" || name === "audio") return true;
+    return name === "source" && element.closest("video, audio") !== null;
+  };
+  for (const script of scriptBodies) {
+    for (const mutation of extractMediaSrcMutations(script)) {
+      try {
+        const id = /^#[A-Za-z_][\w-]*$/.test(mutation.selector) ? mutation.selector.slice(1) : null;
+        const idTarget = id ? document.getElementById(id) : null;
+        const targets = id
+          ? idTarget
+            ? [idTarget]
+            : []
+          : [...document.querySelectorAll(mutation.selector)];
+        if (targets.some(isManagedMedia)) return true;
+      } catch {
+        // Invalid selectors are diagnosed by lint and cannot prove a media target here.
+      }
+    }
+  }
+  return false;
 }
 
 /**
@@ -214,9 +240,9 @@ function hasRuntimeInsertedMedia(html: string): boolean {
  * `resolveCompositionElementCount` / `resolveDeShortBand`). Notably NONE of
  * these conditions fire for a known-duration, media-free composition that
  * builds thousands of `div`/`span` nodes in its own init script —
- * `hasRuntimeInsertedMedia` matches only `createElement("video"|"audio")` —
- * so that shape is measured statically and must never reach the band's
- * `applied` cohort (review finding, R4).
+ * `hasRuntimeMediaChanges` matches only media creation/source changes — so
+ * that shape is measured statically and must never reach the band's `applied`
+ * cohort (review finding, R4).
  */
 export function probeRequiresBrowser(args: {
   durationSeconds: number;
@@ -269,7 +295,7 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
     composition.audios.length,
   );
   const hasVariableMedia = hasVariableBoundMedia(compiled.html, job.config.variables);
-  const hasInsertedMedia = hasRuntimeInsertedMedia(compiled.html);
+  const hasInsertedMedia = hasRuntimeMediaChanges(compiled.html);
   const needsBrowser = probeRequiresBrowser({
     durationSeconds: composition.duration,
     unresolvedCompositionCount: compiled.unresolvedCompositions.length,
@@ -286,7 +312,7 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
       reasons.push(`${compiled.unresolvedCompositions.length} unresolved composition(s)`);
     if (hasAutoStart) reasons.push("auto-start video(s)");
     if (hasScriptedAudio) reasons.push("scripted audio volume");
-    if (hasInsertedMedia) reasons.push("runtime-inserted media");
+    if (hasInsertedMedia) reasons.push("runtime-created or source-mutated media");
     if (hasVariableMedia) reasons.push("variable-bound media source(s)");
 
     log.info("Launching browser for composition probe...", {
diff --git a/packages/producer/src/services/render/videoFrameCoverage.ts b/packages/producer/src/services/render/videoFrameCoverage.ts
index 02b302bd6a..d31e5aa9db 100644
--- a/packages/producer/src/services/render/videoFrameCoverage.ts
+++ b/packages/producer/src/services/render/videoFrameCoverage.ts
@@ -1,7 +1,7 @@
 /**
  * Per-clip render-time frame-coverage accounting + threshold fail-loud gate.
  *
- * Sibling to #2474's `hasRuntimeInsertedMedia` probe: that PR guarantees the
+ * Sibling to #2474's `hasRuntimeMediaChanges` probe: that PR guarantees the
  * DISCOVERY of runtime-inserted media (so the browser probe launches and
  * reconciles element identity). This module owns the DELIVERY side —
  * for each authored/discovered video clip on the timeline, did the
@@ -258,7 +258,7 @@ export function assertVideoFrameCoverage(
  * composition is queryable in telemetry (the ts=1784144554 field signal
  * shape). Runtime `syncTimedElementVisibility` iterates the same set at
  * render time; counting statically here is a coarse proxy — dynamic
- * script-inserted `[data-start]` divs land in `hasRuntimeInsertedMedia`'s
+ * script-inserted `[data-start]` divs land in `hasRuntimeMediaChanges`'s
  * probe path (PR #2474), not this static scan.
  */
 export function countAuthoredTimedClips(html: string): number {
